Cell Biology: Centrifugation and Cell-Free Systems

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Questions and Answers

What is the term used to describe the living substance of plants and animals?

Nucleus

Protoplasm (correct)

Cytoplasm

Cellula

What is the function of centrifuging at high speeds (100 000 x g for 30 mins) in cellular biology?

To study DNA replication

To pellet ER, golgi apparatus and other membrane fragments (correct)

To isolate myofibrils from skeletal muscle cells

To analyze the molecular details of protein synthesis

Who is credited with introducing the term 'nucleus' in 1833?

R.Brown (correct)

Schleiden

Leeuwenhoek

Robert Hooke

What was isolated in 1949 from skeletal muscle cells?

Myofibrils Signup and view all the answers

What is the term used to describe the small chamber-like structures observed by Robert Hooke in 1665?

Cellula Signup and view all the answers

Who made the first microscopic observations of protozoa, bacteria, and sperm in 1674?

Leeuwenhoek Signup and view all the answers

What is the term for fractionated cell extracts that maintain a biological function?

Cell-free systems Signup and view all the answers

What was achieved in 1954 in the field of cell-free systems?

First cell-free system to carry out protein synthesis Signup and view all the answers

What is the fundamental unit of protoplasm that is capable of independent existence?

Cell Signup and view all the answers

Who developed the cell theory in 1838 and 1839?

Schleiden and Schwann Signup and view all the answers

What is the primary function of intercellular or extracellular substances in mammalian tissue?

To nourish and support cells Signup and view all the answers

What is the term for the basic building blocks of all living organisms?

Cells Signup and view all the answers

What is the purpose of centrifuging at very high speeds (300 000 x g for 3hrs) in cellular biology?

To pellet ribosomes Signup and view all the answers

What is the name of the book referenced in the content that is authored by RJ Epstein?

Human Molecular Biology Signup and view all the answers

What is the application of microdissection techniques in cell culture?

Isolating selected cells from tissue slices Signup and view all the answers

What is the significance of embryonic stem cells in medical research?

They can give rise to all cell types in the body Signup and view all the answers

What is the main advantage of using Escherichia coli in biological research?

It is the best understood cell in the world of biology Signup and view all the answers

What is the purpose of cell fusion in biological research?

To produce hybrid cells Signup and view all the answers

What is the significance of Saccharomyces cerevisiae in biological research?

It is the most primitive eukaryotic cell Signup and view all the answers

What is the application of cytogenetic research in medicine?

Diagnosing diseases caused by chromosomal abnormalities Signup and view all the answers

What is the potential application of embryonic stem cells in therapy?

All of the above Signup and view all the answers

What is the result of cell fusion?

Membrane merging and cytoplasmic mixing Signup and view all the answers

What is the primary purpose of centrifugation in cell fractionation?

To separate components that differ greatly in size Signup and view all the answers

What type of homogenization uses enzymes such as lysozyme?

Enzymatic Signup and view all the answers

What is the purpose of using a shallow gradient of sucrose in centrifugation?

To prevent convective mixing Signup and view all the answers

What type of homogenization uses detergents?

Chemical Signup and view all the answers

What is the term used to describe the movement of components in a centrifugation process?

Velocity sedimentation Signup and view all the answers

What type of instrument is used to achieve a finer degree of separation in centrifugation?

Ultracentrifuge Signup and view all the answers

What is the purpose of cell fractionation?

To study certain organelles from a cell Signup and view all the answers

How are cells typically disrupted in cell fractionation?

Using osmotic shock or ultrasonic vibration Signup and view all the answers

What is the result of cell disruption procedures if carefully applied?

Organelles such as nuclei and mitochondria are largely intact Signup and view all the answers

What is the term for the process of breaking up a cell and its components?

Homogenization Signup and view all the answers

What is the purpose of differential centrifugation in cell fractionation?

To separate components based on mass Signup and view all the answers

What is the final step in the subcellular fractionation process?

Collection of fractions Signup and view all the answers

What is the purpose of density gradient centrifugation in cell fractionation?

To separate components based on specific gravity Signup and view all the answers

What is the result of cell fractionation?

A separation of organelles and macromolecules Signup and view all the answers

What happens to the cell membranes during homogenization?

They are broken Signup and view all the answers

What is the purpose of low speed centrifugation in cellular biology?

To separate intact cells from medium Signup and view all the answers

What is the result of centrifuging a cell suspension at high speed?

The heaviest particles are sedimented first Signup and view all the answers

What is the purpose of using an ice cold isotonic buffer in the differential centrifugation method?

To stop enzyme reactions and osmosis Signup and view all the answers

Which organelle is pelleted first in the differential centrifugation method?

Nucleus Signup and view all the answers

What is the purpose of increasing the speed of centrifugation in the differential centrifugation method?

To separate particles of lower densities Signup and view all the answers

What is the result of ultracentrifugation in cellular biology?

The isolation of pure fractions of organelles and macromolecules Signup and view all the answers

What is the purpose of grinding tissue in a blender in the differential centrifugation method?

To break open cells Signup and view all the answers

What is the role of SRP receptor in protein synthesis?

To bind SRP and ribosome complex to the ER Signup and view all the answers

What happens to protein synthesis when SRP binds to the signal peptide and ribosome?

It is arrested Signup and view all the answers

What is the function of the translocon in protein synthesis?

To translocate the polypeptide chain into the ER lumen Signup and view all the answers

What happens to the signal peptide after it is released into the ER lumen?

It is cleaved from the polypeptide chain by signal peptidase Signup and view all the answers

What is the last step in protein synthesis after the polypeptide chain is translocated into the ER lumen?

The ribosome detaches from the ER membrane Signup and view all the answers

What is the purpose of post-translational modifications in the Rough ER?

To modify the polypeptide chain to its final form Signup and view all the answers

What is the role of the ribosomal large subunit during protein synthesis?

To attach to the translocon on the ER membrane Signup and view all the answers

What is the significance of the signal hypothesis proposed by Günter Blobel and David Sabatini?

It explains how proteins are targeted to the ER membrane Signup and view all the answers

What is the name of the hypothesis that explains the mechanism of protein synthesis in GER?

Signal Hypothesis Signup and view all the answers

What is the purpose of protein targeting in cells?

To deliver proteins to their correct subcellular location Signup and view all the answers

What is the process by which newly synthesized proteins are delivered to their correct subcellular location?

Protein targeting Signup and view all the answers

What is the Nobel Prize that Günter Blobel was awarded in 1999?

Nobel Prize in Physiology or Medicine Signup and view all the answers

What is the type of translocation that occurs when a protein is synthesized and transferred to the endoplasmic reticulum (ER) simultaneously?

Co-translational translocation Signup and view all the answers

What is the type of translocation that occurs when a protein is synthesized in the cytosol and later transported to its destination?

Post-translational translocation Signup and view all the answers

What is the result of incorrect protein targeting in cells?

Diseases can occur Signup and view all the answers

What is the receptor that synthesized proteins are transferred to during co-translational translocation?

SRP receptor Signup and view all the answers

What is the function of the endoplasmic reticulum in protein synthesis?

All of the above Signup and view all the answers

What is the characteristic of the surface of the Rough Endoplasmic Reticulum (RER)?

Studded with ribosomes Signup and view all the answers

What is the function of integrin in stem cell biology?

Promoting asymmetric cell division Signup and view all the answers

What is the term used to describe the network of folded membranes that extends from the nuclear envelope to the plasma membrane?

Endoplasmic reticulum Signup and view all the answers

What is the function of the sacs (cisternae) in the endoplasmic reticulum?

Folding of protein molecules Signup and view all the answers

What is the main difference between Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER)?

Presence of ribosomes Signup and view all the answers

What is the function of integrin-mediated mechanotransduction?

All of the above Signup and view all the answers

What is the significance of integrin a6/ITGA6/CD49f in stem cell biology?

It is a marker for cancer stem cells Signup and view all the answers

What is the term 'reticulum' in Endoplasmic Reticulum derived from?

Latin for 'little net' Signup and view all the answers

What is the primary function of the Rough Endoplasmic Reticulum?

Protein biosynthesis Signup and view all the answers

What is the characteristic of the surface of the Rough Endoplasmic Reticulum?

Studded with ribosomes Signup and view all the answers

What is the function of the Endoplasmic Reticulum in cellular transport?

Transport of synthesized proteins in vesicles to the Golgi apparatus Signup and view all the answers

What can happen to the quantity of Rough Endoplasmic Reticulum and Smooth Endoplasmic Reticulum in a cell?

It can interchange from one type to the other, depending on changing metabolic needs Signup and view all the answers

What is the function of the Endoplasmic Reticulum in protein synthesis?

Folding of protein molecules in sacs called cisternae Signup and view all the answers

What is the structure of the Endoplasmic Reticulum?

A network of branching tubules and sacs Signup and view all the answers

What is the function of the Endoplasmic Reticulum in lipid biosynthesis?

Synthesis of phospholipids from fatty acids and glycerol phosphate Signup and view all the answers

What is the name of the structures that can be detected in light microscope by staining with basic dyes?

Nissl bodies Signup and view all the answers

What is the appearance of GER in electron microscope?

Parallel membrane limited flattened sacs or cisternae Signup and view all the answers

Which of the following cells is well developed in GER?

All of the above Signup and view all the answers

What is the function of GER in protein biosynthesis?

Transportation of proteins Signup and view all the answers

What is the relation between the membranes of the ER and the nuclear envelope?

They are connected Signup and view all the answers

In which cell type is the GER well developed for producing digestive enzymes?

Aciner cells Signup and view all the answers

What is the role of GER in protein secretion?

Transportation of proteins Signup and view all the answers

What is the structure of the ER membrane in a pancreatic exocrine cell?

Closely packed sheets of ER membrane Signup and view all the answers

What is the term used to describe the small, closed vesicles that are formed from the ER when a tissue is disrupted by homogenization?

Microsomes Signup and view all the answers

What is the main function of the Smooth ER region that lacks bound ribosomes?

Lipid synthesis and transport Signup and view all the answers

Where does the synthesis of fatty acids and phospholipids occur in the ER?

Smooth ER Signup and view all the answers

What is the role of bile salts in the digestion of fats in the small intestine?

Emulsifying fats, breaking them down into smaller droplets Signup and view all the answers

What is the term used to describe the regions of the ER that have bound ribosomes?

Rough ER Signup and view all the answers

What is the function of the enzyme acyl-CoA synthetase in the smooth ER?

Converting fatty acids into acyl-CoA Signup and view all the answers

What is the role of the Golgi Complex in the transport of lipids?

Packaging lipids into lipoprotein particles Signup and view all the answers

What is the function of SER in the contraction process of muscle cells?

Regulation of muscle contraction Signup and view all the answers

What is the main function of Smooth ER in liver cells?

It is involved in the metabolic reactions Signup and view all the answers

What is the role of Sarcoplasmic Reticulum in muscle cells?

It is involved in the regulation of calcium ions Signup and view all the answers

What is the function of SER in neuronal synapse?

It is involved in the regulation of calcium ions Signup and view all the answers

What is the function of Granular ER?

It is involved in the synthesis of proteins Signup and view all the answers

What is the function of Rough ER?

It is involved in the synthesis of proteins Signup and view all the answers

What is the main function of SER in the liver cells?

It is involved in the metabolic reactions Signup and view all the answers

What is the function of SER in muscle cells?

It is involved in the regulation of calcium ions Signup and view all the answers

What is the function of SER in neuronal synapse?

It is involved in the regulation of calcium ions Signup and view all the answers

What is the term used to describe the small closed vesicles that form from the ER during homogenization?

Microsomes Signup and view all the answers

What is the main function of Smooth Endoplasmic Reticulum (SER) in lipid metabolism?

Lipid biosynthesis and transport Signup and view all the answers

Where are the enzymes that synthesize the lipid component of lipids located?

SER membrane Signup and view all the answers

What is the purpose of pancreatic lipase in the small intestine?

To break down dietary lipids into fatty acids and monoglycerides Signup and view all the answers

What is the role of bile salts in the small intestine?

To emulsify dietary fats into smaller droplets Signup and view all the answers

What is the function of Acyl-CoA synthetase and acyltransferases in the SER?

To synthesize lipids Signup and view all the answers

What is the term used to describe the regions of ER that lack bound ribosomes?

Smooth Endoplasmic Reticulum (SER) Signup and view all the answers

What is the function of the SER in the liver?

Lipid biosynthesis and transport Signup and view all the answers

In which type of cells is the GER particularly well developed?

Protein secreting cells Signup and view all the answers

What appears as parallel membrane-limited flattened sacs or cisternae in an electron microscope?

GER Signup and view all the answers

What is the role of the GER in protein synthesis?

Transportation of proteins within the cell Signup and view all the answers

What is the name of the structures stained with basic dyes in light microscopy?

Nissl bodies Signup and view all the answers

Which type of cells have closely packed sheets of ER membrane studded with ribosomes?

Pancreatic exocrine cells Signup and view all the answers

What is the function of the GER in protein synthesis?

Transportation of proteins Signup and view all the answers

What is the relation between the GER and the nuclear envelope?

The membranes of the GER are continuous with the outer membrane of the nuclear envelope Signup and view all the answers

What type of cells have a high demand for protein synthesis and secretion?

Protein secreting cells Signup and view all the answers

Who formulated the signal hypothesis in 1971?

Günter Blobel and David Sabatini Signup and view all the answers

What is the term used to describe the phenomenon where newly synthesized proteins must be delivered to a specific subcellular location?

Protein Targeting Signup and view all the answers

What is the name of the complex where the nascent protein is inserted during co-translational translocation?

Translocon Signup and view all the answers

What is the purpose of protein targeting?

To ensure correct protein activity Signup and view all the answers

What is the type of translocation where proteins are translated in the cytosol and later transported to their destination?

Post-Translational Translocation Signup and view all the answers

What is the importance of correct sorting in the cell?

It is crucial for correct protein activity Signup and view all the answers

What is the name of the receptor on the endoplasmic reticulum where the nascent protein is transferred during co-translational translocation?

SRP Receptor Signup and view all the answers

What was awarded to Günter Blobel in 1999?

Nobel Prize in Physiology or Medicine Signup and view all the answers

What is the role of BiP in protein synthesis?

To assist in protein folding Signup and view all the answers

Where are disulfide bonds formed in eukaryotic cells?

In the ER lumen Signup and view all the answers

What is the function of calnexin and calreticulin in protein synthesis?

To assist in protein folding Signup and view all the answers

What is the purpose of oligosaccharyl transferases in protein synthesis?

To transfer oligosaccharides to the polypeptide chain Signup and view all the answers

What is the result of the formation of disulfide bonds in protein synthesis?

The tertiary and quaternary structure of the protein is stabilized Signup and view all the answers

What is the function of protein disulfide isomerase in protein synthesis?

To catalyze the rearrangement of disulfide bonds Signup and view all the answers

What type of proteins are synthesized in cells with three different types?

Proteins that are secreted from the cell, lysosomal enzymes, and plasma membrane glycoproteins Signup and view all the answers

What is the role of molecular labels in cells?

To 'address' proteins for delivery to specific locations Signup and view all the answers

What is the characteristic feature of targeting pathways, except for cytosolic and nuclear proteins?

Presence of a signal sequence Signup and view all the answers

Where are proteins with signal sequences synthesized?

On ribosomes attached to the ER membrane Signup and view all the answers

What is the purpose of targeting signals in cells?

To enable the cellular transport machinery to correctly position a protein inside or outside the cell Signup and view all the answers

What is co-translational targeting associated with?

ER, Golgi, lysosomes, and plasma membrane Signup and view all the answers

What is post-translational targeting associated with?

Nucleus, mitochondria, and peroxisomes Signup and view all the answers

Who demonstrated that proteins with signal sequences are synthesized on ribosomes attached to the ER membrane?

George Palade Signup and view all the answers

What is the term for the process of fertilization outside the human body?

In vitro fertilization Signup and view all the answers

What is the term for the surface of epithelial cells that faces the lumen?

Apical surface Signup and view all the answers

What is the function of lateral surface modifications?

To increase the cell surface for absorption Signup and view all the answers

Where are cell-cell junctions found?

On the lateral surface of epithelial cells Signup and view all the answers

What is the function of microvilli?

Increase the surface area for absorption Signup and view all the answers

What is the function of tight junctions?

To form barriers that prevent the free passage of substances Signup and view all the answers

What is the term for the extensions of the cell surface?

Microvilli Signup and view all the answers

What is the location of the microvilli in the epithelial cells?

Apical surface Signup and view all the answers

What is the function of adherens junctions?

To attach cells to each other Signup and view all the answers

What is the diameter of microvilli?

0.1 μm Signup and view all the answers

What is the function of gap junctions?

To facilitate cell-to-cell communication Signup and view all the answers

Where are microvilli found?

On the apical surface of epithelial cells Signup and view all the answers

What is the structure that microvilli contain?

A bundle of actin filaments Signup and view all the answers

What is the term for the border of vertical striations seen in the light microscope?

Striated border Signup and view all the answers

What is the function of desmosomes?

To attach cells to each other Signup and view all the answers

What is the function of hemidesmosomes?

To attach cells to the basement membrane Signup and view all the answers

Where does the addition of carbohydrate chains to proteins start?

Endoplasmic Reticulum (ER) Signup and view all the answers

What is the name of the lipid molecule that holds oligosaccharide chains in the ER membrane?

Dolichol Signup and view all the answers

What type of bond is formed between the oligosaccharide chain and the polypeptide?

Covalent bond Signup and view all the answers

Where is the signal peptide removed from the polypeptide?

Endoplasmic Reticulum (ER) Signup and view all the answers

What is the name of the enzyme that removes the signal peptide from the polypeptide?

Signal peptidase Signup and view all the answers

What type of protein is formed after the signal peptide is removed?

Multimeric protein Signup and view all the answers

What is the primary function of the extracellular matrix in animal tissues?

to provide structural integrity and mechanical strength Signup and view all the answers

What is the name of the thin, sheetlike structure that supports layers of epithelial cells?

Basal lamina Signup and view all the answers

What type of protein is the most abundant in the extracellular matrix?

Collagen Signup and view all the answers

What is the function of proteoglycans in the extracellular matrix?

to cushion cells and bind extracellular molecules Signup and view all the answers

What is the name of the polysaccharide-rich coat on the plasma membrane of endothelial cells or intestinal epithelial cells?

Glycocalyx Signup and view all the answers

What is the main component of tendons in the extracellular matrix?

Fibrous proteins Signup and view all the answers

What is the function of soluble multi-adhesive matrix proteins in the extracellular matrix?

to bind to and cross-link adhesion receptors and ECM components Signup and view all the answers

What is the term used to describe the ensemble of proteins that compose the ECM and associated proteins?

Matrisome Signup and view all the answers

What is the Extracellular Matrix (ECM) composed of?

A variety of specialized structures Signup and view all the answers

What are the two components of all tissues?

Cellular and extracellular components Signup and view all the answers

What type of adhesion is mediated by selectins?

Transient cell-cell adhesions Signup and view all the answers

What is the binding of selectins dependent on?

Calcium ions Signup and view all the answers

What is the function of L-selectin?

Mediating lymphocyte binding to endothelium in lymph nodes Signup and view all the answers

What type of proteins are selectins?

Lectins Signup and view all the answers

What is the name of the ligand for selectin?

Sialyl-Lewis X saccharides Signup and view all the answers

What is the role of selectins in the body?

Initiating the adhesion of leukocytes and platelets during inflammatory and hemostatic responses Signup and view all the answers

What is the primary function of the extracellular matrix in the dermis?

To provide great strength and elasticity to dissipate the stresses impinging on the skin Signup and view all the answers

What is the main characteristic of extracellular matrices in adult organisms?

They exhibit slow turnover and are permanent or semipermanent in nature Signup and view all the answers

What is the role of the extracellular matrix in cell signaling?

It provides the basis for signals transmitted to cells by adhesion receptors Signup and view all the answers

What is the major structural protein of the extracellular matrix?

Collagen Signup and view all the answers

What is the function of glycosaminoglycans (GAGs) in the extracellular matrix?

To absorb water and resist compression Signup and view all the answers

What is the characteristic of glycosaminoglycans (GAGs) that allows them to absorb water and resist compression?

They are strongly negatively charged Signup and view all the answers

What is the function of the basement membrane in the extracellular matrix?

To provide a thin supporting layer for cell attachment Signup and view all the answers

What is the role of the extracellular matrix in the healing of wounds?

It needs to respond to changes such as injury for the healing of wounds Signup and view all the answers

What is the location of the basal lamina in epithelial cells?

Basal surface Signup and view all the answers

What is the function of the basal lamina in epithelial cells?

To attach to the connective tissue Signup and view all the answers

What is the relationship between the basal lamina and the epithelial cells?

The epithelial cells are attached to the basal lamina Signup and view all the answers

What is the main component of the basal lamina?

Laminin Signup and view all the answers

What is the function of the hemidesmosomes in epithelial cells?

To attach to the basal lamina Signup and view all the answers

What is the location of the lamina propria in relation to the epithelial cells?

Below the epithelial cells Signup and view all the answers

What is the function of the cell-cell junctions in epithelial cells?

To form barriers that prevent the free passage of substances Signup and view all the answers

What is the name of the type of junction that is located just below the apical surface of epithelial cells?

Tight junction Signup and view all the answers

What is the thickness of the basal lamina?

60-120 nm Signup and view all the answers

What is the function of basal lamina in embryonic development?

to help four- and eight-celled embryos adhere together Signup and view all the answers

What is the term used to describe the thin, sheetlike structure that epithelial cells rest on?

Basal Lamina Signup and view all the answers

In which type of tissue does the basal lamina surround each cell?

muscle tissue Signup and view all the answers

What is the role of basal lamina in tissue regeneration?

to provide a foundation for tissue assembly Signup and view all the answers

What type of proteins are found in the basal lamina?

Adhesion proteins Signup and view all the answers

What is the main function of proteoglycans in the ECM?

Cushion cells and bind ECM molecules Signup and view all the answers

What type of cells migrate along ECM pathways containing basal laminal components during nervous system development?

neurons Signup and view all the answers

What is the term used to describe the ensemble of proteins that compose the ECM and associated proteins?

Matrisome Signup and view all the answers

What is the shape of the basal lamina?

sheet-like Signup and view all the answers

What is the main component of the ECM in bone?

Calcium phosphate crystals Signup and view all the answers

In which type of epithelia does the basal lamina provide a foundation for only one surface of the cells?

columnar epithelia Signup and view all the answers

What is the function of soluble multi-adhesive matrix proteins?

Bind to and cross-link adhesion receptors and other ECM components Signup and view all the answers

What is the role of basal lamina in tissue assembly?

to provide a foundation for tissue assembly Signup and view all the answers

What is the most abundant protein in the extracellular matrix?

Collagen Signup and view all the answers

What is the main function of collagen fibers in the ECM?

Provide structural integrity, mechanical strength, and resilience Signup and view all the answers

What is the function of cell-cell junctions in epithelial cells?

To form barriers that prevent the free passage of substances and cells Signup and view all the answers

Where are cell-cell junctions found in epithelial cells?

On the lateral surface Signup and view all the answers

What is the function of tight junctions?

Barriers that prevent the free passage of substances and cells Signup and view all the answers

What is the name of the junctions that allow direct cell-to-cell communication?

Gap junctions Signup and view all the answers

What is the name of the junctions that provide cell-to-cell attachment?

Adherens junctions Signup and view all the answers

Where is the tight junction located in epithelial cells?

Below the apical surface Signup and view all the answers

What is the term for the complex formed by multiple cell-cell junctions?

Junctional complex Signup and view all the answers

What is the purpose of epithelial cells showing apical-luminal-free surface polar differentiation?

To form protective layers Signup and view all the answers

What is the role of Cx43, Cx46, and Cx50/α8 in the human body?

They are responsible for the formation of cataracts and heart malformations Signup and view all the answers

What is the primary function of anchoring junctions?

To provide mechanical strength to tissues Signup and view all the answers

What is the name of the skin disease caused by autoantibodies attacking type XVII collagen?

Bullous pemphigoid Signup and view all the answers

What is the function of integrins in focal adhesions?

To bind to laminin protein in the basal lamina Signup and view all the answers

What is the name of the anchor protein that binds to keratin intermediate filaments in hemidesmosomes?

Plectin Signup and view all the answers

What is the function of hemidesmosomes?

To provide mechanical strength to epithelial cells Signup and view all the answers

What is the type of junction that allows for the free exchange of substances between cells?

Gap junction Signup and view all the answers

What is the type of junction that anchors cells to the extracellular matrix?

Focal adhesion Signup and view all the answers

What is the primary function of tight junctions in cell-cell junctions?

To separate the apical and basolateral domains of the plasma membrane Signup and view all the answers

What is the characteristic of tight junctions in terms of the plasma membranes of adjacent cells?

They come together at periodic intervals Signup and view all the answers

What is the location of tight junctions in relation to the apical surface of the cell?

Just below the apical surface Signup and view all the answers

What is the structure of tight junctions, as observed by electron microscopy?

An anastomosing network of protein strands Signup and view all the answers

What is the purpose of tight junctions in preventing the passage of molecules?

To prevent the passage of molecules from the lumen to the blood circulation Signup and view all the answers

What is the result of tight junctions in terms of the intercellular space?

A very narrow intercellular space Signup and view all the answers

What is the type of connection formed by tight junctions between adjacent cells?

Focal connections Signup and view all the answers

What is the characteristic of tight junctions in terms of their distribution around the cell?

They are continuous around the entire periphery of the cell Signup and view all the answers

What is the primary function of cell-cell junctions?

To form barriers that prevent the free passage of substances and cells Signup and view all the answers

What type of junction allows free interchange of substances?

Gap Junctions Signup and view all the answers

What type of filaments are anchored by Hemidesmosomes?

Intermediate Filaments Signup and view all the answers

What type of junction is responsible for cell to cell communication?

Gap junctions Signup and view all the answers

What is the function of Integrins in Focal Adhesions?

Binding to Laminin in the Basal Lamina Signup and view all the answers

Where are tight junctions typically located?

Just below the apical surface Signup and view all the answers

What is the result of mutations in Plectin?

Skin Blisters Signup and view all the answers

What is the term for the process by which cells are bound to one another?

Cell-cell junctions Signup and view all the answers

What is the site of firm adhesion of cells?

Anchoring Junctions Signup and view all the answers

Where are cell-cell junctions typically found?

On the lateral surface of epithelial cells Signup and view all the answers

What is the term for junctions that anchor cells to the basement membrane?

Hemidesmosomes Signup and view all the answers

What is the function of Plectin in Hemidesmosomes?

Binding to Keratin Intermediate Filaments Signup and view all the answers

What type of junctions can be subclassified according to the cytoskeletal element involved?

Anchoring Junctions Signup and view all the answers

What is the function of desmosomes?

To hold cells together Signup and view all the answers

What is the term for the complex of adhesion molecules and other proteins involved in cell-cell junctions?

Junctional complex Signup and view all the answers

What is the main function of tight junctions in cell membranes?

To separate the apical and basolateral domains of the plasma membrane Signup and view all the answers

What is the characteristic of tight junctions in electron microscopy?

Membranes of adjacent cells come together at periodic intervals Signup and view all the answers

What is the location of tight junctions in epithelial cells?

Just below the apical surface of the cell Signup and view all the answers

What is the composition of tight junction strands?

Transmembrane proteins only Signup and view all the answers

What is the purpose of tight junctions in preventing the passage of molecules?

To prevent the passage of ions and molecules from the lumen to the blood circulation Signup and view all the answers

What is the term used to describe the contacts between adjacent cells?

Tight junctions Signup and view all the answers

What is the result of the linkage of rows of protein particles in tight junctions?

Formation of an anastomosing network of protein strands Signup and view all the answers

What is the characteristic of the intercellular space at the level of tight junctions?

Very narrow Signup and view all the answers

What is the name of the hypothesis that explains protein targeting in cells?

Signal Hypothesis Signup and view all the answers

What is the process by which proteins are delivered to a specific subcellular location or exported from the cell?

Protein Targeting Signup and view all the answers

Who is credited with the discovery of the signal hypothesis?

Günter Blobel and David Sabatini Signup and view all the answers

What is the term for the process by which proteins are synthesized and transported to the endoplasmic reticulum?

Co-translational Translocation Signup and view all the answers

What is the purpose of the signal peptide in protein synthesis?

To target proteins to specific subcellular locations Signup and view all the answers

What happens to the ribosome when the signal peptide emerges from it?

It is transported to the ER Signup and view all the answers

What is the function of the SRP receptor in protein synthesis?

To transport the ribosome to the ER Signup and view all the answers

What is the role of the SRP receptor in protein synthesis?

To bind to the signal peptide and ribosome Signup and view all the answers

What is the result of incorrect protein targeting in cells?

Diseases Signup and view all the answers

What is the purpose of the signal sequence in protein synthesis?

To transport the polypeptide into the ER lumen Signup and view all the answers

What is the result of the interaction between the signal sequence and the ER membrane?

A channel is opened in the ER membrane for protein transport Signup and view all the answers

What is the purpose of post-translational translocation?

To transport proteins to the ER Signup and view all the answers

What is the composition of the Signal Recognition Particle (SRP)?

6 proteins and 7S RNA molecule Signup and view all the answers

What is the length of the signal peptide in amino acids?

16-20 amino acids Signup and view all the answers

What is the role of the hydrophobicity of the signal sequence in protein synthesis?

It interacts with the ER membrane Signup and view all the answers

Where is the protein synthesized in the absence of a signal sequence?

On free ribosomes in the cytoplasm Signup and view all the answers

What is the role of SRP receptor in protein synthesis?

To bind to the signal peptide and ribosome, and attach to the ER at specific sites Signup and view all the answers

What happens to protein synthesis when SRP binds to the signal peptide and ribosome?

It is arrested temporarily Signup and view all the answers

What is the function of the translocon in protein synthesis?

To translocate the polypeptide chain into the ER lumen Signup and view all the answers

What happens to the signal peptide after it is released into the ER lumen?

It is cleaved from the polypeptide chain by signal peptidase Signup and view all the answers

What is the last step in protein synthesis after the polypeptide chain is translocated into the ER lumen?

Detachment of the ribosome from the ER membrane Signup and view all the answers

What is the purpose of post-translational modifications in the Rough ER?

To make the polypeptide chain more stable and functional Signup and view all the answers

What happens to SRP after it releases the ribosome from the ER membrane?

It returns to the cytosol and participates in other rounds of protein synthesis Signup and view all the answers

What is the significance of the signal peptide in protein synthesis?

It targets the polypeptide chain to the ER Signup and view all the answers

What is the role of BiP in protein folding?

To assist protein folding in the ER Signup and view all the answers

What is the function of protein disulfide isomerase?

To catalyze the formation of disulfide bonds Signup and view all the answers

Where do disulfide bonds form in eukaryotic cells?

In the ER lumen Signup and view all the answers

What is the role of Calnexin and Calreticulin in protein folding?

To assist protein folding as lectins Signup and view all the answers

What is the primary function of oligosaccharyl transferases in protein synthesis?

To add carbohydrates to proteins Signup and view all the answers

What determines the unique folding pathway of a polypeptide chain?

The sequence of the polypeptide chain Signup and view all the answers

What is the purpose of the KDEL receptors in the Golgi apparatus?

To trap and return ER resident proteins to the ER Signup and view all the answers

How do proteins travel from the ER to the Golgi complex?

In transport vesicles that bud from the ER and fuse with the Golgi complex Signup and view all the answers

What is the function of the Golgi complex in protein synthesis?

To modify and sort proteins Signup and view all the answers

What is the purpose of post-translational modifications in the Golgi complex?

To modify and sort proteins Signup and view all the answers

What is the role of ATP in protein insertion into the ER?

To provide energy for protein translocation Signup and view all the answers

What is the topology of integral membrane proteins synthesized on the rough ER?

Multi-pass transmembrane proteins Signup and view all the answers

What is the function of the signal peptide in protein synthesis?

To provide a binding site for the SRP receptor Signup and view all the answers

What is the difference between co-translational and post-translational protein insertion?

Co-translational insertion occurs during protein synthesis, while post-translational insertion occurs after protein synthesis Signup and view all the answers

What is the term used to describe the process of delivering proteins to a specific subcellular location or exporting them from the cell?

Protein targeting Signup and view all the answers

According to the signal hypothesis, what determines the destination of a protein in the cell?

Information contained in the protein itself Signup and view all the answers

What is the name of the Nobel Prize winner who discovered that proteins have intrinsic signals that govern their transport and localization in the cell?

Günter Blobel Signup and view all the answers

What is the receptor that the synthesized protein is transferred to during co-translational translocation?

SRP receptor Signup and view all the answers

What is the process by which some proteins are translated in the cytosol and later transported to their destination?

Post-translational translocation Signup and view all the answers

What is the result of errors in protein targeting?

Diseases Signup and view all the answers

What is the location where the nascent protein is inserted into the translocation complex during co-translational translocation?

Endoplasmic reticulum Signup and view all the answers

What is the purpose of protein targeting?

To deliver proteins to a specific subcellular location or export them from the cell Signup and view all the answers

What is the primary site of lipid biosynthesis in the cell?

SER Signup and view all the answers

What is the function of acyl-CoA synthetase and acyltransferases in lipid transport?

Synthesis of lipids Signup and view all the answers

What is the result of pancreatic lipase action on dietary lipids in the small intestine?

Fatty acid and monoglyceride formation Signup and view all the answers

What is the role of bile salts in lipid digestion?

Emulsification of lipids Signup and view all the answers

What is the function of microsomes in cellular biology?

Studying ER functions in vitro Signup and view all the answers

What is the shape of the Smooth Endoplasmic Reticulum (SER)?

Tubular or vesicular Signup and view all the answers

What is the function of SER in muscle cells?

Contraction process Signup and view all the answers

What is the origin of SER membranes?

GER Signup and view all the answers

What is the result of complete digestion of a triglyceride?

Fatty acid and monoglycerol molecules Signup and view all the answers

Where are the enzymes required for steroid synthesis abundant?

Leydig's cells of testes and adrenal cortex cells Signup and view all the answers

What is the function of Smooth ER in the liver cells?

It contains enzymes to detoxify drugs, alcohol, and toxic chemicals Signup and view all the answers

What is the role of CYP enzymes in the liver cells?

They catalyze the oxidation of organic substances Signup and view all the answers

What is the function of SER in the liver cells?

It contains enzymes to detoxify drugs, alcohol, and toxic chemicals Signup and view all the answers

What is the result of the detoxification process in the liver cells?

The conversion of toxic substances to safer water-soluble products Signup and view all the answers

What is the structure of GER in an electron microscope?

Parallel membrane limited flattened sacs or cisternae Signup and view all the answers

What is the function of GER in protein synthesis?

Biosynthesis of proteins Signup and view all the answers

Which type of cells have well-developed GER?

Protein secreting cells Signup and view all the answers

What is the composition of Nissl bodies?

GER + Ribosomes Signup and view all the answers

What is the location of GER in relation to the nuclear envelope?

Continuous with the outer membrane of the nuclear envelope Signup and view all the answers

What is the function of GER in the cells of exocrine pancreas?

Secretion of digestive enzymes Signup and view all the answers

What is the name of the cell that manufactures serum albumin and fibrinogen?

Hepatocyte Signup and view all the answers

What is the role of GER in the cells of connective tissue?

Synthesis of collagen and elastin Signup and view all the answers

What can be detected by staining with basic dyes in a light microscope?

Nissl bodies Signup and view all the answers

What is true about the membranes of the ER?

They are continuous with the outer membrane of the nuclear envelope Signup and view all the answers

In which type of cells is GER particularly well developed?

Protein secreting cells Signup and view all the answers

What is the function of GER in cells?

Protein biosynthesis and transportation Signup and view all the answers

What is found in the cytosol of a pancreatic exocrine cell?

Closely packed sheets of ER membrane studded with ribosomes Signup and view all the answers

What is the name of the cell that manufactures serum albumin, fibrinogen, and the prothrombin group of clotting factors?

Hepatocyte Signup and view all the answers

What is true about the structure of GER in an electron microscope?

It appears as parallel membrane limited flattened sacs or cisternae Signup and view all the answers

What type of cells produce and secrete large amounts of digestive enzymes every day?

Pancreatic exocrine cells Signup and view all the answers

What is the function of integrin in stem cell division?

Promoting asymmetric cell division Signup and view all the answers

What is the Endoplasmic Reticulum (ER)?

A netlike labyrinth of branching tubules and sacs that extends from the nucleus to the plasma membrane Signup and view all the answers

What is the function of the Rough Endoplasmic Reticulum (RER)?

Protein synthesis and transport Signup and view all the answers

What is the difference between Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER)?

RER has ribosomes attached, while SER does not Signup and view all the answers

What is the function of integrin-mediated mechanotransduction?

Sensing biophysical cues from the ECM and modulating cellular responses Signup and view all the answers

What is the role of laminin in stem cell division?

Promoting asymmetric cell division Signup and view all the answers

What is the function of the Endoplasmic Reticulum (ER) in protein synthesis?

Folding and transporting proteins Signup and view all the answers

What is the structure of the Endoplasmic Reticulum (ER)?

A single continuous membrane with separate sacs and tubules Signup and view all the answers

What is the primary function of tight junctions?

Separates the apical and basolateral domains of the plasma membrane Signup and view all the answers

What is the correct description of tight junctions?

Focal connections between adjacent cells Signup and view all the answers

What is the structure of tight junctions?

Anastomosing network of protein strands Signup and view all the answers

What is the result of the linkage of rows of protein particles?

Formation of a tight junction Signup and view all the answers

What is the characteristic of the intercellular space at the level of tight junctions?

Very narrow Signup and view all the answers

What is the location of tight junctions?

Just below the apical surface Signup and view all the answers

What is the function of tight junctions in terms of membrane components?

Prevents the free diffusion of membrane proteins Signup and view all the answers

What is the correct description of the structure of tight junctions as seen in EM micrographs?

Membranes of adjacent cells come together at periodic intervals Signup and view all the answers

What is the term used to describe the dysfunction of ER functions?

ER stress Signup and view all the answers

What is the result of cells experiencing ER stress?

Apoptosis Signup and view all the answers

What is the unfolded protein response (UPR) activated in response to?

Accumulation of unfolded or misfolded proteins Signup and view all the answers

What are the four functionally distinct unfolded protein responses (UPR)?

PERK, IRE1, ATF6, and ERN1 Signup and view all the answers

What is the term used to describe the condition in which the ER is unable to properly fold proteins?

ER stress Signup and view all the answers

What is the purpose of the unfolded protein response (UPR)?

To protect the cell against ER stress Signup and view all the answers

What is the name of the hypothesis that explains the mechanism of protein synthesis in GER?

Signal Hypothesis Signup and view all the answers

Who was awarded the 1999 Nobel Prize in Physiology for the discovery of intrinsic signals that govern protein transport and localization?

Günter Blobel Signup and view all the answers

What is the term used to describe the phenomenon of delivering proteins to a specific subcellular location or exporting them from the cell?

Protein targeting Signup and view all the answers

What is the correct sorting of proteins crucial for?

The cell Signup and view all the answers

What is the process called where synthesized protein is transferred to an SRP receptor on the endoplasmic reticulum (ER)?

Co-translational translocation Signup and view all the answers

What is the purpose of the translocation complex?

To insert nascent proteins into the ER Signup and view all the answers

What type of translocation occurs when proteins are translated in the cytosol and later transported to their destination?

Post-translational translocation Signup and view all the answers

What is the receptor that the synthesized protein is transferred to during co-translational translocation?

SRP receptor Signup and view all the answers

Which protein assists protein folding in the ER?

BIP Signup and view all the answers

What is the purpose of disulfide bonds in proteins?

To stabilize the tertiary and quaternary structure of proteins Signup and view all the answers

Where are disulfide bonds formed in eukaryotic cells?

In the GER Signup and view all the answers

What is the role of Calnexin and Calreticulin in the ER?

To assist protein folding Signup and view all the answers

Which enzyme catalyzes the rearrangement of disulfide bonds in the ER?

Disulfide isomerase Signup and view all the answers

What is the result of the Sec61 channel and BiP in the ER?

Protein folding is facilitated Signup and view all the answers

What promotes cell fusion in biological research?

All of the above Signup and view all the answers

What is a result of cell fusion in human-rodent hybrid cells?

Progressive loss of human chromosomes Signup and view all the answers

What is the result of ER stress in mammalian cells?

Apoptosis Signup and view all the answers

What is an application of cell fusion in genetic analysis?

To isolate a single human chromosome Signup and view all the answers

What is the term used to describe the initial stage of cell fusion?

Heterokaryon Signup and view all the answers

What is the term used to describe the mechanism that protects cells from ER stress?

Unfolded protein response (UPR) Signup and view all the answers

What is the purpose of treating cells with polyethylene glycol or inactivated Sendai virus?

To induce cell fusion Signup and view all the answers

What is one of the conditions that can cause ER stress?

Decreased glucose levels Signup and view all the answers

What is the outcome of cell fusion in human-mouse hybrid cells?

Human chromosomes are lost, mouse chromosomes are retained Signup and view all the answers

What is the result of the accumulation of unfolded or misfolded proteins in the ER lumen?

Activation of the UPR Signup and view all the answers

What is associated with ER stress-induced apoptosis?

Ischemia and diabetes Signup and view all the answers

What is a type of mutation that can cause ER stress?

Genetic mutation Signup and view all the answers

What is the main advantage of using Escherichia coli in biological research?

It is the best understood cell in the world of biology. Signup and view all the answers

What can embryonic stem cells give rise to?

All cell types in the body. Signup and view all the answers

What is the result of cell fusion?

Membrane merging and cytoplasmic mixing of two cell types. Signup and view all the answers

What is the application of cytogenetic research in medicine?

To diagnose diseases caused by abnormalities of chromosomes. Signup and view all the answers

What can be cultured using microdissection techniques?

Embryonic organs. Signup and view all the answers

What is the significance of Saccharomyces cerevisiae in biological research?

It is the most well-studied yeast. Signup and view all the answers

What is the potential application of embryonic stem cells in therapy?

To replace and repair damaged mature human tissues. Signup and view all the answers

What can be produced using hybrid cells?

Specialized cells for therapy. Signup and view all the answers

What happens to the signal peptide after it is recognized by the signal recognition particle?

It is attached to the SRP receptor Signup and view all the answers

What is the function of the signal sequence in protein synthesis?

To target the protein to the ER Signup and view all the answers

What is the role of the SRP receptor in protein synthesis?

To anchor the ribosome to the ER membrane Signup and view all the answers

What is the result of the interaction between the signal sequence and the ER membrane?

A channel is opened in the ER membrane for protein translocation Signup and view all the answers

What happens to the mRNA if it lacks a signal sequence?

It is translated on free ribosomes in the cytoplasm Signup and view all the answers

What is the composition of the signal recognition particle (SRP)?

6 proteins and 7S RNA Signup and view all the answers

What is the length of the signal peptide?

16-20 amino acids Signup and view all the answers

What is the function of the ribosome receptor in protein synthesis?

To anchor the ribosome to the ER membrane Signup and view all the answers

What is the enzyme responsible for removing the signal peptide from the polypeptide?

Signal peptidase Signup and view all the answers

Where does the addition of carbohydrate chains to the polypeptide start?

Endoplasmic reticulum Signup and view all the answers

What is the molecule responsible for holding the precursor oligosaccharide chains in the ER membrane?

Dolichol Signup and view all the answers

What is the type of bond formed between the carbohydrate chains and the polypeptide?

Covalent bond Signup and view all the answers

What is the sequence recognized by the enzyme responsible for transferring the carbohydrate chains to the polypeptide?

Asparagine-X-serin/threonin Signup and view all the answers

Where is the final processing of the carbohydrate chains completed?

Golgi complex Signup and view all the answers

What is the role of the SRP receptor in protein synthesis?

To attach the ribosome to the ER membrane Signup and view all the answers

What happens to protein synthesis when SRP binds to the signal peptide and ribosome?

It is inhibited Signup and view all the answers

What is the function of the translocon in protein synthesis?

To translocate the polypeptide chain into the ER lumen Signup and view all the answers

Where is the signal peptide cleaved from the polypeptide chain?

In the ER lumen Signup and view all the answers

What is the last step in protein synthesis after the polypeptide chain is translocated into the ER lumen?

The ribosome detaches from the ER membrane Signup and view all the answers

What is the purpose of post-translational modifications in the Rough ER?

To modify the protein before it reaches its final destination Signup and view all the answers

What happens to SRP when it binds to the docking protein?

It is released from the ribosome Signup and view all the answers

What is the name of the protein complex that helps to translocate the polypeptide chain into the ER lumen?

Sec61p complex Signup and view all the answers

What enzyme is responsible for removing the signal peptide from the polypeptide?

Signal peptidase Signup and view all the answers

Where does the addition of carbohydrate chains to proteins start?

Endoplasmic Reticulum (ER) Signup and view all the answers

What is the purpose of dolichol in glycosylation?

To import precursor oligosaccharides into the ER or Golgi lumen Signup and view all the answers

What type of bonds are formed between carbohydrates and proteins during glycosylation?

Covalent bonds Signup and view all the answers

Where is the final step of glycosylation completed?

Golgi Complex Signup and view all the answers

What is the sequence recognized by the enzyme that transfers oligosaccharide chains to proteins?

Asparagine-X-serine/threonine Signup and view all the answers

What happens to protein synthesis when SRP binds to the signal peptide and ribosome?

It is arrested Signup and view all the answers

What is the function of the translocon in protein synthesis?

To attach the ribosome to the ER membrane Signup and view all the answers

What happens to the signal peptide after it is released into the ER lumen?

It is cleaved by signal peptidase Signup and view all the answers

What is the last step in protein synthesis after the polypeptide chain is translocated into the ER lumen?

Detachment of the ribosome from the ER membrane Signup and view all the answers

What is the role of the SRP receptor in protein synthesis?

To release the SRP from the ribosome Signup and view all the answers

What is the purpose of post-translational modifications in the Rough ER?

To modify proteins before they reach their final destinations Signup and view all the answers

What happens to the SRP after it is released from the ribosome?

It is recycled and can participate in another round of protein synthesis Signup and view all the answers

What is the function of signal peptidase in protein synthesis?

To cleave the signal peptide from the polypeptide chain Signup and view all the answers

What happens to the mRNA in the cytoplasm if it lacks a signal sequence?

It is synthesized entirely in the cytoplasm on free ribosomes as a structural protein of the cell Signup and view all the answers

What recognizes the signal peptide in the cytosol?

Signal recognition particle (SRP) Signup and view all the answers

What is the approximate length of the signal peptide?

16-20 amino acids Signup and view all the answers

What is the function of the SRP receptor?

To anchor the ribosome to the ER membrane Signup and view all the answers

What is the purpose of the signal sequence?

To provide the molecular key for interaction with the ER membrane Signup and view all the answers

What is the result of the interaction between the signal sequence and the ER membrane?

A channel is opened in the membrane for polypeptide transport Signup and view all the answers

What is the composition of the signal recognition particle (SRP)?

6 proteins + 7S RNA molecule Signup and view all the answers

What is the purpose of SRP binding to the signal peptide and ribosome?

To take the ribosome to the ER Signup and view all the answers

Study Notes

Cell Biology Basics

A cell is the basic morphological and functional unit of the body, and is the basic building block of all living organisms.
Cells are too small to be seen with the naked eye and required the development of a magnifying device, such as a light microscope, to be studied.
The term "cellula" was first used by Robert Hook in 1665 to describe small chamber-like structures he observed in a piece of cork under a microscope.

Cell Theory

The cell theory was introduced by Schleiden and Schwann in 1838 and 1839, respectively.
The three main principles of the cell theory are:
- Cells are the fundamental units of both structure and function in all living organisms.
- All living organisms are composed of cells and their products.
- Cells arise only from preexisting cells.

Cell Components

Protoplasm is the living substance of plants and animals, and is made up of the nuclear region and cytoplasm.
The nucleus was introduced as a term by R. Brown in 1833.

Cell Fractionation

Centrifugation at high speeds (100,000 x g for 30 minutes) pellets the endoplasmic reticulum, golgi apparatus, and other membrane fragments.
Centrifugation at very high speeds (300,000 x g for 3 hours) pellets ribosomes.

Cell-Free Systems

Cell-free systems are fractionated cell extracts that maintain a biological function.
Cell-free systems have been used to study molecular mechanisms involved in cellular processes, such as protein synthesis, DNA replication, and DNA transcription.
Many important discoveries in molecular biology have been made using cell-free systems.

Importance of Cell Biology

Cell biology is important for understanding the molecular biology of the cell and has led to many important discoveries.
Cell biology has many practical applications, including embryonic stem cell research, cancer research, and cytogenetic research.

Model Organisms

Escherichia coli is the most well-understood cell in biology and is a popular model organism.
Saccharomyces cerevisiae (yeast) is a frequently used single-celled eukaryote and is a popular model organism for studying molecular biology.
Studies with bacteria and yeast have established the basic principles of molecular biology.

Cell Fusion

Cell fusion is the process of merging the membranes and cytoplasm of two cell types, such as human and mouse cells.
Hybrid cells are produced by cell fusion and can be used to study cell-cell interactions and other biological processes.

Fractionation of Cells and Analyzing their Molecules

Cell fractionation is a method to study certain organelles from a cell, involving the disruption of cells and isolation of organelles and macromolecules in pure form.
Cells can be disrupted in various ways, such as osmotic shock, ultrasonic vibration, or grinding up.

Cell Disruption Methods

Chemical: alkali, organic solvents, detergents
Enzymatic: lysozyme, chitinase
Physical: osmotic shock, freeze/thaw
Mechanical: sonication, homogenization, French press

Steps of Subcellular Fractionation

Homogenization
Differential centrifugation
Further separation and purification by density gradient centrifugation
Collection of fractions
Analysis of fractions

Centrifugation

Centrifugation is the first step in most fractionations, separating components that differ greatly in size.
Low speed centrifugation is used to separate intact cells from medium.
High speed centrifugation can be used to separate subcellular components.
When a centrifugal force is applied to an aqueous mixture, components of larger size and density will sediment faster.

Density Gradient Centrifugation

A finer degree of separation can be achieved by layering the homogenate as an arrow band on top of a dilute salt solution that fills a centrifuge tube.
The various components in the mixture move as a series of distinct bands through the salt solution, each at a different rate, in a process called velocity sedimentation.

History of Mitochondria Isolation

1934: First isolation of mitochondria from liver by Bensley
1937: First chemical analysis of mitochondria

Isolation of Components of Living Cells by Differential Centrifugation

Organelles and macromolecules can be separated by ultracentrifugation.
Pure fractions of nucleus, nucleolus, mitochondria, lysosomes, microsomes, and ribosomes can be isolated.
This method has made it possible to obtain knowledge on the molecular composition of cell components.

Method of Differential Centrifugation

Cut tissue in an ice-cold isotonic buffer to stop enzyme reactions, osmosis, and pH changes.
Grind tissue in a blender to break open cells.
Filter to remove insoluble tissue.
Centrifuge filtrate at low speeds (1000 X g for 10mins) to pellet nuclei.
Centrifuge at medium speeds (10 000 x g for 30 mins) to pellet mitochondria.

Signal Recognition Particle (SRP) Receptor

SRP binds to the signal peptide and ribosome, arresting protein synthesis
The SRP-ribosome complex attaches to the ER at specific sites called SRP receptor (or docking protein)
Upon binding, SRP is released from the ribosome, returns to the cytosol, and may participate in another round of protein synthesis

Protein Synthesis and Translocation

Ribosomal large subunits attach to pore proteins (translocon) on the ER membrane
When SRP is released, protein synthesis starts, and the signal peptide and growing polypeptide chain are released through the translocon into the ER lumen
The signal peptide is cleaved from the polypeptide chain by signal peptidase in the ER lumen

Post-Translational Modifications in the Rough ER

Newly synthesized polypeptides undergo principal modifications, including:
- Specific proteolytic cleavages
- Addition and processing of carbohydrates
- Proper folding
- Binding of integrin to basement membrane constituents (e.g., laminin, fibronectin, and collagen IV)

Structure and Function of the Endoplasmic Reticulum

The ER is an extensive network of folded membranes that extends from the nuclear envelope to the plasma membrane
The ER is composed of interconnected tubules and sacs (cisternae) with a single continuous membrane
The ER serves various functions, including:
- Protein biosynthesis
- Lipid biosynthesis
- Intracellular transport
- Folding of protein molecules in cisternae

Types of Endoplasmic Reticulum

Rough (granular) ER (RER or GER): studded with ribosomes, giving it a "rough" appearance
Smooth (agranular) ER (SER): lacks ribosomes
The quantity of RER and SER in a cell can interchange depending on changing metabolic needs

Signal Hypothesis

Formulated by Günter Blobel and David Sabatini in 1971
Elaborated by Blobel and his colleagues between 1975 and 1980
Günter Blobel was awarded the 1999 Nobel Prize in Physiology for the discovery of protein targeting signals
The signal hypothesis proposes that proteins have intrinsic signals that govern their transport and localization in the cell

Binding of Integrin to Basement Membrane Constituents

Integrin binding to basement membrane constituents (e.g., laminin, fibronectin, and collagen IV) promotes asymmetric cell division in many stem cell types.
Integrin a6/ITGA6/CD49f, which binds to laminin, is expressed in multiple adult stem cell types and is recognized as a marker for different cancer stem cells.
Integrin-mediated mechanotransduction senses biophysical cues from ECM and modulates cellular responses by altering cytoskeleton and changing gene transcription.

Structure and Function of the Endoplasmic Reticulum

Endoplasmic Reticulum (ER) is an extensive network of folded membranes that extends from the nuclear envelope to the plasma membrane.
ER is composed of a netlike labyrinth of branching tubules and sacs (cisternae) that are all interconnected by a single continuous membrane.
ER serves many general functions, including protein synthesis, intracellular transport, lipid biosynthesis, and protein biosynthesis.

Types of Endoplasmic Reticulum

There are two basic kinds of Endoplasmic Reticulum: Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER).
RER is studded with ribosomes, giving it a "rough" appearance, and is involved in protein synthesis.
SER lacks bound ribosomes and is involved in lipid biosynthesis, lipid transport, and other functions.

Smooth Endoplasmic Reticulum (SER)

SER functions vary in different cells, including:
- Lipid biosynthesis (e.g., synthesis of fatty acids, phospholipids, and cholesterol)
- Lipid transport (e.g., transport of dietary lipids in the small intestine)
- Biosynthesis of steroid hormones
- Metabolic reactions in liver cells
- Contraction process in muscle cells
- Regulation in neuronal synapse

Rough Endoplasmic Reticulum (RER)

RER is involved in protein synthesis and is well-developed in protein-secreting cells, such as:
- Digestive enzyme-producing cells of the exocrine pancreas
- Collagen and elastin-producing cells of the connective tissue
- Antibodies-producing cells
- Neurotransmitter-producing cells
- Phagocytic cells containing lysosomal enzymes

Microsomes

Microsomes are small, closed vesicles formed from the ER when tissue is disrupted by homogenization.
Microsomes are useful for studying ER functions in vitro.
RER and SER regions of ER can be separated by centrifugation.

Human CYPs

Humans have 57 cytochrome P450 genes.

Smooth Endoplasmic Reticulum (SER)

SER is involved in metabolic reactions in liver cells.
It is involved in the breakdown of glycogen into glucose.
SER contains the enzyme glucose-6-phosphatase, which converts glucose-6-phosphate to glucose.
The liver is the main site for gluconeogenesis.
SER also participates in the contraction process in muscle cells.
In muscle cells, SER is called the sarcoplasmic reticulum (SR) and stores and pumps calcium ions.
SR membrane contains Ca+2-ATPase pumps, which regulate muscular contraction.
SR's release of Ca+2 upon electrical stimulation of the cell plays a major role in excitation-contraction coupling.
SER regulates neuronal synapse by controlling Ca+2 flow.

Granular Endoplasmic Reticulum (GER)

GER is involved in protein synthesis.
GER is prominent in cells that secrete proteins.
In light microscopy, GER can be detected by staining with basic dyes, forming Nissl bodies.
In electron microscopy, GER appears as parallel membrane-limited flattened sacs or cisternae.
GER is continuous with the outer membrane of the nuclear envelope.
GER is well developed in protein-secreting cells, such as:
- Pancreatic exocrine cells (digestive enzymes)
- Fibroblasts (collagen and elastin)
- Plasma cells (antibodies)
- Motor neurons (neurotransmitters)
- Macrophages (lysosomal enzymes)

Microsomes

RER and SER regions of ER can be separated by centrifugation.
Microsomes are small closed vesicles formed from ER fragments, useful for studying ER functions in vitro.

SER Functions

SER has different functions in specialized cells:
- Biosynthesis of lipids
- Lipid transport
- Biosynthesis of steroid hormones
- Metabolic reactions in liver cells
- Contraction process in muscle cells
- Regulation in neuronal synapse

Protein Folding and Transport

Folding of polypeptide chains occurs in the ER lumen and is facilitated by various folding catalysts.
Each polypeptide has a unique folding pathway determined by its sequence.
Folding catalysts include:
- Binding protein (BiP) that assists protein folding and pulls the polypeptide chain through the Sec61 channel.
- Calnexin and Calreticulin (lectins) that facilitate folding.
- Disulfide isomerase that catalyzes the formation of disulfide bonds.

Disulfide Bonds

Disulfide bonds (-S-S-) help stabilize the tertiary and quaternary structure of many proteins (e.g., insulin).
In eukaryotic cells, disulfide bonds are formed in the ER lumen, but not in the cytosol.
Protein disulfide isomerase, an enzyme localized to the ER lumen, catalyzes the rearrangement of disulfide bonds.

Protein Targeting

Co-translational targeting involves the secretory pathway: ER, Golgi, lysosomes, plasma membrane, and secreted proteins.
Post-translational targeting involves the nucleus, mitochondria, and peroxisomes.
Three types of proteins related to the GER are synthesized in cells: secreted proteins, lysosomal enzymes, and plasma membrane glycoproteins.

Signal Hypothesis

The signal hypothesis proposes that proteins have intrinsic signals that govern their transport and localization in the cell.
Newly synthesized proteins must be delivered to a specific subcellular location or exported from the cell for correct activity.
This delivery process is carried out based on information contained in the protein itself.
Correct sorting is crucial for the cell; errors can lead to diseases.

Protein Synthesis and Transport

There are two types of protein synthesis and transport:
- Co-translational translocation: Synthesized protein is transferred to an SRP receptor on the ER, and then inserted into the translocation complex.
- Post-translational translocation: Proteins are translated in the cytosol and later transported to their destination.
Günter Blobel was awarded the 1999 Nobel Prize in Physiology for the discovery of the signal hypothesis.

In Vitro Fertilization (IVF)

IVF is a process where egg cells are fertilized by sperm outside the body (in vitro).

Cell Surface Modifications

Epithelial cells show polar differentiation with apical, lateral, and basal surfaces.
Apical surface is also known as luminal or free surface.

Apical Surface Modifications

Microvilli are finger-like projections from the cell surface that increase the surface area for absorption.
Found in cells specialized for absorption, such as intestinal epithelium, proximal tubule of the kidney, and epithelium of the gall bladder.
Microvilli are 1 μm in length and 0.1 μm in diameter, enclosed in an extension of the plasma membrane, and contain a bundle of straight parallel actin filaments.

Lateral Surface Modifications

Cell-cell junctions are found on the lateral surface of epithelial cells and are abundant in epithelial tissues.
Functions of cell-cell junctions include cell-to-cell attachment, forming barriers, and cell-to-cell communication.

Cell-Cell Junctions

Can be classified into three functional groups: occluding junctions (tight junctions), anchoring junctions (adhering junctions), and channel-forming junctions (communicating junctions).
Types of cell-cell junctions include tight junction, adherens junction, desmosome, gap junction, and hemidesmosome.

Tight Junction

Located just below the apical surface and assembles into multimeric proteins.
Specific proteolytic cleavages and addition of carbohydrates occur during tight junction formation.

Protein Synthesis and Processing

Signal peptide is removed from the polypeptide by signal peptidase (protease).
Glycosylation involves the addition and processing of carbohydrates, which starts in ER and terminates in the Golgi Complex.
Dolichol, a special lipid molecule, holds oligosaccharide chains in the ER membrane and is involved in the transfer of carbohydrates to the polypeptide.

The Matrisome and Extracellular Matrix (ECM)

The matrisome is the ensemble of proteins that compose the ECM itself and associated proteins that covalently modify (e.g., chemically cross-link, phosphorylate, cleave) the ECM.
The ECM fills the spaces between cells and binds cells and tissues together.
There are several types of extracellular matrices, consisting of a variety of secreted proteins and polysaccharides.

Types of Extracellular Matrices

Basal laminae (previously called basement membranes): thin, sheetlike structures upon which layers of epithelial cells rest.
Matrix structural proteins: tough, fibrous proteins embedded in a gel-like polysaccharide ground substance (e.g., tendons, cartilage, bone).
Glycocalyx: a polysaccharide-rich coat on the plasma membrane of endothelial cells or the apical membrane of intestinal epithelial cells.

Matrix Components

Collagen fibers: provide structural integrity, mechanical strength, and resilience.
Proteoglycans: a group of glycoproteins that cushion cells and bind a wide variety of extracellular molecules.
Soluble multi-adhesive matrix proteins (e.g., laminin, fibronectin): bind to and cross-link adhesion receptors and other ECM components.

Collagen

The most abundant protein in the ECM, secreted mainly by fibroblasts.
Accounts for up to 30% of the total proteins in human bodies.

Functions of ECM

Provides structural integrity, mechanical strength, and resilience to tissues.
Acts as a reservoir for growth factors that bind reversibly to its constituents.
Provides the basis for signals transmitted to cells by adhesion receptors that bind to its components.

Glycosaminoglycans and Proteoglycans

Strongly negatively charged, with most sugars bearing carboxylic acid groups.
Commonly sulphated in chondroitin sulphate, dermatan sulphate, heparan sulphate, and keratan sulphate.
Absorb water and resist compression.

Cell-Matrix Adhesion Proteins

Integrins: mediate cell-matrix adhesion.
Cadherins: mediate cell-cell adhesion (Ca2+ dependent).
Immunoglobulin (Ig)-super family members (e.g., N-CAM, I-CAM, VE-CAM): mediate cell-cell adhesion (Ca2+ independent).
Selectins: mediate transient cell-cell adhesions in the bloodstream.

Selectins

Cell-surface carbohydrate-binding proteins (lectins).
Composed of three members: L- (leukocyte), E- (endothelial), and P- (platelet) selectin.
Adhesive binding is calcium dependent.
Ligands include sialyl-Lewis X saccharides and mucin-like glycoprotein (GP) ligands.
Mediate the adhesion of leukocytes and platelets during inflammatory and hemostatic responses.

Cellular Response to Stress

Transcriptional induction of ER chaperones increases protein folding activity and prevents protein aggregation
Translational attenuation reduces the load of new protein synthesis and prevents further accumulation of unfolded proteins
The ER associated degradation (ERAD) pathway eliminates misfolded proteins by the ubiquitin-proteasome system
If the stress cannot be resolved, severe and prolonged ER stress extensively impairs the ER functions, leading to cell death by apoptosis

Basal Laminae

A sheet-like meshwork of ECM components, no more than 60-120 nm thick
Provides a foundation for assembly of cells into tissue
In columnar and other epithelia, it is a foundation on which only one surface of the cells rests
In other tissues, such as muscle or fat, the basal lamina surrounds each cell
Helps in regeneration after tissue damage and in embryonic development
Assists in the development of the nervous system, where neurons migrate along ECM pathways that contain basal laminal components

Cell-Cell Junctions

Cell-cell junctions are abundant in epithelial tissues
Cell-cell junctions are found on the lateral surface of epithelial cells
Functions of cell-cell junctions include:
- Cell to cell attachment (adhesion)
- Forming barriers that prevent the free passage of substances and cells from lumen to the blood circulation
- Cell to cell communication

Classification of Cell-Cell Junctions

Occluding junctions (tight junctions): form barriers that prevent the free passage of substances and cells
Anchoring junctions (adhering junctions): cell-cell attachment
Channel-forming junctions (communicating junctions): allow for cell-cell communication

Cell-Matrix Junctions

Cell-matrix junctions are found on the basal surface of epithelial cells
Basal lamina is a type of extracellular matrix that provides a foundation for assembly of cells into tissue

Extracellular Matrix (ECM)

Fills the spaces between cells and binds cells and tissues together
Composed of a variety of secreted proteins and polysaccharides
Includes matrix structural proteins, such as collagen fibers, which provide structural integrity, mechanical strength, and resilience
Includes proteoglycans, which cushion cells and bind a wide variety of extracellular molecules
Includes soluble multi-adhesive matrix proteins, such as laminin and fibronectin, which bind to and cross-link adhesion receptors and other ECM components

Cell Junctions

Cell-cell junctions are abundant in epithelial tissues, which cover surfaces and line cavities, forming protective layers.
Epithelial cells show apical-basal polarity, with lateral surface modifications featuring cell-cell junctions.

Types of Cell-Cell Junctions

Occluding junctions (Tight junctions):
- Form barriers preventing passage of substances and cells from lumen to blood circulation.
- Located just below the apical surface.
- Consist of anastomosing network of protein strands.
- Found in epithelial and endothelial cells.

Anchoring Junctions

Zonula adherens (Adherens junctions):
- Involved in cell-cell attachment.
- Found in epithelial and endothelial cells.
Desmosomes:
- Involved in cell-cell attachment.
- Contain cadherins and desmoplakin.
- Found in epithelial, endothelial, and connective tissue cells.

Channel-Forming Junctions

Gap junctions (Communicating junctions):
- Allow direct cell-cell communication.
- Found in epithelial, endothelial, and connective tissue cells.

Cell-Matrix Junctions

Hemidesmosomes:
- Anchor cells to the basal lamina.
- Contain integrins and plectin.
- Important in skin blistering disease bullous pemphigoid.
Focal adhesions:
- Bind cells to the extracellular matrix.
- Contain integrins.

Importance of Cell Junctions

Cell-cell junctions: cell to cell attachment, forming barriers, and cell-cell communication.
Cell-matrix junctions: anchoring cells to the extracellular matrix.

Cx43, Cx46, and Cx50/α8

Related to cataract and heart malformations
Involved in firm adhesion of cells and free interchange of substances
Play a crucial role in regulating intrauterine development and differentiation
Important for coordinating function among groups of cells

Anchoring Junctions

Include hemidesmosomes and focal adhesions
Hemidesmosomes:
- Have a single dense plaque on the cytoplasmic surface
- Anchor loops of intermediate filaments
- Involve integrins binding to laminin protein in the basal lamina
- Intracellular domain binds to an anchor protein (plectin) that binds to keratin intermediate filaments
Focal adhesions:
- Bind cells to the extracellular matrix
- Involve integrins responsible for binding to the matrix
- Cytoplasmic domain of the integrin binds indirectly to actin filaments

Hemidesmosomes

Involved in a blistering skin disease called bullous pemphigoid, where autoantibodies attack type XVII collagen within the basal lamina
Mutations in plectin cause skin blisters

Focal Adhesions

Integins' extracellular domains bind to components of extracellular matrix
Cytoplasmic tail of the β subunit binds indirectly to actin

Anchoring Junctions Subclassification

Anchoring junctions can be subclassified according to the cytoskeletal element involved
Actin filament attachment sites:
- Cell-cell junctions (adherens junctions)
- Cell-matrix junctions (focal adhesions)
Intermediate filament attachment sites:
- Cell-cell junctions (desmosomes)
- Cell-matrix junctions (hemidesmosomes)

Adhesion Proteins

No basal body
Function: increase the cell surface for absorption
Localization: ductus epididymis, ductus deferens

Cell-Cell Junctions

Abundant in epithelial tissues
Found on lateral surface of epithelial cells
Functions:
- Cell-to-cell attachment (adhesion)
- Form barriers that prevent the free passage of substances and cells from lumen to the blood circulation
- Cell-to-cell communication

Cell-Cell Junction Classification

Classified into three functional groups:
- I-Occluding junctions (tight junctions) (barrier)
- II-Anchoring junctions (adhering junctions) (cell-cell attachment)
  - Z. adherens and desmosomes
- III-Channel-forming junctions (communicating junctions)
  - Gap junction

Tight Junctions

Also known as zonula occludens or occluding junction
Located just below the apical surface
Functions:
- Seals neighboring cells together, preventing passage of molecules (including ions) from lumen to the blood circulation
- Forms a barrier
- Separates the apical and basolateral domains of the plasma membrane
- Prevents the free diffusion of membrane components (lipids and proteins)
Characteristics:
- The closest contacts between adjacent cells
- Outer leaflets of the plasma membranes of adjacent cells come together at periodic intervals (focal connections)
- Membranes do not fuse

Protein Synthesis and Targeting

Signal Recognition Particle (SRP) binds to the signal peptide and ribosome, arresting protein synthesis, and attaches to the SRP receptor on the ER membrane.
Upon binding, SRP is released from the ribosome, and protein synthesis resumes.
The signal peptide and growing polypeptide chain are released through the translocon into the ER lumen.
The signal peptide is cleaved from the polypeptide chain by signal peptidase.

ER Lumen and Post-Translational Modifications

Newly synthesized polypeptides in the ER undergo principal modifications, including:
- Specific proteolytic cleavages
- Addition and processing of carbohydrates
- Proper folding

Signal Hypothesis

Formulated by Günter Blobel and David Sabatini in 1971, and elaborated by Blobel and his colleagues between 1975 and 1980.
Günter Blobel was awarded the 1999 Nobel Prize in Physiology for the discovery that proteins have intrinsic signals that govern their transport and localization in the cell.
The signal hypothesis states that proteins have intrinsic signals that govern their transport and localization in the cell.

Protein Targeting

Protein targeting is necessary for proteins destined to work outside the cytoplasm.
Correct sorting is crucial for the cell, and errors can lead to diseases.
Protein targeting can occur through:
- Co-translational translocation: Synthesized protein is transferred to an SRP receptor on the ER.
- Post-translational translocation: Some proteins are translated in the cytosol and later transported to their destination.

SRP and Signal Sequence

SRP is composed of 6 proteins and a 7S RNA molecule.
The signal sequence is about 16-20 amino acids and appears at the beginning of the polypeptide chain.
The signal sequence is recognized by SRP, which takes the ribosome to the ER.
The hydrophobicity of the signal sequence is believed to be the molecular key for the polypeptide's interaction with the ER membrane.

Protein Synthesis and Folding in the ER

Oligosaccharyl transferases catalyze protein synthesis in the ER.
Newly synthesized protein chains fold and assemble in the ER lumen, with each polypeptide having its own unique folding pathway determined by its sequence.

Folding Assistants in the ER

Folding catalysts in the ER include:
- BIP (Binding protein), which assists protein folding and pulls polypeptide chains through the Sec61 channel.
- Calnexin and Calreticulin (lectins), which facilitate folding.
- Disulfide isomerase, which forms disulfide bonds (-s-s-) that stabilize tertiary and quaternary protein structures.

Co-Translational Protein Insertion

In mammalian cells, most proteins enter the ER co-translationally, with the polypeptide chain being inserted into the ER as it is being synthesized.
Topologies of integral membrane proteins synthesized on the rough ER can have multiple, alternating start-transfer and stop-transfer signals, resulting in multipass transmembrane proteins.

Post-Translational Protein Insertion

Some proteins enter ER membranes post-translationally, requiring ATP.
Hydrophobic portions (signal peptides) of the protein can act as start-transfer or stop-transfer signals during translocation.

Export of Proteins from the ER to Golgi Complex

Newly synthesized proteins travel along the secretory pathway in transport vesicles, which bud from the ER membrane and fuse with the Golgi apparatus membrane.
Resident ER proteins containing KDEL sequences can escape from the ER but are returned to the ER from the Golgi apparatus by KDEL receptors.

The Signal Hypothesis

The signal hypothesis proposes that proteins have intrinsic signals that govern their transport and localization in the cell.
This hypothesis was formulated by Günter Blobel and David Sabatini in 1971 and elaborated by Blobel and colleagues between 1975 and 1980.
Günter Blobel was awarded the 1999 Nobel Prize in Physiology or Medicine for this discovery.
Protein targeting is necessary for proteins that are destined to work outside the cytoplasm, and correct sorting is crucial for the cell to prevent diseases.

Microsomes

Microsomes are small, closed vesicles (100-200 nm in diameter) formed from the ER when tissue is disrupted by homogenization.
They are useful for studying ER functions in vitro.

Smooth Endoplasmic Reticulum (SER)

Regions of ER that lack bound ribosomes are called smooth endoplasmic reticulum.
SER is tubular or vesicular in form and arises from the Golgi apparatus.
SER is not involved in protein synthesis.

Functions of Smooth Endoplasmic Reticulum (SER)

SER functions differently in specialized cells.

Biosynthesis of Lipids

SER is involved in the synthesis of fatty acids, phospholipids, and cholesterol.
The enzymes that synthesize lipid components are located in the SER membrane.
SER is the principle site of production of lipoprotein particles in the liver.

Lipid Transport

Dietary lipids are broken down into fatty acids and monoglycerides by pancreatic lipase in the small intestine.
SER is involved in the absorption and re-synthesis of lipids.
The process of lipid absorption involves emulsification, lipolysis, micellar formation, membrane translocation, intracellular re-synthesis, chylomicron formation, and lymphatic drainage.

Biosynthesis of Steroid Hormones

SER contains enzymes required for steroid synthesis, which are abundant in Leydig's cells of testes, adrenal cortex cells, and corpus luteum cells of ovaries.
SER produces testosterone, corticosteroids, and progesterone in these cells.

Metabolic Reactions in Liver Cells

SER is abundant in hepatocytes and contains specific enzymes to detoxify drugs, alcohol, steroid hormones, and toxic chemicals.
Cytochrome P450 enzymes (CYP enzymes) catalyze the oxidation of organic substances and are involved in detoxification.

Rough Endoplasmic Reticulum (RER)

RER arises from the nuclear envelope and is particularly well developed in protein-secreting cells.
RER is involved in protein biosynthesis and transportation within the cell.
RER is abundant in cells that produce digestive enzymes, collagen, elastin, antibodies, and neurotransmitters.

Functions of Tight Junctions

Seals neighboring cells together, preventing passage of molecules (including ions) from lumen to blood circulation
Forms a barrier and separates apical and basolateral domains of the plasma membrane
Prevents free diffusion of membrane components (lipids and proteins) and restricts lateral movement of membrane proteins

Characteristics of Tight Junctions

Closest contacts between adjacent cells
Located just below the apical surface and continuous around the entire periphery of the cell (zonula)
Very narrow intercellular space at the level of tight junctions
Membranes of adjacent cells come together at periodic intervals (focal connections) but do not fuse
Consist of anastomosing network of protein strands

GER (Rough Endoplasmic Reticulum)

Particularly well developed in protein-secreting cells
Found in cells such as digestive enzyme-producing cells of the exocrine pancreas, collagen and elastin-producing cells of the connective tissue, antibody-producing cells, and neurotransmitter-producing cells
Appears as parallel membrane-limited flattened sacs or cisternae in the electron microscope
The membranes of the ER are continuous with the outer membrane of the nuclear envelope

Functions of GER

Has a central role in protein biosynthesis and their transportation within the cell
Involved in the synthesis of proteins such as serum albumin, fibrinogen, and the prothrombin group of clotting factors

Structure and Function of Endoplasmic Reticulum

Extensive network of folded membranes that extends from the nuclear envelope to the plasma membrane
Consists of a netlike labyrinth of branching tubules and sacs
The sacs and tubules are all interconnected by a single continuous membrane
Serves many general functions, including the folding of protein molecules, intracellular transport, lipid biosynthesis, and protein biosynthesis

Types of Endoplasmic Reticulum

Rough (granular) ER (RER or GER): surface is studded with ribosomes, giving it a "rough" appearance
Smooth (agranular) ER (SER): lacks ribosomes

ER Stress

ER dysfunction can occur due to various conditions, including:
- Inhibition of protein glycosylation
- Reduction of disulfide bond formation
- Calcium depletion from the ER lumen
- Impairment of protein transport from the ER to the Golgi
- Expression of misfolded proteins
This ER dysfunction leads to proteotoxicity in the ER, known as "ER stress"

Causes of ER Stress

ER stress can occur under various conditions, including:
- Pharmacological chemicals
- Decreases in oxygen, glucose, ATP, and calcium ions
- Nutrient deprivation
- Developmental processes
- Genetic mutations
- Pathogenic insult

Unfolded Protein Response (UPR)

The UPR is a self-protective mechanism against ER stress
The UPR is activated in response to an accumulation of unfolded or misfolded proteins in the ER lumen
There are four functionally distinct UPR pathways

ER Stress and Disease

ER stress-induced apoptosis is associated with various pathophysiological conditions, including:
- Ischemia
- Diabetes
- Neurodegenerative diseases

Signal Hypothesis

The signal hypothesis proposes that proteins have intrinsic signals that govern their transport and localization in the cell
This hypothesis was formulated by Günter Blobel and David Sabatini in 1971
Günter Blobel was awarded the 1999 Nobel Prize in Physiology for this discovery

Protein Targeting

Protein targeting is necessary for proteins that are destined to work outside the cytoplasm
Correct sorting is crucial for the cell; errors can lead to diseases
Protein targeting is carried out based on information contained in the protein itself

Translocation of Proteins

There are two types of translocation:
- Co-translational translocation: protein is transferred to an SRP receptor on the ER during translation
- Post-translational translocation: protein is translated in the cytosol and later transported to its destination

Protein Folding

Protein folding occurs in the ER lumen and is facilitated by various folding catalysts, including:
- BIP (binding protein)
- Calnexin and Calreticulin (lectins)
- Disulfide isomerase (for formation of disulfide bonds)

Disulfide Bonds

Disulfide bonds help stabilize the tertiary and quaternary structure of many proteins
Disulfide bonds are formed in the ER lumen, but not in the cytosol
Protein disulfide isomerase catalyzes the rearrangement of disulfide bonds

ER Stress

Disturbances in ER function can lead to ER stress, including inhibition of protein glycosylation, reduction of disulfide bonds, calcium depletion, and impairment of protein transport.
ER stress can be caused by various conditions, including pharmacological chemicals, decrease in oxygen, glucose, ATP, and calcium ions, nutrient deprivation, developmental processes, genetic mutations, and pathogenic insult.
ER stress can lead to proteotoxicity in the ER and trigger the unfolded protein response (UPR).

Unfolded Protein Response (UPR)

The UPR is a self-protective mechanism against ER stress, activated in response to an accumulation of unfolded or misfolded proteins in the ER lumen.
There are four functionally distinct UPR pathways.

ER Stress and Disease

ER stress-induced apoptosis is associated with various pathophysiological conditions, including ischemia, diabetes, and neurodegenerative diseases.

Cell Fusion

Cell fusion is the process of membrane merging and cytoplasmic mixing of two cell types.
Fusion proteins, such as viral glycoproteins, and polyethylene glycol can promote cell fusion.
Applications of cell fusion include:
- Genetic analysis, such as isolating a single human chromosome or a part of a chromosome.
- Production of hybrid cells, which can be used to study cancer cells, cell-virus relations, and cytogenetic research.

Applications of Cell Culture

Embryonic organs can be cultured, allowing for morphogenesis and study of tissue development.
Cell culture can be used to study cancer cells, cell-virus relations, and cytogenetic research.
Microdissection techniques allow for the isolation of selected cells from tissue slices.

Embryonic Stem Cells

Embryonic stem (ES) cells are derived from the inner cell mass of early mouse embryos and can proliferate indefinitely.
ES cells can give rise to all cell types in the body and have potential therapeutic applications, such as replacing damaged tissues in muscular dystrophy, Parkinson's disease, and type I diabetes patients.

Model Organisms

Escherichia coli is the most well-understood cell in biology and a popular bacterium for research.
Saccharomyces cerevisiae (yeast) is the most frequently used single-cell eukaryote and has contributed significantly to the understanding of molecular biology.

SRP Receptor and ER Lumen

SRP binds to the signal peptide and ribosome (A site), arresting protein synthesis.
The SRP-ribosome complex attaches to the ER at specific sites called SRP receptor (or docking protein).
Upon binding, SRP is released from the ribosome and returns to the cytosol, allowing protein synthesis to resume.

Translocation and Signal Peptide Cleavage

The ribosomal large subunits attach to pore proteins (translocon) on the ER membrane.
The signal peptide and growing polypeptide chain are released through the translocon into the ER lumen.
The signal peptide is cleaved from the polypeptide chain by signal peptidase.

Post-Translational Modifications in the Rough ER

Newly synthesized polypeptides undergo principal modifications in the ER:
- Specific proteolytic cleavages
- Addition and processing of carbohydrates
- Proper folding

Signal Hypothesis and Protein Synthesis

The signal hypothesis proposes that proteins with a signal sequence are synthesized on ribosomes in the cytoplasm and then transported to the ER.
The signal peptide is recognized by signal recognition particles (SRP) in the cytosol, which takes the ribosome to the ER.
The signal sequence interacts with the ER membrane, opening a channel for polypeptide transport into the ER lumen.

Addition and Processing of Carbohydrates

Glycosylation occurs in the ER and is completed in the Golgi Complex.
Precursor oligosaccharides are synthesized in the cytosol and imported into the ER or Golgi lumen by Dolichol.
Oligosaccharide chains are transferred to specific sites on the polypeptide in the ER membrane.

SRP Receptor and Protein Synthesis

SRP binds to the signal peptide and ribosome (A site), arresting protein synthesis.
The SRP-ribosome complex attaches to the ER at specific sites called SRP receptor (or docking protein).
Upon binding, SRP is released from the ribosome, returns to the cytosol, and may participate in other rounds of protein synthesis.

Protein Synthesis and Translocation

The signal peptide and growing polypeptide chain are released through the translocon into the ER lumen.
The signal peptide is cleaved from the polypeptide chain by signal peptidase.
When protein synthesis is complete, the ribosome detaches from the ER membrane, and mRNA and ribosomes may participate in another round of protein synthesis.

The Signal Hypothesis

The signal hypothesis was proposed by Günter Blobel and David Sabatini in 1971.
The signal peptide is a critical component of the process, guiding the protein to the ER membrane.

Post-Translational Modifications in the Rough ER

Newly synthesized polypeptides in the ER undergo principal modifications before reaching their final destinations:
- Specific proteolytic cleavages
- Addition and processing of carbohydrates
- Proper folding
- Assembly into multimeric proteins

Specific Proteolytic Cleavages

The signal peptide is removed from the polypeptide by signal peptidase.

Addition and Processing of Carbohydrates (Glycosylation)

Many secreted and membrane proteins contain covalently attached carbohydrates.
Addition of carbohydrate chains starts in the ER and terminates in the Golgi Complex.
Precursor oligosaccharides (OlSc) are synthesized in the cytosol, then imported into the ER or Golgi lumen by Dolichol (a special lipid molecule).

Importance of Targeting Signals

In the absence of targeting signals, a protein will remain in the cytoplasm.
If an mRNA lacks a signal sequence, the protein will be synthesized entirely in the cytoplasm on free ribosomes as a structural protein of the cell.
If an mRNA contains a signal sequence, the signal peptide is synthesized on the ribosome in the cytoplasm, and SRP binds to the signal peptide and ribosome, guiding it to the ER.

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This quiz covers the topic of centrifugation and its applications in cell biology, including the isolation of cellular components and the study of molecular mechanisms in cell-free systems.

Cell Biology: Centrifugation and Cell-Free Systems

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Questions and Answers

What is the term used to describe the living substance of plants and animals?

What is the function of centrifuging at high speeds (100 000 x g for 30 mins) in cellular biology?

Who is credited with introducing the term 'nucleus' in 1833?

What was isolated in 1949 from skeletal muscle cells?

What is the term used to describe the small chamber-like structures observed by Robert Hooke in 1665?

Who made the first microscopic observations of protozoa, bacteria, and sperm in 1674?

What is the term for fractionated cell extracts that maintain a biological function?

What was achieved in 1954 in the field of cell-free systems?

What is the fundamental unit of protoplasm that is capable of independent existence?

Who developed the cell theory in 1838 and 1839?

What is the primary function of intercellular or extracellular substances in mammalian tissue?

What is the term for the basic building blocks of all living organisms?

What is the purpose of centrifuging at very high speeds (300 000 x g for 3hrs) in cellular biology?

What is the name of the book referenced in the content that is authored by RJ Epstein?

What is the application of microdissection techniques in cell culture?

What is the significance of embryonic stem cells in medical research?

What is the main advantage of using Escherichia coli in biological research?

What is the purpose of cell fusion in biological research?

What is the significance of Saccharomyces cerevisiae in biological research?

What is the application of cytogenetic research in medicine?

What is the potential application of embryonic stem cells in therapy?

What is the result of cell fusion?

What is the primary purpose of centrifugation in cell fractionation?

What type of homogenization uses enzymes such as lysozyme?

What is the purpose of using a shallow gradient of sucrose in centrifugation?

What type of homogenization uses detergents?

What is the term used to describe the movement of components in a centrifugation process?

What type of instrument is used to achieve a finer degree of separation in centrifugation?

What is the purpose of cell fractionation?

How are cells typically disrupted in cell fractionation?

What is the result of cell disruption procedures if carefully applied?

What is the term for the process of breaking up a cell and its components?

What is the purpose of differential centrifugation in cell fractionation?

What is the final step in the subcellular fractionation process?

What is the purpose of density gradient centrifugation in cell fractionation?

What is the result of cell fractionation?

What happens to the cell membranes during homogenization?

What is the purpose of low speed centrifugation in cellular biology?

What is the result of centrifuging a cell suspension at high speed?

What is the purpose of using an ice cold isotonic buffer in the differential centrifugation method?

Which organelle is pelleted first in the differential centrifugation method?

What is the purpose of increasing the speed of centrifugation in the differential centrifugation method?

What is the result of ultracentrifugation in cellular biology?

What is the purpose of grinding tissue in a blender in the differential centrifugation method?

What is the role of SRP receptor in protein synthesis?

What happens to protein synthesis when SRP binds to the signal peptide and ribosome?

What is the function of the translocon in protein synthesis?

What happens to the signal peptide after it is released into the ER lumen?

What is the last step in protein synthesis after the polypeptide chain is translocated into the ER lumen?

What is the purpose of post-translational modifications in the Rough ER?

What is the role of the ribosomal large subunit during protein synthesis?

What is the significance of the signal hypothesis proposed by Günter Blobel and David Sabatini?

What is the name of the hypothesis that explains the mechanism of protein synthesis in GER?

What is the purpose of protein targeting in cells?

What is the process by which newly synthesized proteins are delivered to their correct subcellular location?

What is the Nobel Prize that Günter Blobel was awarded in 1999?

What is the type of translocation that occurs when a protein is synthesized and transferred to the endoplasmic reticulum (ER) simultaneously?

What is the type of translocation that occurs when a protein is synthesized in the cytosol and later transported to its destination?

What is the result of incorrect protein targeting in cells?

What is the receptor that synthesized proteins are transferred to during co-translational translocation?

What is the function of the endoplasmic reticulum in protein synthesis?

What is the characteristic of the surface of the Rough Endoplasmic Reticulum (RER)?

What is the function of integrin in stem cell biology?

What is the term used to describe the network of folded membranes that extends from the nuclear envelope to the plasma membrane?

What is the function of the sacs (cisternae) in the endoplasmic reticulum?

What is the main difference between Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER)?

What is the function of integrin-mediated mechanotransduction?

What is the significance of integrin a6/ITGA6/CD49f in stem cell biology?

What is the term 'reticulum' in Endoplasmic Reticulum derived from?

What is the primary function of the Rough Endoplasmic Reticulum?

What is the characteristic of the surface of the Rough Endoplasmic Reticulum?

What is the function of the Endoplasmic Reticulum in cellular transport?

What can happen to the quantity of Rough Endoplasmic Reticulum and Smooth Endoplasmic Reticulum in a cell?

What is the function of the Endoplasmic Reticulum in protein synthesis?

What is the structure of the Endoplasmic Reticulum?

What is the function of the Endoplasmic Reticulum in lipid biosynthesis?