Plasma Membrane Notes (Rayana) PDF

## MEMBRANE PLASMIQUE ### **What is the purpose of the plasma membrane?** - Separates the cytoplasm from the extracellular medium. ### **What is the plasma membrane composed of?** - A mosaic of proteins and lipids. ### **What characterizes the lipids that make up the plasma membrane?** - They are...

## MEMBRANE PLASMIQUE ### **What is the purpose of the plasma membrane?** - Separates the cytoplasm from the extracellular medium. ### **What is the plasma membrane composed of?** - A mosaic of proteins and lipids. ### **What characterizes the lipids that make up the plasma membrane?** - They are amphiphilic (one part is hydrophilic and the other is hydrophobic) and the membrane separates two mediums. - Organized in a bilayer. ### **What are the types of proteins that compose the plasma membrane?** - **Integrated proteins:** transmembrane or intrinsic, they cross the lipid bilayer. - **Extrinsic proteins:** peripheral, they only cross one layer. ### **What are the types of integrated proteins?** - **Bitopic:** cross once. - **Monotopic:** cross only one layer. - **Polytopic:** cross multiple layers. ### **Is the plasma membrane static?** - No, it is fluid. ### **Why is the plasma membrane fluid?** - The lipids are in constant movement: - **Rotation** - **Lateral diffusion** (along the same hemi-layer) - **Flexion** (of the hydrophobic tail) - **Flip-flop** (from one hemi-layer to the other, from extracellular to intracellular) - The proteins are also in movement: - **Rotation** - **Lateral diffusion** - **Flip-flop** is impossible. ### **Why is the plasma membrane fluid?** - To allow the transport of small molecules without deformation. ### **Is the plasma membrane visible under a light microscope?** - No. ### **How can the plasma membrane be identified under a light microscope?** - Using dyes that reveal the membrane as a limit. ### **What are two differences between a light microscope (LM) and an electron microscope (EM)?** - LM uses photons (light), whereas EM uses electrons. - LM requires the use of dyes, whereas EM does not, because the electrons must cross the specimen. ### **What is the appearance of the plasma membrane under an electron microscope?** - A three-layered structure: - Outer dense (osmophilic) layer - Inner dense (osmophilic) layer - Middle clear (osmophobic) layer ### **Why does the plasma membrane have this appearance?** - Hydrophilic parts of the lipids and proteins are osmophilic, whereas hydrophobic parts are osmophobic. ### **Why does the outer layer appear thicker than the inner layer?** - Because of the presence of glycoproteins, only in the extracellular side and due to the absence of flip-flop. ### **What do we refer to when we talk about the activity of the plasma membrane?** - The different transportation mechanisms: - **Uniport:** transports a single molecule. - **Symport:** transports two molecules in the same direction. - **Antiport:** transports two molecules in opposite directions. ### **Give an example of a transportation mechanism.** - Absorption of glucose by enterocytes. ### **What is the advantage of polytopic proteins?** - They can form channels for the transport of ions or water. ### **How can a bitopic protein allow penetration of a molecule?** - By changing its shape. ###**. How can macromolecules penetrate the plasma membrane?** - Through endocytosis and exocytosis: - **Pinocytosis:** for liquids - **Phagocytosis:** for solids - **Transcytose:** endocytosis followed by exocytosis, without degradation. ### **What is a plasma membrane differentiation?** - A specialization in a specific region of the cell, such as apical, basal, or lateral. ### **Give examples of plasma membrane differentiations.** - **Apical membrane:** - **Microvilli:** (ME) / striated plateau (MO) on enterocytes, increase the surface area for exchange. Contain actin filaments. - **Stereocilia:** in the epididymis, long and branched microvilli, lack actin filaments. - **Lateral membrane:** - In the bladder, epithelial cells have interdigitations, which allow for the attachment of cells during changes in volume. - **Basal membrane:** - **Hemidesmosome:** for epithelial cells, half of a desmosome, allowing for attachment to the basal lamina. - **Invaginations:** in renal cells, where mitochondria (Hendenhein’s rods) are located and generate energy. - **Cell coat:** contribute to cell recognition, selection, and sorting of nutrients. ## **CYTOSKELETON** ### **What is the structure of the cytoskeleton?** - Composed of actin filaments, microtubules, and intermediate filaments. ### **Why is the cytoskeleton considered a dynamic structure?** - It is constantly assembling and disassembling, allowing for shortening and lengthening. ### **What are the four roles of the cytoskeleton?** - Maintaining cell shape. - Anchoring organelles. - Formation of pseudopods. - Intracellular transportation. ### **What is the localization of the components of the cytoskeleton?** - **Microtubules:** from the center to the periphery. - **Actin filaments:** located at the periphery. - **Intermediate filaments:** from the periphery to the center. ### **What are the basic molecules that compose microtubules?** - Alpha-tubulin and beta-tubulin, forming heterodimers. 13 protofilaments form a MT. ### **What is the difference between cilia and flagella?** - **Ultrastructure:** they are identical. - **Number:** cilia are more numerous. - **Size:** flagella are longer. - **Movement:** cilia are pendular, flagella are sinusoidal. ## **ENDOPLASMIC RETICULUM** ### **What is the endoplasmic reticulum (ER)?** - A network of membrane-bound sacs and tubules located in the cytoplasm: - **Rough ER (RER):** with ribosomes on the cytosolic side; involved in protein synthesis. - **Smooth ER (SER):** lacks ribosomes; involved in lipid synthesis, calcium storage, and detoxification. ### **What are the different functions of the RER?** - **Endocrine activity:** secretion of hormones, such as insulin and glucagon. - **Exocrine activity:** secretion of pancreatic juice, containing digestive enzymes. ### **What evidence supports the connection between the nuclear membrane and the RER?** - The outer membrane of the nuclear envelope is continuous with the RER. - Ribosomes are present on both membranes. - The perinuclear space is continuous with the RER lumen. ### **What are the different forms of the RER?** - **Flattened cisternae:** for synthesis and elimination, no storage. - **Dilated cisternae:** for synthesis, there is storage, such as in follicle cells. ### **What is the relationship between the RER and the Golgi apparatus (AG)?** - They are continuous through transport vesicles. ### **What is the Smooth Endoplasmic Reticulum (SER)?** - Composed of a lipid bilayer and transmembrane proteins, including cytochrome P450. ### **What are the different functions of the SER?** - **Lipid synthesis:** steroid hormones. - **Calcium storage:** in collaboration with mitochondria. - **Glycogen metabolism:** via glucose-6-phosphatase, located in the membrane. - **Detoxification:** via cytochrome P450. ### **What is the functional relationship between the RER, Golgi apparatus, Lysosome, and mitochondria?** - **RER:** Produces proteins, lipids, and carbohydrates. - **AG:** Processes and packages proteins, lipids, and carbohydrates, also involved in cell detoxification. - **Lysosomes:** Digest macromolecules. - **Mitochondria:** Provide energy for the cell. ## **LYSOSOMES** ### **What are lysosomes?** - Membrane-bound organelles containing digestive enzymes, responsible for intracellular digestion. ### **How are lysosomes involved in digestion?** - **Heterophagy:** Digestion of materials brought into the cell by endocytosis: - **Pinocytosis:** ingestion of liquids. - **Phagocytosis:** ingestion of solids, such as bacteria. - **Autophagy:** Digestion of worn-out organelles or cellular components for recycling. ### **What is an endosome?** - A vesicle that transports materials from the plasma membrane to the lysosome. - There different endosomes: - **Early endosome:** pH slightly acidic (6.5). - **Late endosome:** pH more acidic (5.5). - **Endolysosome:** pH even more acidic (5). - **Phagosome:** a vesicle that transports solids engulfed by phagocytosis (e.g., bacteria) - **Autolysosome:** fuses with autophagosomes, digests cellular components. ### **What are the functions of lysosomes?** - Heterophagy - Autophagy - Removal of worn out organelles or cellular components. ### **What is the relationship between lysosomes and the Golgi apparatus?** - The AG produces enzymes for the lysosomes, allowing for their full function. ## **MITOCHONDRIA** ### **What are mitochondria?** - Organelles responsible for cellular respiration and ATP production. ### **What makes mitochondria semi-autonomous?** - They have their own DNA and synthesize about 10% of their proteins. ### **What are the structures of mitochondria?** - **Outer membrane:** Permeable. - **Inner membrane:** Forms folds called cristae, increasing the surface area for ATP production. - **Matrix:** Contains mitochondrial DNA, ribosomes, and enzymes. ### **What are ATPosomes?** - Structures located on mitochondrial cristae, responsible for ATP production. ### **What are the different types of cristae?** - **Lamellar:** parallel or perpendicular. - **Tubular:** found in cells of the adrenal cortex. - **Prismatic:** found in astrocytes and glial cells. ### **Where are mitochondria located?** - Spread throughout the cytoplasm, although concentrated in certain areas, such as: - **Basal pole** of proximal tubule cells in the kidneys. - **Basal and apical poles** of intestinal cells. - **Intermediate piece** of the sperm. - **Globule form** in brown adipose tissue of hibernating animals. ### **How are mitochondria divided?** - By **fission** or **constriction**. ### **What is the origin of mitochondria?** - Maternal, as only the nucleus and the proximal centriole of the sperm enter the egg. ## **THE NUCLEUS IN INTERPHASE, MITOSIS, AND THE KARYOTYPE** ### **What is the nucleus?** - The control center of the cell, containing genetic information. ### **What is chromatin?** - The complex of DNA and proteins that make up chromosomes. ### **What are the different types of chromatin?** - **Heterochromatin:** Highly condensed, transcriptionally inactive, located at the periphery of the nucleus. - **Euchromatin:** less condensed, transcriptionally active, located in the central region of the nucleus. - **Chromatin associated with the nucleolus:** involved in ribosome synthesis. ### **What is the nuclear envelope?** - A double membrane that surrounds the nucleus, allowing for communication with the cytoplasm and control of nuclear transport. ### **What are nuclear pores?** - Channels through the nuclear envelope that allow for the passage of macromolecules, such as RNA, proteins, and ribosomes. ### **What is the nucleolus?** - A region within the nucleus, not enclosed by a membrane, where ribosomes are assembled: - **Fibrillar dense component (CFD):** Contains DNA for the synthesis of ribosomal RNA. - **Fibrillar clear component (CF):** Where the processing of ribosomal RNA occurs. - **Granular component (CG):** Where proteins are assembled with ribosomal RNA. ### **What is a ribosome?** - An organelle involved in protein synthesis, consisting of two subunits: ### **Explain how DNA is transcribed and translated in the nucleus.** - DNA is transcribed into mRNA in the nucleus. - mRNA is then processed and exported to the cytoplasm, where it is translated into protein. ### **What is a karyotype?** - A complete set of chromosomes in a cell. - Human karyotype: 46 chromosomes, divided into 22 pairs of autosomes and one pair of sex chromosomes (X, Y). ### **What is the role of the kinetochore?** - A protein structure attached to centromeres, where spindle fibers connect to chromosomes during mitosis. ### **What is the significance of the cell cycle?** - The cell cycle is a series of events that lead to cell growth and division. ### **What are the phases of the cell cycle?** - **Interphase:** The cell grows and copies its DNA. - **G1:** Growth and preparation for DNA replication. - **S:** DNA replication. - **G2:** Growth and preparation for mitosis or meiosis. - **Mitosis:** The cell divides. - **Prophase:** Chromatin condenses into chromosomes, the nuclear envelope breaks down, and the mitotic spindle forms. - **Metaphase:** Chromosomes align at the metaphase plate - **Anaphase:** Sister chromatids separate and move to opposite poles. - **Telophase:** Chromosomes decondense, nuclear envelopes reform, and cytokinesis (division of the cytoplasm) occurs. ### **What are the different types of mitotic spindles?** - **Kinetochore microtubules:** Attach to the kinetochores of chromosomes. - **Polar microtubules:** Extend from one pole to the other, pushing the poles apart. - **Aster microtubules:** Radiate from the poles and help to position the spindle. ### **What are homologous chromosomes?** - Pairs of chromosomes, one from each parent, that carry genes for the same traits, but may have different alleles. ### **What is a sex chromosome?** - A chromosome that determines an individual's sex. - Females have two X chromosomes (XX). - Males have one X and one Y chromosome (XY). ### **What is aneuploidy?** - A condition where an individual has an abnormal number of chromosomes. ### **What are the different types of aneuploidy?** - **Trisomy:** Having three copies of a particular chromosome. - **Monosomy:** Having only one copy of a particular chromosome. - **Klinefelter syndrome:** XXY (males). - **Turner syndrome:** XO (females). ### **What is polyploidy?** - A condition where an individual has more than two sets of chromosomes. - In humans, polyploidy is lethal. ### **What is the Barr body?** - An inactive X chromosome in females. - It appears as a dense, darkly staining structure in the nucleus. - The number of Barr bodies in a cell is one less than the number of X chromosomes, so women (XX) have one, men (XY) have none. - Because the Barr body is condensed, genes on that chromosome are not expressed.