1.2 The Cell and Its Function1 (1).pptx

1.2 The Cell and Its Function Lecture Outline I. Organization of the cell – Cell constituents (proteins, lipids, carbohydrates) II. Cell structure: Membrane structure • Phospholipid bilayer • Membrane proteins • Glycocalyx III. Cytoplasm and its organelles • Endoplasmic reticulum – Rough ER, Ribosom...

1.2 The Cell and Its Function Lecture Outline I. Organization of the cell – Cell constituents (proteins, lipids, carbohydrates) II. Cell structure: Membrane structure • Phospholipid bilayer • Membrane proteins • Glycocalyx III. Cytoplasm and its organelles • Endoplasmic reticulum – Rough ER, Ribosomes, Smooth ER • Golgi apparatus • Lysosomes • Peroxisomes • Secretory vesicles • Mitochondria and ATP production • Cell Cytoskeleton-filament and tubular structures • Proteasomes and Protein Degradation • Nucleus IV. Functional Systems of the Cell – Endocytosis – Exocytosis – ATP V. Locomotion of the cell 1 The Cell and Its Function Objectives 1. Describe the basic organization of the animal cell 2. Describe the contributions of proteins, lipids, and carbohydrates to cells 3. Describe the phospholipid bilayer including the contribution of proteins, lipids, and carbohydrates 4. Identify the membranous and non-membranous organelles and their functions 5. Explain energy production relative to the mitochondria 6. Describe the structure and function of the cytoskeleton 7. Explain protein degradation of the cell 8. Describe the structure of the nucleus 9. Describe the processes and purposes of endocytosis (3 types) and exocytosis 10.Identify types of cell movements 2 References Assigned reading from your text: Hall Chapter 2 pages 13-28 Hall Chapter 68 pages 843-845 3 I. Organization of the Cell 4 Cell Organization  Two major parts of cell: – Nucleus • is separated from cytoplasm by a nuclear membrane – Cytoplasm • is separated from surrounding fluids by a cell (plasma) membrane Cell Composition- Components of Protoplasm  Five basic substances comprise protoplasm:  Water- Most cells, except adipocytes, are 70-85% water  Ions are inorganic chemicals  Proteins comprise 10-20% of cell mass  Lipids (phospholipids and cholesterol) account for ~ 2% of most cells • Important part of membrane barriers that separate different cell compartments – Cell membrane, organelles, nuclear membrane • Adipocytes are anhydrous storage- triglycerides account for ~ 95% of cell mass  Carbohydrates account for 1% of most cells • 3% in myocytes (muscle cells) • 6% in hepatocytes (liver cells) • Important for cell nutrition/energy , glycoproteins, and structural functions. Cell Proteins  Second most abundant substance in cells  Proteins are made up large numbers of amino acids – Translation is the process of protein synthesis – mRNA into a protein – Small chains are peptides (2-100 amino acids) and larger chains are polypeptides – Linked by peptide bonds joining the amino group of one amino acid to the carboxyl group of the next • • • • Primary structure is the amino acid sequence into a long polypeptide Secondary structure is the spatial arrangement of twisting and folding Tertiary and quaternary structures produce the functional structure Primary structure not affect by denaturing- others are – Simple proteins are made of polypeptides containing only amino acids only • eg albumin, insulin – Conjugated proteins are amino acids plus prosthetic groups (nonamino acid group) • Heme groups in hemoglobin • Glycosaminoglycans in proteoglycans Two Types of Cell Proteins  Two types of proteins exist in cells: • Structural proteins are present in long polymers of many individual protein molecules – Microtubules provide the cytoskeleton of organelles such as cilia, nerve axons, and mitotic spindles – Fibrillar proteins are found in collagen and elastin fibers of connective tissue – Structural proteins are typically fixed • Functional proteins are composed of a few molecules of tubular-globular proteins – Enzymes are often mobile in the cell fluid – Enzymes may be bound to the membranous structures inside the cell to catalyze reactions Cell Lipids  Lipids are grouped due to the property of being soluble in fat solvents – Two main types of lipids: • Structural lipids are inherent part of membranes and progenitors for cell signaling molecules • Neutral fat is stored in adipose- mobilized during starvation – Insoluble in aqueous solution; do not circulate in free form – Cholesterol, triglyceride, and phospholipids transported in lipoprotein complexes to increase the solubility of their lipids  Triglycerides (neutral fats) are source of anhydrous energy storage in adipose – Composed of a glycerol molecule bonded to three fatty acids – A precursor of other lipids – Composed of: • A glycerol molecule • Three fatty acids • May be saturated or unsaturated Carbohydrates • Carbohydrates are made of equal amounts of carbon and H2O • Held together by glycosidic bonds • The monosaccharides are simple sugars • glucose is the major source of energy for cells • Fructose is also a source of energy • Includes Pentoses (5 Carbon, eg ribose); and hexoses (6 Ceg glucose) • Structural (part of nucleotides) and functional (IP3 is a signaling molecule) • Disaccharides- two monosaccharides bonded together • Sucrose and lactose are acquired from diet • Oligosaccharides: 2-15 monosaccharides bonded together • Can attach to proteins = glycoproteins • Can attach to lipids = glycolipids • A and B antigens of ABO system 10 Example of Cell Carbohydrates- ABO Blood Types • ABO blood types • Presence/absence of oligosaccharide antigens • ABO Blood Groups: The human blood groups O, A, B, and AB are determined by specific glycosphingolipids found in the red cell membrane. • The addition of specific, extra carbohydrate molecule to a galactose residue on Hsubstance produces the four common blood types. Janson, Lee W.. Medical Biochemistry: The Big Picture (LANGE The Big Picture) . McGraw-Hill Education. Kindle Edition. Cell Carbohydrates- Glycogen • Polysaccharides- very large compounds • Glycogen is a homopolysaccharide- all units are the same monosaccharide • Stored in liver and muscle as polymerized glucose • Stored chemical energy • 400 gms glycogen stored in muscle • ~ 1600 kcal • 100 gms glycogen stored in liver • ~ 400 kcal • When glucose enters the cell, it is phosphorylated by a kinase to form glucose-6-phosphate (G6P) • Irreversible (except for liver cells) and is catalyzed by: • Hexokinase most cells - not affected by insulin • Glucokinase in liver cells- is affected by insulin 12 Cell Carbohydrates: emical Reactions of Glycogenesis and Glycogenolysis in Hepatoc • Liver cells have glucokinase • Glycogenesis- Insulin increases glucokinase and polymerizes G6P into glycogen • Gycogenolysis- starvation and diabetes decrease glucokinase and breakdown glycogen into glucose 13 l Carbohydrates- Glycosaminoglycans • GAGs are repeating units of different disaccharides – heteropolysaccharides • GAGs form the hydrated gels of the extracellular matrix (ECM gel) • The sugars of most GAGs are modified with sulfate groups (except hyaluranon) • Sulfate groups: • make them highly negatively charged • bind positively charged ions and trap water molecules to form hydrated gels • Hyaluranon is different: • A single long polysaccharide chain • Without sulfate groups • Synthesized at the plasma membrane by a transmembrane enzyme 14 Cell Carbohydrates - Major GAGs Major GAGs- Note lack of sulfate group on hyaluranon Cell Carbohydrates: Proteoglycans • All GAGS except hyaluronan are linked to proteins to form proteoglycans • Proteoglycans are 95% glycosaminoglycans (sugars) • Proteoglycans interact with hyaluronan to act as a filler between cells in the ECM • For example, aggrecan is a large proteoglycan consisting of more than 100 chondroitin sulfate chains jointed to a core protein. • Multiple aggrecan molecules bind to long chains of hyaluronan (green) forming large complexes in the ECM of cartilage. • The association is stabilized by link proteins (orange) 16 II. Cell Structure: Membrane Structure 17 Cell Structure  Intracellular organelles are highly organized  Cell membrane is composed primarily of lipids and proteins  Structures and organelles with these membranes include: • Cell membrane • Nuclear membrane • Endoplasmic reticulum • Mitochondria • Lysosomes • Golgi apparatus Cell Membrane: Bilayer of Phospholipids with Proteins  Cell membrane is also known as the Plasma Membrane (PM)  Plasma Membrane is found at the surface of every cell – Composed of: • Phospholipids (phosphoglycerides) • Cholesterol • Glycolipids • Proteins • Glycoproteins 19 Membrane Lipids- Phospholipids  Lipids crucial to membrane structure: are phospholipids and sterols  Phospholipids (Phosphoglycerides) are essential for cellular membranes – Composed of the following: • a glycerol molecule • 2 fatty acids (non-polar groups) • A phosphate group • A polar group (choline, serine, ethanolamine, and usually inositol) Membrane Lipids- Cholesterol  Cholesterol also important in cell membranes – Precursor to steroid/sex hormones, Vitamin D, bile acids – 80% synthesized in liver – 20% from diet  Cholesterol is interspersed in the phospholipid bilayer; – Present in varying amounts – Generally decreases membrane fluidity and permeability • Increases membrane flexibility and stability • Prevents small water-soluble molecules from penetrating the membrane Membrane Lipid Barrier Lipids in membranes: • Provide a barrier to water and water-soluble substances • Organized in a bilayer of phospholipid molecules • Polar, hydrophilic head of outer layer interacts with water of ECF • Polar, hydrophilic head of inner layer interacts with water ICF (Cytosol) • Nonpolar, hydrophobic tails interact O2, CO2 H O Glucose 2 Ions Urea Halothane Hydrophilic “head” Hydrophobic FA “tail” Cell Membrane- Fluid Mosaic Structure of the Phospholipid Bilayer  Lipids and proteins move easily about in the overall membrane structure  3rd type of membrane lipid- glycolipid • Oligosaccharide attached to the polar end of an ECF polar head • Participate in cell recognition for cell-cell interactions Two Types of Cell Membrane Proteins  Defined by their association with the lipid bilayer- integral or peripheral • Proteins provide specificity since cell and organelle membranes each have unique proteins  Integral proteins protrude through the membrane • Transmembrane proteins extend into both the ECF and ICF • Can be single or multiple pass proteins  Peripheral proteins do not penetrate the membrane • Most (not all) located in the ICF • May be attached to integral proteins K+ Integral Membrane Proteins  Integral proteins in a selectively permeable membrane: • Form channels or pores for water and ions to diffuse through the selectively permeable membrane • Function as carrier proteins that transport substances through membranes • With their electrochemical gradients- facilitated diffusion • Against their electrochemical gradient -“Active transport” uses these “pumps” • Serve as receptors that produce an intracellular response when a ligand binds • e.g. Hormones promote intracellular reactions • G Protein-could receptors (GPCRs) catalyze reactions in the plasma membrane associated with the production of second messengers • Can be enzymes that catalyze reactions • In the ECF- Small intestine enzymes digest disaccharides • In the ICF- Phospholipase A and C produce intracellular second messengers eripheral Membrane Proteins  Peripheral proteins are usually located in the ICF just under the plasma membrane • They interconnect integral membrane proteins to the cytoskeleton system of the cell • G-protein component of GPCR and signal transduction. Moves in the cell membrane on its intracellular side next to the cytosol. 26 Example of Membrane Proteins- G Protein Coupled Receptor (GP • Large class of cell receptor • 7-pass integral, transmembrane protein • Peripheral protein subunit part of the second-messenger response 27 Cell Adhesion Proteins  Adhesion molecules hold animal cells together  Major cell adhesion molecules (CAMs): • Integrins are transmembrane proteins that anchor cells to ECM via cell-matrix junctions: • Focal adhesions Anchor actin to ECM • Hemidesmosomes Anchor epithelial cells to basal laminae • Cadherins link cytoskeletons of adjacent cells: • Adherens junctions • Desmosomes  The CAM integrin connects to glycoproteins: • Fibronectin- the main adhesion protein of ECM and plasma that links ECM components to cell surfaces 28 Membrane Carbohydrates- the Glycocalyx  The glycocalyx- a loose carbohydrate coat that covers the entire outside of the cell • Membrane carbohydrates typically appear with either proteins or lipids • Glycoproteins and glycolipids • ~90% integral proteins are glycoproteins • “Glyco” portion extends to the outside of the cell • Proteoglycans also loosely attached to the outside of cell Glycocalyx Membrane Carbohydrates- Glycocalyx and Microvilli  Carbohydrate moieties have several important functions: • Negative charge of the carbohydrate chains repels other negative charges • Glycocalyx of some cells adheres to the glycocalyx of other cells • Carbohydrates act as receptors for binding hormones, such as insulin • Play a role in immune reactions ECF III. Cytoplasm and its Organelles 31 Cytoplasm  Cytoplasm contains: • Cytosol- the jelly-like fluid • 5 main organelles - Endoplasmic reticulum - Golgi apparatus - Lysosomes - Peroxisomes - Mitochondria • Cytoskeleton- 3 types of filaments • Other inclusions - Fat globules - Glycogen granules - Ribosomes - Secretory vesicles  Regions include: • Endoplasm- inner layer • Ectoplasm- outer layer Overview of Protein Processing by Organelles 33 Endoplasmic Reticulum 34 Endoplasmic Reticulum (ER) • Network of double layered membrane made of tubular and flat vesicular structures • Membrane is similar to (and contiguous with) the nuclear membrane • Space inside the tubules is called the endoplasmic matrix Nuclear Envelope and Rough Endoplasmic Reticulum 36 Rough or Granular Endoplasmic Reticulum • Outer membrane cytosolic surface covered with ribosomes • Proteins are synthesized on ribosomes and extruded into the ER matrix • Proteins are “processed” inside the matrix 1. Crosslinking of peptide bonds (primary structure) 2. Folded (secondary structure) 3. Glycosylated (N-linked) with oligosaccharides into a glycoprotein 4. Glycoproteins packaged in transport vesicles and sent to Golgi Figure 214 Rough Endoplasmic Reticulum 38 Ribosomes  Ribosomes are the site of protein synthesis in cells • Composed of Ribosomal RNA (3) and many proteins (~75) • Attached to ER: • Proteins become components of membranes or will be secreted out of cell • Free in cytosol: • Synthesize proteins that remain in cytosol – polyribosomes • e.g. hemoglobin, proteins in peroxisomes Newly Synthesized Protein Entering RER Cisterna 39 Glycoprotein in Rough Endoplasmic Reticulum Cisterna 40 Smooth or Agranular ER • Has no ribosomes attached • Found mostly in specialized cells • Site for synthesis of: - Phospholipids & cholesterol in liver − Synthesis steroid hormones (adrenal gland, ovary, testis) − Active vitamin D (epidermis) − Detoxification of drugs (liver) − Storage of calcium in most cellsespecially muscle − Glycogenolysis (liver) • Growing ER membrane buds are continuously forming transport vesicles, most of which migrate to the Golgi apparatus Endoplasmic Reticulum 42 Golgi Apparatus 43 Golgi Apparatus (Complex)  Components: • Flattened, curved, smooth membranous cisternae with internal matrix • Composed of four or more stacked layers of flat vesicular structures situated on microtubule tracks • Golgi vesicles (single membrane) at periphery of cisternae • Often cited as part of the ERGolgi System Functions of the Golgi Apparatus- Processing  The Golgi complex receives/transports, modifies, packages and sorts proteins and lipids  In the Golgi, proteins and lipids are: • Processed − Modifies hormones from inactive to active state (proinsulin to insulin) − Condenses secretory proteins and produces secretory vesicles − Produce vesicles that become part of the plasma membrane − Produces lysosomes containing digestive enzymes • Glycosylated - Most glycosylation occurs in Golgi - Adds sugar to glycolipids, glycoproteins, • and proteoglycans Function of Golgi Apparatus- Sorting and Transporting  Sorts and transports proteins and lipids: • to lysosomes • to recycling endosomesmolecule transport to the cell surface • directly to plasma membraneglycolipids and sphingomyelin synthesized • for secretion immature preprohormones from ER modified to Polarity of the Golgi  Polarity • Proteins and lipids: • Enter the Cis face- faces ER • Exit the Trans face- faces PM • Cis compartment • receives molecules from the ER • Medial and trans compartments • modify molecules • Trans-Golgi network sorts and distributes  The Golgi generates a subset of microtubules that keeps cisternae clustered together and provides tracks for vesicular movements • It is associated with a microtubule organizing center (MOTC) Golgi Vesicle Transport  Vesicles are transported between the ER-Golgi and through Golgi • By coat protein complexes • Along microtubule tracks by motor proteins • COP II Vesicles – antegrade movement to TGN • COP I Vesicles – retrograde movement of contents to ER Lysosomes 49 Lysosomes • • • • • Digestive organs of the cell Large vesicular organelle formed from budding Golgi Bounded by a single membrane Interior is kept acidic by a proton pump (H+ ATPase) • Contains 40 digestive, hydrolytic enzymes (acid hydrolases) – phosphatases – nucleases – Proteases – lipid-degrading enzymes – lysozymes digest bacteria Fuse with pinocytotic or phagocytotic vesicles to form digestive vesicles • The indigestible substances, the residual body, is excreted through the cell membrane via exocytosis Figure 2-12 Functions of Lysosomes  Lysosomes digest pinocytotic and phagocytotic foreign substances inside the cell  Regression of tissues to a smaller size – Uterus after pregnancy – Disuse atrophy of skeletal muscle when limb in a cast  Autolysis of damaged cells – Lysosomes release enzymes and destroy damaged cells – Apoptosis is orderly, planned “programmed” cell death (caspases) – Bactericidal agents phagocytize bacteria before they cause damage • Lysozyme- dissolves bacterial wall • Lysoferrin- binds iron and prevents bacterial growthFigure 2-12 • Acid (pH ~5.0) activates hydrolases and inactivates Lysosomes 52 Example of a Lysosomal Storage Disease • Absence of one or more hydrolases • not synthesized • inactive • not properly sorted and packaged • Result: Lysosomes become engorged with undigested substrate • Example: Tay-Sachs disease – neurodegenerative disorder • Genetic mutations result in the loss of an enzyme that catalyzes the biodegradation of fatty acid derivatives • Results in blindness and intellectual disabilities • Mortality commonly occurs by age 5 Autophagy • An autophagosome transfers worn-out organelles to lysosomes for autophagy • Housekeeping process for degrading and recycling obsolete organelles and large protein aggregates • Important for tissue development and cell survival when nutrients are scarce 55 Cell Turnover  Autophagy • Single membrane vacuole forms in the cytoplasm around a dying mitochondrion or other component • A primary lysosome fuses to form a secondary lysosome where cellular components digested • Chemical products recycled by the cell Figure 2-13 Peroxisomes 57 Peroxisomes • Similar physically to lysosomes- single membrane vesicles • Major functions: • To oxidize harmful substances (e.g. alcohol) • Breakdown of very long chain fatty acids (>C22) from triglycerides by beta-oxidation to shorter chain FAs for beta oxidation in mitochondria • Only in peroxisomes since catalase required • Specificity for longer chain FAs • Shorter chain FAs sent to mitochondria to complete beta-oxidation • Two major differences from lysosomes: – Formed by self-replication (division) or by budding off ER/Golgi – They contain oxidases and catalase Peroxisomal • Oxidation Ethanol metabolism (yellow): • • Oxidases combine with O2 and H to form hydrogen peroxide (H2O2) • Oxidize ethyl alcohol into acetaldehyde Catalases break down H2O2 • VLCFAs • Catalase breaks into shorter FA chain • Diseases linked to peroxisome deficiency (inability to breakdown VLCFA) Secretory Vesicles 60 Secretory Vesicles or Granules • Secretion is an important cell function • Secretory products formed by the ER-Golgi system and stored in secretory vesicles (granules) • Typically store proenzymes that are not yet activated Mitochondria 62 Mitochondria  Primary function: extraction of energy from nutrients • Form 95% of cell’s ATP- mostly through oxidative phosphorylation • Also function to regulate apoptosis  Contain their own DNA, RNA and synthesize own proteins • Presence of DNA = Self-replicating • Cells with increased energy demands can increase the density of mitochondria  Plentiful in active cells • Skeletal and heart muscle  RBCs have no mitochondria • All other cells do MITOCHONDRION 64 Dividing Mitochondria 65 Mitochondria and ATP Production  Structure facilitates oxidative phosphorylation • Elongated, double-membrane structure (shapes can vary) has an: • Outer membrane • Intermembrane space lies between the inner and outer membranes • Inner membrane exists in folds/shelves of “cristae” • Cristae contain the proteins of the electron transport system • a central Matrix space • Matrix contains the Krebs (TCA) cycle enzymes Mitochondrion Contains Electron Transport System Intermembrane Space Contains Krebs Cycle Enzymes Krebs Cycle Metabolizes Pyruvic Acid Produced In Cytosol, Fatty Acids and Amino Acids; ETS Produces ATP 67 Adenosine Triphosphate • ATP is the most important energy-rich phosphate compound • Energy used in cellular processes is primarily stored in bonds • Between phosphoric acid residues and organic compounds • High-energy phosphate that releases 10-12 kcal/mol when hydrolyzed • On hydrolysis to ADP, it liberates energy 68 ATP Links Energy Production and Utilization 69 Overview of Carbohydrate Metabolism 70 Use of ATP for Cellular Function 1. Membrane transport 2. Synthesis of chemical compounds 3. Mechanical work Figure 216 ATP Production Step 1 - breakdown to substrates Figure 215 • Carbohydrates are converted into glucose • Proteins are converted into amino acids • Fats are converted into fatty acids Step 2 – pyruvic acid then acetyl CoA • Glucose, AA, and FA are processed into Acetyl-CoA Step 3 • Acetyl-CoA condenses with oxaloacetate to enter TCA cycle then oxidative A maximum of 38 molecules of ATP are formed per molecule of glucose degraded. Citric Acid Cycle – aka Tricarboxylic Acid Cycle – aka Krebs Cycle  Energy source for most cellular energy-requiring activities  Metabolic reactions for glycolysis (pyruvic acid) occurred in cytosol  TCA cycle enzymes metabolize pyruvic acid, fatty acids, and amino acids  Each of the intermediate steps serve multiple other metabolic functions  As the compounds are metabolized, ETS is producing ATP Electron Transport and Synthesis of ATP 74 Cytoskeleton 75 Cytoskeleton (aka Microtrabecular System)  Eukaryotic cells have a cytoskeleton: • A system of protein filaments extending through the cytoplasm • Provides a structural framework that determines cell shape, positions of organelles, and general organization of the cytoplasm • Maintains flexible support to cell structure • Permits cell movements Three Main Types of Protein Filaments Cytoskeleton- Microfilaments  Microfilaments – F-Actin – Occur in ectoplasm, microvilli – Along with thicker myosin filaments • Serve as the contractile unit of muscle cells • Produce cytoplasmic movements of phagocytosis and cytokinesis – Actin filaments assembly via treadmilling- dynamic in the cell 78 Figure 13.3 Treadmilling and the role of ATP in actin filament polymerization Cytoskeleton- Intermediate Filaments  Intermediate Filaments – Comprised of cell-specific fibrillar monomers • e.g. desmin, vimentin, keratins in epithelium, nuclear lamins, neurofilaments in neurons – Resist stretch – More stable than microfilaments – Maintain cell shape – Together with plasma membrane form: • Hemidesmosomes • Desmosomes 80 DESMOSOME 81 Cytoskeleton- Microtubules  Microtubules – Hollow tubules of protein called tubulin – Contribute to the strength of the cytoskeleton – Highly dynamic because of rapid disassembly and reassembly that contribute to change in cell shape – Provide movement of chromosomes during cell division – Provide “railroad tracks” in cytosol for movement of mitochondria and vesicular structures by means of “motor proteins” • Particularly in axons of neurons 82 Molecular Motors • Motor proteins move “cargo” • proteins, organelles, other cell parts • ATPases- convert the energy of ATP into movement • Attach to: • Cargo at one end of the molecule • Microtubule or actin at the other end with their “head” • Three families: Kinesin- most move toward the (+) end Dynein moves toward the (-) end Myosins assist with muscle contraction 83 The role of GTP in microtubule polymerization Cytoskeleton- Comparison of Filaments 85 Cytoskeleton and Cell Junctions 86 Centrosomes and Centrioles  Centrosomes are located near the nucleus • Made of two microtubular structures (centrioles) at right angles to one another • “9+0” arrangement of microtubules 87 Microtubule Organizing Center (MTOC) • Microtubule organizing centers (MTOCs) are where microtubules emerge • Two functions: • Organize the mitotic and meiotic spindle apparatus which separate chromosomes during cell division • Organize flagella and cilia • Most animal cells have one pair of centrioles at their center during interphase- closely associated with the Golgi apparatus • MTOCs organize microtubules into the mitotic spindle that causes the movement of chromosomes during cell division • Are self-replicating: recently RNA has been found associated with centrioles, and this may be the source of The self-replicating mechanism 88 “9+0 Arrangement” 89 Cilia and Flagella  Cilia and flagella extend out of the cell's cytoplasm into ECF  Cilia • Composed of microtubules that are surrounded by plasma membrane; “9+2” arrangement of microtubules • Cilia are short and propel mucus and other substances over the surfaces of epithelia • Most cells contain a single non-motile primary cilium that serves as a sensory organelle that receives both mechanical and chemical signals from other cells  Flagella • Are 10-100x longer than cilia; • One per cell • Cause movement of a cell • Spermatozoa are the only flagellated cells in humans 90 Cilia 91 92 Proteasomes and Protein Degradation 93 Protein Degradation & Ubiquination  Protein degradation and recycling is carefully regulated  Reasons for protein degradation includes: • ~ 30% new proteins are abnormal due to misfolding • Aged normal proteins need to be replaced • Some proteins are part of an invading virus  Proteasomes degrade misfolded, old, and viral proteins • Conjugation with the polypeptide ubiquitin marks them for degradation • Degradation occurs in multisubunit proteolytic particles- proteasomes • Ubiquinated membrane proteins can also be degraded by lysosomes • Proteasomes inside the cell/ lysosomes outside the cell • Post-translational modifications by ubiquitin important for cell interactions and signaling pathways – not just destruction 94 Proteasomes • Proteasomes are made of multiple protein units • Ubiquinated protein enters the proteasome and is enzymatically broken down into small peptides • Ubiquitin is liberated in the cytosol • Peptides resulting from proteasome activity are further broken down into amino acids by endopeptidases in the cytosol 95 P-Bodies (Processing Bodies) These are sites in the cell where “used” m-RNA is inactivated, stored, reactivated for future use or destroyed after use P-BODIES DISSEMINATED IN THE CYTOSOL AROUND A NUCLEUS 96 Nucleus 97 The Nucleus • The nucleus is the control center of the cell • Controls biochemical reactions in the cell via information coded in DNA • The double nuclear membrane (composed of phospholipid bilayers) and matrix are contiguous with the endoplasmic reticulum Figure 29 Nuclear Membrane and Nuclear Pores • Nuclear membrane is permeated by thousands of nuclear pores • Separates the contents of the nucleus from the cytoplasm • Contains pores that restrict movement of large molecules between the nucleus and cytoplasm • Selectively permeable to molecules up to 44,000 MW • 100 nm in diameter yet functional diameter is ~9 nm NUCLEAR PORES 100 Chromatin and Nucleolus • Nucleoplasm is a suspension and reaction medium located in the nuclear envelope • Chromatin is found in the nucleoplasm • • • Composed of DNA, histones (basic proteins), and non-histones (acidic proteins) During interphase, chromatin is located throughout neoplasm • In non-dividing cells- chromatin is attenuated (stretched out) During mitosis, chromatin organizes into chromosomes • In dividing cells- chromatin is condensed into chromosomes • 23 pairs in humans; 44 autosomes, 2 sex chromosomes (XX or XY) Nucleolus Nucleolus • One or more per nucleus- found in nucleoplasm • High content of r-RNA (three types) and proteins (~75) • Precursors to ribosomes • Nucleolar pre-ribosomes migrate to cytoplasm and are converted into mature ribosomes during migration • • Not membrane delimited Functions to form the granular “subunits” of ribosomes Cell Nucleus Chromatin Nucleolus Nuclear Envelope 103 IV. Functional Systems of the Cell 104 Exocytosis and Endocytosis Transport Into the Cell – 2 Forms of Endocytosis  Endocytosis is ingestion by the cell • Endocytosis is the way large particles enter the cell (active transport) Two principle forms are:  Pinocytosis- “cell drinking” ingestion of minute water-soluble particles • 2 Types of pinocytosis: • Fluid-phase or constitutive pinocytosis occurs in most cells • Clathrin-mediated endocytosis occurs at membranes with clathrin • Regions of the plasma membrane are slightly invaginated (coated pits) and cytosolic side of the membrane is coated with proteins (mainly clathrin  Phagocytosis- “cell eating”is the process by which bacteria, dead cells, etc are engulfed by cells  All are active transport that require ATP Transport Into the Cell- Constitutive Pinocytosis  Constitutive pinocytosis- continually occurs to replace cell fluid volume • Vesicles of ECF and particulate constituents form inside cytoplasm • Deals with uptake of small quantities of ECF and dissolved substancesincluding water soluble molecules- but not particulates • e.g. 3% of total macrophage membrane is engulfed each minute • Rate increases as macromolecules present to the cell Transport Into the Cell- Lysosomes Secretory vesicles diffuse through the cytosol and fuse to the plasma membrane. Lysosomes fuse with internal endocytotic vesicles. Figure 2-14 Transport Into the Cell- Receptor-Mediated Pinocytosis  Clathrin-mediated or receptor-mediated is used to migrate vesicles through cytosol • • • • • • Pits occur at membrane indentations where the protein clathrin accumulates As endocytosis progresses, clathrin molecules form a geometric array that surrounds the endocytotic vesicle before it pinches away Once the complete vesicle is formed, clathrin falls off and is recycled to form another vesicle At the destination, clathrin is removed from the early endosome, the vesicle becomes “smooth” and fuses with another early endosome produced from fusion of previously uncoated vesicles • These are now called late endosomes Primary lysosomes from the Golgi that contain hydrolytic enzymes fuse with late endosomes Enzymes digest substances that produce amino acids and other small molecules that will be released to the cytoplasm for utilization by the cell Clathrin-Mediated Endocytosis • Molecules attach to cell-surface receptors concentrated in clathrin-coated pits • Receptor binding induces invagination • Also ATP-dependent and involves recruitment of actin and myosin CLATHRIN-COATED VESICLES 111 Example of cellular uptake of cholesterol Transport Into the Cell- Phagocytosis  Cell engulfs a bacterium, abnormal cell, or debris by extending pseudopods • Characteristic of neutrophils, monocytes, and macrophages • Neutrophils recognize bacteria or dead cell and tissue debris • The plasma membrane of the neutrophil is stimulated • Results in evagination of pseudopods to surround a particulate and to engulf it by action of actin and myosin microfilaments at the inner surface of the plasma membrane Transport Into the Cell- Phagocytosis (continued) • ATP energy expended for movement of pseudopods • Large vacuole pinched off and is free in cytoplasm • Primary lysosome will fuse with the vacuole and release its digestive enzymes into the vacuole • After digestion, small molecules enter cytosol • Undigested substances remain inside (residual body) that are eventually released by exocytosis or may accumulate within the cell PHAGOCYTOSIS 115 Phagocytosis - Lysosome Digesting Food 116 Digestion of Substances in Pinocytotic or Phagocytic Vesicles Ends in excretion via exocytosis of waste products (residual bodies) to the exterior of the cell Allows for new portions of membrane to be added to the PM to offset the loss of PM resulting from endocytosis Secretion by Exocytosis • Secretory vesicles containing proteins synthesized in the RER bud from the Golgi apparatus • Exocytosis- Fusion of vesicles with plasma membrane and the extrusion of contents to outside • Exocytosis is typically stimulated by the entry of calcium ions into the cell − constitutive secretion— happens randomly − stimulated secretion— requires trigger • Exocytosis is a mechanism that releases polar substances from cells to ECF (e.g. ACh, NE, hormones, enzymes) Figure 2-14 V. Locomotion of the Cell 119 Ameboid Locomotion • Continual endocytosis at the “tail” and exocytosis at the leading edge of the pseudopodium • Attachment of the pseudopodium is facilitated by receptor proteins carried by vesicles • Forward movement results through interaction of actin and myosin (ATPdependent) Figure 2-17 Cell Movement Is Influenced by Chemical Substances • • • An important mediator of ameboid locomotion is chemotaxis Chemotaxis results from the appearance of certain substances in the tissues Chemotactic substances cause chemotaxis Chemotaxis Low concentration (negative) high concentration (positive) Cilia and Ciliary Movements Movement occurs by the protein Dynein- which has ATPase activity 122 123