Cellular Structures and Processes Notes

Contents

Osmosis High-Yield Notes

This Osmosis High-Yield Note provides an overview of Cellular Structures and Processes essentials. All Osmosis Notes are clearly laid-out and contain striking images, tables, and diagrams to help visual learners understand complex topics quickly and efficiently. Find more information about Cellular Structures and Processes:

Cellular structure and function

Resting membrane potential

Cell signaling pathways

Cytoskeleton and intracellular motility

Nuclear structure

Cell membrane

Selective permeability of the cell membrane

Extracellular matrix

Cell-cell junctions

Endocytosis and exocytosis

Osmosis

NOTES NOTES CELLULAR STRUCTURES & PROCESSES CELLULAR STRUCTURE & FUNCTION osms.it/cellular-structure-and-function CELL STRUCTURE BASICS ▪ Basic structural, biological, functional unit that comprise organism ▪ Smallest self-replicating life-form ▪ Over 200 types in human body ▪ Cells → tissue → organ → organ systems → organism Basic constituents ▪ Plasma membrane ▪ Cytoplasm ▫ Fluid suspension ▫ Composition: cytosol, organelles CYTOSOL ▪ Intracellular fluid ▫ Composition: water; dissolved/ suspended organic, inorganic chemicals; macromolecules; pigments; organelles ▪ Site of most cellular activity ORGANELLES ▪ Specialized cellular subunits carry out essential functions Ribosomes ▪ Composition: rRNA, ribosomal proteins ▪ Can exist freely in cytoplasm/bound to endoplasmic reticulum (forms rough endoplasmic reticulum) ▪ Turns mRNA into protein via translation ▪ Organized into two subunits (40s, 60s) ▫ Small subunit: binding sites for mRNA, tRNA 160 OSMOSIS.ORG ▫ Larger subunit: has ribozyme to catalyze peptide bond formation (for bonds between amino acids) Endoplasmic reticulum ▪ Membrane-enclosed organelle ▪ Appearance: stack of membranous, flattened disks (cisterns) ▪ Rough endoplasmic reticulum (RER) ▫ Contains bound ribosomes on surface ▫ Site of packaging, folding of proteins designated for secretion, lysosomal degradation, plasma membrane insertion; proteins packed into vesicles, sent to Golgi apparatus for further modification ▫ RER cisterna continuous with nuclear envelope ▪ Smooth endoplasmic reticulum (SER) ▫ No ribosomes ▫ Site of lipid, steroid synthesis, Ca2+ ions storage (muscles), glycogen metabolism, detoxification (liver) Golgi apparatus (complex) ▪ Membrane-enclosed organelle ▫ Appearance: collection of fused, flattened sacs (cisterns) with associated vesicles, vacuoles ▪ Two sides ▫ Cis side: receives proteins from RER (entry) ▫ Trans side: opposite side, releases vesicles towards plasma membrane (exit)
Chapter 23 Cellular Physiology: Cellular Structures & Processes ▪ Post-translational modification site (e.g. phosphorylation, glycosylation, sulfonation) of proteins, lipids, hormones → sorted, packaged into secretory vesicles → secreted out of cell/lysosomal fusion/plasma membrane insertion ▫ In glucose absence, mitochondria can use fatty acids as fuel via beta oxidation (only medium sized fatty acids used; longer ones chopped by peroxisome) ▪ Mitochondria number: correlates with cell activity/energy requirements Mitochondria ▪ Double membrane-enclosed organelle; synthesizes ATP for cell via aerobic respiration ▫ Outer smooth membrane: encloses whole organelle ▫ Inner membrane: forms folds, caverns called cristae (contain proteins needed for aerobic respiration); encloses mitochondrial matrix (contains mitochondrial DNA, ribosomes) ▪ Intermembrane space: space between inner, outer membrane ▪ In cytoplasm glucose undergoes glycolysis, glucose cleaved into pyruvate ▫ Pyruvate enters mitochondria → citric acid cycle (Krebs cycle), electron transport chain (require oxygen) Nucleus ▪ Large, membrane-enclosed organelle present in all cells except mature erythrocytes ▪ Contains genetic material (DNA, tightly packed into chromatin); coordinates cellular activities ▪ Most cells contain one nucleus; some cells have more (e.g. skeletal muscle cells, osteoclasts, hepatocytes) ▪ Usually spherical, may take on other shapes ▫ Lobulated (e.g. polymorphonuclear leukocytes) ▫ Elongated (e.g. columnar epithelium) Figure 23.1 Cellular structures and their functions. OSMOSIS.ORG 161
CELL MEMBRANE osms.it/cell-membrane ▪ Semipermeable membrane made from phospholipid bilayer; surrounds cell cytoplasm Phospholipid bilayer ▪ Two-layered polar phospholipid molecules comprising two parts ▫ Negatively charged phosphate “head” (hydrophilic; oriented outwards) ▫ Fatty acid “tail” (hydrophobic; oriented inwards) ▪ Semipermeable ▫ Allows passage of certain molecules through membrane (O2, CO2, etc.) ▫ Denies passage of others (large molecules such as proteins, glucose) ▪ Certain molecule transportation (ions, H2O) allowed through embedded membrane proteins (ion channels, pumps) Figure 23.3 Transport proteins move molecules that can’t freely diffuse across the cell membrane. Channels form a tunnel through which water and ions flow. Carriers have a binding site for a specific molecule and gates at both ends that open sequentially. Enzymes, or ATPases, actively pump ions in/out of the cell against their concentration gradients. Figure 23.2 Phospholipid parts and their arrangement in a cell membrane. 162 OSMOSIS.ORG
Chapter 23 Cellular Physiology: Cellular Structures & Processes SELECTIVE PERMEABILITY OF THE CELL MEMBRANE osms.it/cell-membrane-selective-permeability ▪ Cell membrane controls which molecules enter, leave ▫ Passive transport: no energy required ▫ Active transport: energy required → adenosine triphosphate (ATP) PASSIVE TRANSPORT Simple diffusion ▪ Random molecular motion ▪ Small, nonpolar molecules move from ↑ concentration → ↓ concentration Fick’s law ▪ Three factors affect diffusive flux ▪ Concentration gradient ▫ Larger differences in solute concentration on each side of membrane → ↑ driving force → ↑ net diffusion ▫ Equal concentrations → no net diffusion (e.g.CO2, O2 movement between alveoli, blood) ▪ Membrane surface area ▫ ↑ surface area available for diffusion → ↑ diffusion rate; vice versa (e.g. microvilli in small intestines amplify surface area → ↑ nutrient, water absorption) ▪ Distance separating each side of membrane (e.g. thickness) ▫ ↑ distance molecules must travel → ↓ net diffusion; vice versa (e.g. pulmonary edema → ↑ distance between compartments → ↓ net diffusion) Facilitated diffusion ▪ Uses transport proteins (e.g. channels, carrier proteins) ▪ Allows larger/polar molecules to move across membrane Channels ▪ Non-specific; open to allow water, small polar molecules through (e.g. voltage-gated calcium channel) Carrier proteins ▪ Very specific, only allow certain molecules to bind (e.g. glucose transporter protein GLUT4) ACTIVE TRANSPORT Primary ▪ Uses ATP ▫ Enzymes called ATPases use ATP as fuel; (e.g. Na+-K+ ATPase, Ca2+ ATPase, H+-K+ ATPase) ▫ May create concentration/ electrochemical gradients Secondary ▪ Uses existing electrochemical gradients ▫ One solute, normally Na+, moves with concentration gradient through transporter → supplies energy transporter needs to → another solute against concentration gradient in same/ opposite direction as Na+ (e.g. sodiumglucose SGLT1 transporter) Bulk transport ▪ AKA vesicular transport ▪ Endocytosis ▫ Cell membrane invaginates, pulling something in from outside (e.g. pathogen phagocytosis) ▪ Exocytosis ▫ Vesicle inside cell pushes something out (e.g. hormone secretion) OSMOSIS.ORG 163
Figure 23.4 Endocytosis and exocytosis. EXTRACELLULAR MATRIX osms.it/extracellular-matrix ▪ Environment surrounding cells ▪ Varies between tissues (epithelial, connective, muscular, and nervous) THREE MAJOR MOLECULES Adhesive proteins ▪ Adhere cells together (communication with extracellular fluid) ▫ E.g. integrins, cadherins Structural proteins ▪ Give tissues tensile, compressive strength ▪ Collagen ▫ Resists tension, can stretch 164 OSMOSIS.ORG ▫ Starts as procollagen → cleaved into tropocollagen → arranged into collagen fibrils ▫ Four types: type I (bone, skin, tendon), type II (cartilage), type III (reticulin, blood vessels), type IV (basement membrane) ▪ Elastin ▫ Elastic, returns tissue to original shape ▪ Keratin ▫ Tough, found in hair, nails Proteoglycans ▪ Fill space between cells, hydrate, cushion cells ▫ Consists of protein core with sugar chains
Chapter 23 Cellular Physiology: Cellular Structures & Processes Figure 23.6 The three kinds of structural proteins in the extracellular matrix and their functions. Figure 23.5 Cadherins and integrins are both adhesive proteins which hold cells together. Figure 23.7 Collagen production steps. Figure 23.8 Structure of proteoglycans, which hydrate and cushion cells. OSMOSIS.ORG 165
CELL-CELL JUNCTIONS osms.it/cell-cell_junctions ▪ Protein structures that physically connect cells ▪ Improve cellular communication, tissue structure; allow transport of some substances between cells, create impermeable barrier for others ▪ Only found between immobile cells; abundant in epithelial tissue (e.g. in skin) THREE JUNCTION TYPES Tight junctions ▪ E.g. in gastrointestinal tract/brain ▪ Seal adjacent-cell plasma membranes, especially near apical surface; prevent passage of water, small proteins, bacteria ▫ Formed by claudins, occludins embedded in cellular plasma membranes ▫ In “leaky” epithelia, tight junctions may allow certain molecules to pass (e.g. K+, Na+, Cl- in kidney’s proximal tubules— due to ion pores) Figure 23.9 The three types of cell junctions. 166 OSMOSIS.ORG Adherens junctions ▪ E.g. in skin ▪ Anchor cells together, provide strength; consist of three major components ▫ Actin filaments: provide cellular shape ▫ Protein plaques: anchor membrane, bind to actin filaments ▫ Cadherins: attach to protein plaques, connect to cadherins on other cells Gap junctions ▪ E.g. in heart ▪ Connect adjacent cells, allow rapid communication; formed by connexins → create tubular structure (allows charged particles to pass) ▫ In cardiac myocytes: gap junctions create coordinated heart contractions ▫ In infected cells: gap junctions send cytokines to neighboring cells, triggering apoptosis, preventing infectious spread (“bystander effect”)
Chapter 23 Cellular Physiology: Cellular Structures & Processes ENDOCYTOSIS & EXOCYTOSIS osms.it/endocytosis-and-exocytosis ▪ Transports material in/out of cell ▪ Requires adenosine triphosphate (ATP) for energy ENDOCYTOSIS ▪ Cells engulf extracellular material ▪ Edges of pit come together, clathrin proteins link up ▪ Vesicle pinches off; clathrin detaches, returns to cell membrane ▪ Vesicle merges with endosome to separate receptors into second vesicle PHAGOCYTOSIS ▪ AKA cell eating ▪ Used by white blood cells (e.g. macrophages, neutrophils) Process ▪ Cell extends arm-like projects (AKA pseudopods) around target ▪ Cell membrane slowly engulfs target, invaginates to form vesicle ▪ Vesicle separates from cell membrane to form phagosome ▪ Phagosome fuses with lysosome, target is digested ▪ Debris released by exocytosis PINOCYTOSIS ▪ AKA cell drinking ▪ Used by most cells to take in extracellular fluid; non-specific Process ▪ Cell membrane invaginates around extracellular fluid ▪ Edges of invagination come together to form vesicle ▪ Motor proteins use ATP to carry vesicle into cytosol Figure 23.10 The three types of endocytosis. EXOCYTOSIS ▪ Cells expel material into extracellular space (e.g. neurotransmitters, hormones) ▪ Last phagocytosis step Process ▪ Golgi apparatus creates vesicle from various proteins, lipids, hormones ▪ Motor proteins use ATP to carry vesicle along cytoskeleton ▪ Vesicle is pressed against cell membrane until rupture → spills contents into extracellular space RECEPTOR-MEDIATED ENDOCYTOSIS ▪ Used by cells to take in specific molecules (e.g. iron, cholesterol) Process ▪ Clathrin-covered pits/coated pits with receptors bind certain molecules Figure 23.11 Exocytosis: expulsion of material into extracellular space. OSMOSIS.ORG 167
OSMOSIS osms.it/osmosis ▪ Passive water-flow across selectively permeable (semipermeable) cellular membrane; primarily determined by solute concentration differences (osmotic pressure) Factors affecting water movement across membrane ▪ Molecules (e.g. water molecules, ions) tend to move around (kinetic energy) + movement is disordered, random (entropy) → larger solutes tend to block openings in semipermeable membrane ▪ If solute ions positively charged, they attract slightly negatively charged oxygen atom in water molecule; if solute ions are negatively charged, they attract slightly positively charged hydrogen atoms in water molecule → water molecules partially attached to ion → movement through membrane impeded ▪ Water molecules tend to move from hypotonic side (more water/less solutes) to hypertonic side (less water/more solutes) SELECTIVELY-PERMEABLE MEMBRANE ▪ Allows small molecules (e.g. water) across, but not larger molecules/ions Isotonic solution ▪ Side A = side B ▪ If solute concentration is same on each side of membrane → net water movement across membrane is zero (equilibrium) Hypertonic/hypotonic solution ▪ Side A > side B or side B > side A ▪ If solute concentration is greater on one side (hypertonic) → net water migration across membrane is from hypotonic side toward hypertonic side CELLULAR EFFECT ▪ Red blood cell in hypertonic solution → net movement of water molecules out of cell → cell shrinks (crenation) ▪ Red blood cell in hypotonic solution → net movement of water molecules into cell → cell swells, may burst (lyses) Figure 23.12 Net water molecule movement between isotonic, hyper/hypotonic solutions. 168 OSMOSIS.ORG
Chapter 23 Cellular Physiology: Cellular Structures & Processes RESTING MEMBRANE POTENTIAL osms.it/resting-membrane-potential ▪ Electric potential across cell membrane ▫ Given by weighted (based on membrane permeability) sum of equilibrium potentials for all ions ▪ High concentrations of Na+, Cl-, Ca2+ outside cell; high concentrations of K+, A(various anions) inside cell → concentration gradients are established ▫ Sodium-potassium pump uses ATP to move two K ions into cell, three Na ions out ▫ Potassium concentration = 150mMol/L inside cell, 5mMol/L outside ▪ Concentration gradients establish electrostatic gradients ▫ Concentration gradient pushes potassium out through potassium leak channels, inward rectifier channels ▫ Anions remain in cell → negative charge builds up → potassium is pulled back into cell ▪ Equilibrium (Nernst) potential: electrostatic gradient equal to concentration gradient (-92mV for potassium) ▪ Nernst equation: equilibrium potential for an ion ⎛ [ION]out ⎞ Vm = 30.75 × log ⎜ ⎟ ⎝ [ION]in ⎠ ▫ Double charge: ▫ Value is flipped for negative ions ▪ Resting membrane potential is sum of equilibrium potentials of major ions multiplied by their membrane permeabilities Figure 23.13 Equilibrium potential = electric potential for attracting K+ back into the cell that’s needed to balance the concentration gradient pushing K+ out of the cell. ⎛ [ION]out ⎞ ▫ Single charge: Vm = 61.5 × log ⎜ ⎟ ⎝ [ION]in ⎠ Figure 23.14 The resting membrane potential is closest to the equilibrium potential of the most permeable ion (K+). Change in permeability → change in resting membrane potential. OSMOSIS.ORG 169
CELL SIGNALING PATHWAYS osms.it/cell-signaling-pathways INTRACELLULAR SIGNAL CLASSIFICATION ▪ Classified according to distance between signaling, target cells ▫ Autocrine: cell signals nearby cells of same type, including itself (e.g. monocytes secrete interleukin-1 β) ▫ Paracrine: cell signals nearby cells of different type (e.g. ECL cells secrete histamine → signals D cells to secrete somatostatin) ▫ Endocrine: cell signals distant cells (e.g. pituitary gland secretes TSH → signals thyroid gland) ▪ Signalling molecules (ligands) bind to receptors; can be hydrophobic/hydrophilic ▫ Hydrophobic: can’t float in extracellular space → brought to target cells by hydrophilic carrier proteins; can diffuse over cell membranes → bind to receptors inside cell ▫ Hydrophilic: can float in extracellular space → reach target cells themselves; can’t diffuse over cell membranes → bind to cell surface (transmembrane) receptors Cell signalling pathway stages 1. Reception: ligand binds to receptor 2. Transduction: receptor changes activating intracellular molecules 3. Response: signal triggers a response in the target cell MAJOR TRANSMEMBRANE RECEPTOR CLASSES G protein-coupled receptors ▪ Seven-pass transmembrane receptors ▪ Activate guanine nucleotide-binding (G) proteins inside cell ▫ G proteins have three subunits: alpha, beta, gamma ▫ Alpha binds guanosine diphosphate (GDP) when inactive ▫ When ligand binds, alpha releases GDP, binds guanosine triphosphate (GTP) instead → alpha separates from beta, gamma → alpha interacts with proteins turning GTP back into GDP → reattaches Figure 23.15 Autocrine, paracrine, and endocrine signals refer to signal distance from its target cell. Hydrophobic and hydrophilic ligands refer to the affinity of the ligand for water. 170 OSMOSIS.ORG
Chapter 23 Cellular Physiology: Cellular Structures & Processes Figure 23.16 Mechanism of action of G-protein coupled receptors. ▪ Three types of G protein with different pathways ▫ Gq: activates phospholipase C in cell membrane → phospholipase C cleaves phosphatidylinositol 4,5-bisphosphate into inositol trisphosphate, diacylglycerol → inositol trisphosphate opens calcium channels in endoplasmic reticulum (calcium flows to cytoplasm, changing electrical charge distribution in cell → cell depolarization); diacylglycerol binds to protein kinase C which phosphorylates target proteins ▫ Gs: stimulates adenylate cyclase → adenylate cyclase removes phosphate from adenosine triphosphate (ATP) creating cyclic adenosine monophosphate (cAMP) → cAMP binds to regulatory subunit of protein kinase A → catalytic subunit of protein kinase A phosphorylates target proteins ▫ Gi: inhibits adenylate cyclase → negative feedback on Gs Enzyme-coupled receptors ▪ Single-pass transmembrane receptors ▪ Trigger enzymatic activity inside cell when specific ligands bind ▪ Composition: extracellular, ligand-binding domain; intracellular, enzymatic domain ▪ Three main enzyme-coupled receptor types ▫ Receptor tyrosine kinases: when ligand binds, these phosphorylate their own tyrosine residues → conformational change creates binding site for other signalling proteins ▫ Tyrosine kinase associated receptors: when ligand binds, these phosphorylate various proteins to relay signal to tyrosine kinases inside cell ▫ Receptor serine/threonine kinases: when ligand binds, type II receptors of this kind phosphorylate type I receptors, which in turn phosphorylate various proteins to relay signal to serine/ threonine kinase domain inside cell Ion channel receptors ▪ Ion channels which open when specific ligands bind ▪ Allow ions (e.g. chloride, calcium, sodium, potassium) to flow through ▪ Resulting shift in electric charge distribution triggers response OSMOSIS.ORG 171
Figure 23.17 Gq pathway. Figure 23.18 Gs pathway. Figure 23.19 Gi pathway. 172 OSMOSIS.ORG
Chapter 23 Cellular Physiology: Cellular Structures & Processes Figure 23.20 Types of enzyme-coupled receptors and their pathways. Figure 23.21 Mechanism of action for ion channel receptors. OSMOSIS.ORG 173
HORMONAL MECHANISMS ▪ All cells receive, process outside signals via specific proteins (receptors) ▫ Ligand (signalling molecule—e.g. hormone) binds to receptor → physiological response ▪ Target tissue sensitivity to hormone effect controlled by receptor quantity/affinity ▫ ↑ receptor quantity → ↑ maximal response ▫ ↑ receptor affinity → ↑ response likelihood HORMONE RECEPTOR UPREGULATION/DOWNREGULATION Downregulation ▪ External stimulus → cell ↓ hormonal receptor quantity/affinity ▫ Chronic exposure to excessive signalling molecules (e.g. neurotransmitters/ drugs → ligand-induced target receptor desensitization/internalization) ▫ Hormones may alter other hormonal receptor sensitivity (e.g. in uterus— progesterone downregulates its own receptor, estrogen receptor) ▫ Mechanisms: ↓ new receptor synthesis, ↑ existing receptor degradation, inactivating receptors Upregulation ▪ External stimulus → cell ↑ hormonal receptor quantity/affinity ▫ Repeated exposure to receptor antagonists/prolonged ligand absence → upregulation ▫ Hormone may upregulate receptors for other hormones (e.g. in uterus estrogen upregulates its own receptor, also luteinizing hormone (LH) receptors in ovaries) ▫ Mechanisms: ↑ new receptor synthesis, ↓ existing receptor degradation, activating receptors SECOND MESSENGER SYSTEMS ▪ Primary extracellular signalling molecules often hydrophilic → cannot cross cell membrane → second messenger system carries, amplifies signal across cell membrane 174 OSMOSIS.ORG ▪ Second messengers: intracellular signalling molecules released by cells → triggers physiological changes in response to hormone/ligand–receptor interaction ▫ Include: cyclic AMP (cAMP), cyclic GMP (cGMP), inositol trisphosphate (IP3), diacylglycerol (DAG), Ca2+ ▫ Involved in cellular processes: proliferation, differentiation, migration, survival, apoptosis G PROTEINS ▪ Membrane-bound proteins: act as molecular switches, couple hormone receptors to effector enzymes ▪ Heterotrimeric proteins → three subunits → alpha (α), beta (β), gamma (γ) ▪ Can be stimulatory (Gs)/inhibitory (Gi) ▫ Activity determined by α subunit (αs/αi), that contains GTPase activity Binding ▪ α subunit binds guanosine diphosphate (GDP)/triphosphate (GTP) ▫ GDP binding → inactive state ▫ GTP binding → active state → coupling ▫ Guanosine nucleotide-releasing factors (GRFs) facilitate GDP dissociation ▫ GTPase-activating factors (GAPs) facilitate GTP hydrolysis ▪ GRFs, GAPs relative activity ▫ ↑ G protein activation rate ▪ Final signal transduction occurs via cyclic adenosine monophosphate (cAMP) signal pathway/phosphatidylinositol signal pathway ADENYLYL CYCLASE MECHANISM ▪ Hormones acting via cAMP mechanism: adrenocorticotropic hormone, luteinizing hormone, follicle-stimulating hormone, thyroid-stimulating hormone, antidiuretic hormone (V2 receptor), human chorionic gonadotropin, melanocyte-stimulating hormone, corticotropin-releasing hormone, calcitonin, parathyroid hormone, glucagon ▪ Hormone binds to receptor coupled to Gs/ Gi protein → adenylyl cyclase activation/ inhibition → intracellular cAMP ↑/↓ ▪ Stimulatory receptor events ▫ Hormone binds to receptor →
Chapter 23 Cellular Physiology: Cellular Structures & Processes conformational change in αs subunit → αs subunit releases GDP → replacement by GTP → αs subunit detaches from Gs protein ▫ αs subunit-GTP complex migrates within cellular membrane → binds → activates adenylyl cyclase ▫ Activated adenylyl cyclase catalyzes adenosine triphosphate (ATP) → ↑ cAMP (second messenger) ▫ Intrinsic GTPase activity in G protein → GTP converts → GDP → αs subunit inactive again ▪ cAMP acts as second messenger → hormonal signal amplification → final physiological reaction ▪ Intracellular cAMP → protein kinase A activation → intracellular protein phosphorylation → physiological response ▪ Phosphodiesterase degrades intracellular cAMP → 5’ adenosine monophosphate (inactive metabolite) → hormonal response cessation PHOSPHOLIPASE C MECHANISM ▪ Hormones acting via phospholipase C mechanism: gonadotropin-releasing hormone, thyrotropin-releasing hormone, growth hormone-releasing hormone, angiotensin II, antidiuretic hormone (V1 receptor), oxytocin ▪ Receptor Gq phospholipase C complex: embedded in cell membrane ▪ In neutral state (no bound hormone) αq subunit binds GDP → inactive Gq protein ▪ Hormone binding → GDP release from αq subunit → GTP binding → αq subunit detaches from Gq protein ▫ αq-GTP complex migrates within cell membrane → activates phospholipase C → DAG, IP3 released from phosphatidylinositol 4,5-diphosphate (PIP2) ▫ IP3 → Ca2+ intracellular stores released (from endoplasmic/sarcoplasmic reticulum) ▫ DAG, IP3 → activate protein kinase C → protein phosphorylation → physiological response STEROID HORMONE MECHANISM ▪ Hormones acting via steroid hormone mechanism: glucocorticoids, estrogens, progesterone, testosterone, aldosterone, 1,25-dihydroxycholecalciferol, thyroid hormone ▪ No cell membrane-mediated transduction step ▫ Steroid hormone diffuses across cell membrane → binds to cytosolic (or nuclear) receptor proteins (monomeric phosphoproteins) → DNA transcription, protein synthesis initiated ▪ Receptor proteins ▫ Part of intracellular receptor gene superfamily ▫ Each receptor protein has six domains (A–F) ▫ Steroid hormone binds E domain near C terminus (central C domain binds to DNA via zinc fingers) ▪ Steroid-receptor protein complex → conformational change in receptor protein → activation → enters nucleus ▪ Hormone-receptor complex combines with similar hormone-receptor complex (dimerization) ▪ New complex binds at C-domain via zinc fingers to specific DNA sequences (steroidresponsive elements), located in target genes’ 5’ region ▪ DNA-bound active hormone-receptor complex acts as transcription factor for specific genes → messenger RNA (mRNA) transcription ▪ mRNA leaves nucleus → translated into new protein with physiological action specific to original hormone TYROSINE KINASE MECHANISM ▪ Hormones acting via tyrosine kinase mechanism: insulin, insulin-like growth factor 1, growth hormone, prolactin ▪ Primary mechanism: tyrosine kinases phosphorylates protein tyrosine residues ▪ Two main categories ▫ Receptor tyrosine kinases → intrinsic kinase activity within receptor OSMOSIS.ORG 175
▫ Tyrosine kinase–associated receptors → no intrinsic kinase activity, associated noncovalently with proteins without kinase activity Receptor tyrosine kinases (RTKs) ▪ Three structural domains ▫ Extracellular binding domain: binds hormone ▫ Hydrophobic transmembrane domain: membrane anchor ▫ Intracellular domain: tyrosine kinase activity ▪ Hormone binding → activation ▫ Activation → phosphorylates itself, other proteins ▪ Monomer-type RTKs ▫ E.g. epidermal growth factor receptors, nerve growth factor ▫ Hormone binding to extracellular domain → receptor dimerization → intrinsic tyrosine kinase activation → tyrosine moieties phosphorylation of itself, other proteins → physiological response ▪ Dimer-type RTKs ▫ E.g. insulin, insulin-like growth factor receptors ▫ Hormone binding → intrinsic tyrosine kinase activation → tyrosine moieties phosphorylation of itself, other proteins → physiological response Tyrosine kinase-associated receptors ▪ E.g. growth hormone ▪ Three structural domains ▫ Extracellular binding domain: binds hormone ▫ Hydrophobic transmembrane domain: membrane anchor ▫ Intracellular domain: no tyrosine kinase activity; non-covalently associated with tyrosine kinase (e.g. Janus kinase family) ▫ Hormone binds to extracellular domain → receptor dimerization → associated protein’s tyrosine kinase activated → tyrosine moieties phosphorylation of associated protein, hormone receptor, other proteins 176 OSMOSIS.ORG GUANYLYL CYCLASE MECHANISM ▪ Hormones acting via guanylyl cyclase mechanism include: atrial natriuretic peptide, nitric oxide (NO) ▪ Extracellular receptor domain binds ligand; intracellular domain has guanylyl cyclase activity ▪ Ligand binding → guanylyl cyclase activation → GTP to cGMP conversion ▪ cGMP activates cGMP-dependent kinase → protein phosphorylation (proteins responsible for physiological response) Intracellular forms (e.g. NO receptor) ▪ Cytosolic guanylyl cyclase mediates signal conversion ▪ NO synthase cleaves arginine (in vascular endothelial cells) → citrulline, NO ▪ NO diffuses from endothelial cells into adjacent vascular smooth muscle → binds, activates soluble (cytosolic) guanylyl cyclase → GTP conversion → cGMP → smooth muscle relaxation SERINE/THREONINE KINASE MECHANISM ▪ Involved in cell proliferation regulation, apoptosis, cell differentiation, embryonic development ▪ G protein-linked receptors → adenylyl cyclase, phospholipase C-linked mechanism ▪ Hormone binding → protein kinase activation → serine, threonine moieties phosphorylation → physiological response ▫ Ca2+-calmodulin-dependent protein kinase (CaMK), mitogen-activated protein kinases (MAPKs) phosphorylate serine, threonine in subsequent reaction cascade
Chapter 23 Cellular Physiology: Cellular Structures & Processes CYTOSKELETON & INTRACELLULAR MOTILITY osms.it/cytoskeleton-and-intracellular-motility ▪ Non-membrane-bound organelles comprising complex protein filament network ▪ Provide structural stability, shape, organization, intracytoplasmic motility, cell motility TYPES Microfilaments ▪ Actin filaments: approx. 7nm ▪ Dynamic structures made of actin monomers ▫ Arranged in long twisting chain ▪ Form network just below cell membrane ▪ Functions ▫ Muscle contraction: slide closer together, further apart ▫ Diapedesis: create pseudopodia for white blood cells (like neutrophils) ▫ Cell division: allows cell to pinch-off, divide into two cells during mitosis ▫ Microvilli function ▫ Mechanical cell membrane support Microtubules ▪ Approx. 25nm ▪ Dynamic structures made of alternating proteins ▫ α- and β-tubulins; polymerize to form microtubules ▪ Stretch across cell ▪ Functions ▫ Intracellular transport (e.g. vesicle movement, melanin transport within pigmented cells) ▫ Structural integrity ▫ Cell division (form mitotic spindle) ▫ Cilia, flagella structural components Intermediate filaments ▪ Approx. 8–10nm ▪ Static structures made of various fibrous proteins (e.g. keratin, desmin, vimentin) depending on cell type ▪ Rope-like structure; forms branching network ▪ Functions ▫ Organelle, cell-cell anchoring ▫ Play key role in providing structural integrity, cell shape OSMOSIS.ORG 177
Figure 23.22 Cytoskeleton components and their functions. 178 OSMOSIS.ORG
Chapter 23 Cellular Physiology: Cellular Structures & Processes NUCLEAR STRUCTURE osms.it/nuclear-structure NUCLEAR ENVELOPE ▪ Encloses, separates nucleus from cytoplasm ▪ Composed of selectively permeable membrane phospholipid bilayer Nuclear pores ▪ Form where membranes fuse together at various intervals ▪ Each pore lined with nuclear pore complex (nucleoporin) to facilitate communication between nucleus, cytoplasm ▪ Allow bidirectional macromolecule movement Outer membrane ▪ Anchoring proteins that hold nucleus in place within cytoplasm ▪ Continuous with RER Inner membrane ▪ Covered by nuclear lamina ▪ Thin filamentous protein network, creates web within nucleus; provide support for chromatin NUCLEOLUS ▪ Dense non-membrane-bound structure; some cells have more than one nucleolus ▪ Contains rDNA → transcribed into rRNA ▪ Assembles ribosomal subunits NUCLEOPLASM ▪ Protoplasmic material ▫ Composed of complex water, molecule, ion mixture ▪ Contains nucleolus, chromatin CHROMATIN ▪ Helical fiber ▫ Composed of 46 DNA molecules wrapped around proteins (histones) ▪ Histones help regulate DNA, gene expression ▪ Chromosomes become visible as chromatin fibers become tightly coiled during cellular division Figure 23.23 Nuclear envelope components. OSMOSIS.ORG 179
Nucleosome ▪ Eight histones packed together in four stacks of two; DNA wraps around them twice ▪ Strung on strand of DNA-like “beads on string” Two chromatin types ▪ Euchromatin: loosely packed DNA, actively being transcribed into RNA ▪ Heterochromatin: densely packed DNA, inactive (not being transcribed) Figure 23.24 The nucleoplasm contains the nucleolus and chromatin. Figure 23.25 In the nucleus, DNA wraps around collections of histone proteins to form nucleosomes. Figure 23.26 During cell division, chromosomes make an exact copy of themselves. The two are connected at the centromere. Each copy is called a sister chromatid. During cell division, the sister chromatids separate so that there is one copy of their genetic material in each daughter cell. 180 OSMOSIS.ORG

Osmosis High-Yield Notes

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