Plasma Membrane Handouts PDF

PLASMA MEMBRANE: STRUCTURE AND FUNCTION SHERWOOD: Chapters 3 & 4  FUNCTIONS OF CELL MEMBRANE Homeostasis & cell survival: maintain intracellular contents of cells & coordinate activity with others – Mechanical barrier – Adhere together to form tissues – Exchange nutrients, was...

PLASMA MEMBRANE: STRUCTURE AND FUNCTION SHERWOOD: Chapters 3 & 4  FUNCTIONS OF CELL MEMBRANE Homeostasis & cell survival: maintain intracellular contents of cells & coordinate activity with others – Mechanical barrier – Adhere together to form tissues – Exchange nutrients, wastes & secretions – Respond to changes in environment or signals – Maintain ionic gradient for electrical activity  Sherwood, Fig 3-2 COMPONENTS OF CELL MEMBRANE Lipids: phospholipids and cholesterol – Barrier to passage of water-soluble substances – Provides fluidity & stability to the membrane  Siefter, Fig 2-3 & 2-4 COMPONENTS OF CELL MEMBRANE Proteins: transmembrane or one surface only I III II COMPONENTS OF CELL MEMBRANE Proteins: several types for specific functions – Aquapoins: – Ion channels: – Carrier molecules: – Membrane receptors: – Docking-marker acceptors: – Membrane-bound enzymes: – Cell adhesion molecules (CAMs):  Sherwood , Fig 3-3 COMPONENTS OF CELL MEMBRANE Carbohydrates: present on outer surface only – Glycolipids: Glycocalyx – Glycoproteins: – Function: self-identity markers  Sherwood, Fig 3-1, Fig 3-3 STRUCTURE OF CELL MEMBRANE The fluid mosaic model: Tri-laminar structure  Sherwood, Fig 3-3 STRUCTURE OF CELL MEMBRANE The fluid mosaic model: Tri-laminar structure  CELL-CELL ADHESIONS Cells Tissues Organs System Organism Extracellular matrix (biological glue secreted by cells) Specialized cell junctions: – Desmosomes – Tight junctions – Gap junctions Cell adhesion molecules: – Occurs between cells in close proximity – Loop and hook shaped proteins that “velcro” together  EXTRACELLULAR MATRIX AKA Interstitial fluid, present in intercellular space Meshwork of fibrous proteins in a watery gel – Collagen: – Elastin: – Fibronectin:  EXTRACELLULAR MATRIX Secreted by cells, important for their functioning Pathway for diffusion of water-soluble substances Regulates behavior and functions of the cells: – Amount & composition of ECM varies with cell type Can become highly specialized for specific functions: – Examples: cartilage, tendons, hardness of bones & teeth etc.  Sherwood, Fig 3-4 DESMOSOMES/ADHERING JUNCTIONS “Spot rivets” Plaque Glycoprotein filaments Keratin filaments Present in skin, heart, uterus etc.  Sherwood, Fig 3-5 TIGHT/IMPERMEABLE JUNCTIONS Cells adhere firmly, seals formed at kiss sites by junctional proteins Found b/w epithelial cells, separate 2 compartments of diverse chemical compositions e.g., intestines, kidneys Passage of materials takes place through cells, not between cells, via channels and carriers  Sherwood, Fig 3-6 GAP/COMMUNICATING JUNCTIONS Gaps/tunnels between cells Connexons: six subunits in a hollow tube-like structure, 2 connexons join together Only small particles pass Present in electrically active cardiac/smooth muscle Enables synchronized action Metabolic/communication link  MEMBRANE TRANSPORT Essential for homeostasis: nutrients in, wastes out Plasma membrane is selectively permeable Factors affecting membrane permeability: – Lipid solubility: – Particle size: Forces are required for membrane transport – Passive force: no energy expenditure by cells for transport – Active force: energy (ATP) expenditure by cells for transport  MEMBRANE TRANSPORT Unassisted Assisted Diffusion Osmosis Vesicular transport Carrier mediated Movement along electrical gradient Endocytosis Exocytosis Facilitated diffusion Active transport Primary Active transport Secondary Active transport Caveolae: membrane transport and signal transduction  Sherwood, Fig 3-7 DIFFUSION  Sherwood, Fig 3-8 DIFFUSION  Sherwood, Table 3-1 DIFFUSION Rules/Properties: – Occurs only if substances can cross the membrane – Always occurs from area of high to low concentration – No energy required, a passive mechanism, e.g.O2-CO2 Factors influencing rate of diffusion (Fick’s law):  Sherwood, Fig 3-13 ELECTRICAL GRADIENT EG: charge difference between adjacent areas Promotes movement towards opposite charge Electrical and concentration gradient = electrochemical gradient Positively Negatively charged area charged area Cations (positively charged ions) attracted toward negative area Anions (negatively charged ions) attracted toward positive area  Sherwood, Fig 3-9 & 3-15 OSMOSIS Net diffusion of water down its own concentration gradient separated by semi-permeable membrane Membrane H2 O Higher H2O Lower H2O concentration, concentration, lower solute higher solute concentration concentration  Sherwood, Fig 3-16, 3-17 & 3-18 OSMOSIS Membrane (permeable to Membrane (permeable to H2O Membrane (permeable to H2O both water and solute) but impermeable to solute) but impermeable to solute) H2 O H2 O H2 O Solute Original Original Osmosis Hydrostatic level of level of (fluid) solutions solutions Hydrostatic pressure pressure difference Water concentrations equal Water concentrations equal Water concentration not equal Solute concentrations equal Solute concentrations equal Solute concentration not equal No further net diffusion No further net diffusion Osmosis ceases when osmotic Steady state exists Steady state exists pressure is exactly balanced by opposing hydrostatic pressure  Sherwood, Fig 3-13 OSMOSIS Important for water movement of in/out of cells – Intravenous administration, eye drops etc. Tonicity: conc. of non-penetrating solutes Osmolarity: concentration of non-penetrating and penetrating solutes 300mOsm 0.9% NaCl  CARRIER-MEDIATED TRANSPORT Utilize carriers: membrane spanning proteins Able to flip-flop, a reversible change in shape Binding sites of substance on carriers exposed alternatively to either side of the membrane Conformation X of carrier Conformation Y of carrier On binding with molecules to be transported, carrier changes its conformation  Seifter, Fig 2-11 CARRIER-MEDIATED TRANSPORT