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ANP1105 (101)
Chapter 3

ANP1105 Chapter 3 (p. 63-79) - The Plasma Membrane: Structure, Membrane Transport, and Generation of a Resting Membrane Potentail

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Anatomy and Physiology
Jacqueline Carnegie

(Pg. 63-79) Plasma membrane: separates intracellular fluid within cells and extracellular fluid outside cells Fluid mosaic model: depicts PM as a bilayer with lipid and protein molecules dispersed in it. The proteins which float in the lipid bilayer create a constantly changing mosaic characteristic. PM is in dynamic constant flux.At body temperature, very oily. Lipid bilayer is composed of: phospholipids, glycolipids, cholesterol, and lipid rafts (and proteins) Phospholipid: • Polar head (hydrophilic) and uncharged, nonpolar tail made of two fatty acid chains (hydrophobic) • Polar heads are attracted to water so they lie on the inner and outer surfaces of the membrane • Nonpolar tails line up in the middle of the PM • All PM’s are composed of two parallel sheets of phospholipid molecules lying tail to tail • PM is a fluid structure with lipid molecules that move from side to side (but not upside down due to polar-nonpolar interaction) • Most phospholipids are unsaturated, which kinks their tails and creates more space between them Glycolipid: • Lipids with attached sugar groups • Found only on the outward-facing layer of phospholipids • The sugar group makes that end of the glycolipid polar and the fatty acid tails are nonpolar Cholesterol: • Has a polar region (hydroxyl group) and a nonpolar region (its fused ring system) • It wedges between phospholipid tails which stabilizes the membrane, reduces mobility of the membrane, and decreases fluidity of the PM • Used for steroid hormones and biosalts • Not enough cholesterol: cell will become to oily, spread, and fall apart • Too much cholesterol: cell will not be able to change shape Membrane proteins: • Make up half the PM’s mass • Responsible for PM’s special functions Integral proteins: • Firmly inserted into lipid bilayer • Most are transmembrane proteins that span the entire membrane • Have both hydrophobic and hydrophilic ends so can interact with nonpolar lipid tails and water inside and outside the cell • Involved in transport- form channels through which small, water-soluble molecules or ions can move through • Act as carriers that bind to a substance and move it through the PM • Some are enzymes, receptors for hormones, or other chemical messengers that relay messages to the cell interior Peripheral proteins: • Not embedded in lipid bilayer • Attach loosely to integral proteins • Easily removed without disrupting PM • Network of filaments that help support the membrane • Some are enzymes, some link cells together, and others are motor proteins which are involved in changing cell shape and cell contraction Lipid rafts: • Dynamic assemblies of saturated phospholipids which are packed tightly • More stable and less fluid than PM and can include proteins • Are platforms for receptor molecules (protein molecules needed for cell signalling) Glycocalyx: • Fuzzy, sticky, carb-rich part of cell surface • Because each cell contains a different sugar pattern, glycocalyx are used as specific biological markers for cells to recognize each other • Acancer cell’s glycocalyx may continuously change, allowing it to keep ahead of immune system recognition mechanisms and avoid destruction Cells junctions: bind cells together Tight junctions: • Integral proteins of adjacent cells fuse together • Prevent molecules from passing through extracellular space between adjacent cells • Ex: tight junctions between epithelial cells lining the digestive tract keep enzymes and microorganisms from seeping from the intestine into the bloodstream Desmosomes: • Scattered like rivets along the sides of cells to prevent separation (scattered like Velcro or a zipper) • On each PM there is a plaque.Adjacent cells are held together by caherins (linker protein filaments) that extend from the plaques and link them together. Thick keratin filaments attach to the cytoplasmic side (inside) of the plaque to the opposite side of the cell, binding cells together and creating an internal network of filaments • The network of filaments reduces tension which allows pulling forces • Most subjected to mechanical stress Gap junctions: • Communicating junction between cells • At gap junctions, adjacent cells are very close and cells are connected by hollow cylinders called connexions (composed of transmembrane proteins) • Ions, simple sugars and other small molecules pass through the channels from cell to cell • Present in electrically excitable tissue where ion passage synchronizes electrical activity (ex: in heart and smooth muscle) PM: Membrane transport Interstitial fluid: extracellular fluid; contains whatever comes out of cell into the blood Selectively permeable membrane: membrane that allows certain substances to pass while restricting the movement of others *Oily PM flows and closes holes Passive Transport: • Substances cross PM without any energy input from the cell • Two main types: diffusion and filtration Diffusion: • Molecules move from an area of high concentration to an area with lower concentration (down the concentration gradient) • Constant and random high-speed collisions, due to kinetic energy, result in collisions. The particles bounce off each other spreading evenly. The greater the difference of concentration of particles in the two areas, the more collisions occur and the faster the net diffusion of particles. • Speed of diffusion is influenced by particle size (smaller = faster) and temperature (hotter = faster) • The PM is a barrier to diffusion because of its hydrophobic core, but a molecule or ion will diffuse through the membrane if it is (1) lipid soluble, (2) small enough to pass through membrane channels, or (3) assisted by a carrier molecule Simple Diffusion: • Nonpolar and lipid molecules diffuse directly through the lipid bilayer (unassisted) • Substances include O, C and fat-soluble vitamins: O concentration is higher in the bloodstream, so it diffuses into the cell, and C concentration is higher in the cell, so it diffused into the bloodstream Facilitated diffusion: • Certain molecules, notably glucose, which are unable to move through the membrane, pass through in a passive process in which the substance either (1) binds to protein carriers in the membrane and is ferried across, or (2) moves through water-filled channels • The rate of diffusion is controllable by regulating the activity or number of carriers or channels (1) Carrier-mediated facilitated diffusion: • Carriers are transmembrane integral proteins that transport molecules that are too large to pass through channels • Changes in shape of the carrier allow it to envelop and then release the transported substance shielding it from nonpolar regions of the membrane • Substance transported by carrier-mediated diffusion moves down its concentration gradient • Carrier-mediated transport is limited by the number of protein carriers present. When all carriers are engaged, they are said to be saturated and transport is occurring at its maximum rate • G-6-P helps wh
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