chapter3 - IPHY 3430 Human Physiology Fall 2007 REVIEW...

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IPHY 3430 – Human Physiology Fall 2007 REVIEW TOPICS & STUDY QUESTIONS – Chapter 3: Membrane Physiology Lecture Summary 1. The plasma membrane is a lipid bilayer consisting of phospholipids, cholesterol, proteins, and carbohydrates. The dynamic arrangement of membrane components within the lipid bilayer is often referred to as the “fluid mosaic model”. The inner portion of the lipid bilayer contains hydrophobic (“water-fearing”), non-polar fatty acids chains which prevents charged particles, such as ions, and polar substances from freely diffusing across the membrane. Charged substances and polar molecules enter the cell via facilitated diffusion, through integral membrane proteins that are embedded in the membrane and act as selective channels. Many membranes also contain specialized active transport exchangers that pump molecules against their concentration gradients. Other functions of intra- and extracellular membrane proteins include vesicle docking markers, neurotransmitter and peptide hormone receptors, and enzymes. 2. Solvent (water, in biological systems) moves across the membrane by one of two processes. Osmosis is the movement of water down its concentration gradient into an area of higher solute concentration. Tonicity is a relative term used to determine the direction of water flow. Filtration is the second process involving movement of water down a hydrostatic pressure gradient (e.g., blood pressure) due to a higher pressure on one side of the membrane. 3. Movement of solute (particles dissolved in water, such as a sugar or an ion) across a membrane occurs by one of three processes. Diffusion is the passive movement of a solute down a concentration gradient. Facilitated diffusion is also passive movement down a concentration gradient but requires a specific membrane-bound carrier or channel protein. Diffusion down a concentration gradient is a passive process that does not require ATP hydrolysis. A number of factors, including concentration gradient, membrane permeability, membrane surface area, and membrane thickness, determine the rate of diffusion. Assisted transport also involves carrier molecules. The rate of carrier-mediated transport is dependent on the density of carriers within the membrane. Because the number of membrane carriers is limited, solute concentration may exceed the number of transporters. For a given membrane, transport maximum (T m ) refers to the maximum rate of transport across the membrane. T m occurs when all carriers for a specific substrate are saturated. At T m , the rate of
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