Nutrient Transport across Membranes Except for H2O, most polar molecules do NOT move across the lipid bilayers Membranes as barriers Relative speed of nutrient movement across bacterial membranes substance permeability water glycerol Tryptophan (色氨酸) glucose Cl- K+ Na+ 100
10-6 10-7 10-8 Note that many nutrients are polar Membrane transport systems are the systems to move nutrients and waste products across membranes Uniporter transport Passive Active Antiport Passive diffusion Facilitated diffusion Channel proteins Symport ABC system group translocation Molecules move along a concentration gradient (from region of higher concentration to lower concentration) Movement is driven by random thermal action - no energy output anism Concentration reaches equilibrium Water, gasses, lipids, small uncharged polar molecules Not primary mode for hydrophilic ions What is diffusion? Examples of gases that cross membranes by passive diffusion include N2, O2, and CO2; examples of small polar molecules include ethanol, H2O, and urea. Passive 1 Passive diffusion Channel proteins蛋白通道 transport water or certain ions down either a concentration gradient, in the case of water, or an electric potential gradient, in the case of certain ions from an area of higher concentration to lower concentration. While water molecules can directly cross the membrane by simple diffusion, as mentioned above, their transport can be enhanced by channel proteins called aquaporins(运水蛋白). Passive 2. Channel proteins Diffusion aided by a carrier protein - permease - in cell membrane Carrier provides specificity and increases rate to equilibrium is powered by the potential energy of a concentration gradient and does not require the expenditure of metabolic energy. Not highly important in prokaryotes What is facilitated diffusion? Passive 3. facilitated diffusion Passive 3 Facilitated diffusion Diffusion facilitator protein Nutrient molec in high conc Specific b
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