oct 30 mem1 - C e ll m e m b r a n e s f u n c tio n a s b...

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1 ANIMAL CELL P urve s , F ig. 4.7 Cell membranes function as barriers. The plasma membrane separates the cytosol from the external environment, and internal membranes separate the cytosol from the lumen of organelles. Cell membranes are selectively permeable barriers - i.e. a membrane permits some specific chemical compounds to cross, while other compounds cannot. This selectivity is determined by the chemical composition of the membrane. P urve s , F The most prevalent molecules in cell membranes are the phospholipids, which have hydrophobic fatty acid tails covalently bound to a hydrophilic head. Phosphatidylcholine, a common phospholipid In a phospholipid bilayer, the two layers of fatty acid tails create a hydrophobic barrier zone in the center of the membrane. P urve s , F ig. 5.1 Cell membranes contain many other lipids as well, such as cholesterol . Like the phospholipids, cholesterol orients itself in the bilayer with its hydrophobic tail pointing into the membrane. phospholipid bilayer cholesterol In addition to lipids, cell membranes have integral membrane proteins which are embedded in the lipid bilayer. If an integral membrane protein spans the bilayer, it is said to be a ‘transmembrane’ protein. P urve s , F ig. 5.1 In a living cell, the phospholipid bilayer is fluid , and as a result membrane proteins can diffuse within the plane of the membrane.
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2 Fick’s Law of Diffusion: Q = DA [ ] C 1 - C 2 L Rate of Diffusion Cross- Sectional Area Concentration Gradient: C 1 ,C 2 = concentration at L = distance P urve s , F ig. 5.7 P urve s , F ig. 5.7 NOTE: there is potential energy stored in a concentration gradient, and gradients spontaneously diffuse towards equilibrium (= uniformity). Some chemical compounds cross the cell
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oct 30 mem1 - C e ll m e m b r a n e s f u n c tio n a s b...

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