12_Membrane_Transport-1

12_Membrane_Transport-1 - X X X I I I I I I . . . M M M e e...

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Unformatted text preview: X X X I I I I I I . . . M M M e e e m m m b b b r r r a a a n n n e e e t t t r r r a a a n n n s s s p p p o o o r r r t t t Transport of nutrients, ions, and excretory substances from one side to the other is a major function of the cell membrane. A number of different means have been developed to fulfill this function. Generally, the permeation of small molecules across the membrane is quite different from engulfing molecules too large to penetrate membrane. Membrane transport Small molecules Large molecules water nonpolar simple diffusion facilitated diffusion active transport cootransporters exocytosis pinocytosis endocytosis pumps passive transport ions permeases channels T T T r r r a a a n n n s s s p p p o o o r r r t t t o o o f f f s s s m m m a a a l l l l l l m m m o o o l l l e e e c c c u u u l l l e e e s s s Depending on whether a cell pays for the transport (energetically) or not, we talk about passive (free) and active transport. P P P a a a s s s s s s i i i v v v e e e t t t r r r a a a n n n s s s p p p o o o r r r t t t S S S i i i m m m p p p l l l e e e d d d i i i f f f f f f u u u s s s i i i o o o n n n The simplest form of transport is passive diffusion. It is a real freebie; it does not even need helpers. W W W a a a t t t e e e r r r d d d i i i f f f f f f u u u s s s i i i o o o n n n : : : o o o s s s m m m o o o s s s i i i s s s Lipid membranes are semi-permeable; some substances pass through freely (water) some dont (ions). Consider two water solutions, one rich in ions and the other not, which are separated by a semi-permeable membrane. Water can move across the membrane in both directions, but because ions attract water and impede its random diffusion, water is retarded on the ion- rich side, therefore the rate from the ion-rich side is less than the rate of ions permeating the membrane from the other side. The net movement of water toward the ion-rich solution builds up hydrostatic pressure, called osmotic pressure, which at some point will counteract the attraction of ions. The two sides will then be at equilibrium. Whenever two solutions are separated by a semi-permeable membrane, net movement of water will be toward the solution more concentrated in substances that do not permeate the membrane. Diffusion through semipermeable membrane We say, the more concentrated solution is hypertonic with respect to solution less rich in the impermeant substance. The water will always try to rush in to make the more concentrated solution less hypertonic. The less concentrated solution is referred to as hypotonic, water will attempt to leave this compartment and thereby decrease concentration of impermeant solute....
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12_Membrane_Transport-1 - X X X I I I I I I . . . M M M e e...

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