Chapter 8 --- transport across membanes, overcoming permeability barrier

simple diffusion

• small, nonpolar

• down concentration gradient

facilitated diffusion

• can be channel or carrier

• large, polar

• down gradient

active transport

• large, polar

• against gradient

• ex: sodium potassium pump

describe simple diffusion vs. osmosis

• depends if permeable to solute or not

• solute is the thing being dissolved in the solvent

• the solvent is usually water

• if the solutes cannot move, then the water will move in order to keep the concentration the same

what are the responses of animal cells to changes in osmolarity?

• hypertonic solution = shriveled

  • high extracellular solute, so water moves out of cell

  • dehydration

• isotonic solution = normal

• hypotonic solution = lysed

  • low extracellular solute, so water moves in to cell and explodes

  • hyponatremia: low blood Na+

what mechanism does glucose transporter use for movement?

• facilitated diffusion

• passive transport

sodium-potassium pump - initial state and 6 steps

• pump open to inside (E1 conformation)

1. 3 Na+ from inside the cell bind to E1

2. Na+ binding triggers autophosphorylation of the alpha subunit using ATP and ADP is released

3. a conformational change to E2 expels three Na+ to the outside of cell

• pump open to outside, ready to start second half of cycle (E2 conformation)

4. 2 K+ from outside the cell bind to E2

5. K+ binding triggers dephosphorylation, causing a conformational change back to E1

6. two K+ expelled to inside as ATP binds and the pump returns to initial state

   

initial state and 6steps of the Na+-Glucose symporter

• active transport of glucose

• symporter open to outside

1. two Na+ from outside of cell are bound

2. binding of Na+ allows glucose binding and a subsequent conformational change

3. symporter opens to inside

4. Na+ are released inside, but are continually extruded to outside by a separate Na-K pump

5. loss of Na+ is followed by glucose release to inside

6. release of glucose allows empty symporter to return to initial state