Lecture 8: Membrane transport

Question 1

which molecules must cross membranes through membrane transport proteins

Answer 1

larger, polar molecules & ions

Question 2

channels discriminate based on

Answer 2

size and charge

Question 3

transporters discriminate based on

Answer 3

direct binding

Question 4

passive transport

Answer 4

uncharged molecules; can use transporters and or channels

Question 5

electrochemical gradient is a combination of what 2 gradients?

Answer 5

1) the voltage (electrical) gradient & 2) the concentration (chemical) gradient

outside of cell tends to have a net positive charge; cytoplasm tends to have a negative charge

Question 6

what is passive transport

Answer 6

molecules moving DOWN their electrochemical gradient; no input of energy

Question 7

channels

Answer 7

only used in PASSIVE transport; can open and close

“Gated” channels respond to different types of stimuli (i.e. voltage, ligand-gated, stress)

Question 8

glucose transporter

Answer 8

requires conformation changes of the transporter (passive transport)

Question 9

why did cells evolve transporters, rather than channels, for molecules like glucose?

Answer 9

a large channel opening is likely to be less selective

Question 10

active transport

Answer 10

uses energy to move molecules against their electrochemical gradients; use the energy “coupling” principle (traport proteins: gradient-driven pump, ATP-driven pump, light-driven pump)

Question 11

Na+-K+ Pump steps

Answer 11

1. Na+ binds
2. Pump phosphorylates itself (adds P) by hydrolyzing ATP
3. Phosporylation triggers conformational change and Na+ is ejected
4. K+ binds
5. Pump dephosphorylates itself (removes P)
6. Pump returns to original conformation and K+ is ejected

3Na+ out & 2K+ in for every cycle

Question 12

symport

Answer 12

both molecules move in the same direction; can be active or passive

active if at least ONE molecule is moving UP its e.c. gradient; passive if BOTH molecules are moving DOWN its e.c. gradient

Question 13

antiport

Answer 13

molecules are moving in the opposite direction; can be active or passive (example: Na+/K+ pump)

active if at least ONE molecule is moving UP its e.c. gradient; passive if BOTH molecules are moving DOWN its e.c. gradient

Question 14

uniport

Answer 14

one molecule is moving down its concentration gradient; always passive

Question 15

glucose-Na+ coupled transport

Answer 15

transport of glucose UP its conenctration gradient is driven by Na+ moving DOWN its electrochemical gradient; both molecules are moved into the cell - symport

steps of glucose-Na+ coupled transport:
1. transporter starts off occluded (empty)
2. transporter opens and Na+ and glucose enter
3. transporter is occluded (occupied)
4. transporter opens and releases Na+ and glucose
5. transporter occludes and is empty again

Question 16

which transporters are involved in glucose uptake from food

Answer 16

1. glucose-Na+ symport (active)
2. glucose uniport (passive)
3. Na+-K+ pump (Active)

Question 17

k+ leak channels purpose

Answer 17

passive

help generate the membrane potential in animal cells, together with the Na+/K+ pump

the Na+/K+ pump generates a high concentration of K+ and low concentration Na+ inside of the cell

the open K+ channel balances the K+ e.c. gradient, allowing K+ to leave and resulting in an overall charge imbalance (membrane potential)

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