Membrane Transporters

Question 1

Facilitated Diffusion

Answer 1

Uses no energy. Mlcs that are too large/charged.
Ex: glucose transporter.
Diffuses in and is then phosphorylated into G-6-P so it can’t bind to transporter (trapped).
Insulin triggers cascade so transporter exocytoses and ends up on the membrane. No glucose? no insulin, no trasnporter.

Question 2

Pumps

Answer 2

Move solute against energy gradient, require energy, can be electrogenic (Na/K) or non-electrogenic (Na/K/2Cl).. non-electrogenic want to balance ion conc.
If electrogenic activity is governed by Vm.
Primary active or secondary active transporters.

Question 3

Water

Answer 3

Has no pumps, it always moves down the concentration gradient

Question 4

Primary active

Answer 4

Energy from splitting ATP.
Na/K pump
Inside cells: pump H into organelles (lysosomes etc)
Pump Ca into mem-bound compartments.
Inside mitochondria: special proton pump F1-ATPase. Runs backwards, allowing H leak across membrane and synthesize ATP.

Question 5

Secondary active

Answer 5

Energy not from ATP hydrolysis, but from secondary source… often the downhill leak of Na into cells (captured and used).
MOST substances pumped this way. Electrogenic can reverse directions. Tap bigger leak to drive smaller pump (ex: Na/Ca exchanger in heart beats)
2 types.

Question 6

Types of Secondary active?

Answer 6

1. ) Co-transport: move diff. solutes in SAME DIRECTION

2. ) Antiport/exchange: move diff solute in DIFF DIRECTION

Question 7

Na/K pump importance?

Answer 7

All secondary transport mechanisms depend on Na/K pump and therefore ATP is some way!

Question 8

Calcium transport

Answer 8

There is a large electrochemical gradient for CA across mem (it is furthest ion from equil). Way higher outside. Electrical and conc gradients are inwards.
Eca= +111mV so Ca will always leak in and get PUMPED OUT

Question 9

Na/Ca exchange pump

Answer 9

In heart it switches direction with each beat.
At rest exchanger runs forward: Ca out (muscle relaxed) na in.
contract: Ca in (muscle contract) and Na out.
Direction of Ca movement is controlled by Vm (when Vm is more negative than -60 the Na leak rules and drives Ca out, when more positive reverse directions).

Question 10

Digitalis

Answer 10

Foxglove plant extract. Can make heart beat stronger.
It blocks the Na/K pump
(intracell Na increaes, so reduced energy to all Na driven secondary active transporters, including Na/Ca exchanger. So it inhibits this. and Ca gets trapped in cell… get increased cardiac contractility).

Question 11

Hydrogen ions

Answer 11

Usually pumped OUT by Na/H exchange pump. H binds buffers (so hardly ever free). Eh= -24 (most cells more neg than this… so H will be pumped out).
H must be pumped out as Na leaks in. (secondary active!)

Question 12

Chloride ions

Answer 12

They are pumped into some cells by secondary active transport.
Na/K/2Cl cotransporter.
So Ecl becomes more positive.

Question 13

H-K exchanger

Answer 13

Probably non-existent.
Add K and get acidemia, K goes into cells H out.
Add H get hyperkalemia: H into cells K out.

In reality hyperkalemia causes increased K uptake through Na/K pump (not H/K exchanger), this depolarizes the cells by making ek more positive, this affects electrogenic transporters and INHIBITS 3 bicarb/Na from ICF–> ECF, get acidemia (b/c less bicarb outside)

Question 14

How can you get cells to take up K from the ECF?

Answer 14

Give glucose and insulin or bicarb.

Question 15

What happens when pK falls?

Answer 15

Vm depolarizes. Very important mechanism in cardiac action potential.