Lesson test: Crossing the membrane

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Crossing the membrane

1.[2p]
A flux measurement gives a straight line through the origin when plotted against external concentration, with no maximum. What does that indicate?

Simple diffusion through the lipid, with no protein involved

Correct

A carrier protein working at saturation

The answer is: Simple diffusion through the lipid, with no protein involved

A pump spending ATP

The answer is: Simple diffusion through the lipid, with no protein involved

A cotransporter driven by a sodium gradient

The answer is: Simple diffusion through the lipid, with no protein involved

2.[3p]
How does a potassium channel pass potassium over the smaller sodium ion by more than a thousand to one?

Its selectivity filter presents carbonyl oxygens spaced to replace potassium's hydration shell exactly, which does not compensate a sodium ion for the water it loses

Correct

Its pore is too narrow to admit sodium

The answer is: Its selectivity filter presents carbonyl oxygens spaced to replace potassium's hydration shell exactly, which does not compensate a sodium ion for the water it loses

It binds sodium irreversibly and removes it from solution

The answer is: Its selectivity filter presents carbonyl oxygens spaced to replace potassium's hydration shell exactly, which does not compensate a sodium ion for the water it loses

It spends one ATP to reject each sodium ion

The answer is: Its selectivity filter presents carbonyl oxygens spaced to replace potassium's hydration shell exactly, which does not compensate a sodium ion for the water it loses

3.[3p]
Using $R T / F = 26.7$ mV at 310 K, what is the Nernst potential for potassium in millivolts, with 140 mM inside and 5 mM outside? Give a negative number.

CorrectNot quite: -89

4.[1p]
The sodium potassium ATPase moves three sodium ions out and two potassium ions in per ATP, so it contributes directly to the membrane voltage.

True

Correct

False

The answer is: True

5.[3p]
Match each transport protein to its behaviour.

A potassium channel
GLUT1
The sodium potassium ATPase
SGLT1

about 1000 molecules per second, downhill only
about 100 ions per second, uphill, one ATP per cycle
uphill for glucose, powered by two sodium ions falling downhill
passes about $10^{8}$ ions per second, downhill only

Show the answer

A potassium channel: passes about $10^{8}$ ions per second, downhill only GLUT1: about 1000 molecules per second, downhill only The sodium potassium ATPase: about 100 ions per second, uphill, one ATP per cycle SGLT1: uphill for glucose, powered by two sodium ions falling downhill

6.[3p]
Roughly what fraction of a cell's potassium has to cross the membrane to establish a resting potential of 70 mV?

About three in a hundred thousand

Correct

About one in ten

The answer is: About three in a hundred thousand

About half

The answer is: About three in a hundred thousand

All of it

The answer is: About three in a hundred thousand

7.[2p]
Which of these are ways cells avoid bursting from the osmotic pressure of their own contents?

Select all that apply

Pumping sodium out, so that it acts as an impermeant external solute

Correct

Building a rigid wall and allowing turgor pressure to rise

Correct

Expelling water with a contractile vacuole

Correct

Making the membrane impermeable to water

The answer is: Pumping sodium out, so that it acts as an impermeant external solute, Building a rigid wall and allowing turgor pressure to rise, Expelling water with a contractile vacuole

8.[3p]
Why does digoxin, which blocks the sodium potassium ATPase, strengthen heart muscle contraction?

Internal sodium rises, so the sodium calcium exchanger expels less calcium, and internal calcium rises

Correct

It binds directly to the contractile proteins

The answer is: Internal sodium rises, so the sodium calcium exchanger expels less calcium, and internal calcium rises

It increases the cell's ATP supply

The answer is: Internal sodium rises, so the sodium calcium exchanger expels less calcium, and internal calcium rises

It opens calcium channels in the membrane

The answer is: Internal sodium rises, so the sodium calcium exchanger expels less calcium, and internal calcium rises

9.[2p]
Why does oral rehydration therapy need both salt and sugar rather than either alone?

SGLT1 carries sodium and glucose together, so neither is absorbed efficiently without the other, and water follows both

Correct

Sugar makes the salt palatable and has no other role

The answer is: SGLT1 carries sodium and glucose together, so neither is absorbed efficiently without the other, and water follows both

Salt kills the bacteria and sugar feeds the patient

The answer is: SGLT1 carries sodium and glucose together, so neither is absorbed efficiently without the other, and water follows both

Glucose is needed to supply the ATP that pumps the sodium

The answer is: SGLT1 carries sodium and glucose together, so neither is absorbed efficiently without the other, and water follows both

1.[2p] A flux measurement gives a straight line through the origin when plotted against external concentration, with no maximum. What does that indicate?

2.[3p] How does a potassium channel pass potassium over the smaller sodium ion by more than a thousand to one?

3.[3p] Using RT/F=26.7 mV at 310 K, what is the Nernst potential for potassium in millivolts, with 140 mM inside and 5 mM outside? Give a negative number.

4.[1p] The sodium potassium ATPase moves three sodium ions out and two potassium ions in per ATP, so it contributes directly to the membrane voltage.

5.[3p] Match each transport protein to its behaviour.

6.[3p] Roughly what fraction of a cell's potassium has to cross the membrane to establish a resting potential of 70 mV?

7.[2p] Which of these are ways cells avoid bursting from the osmotic pressure of their own contents?

8.[3p] Why does digoxin, which blocks the sodium potassium ATPase, strengthen heart muscle contraction?

9.[2p] Why does oral rehydration therapy need both salt and sugar rather than either alone?

1.[2p]
A flux measurement gives a straight line through the origin when plotted against external concentration, with no maximum. What does that indicate?

2.[3p]
How does a potassium channel pass potassium over the smaller sodium ion by more than a thousand to one?

3.[3p]
Using $R T / F = 26.7$ mV at 310 K, what is the Nernst potential for potassium in millivolts, with 140 mM inside and 5 mM outside? Give a negative number.

4.[1p]
The sodium potassium ATPase moves three sodium ions out and two potassium ions in per ATP, so it contributes directly to the membrane voltage.

5.[3p]
Match each transport protein to its behaviour.

6.[3p]
Roughly what fraction of a cell's potassium has to cross the membrane to establish a resting potential of 70 mV?

7.[2p]
Which of these are ways cells avoid bursting from the osmotic pressure of their own contents?

8.[3p]
Why does digoxin, which blocks the sodium potassium ATPase, strengthen heart muscle contraction?

9.[2p]
Why does oral rehydration therapy need both salt and sugar rather than either alone?