Carrier-mediated Transport

Question 1

On a Michaelis-Menten Kinetics graph, is on the X and Y axis?

Answer 1

X axis: Solute concentration
Y axis: Rate of transport / enzyme activity

Question 2

What does a typical Michaelis-Menten Kinetics graph look like?

Answer 2

Steep increase at the beginning when Transport Rate is dependent on Substrate availability / Concentration.

The Rate of Transport continues to increase but the gradient becomes shallower until it plateaus at Vmax

Question 3

In a Michaelis-Menten Kinetics graph what does the Vmax reflect?

Answer 3

Vmax is when the transport rate is independent of substrate/solute concentration and reflects the rate of carrier reorientation

Question 4

In a Michaelis-Menten Kinetics graph what does the Km point reflect?

Answer 4

This is the solute concentration at 1/2 Vmax and it reflects the concentration for binding and release of the solute (needs to be correct for environment so it can bind and release it on correct sides of membrane)

Question 5

What are the general, and specific to carrier-mediated transport, Michaelis-Menten Kinetics equations?

Answer 5

E + S <> ES <> E + P
“Enzyme + Substrate <>
Enzyme-substrate complex
<> Enzyme + Product”

C + So <> CS <> C + Si
“Carrier + Solute (outside-facing) <>
Carrier-solute/substrate complex <>
Carrier + Solute (inward-facing)”

Question 6

Why might plants, bacteria and fungi need multiple types of mechanisms for uptake of a nutrient?

Answer 6

Their environment nutrients can differ, eg; roots of a plant might grow through pockets of high and low levels of nutrient.

In low levels of nutrient high affinity uptake (low capacity) mechanisms are important, but these are costly, don’t want to use this if not needed such as when in high levels of nutrient, where its more efficient to use low affinity uptake (high capacity) mechanisms, so have both mechanisms available

Question 7

How did chilling experiments prove valinomycin carries solutes by diffusing across the membrane?

Answer 7

Lowering temperature of artificial bilayer (membrane) causes it to stiffen, decreasing efficiency of diffusion and valinomycin transport

(Cold temperatures do not affect ion channels so much this way)

Question 8

What is a practical starting point when trying to learn about a carrier’s secondary structure?

Answer 8

Look at its amino acids, observe how hydrophobic or hydrophilic a string of them are, create a hydropathy plot to predict where they are positioned with respect to the membrane

Question 9

What carrier is the main transporter of glucose between liver and blood?

Answer 9

GLUT2

Question 10

What carrier is the main transporter for glucose absorbance from gut?

Answer 10

GLUT2

Question 11

What carrier forms part of the glucose sensor in pancreatic beta cells?

Answer 11

GLUT2

Question 12

What action does insulin have on facilitators?

Answer 12

Recruits more GLUT2 to the plasma membrane

Question 13

Dysregulation of what carrier is associated with diabetes?

Answer 13

GLUT2

Question 14

What is the main transporter of glucose across the blood-brain barrier?

Answer 14

GLUT1

Question 15

Deficiency in what carrier can cause epilepsy?

Answer 15

GLUT1

Question 16

Over-expression in what carrier can indicate cancer?

Answer 16

GLUT1 (cancer needs energy source for uncontrolled growth)

Question 17

There are two stereoisomers for glucose, what are they called?

Answer 17

L-glucose and D-glucose

Question 18

Which stereoisomer of glucose is the biologically active form?

Answer 18

D-glucose

Question 19

Uptake of L-glucose is due to…?

Answer 19

Diffusion across membrane via lipids (not facilitated diffusion)

Question 20

Uptake of D-glucose is due to…?

Answer 20

Facilitated diffusion (with some lipid-dependent diffusion but this is significantly less of a contribution)

Question 21

When a graph follows a typical Michaelis Menten Kinetics curve, what does this indicate?

Answer 21

Involvement of enzyme activity

Question 22

What organism is Saccharomyces cerevisiae ? (S. cerevisiae)

Answer 22

Yeast

Question 23

What is special about glucose transport in yeast?

Answer 23

Doesn’t have carriers, the glucose just diffuses across the membrane and this is sufficient for the cell, since yeast evolved in glucose-rich environments such as rotting fruits

Question 24

How do most fungi transport / uptake glucose?

Answer 24

Uptake is “energised” via being coupled to H+ electrochemical gradient, glucose symported due to low-glucose environment

Question 25

In the cells transporting glucose from the intestinal lumen into the blood, what is the name of the membrane facing the intestinal lumen?

Answer 25

Apical membrane

Question 26

In the cells transporting glucose from the intestinal lumen into the blood, what is the name of the membrane facing the blood?

Answer 26

Basolateral membrane

Question 27

What transporters do you find in the cells transporting glucose from the intestinal lumen into the blood?

Answer 27

2 Na+ / glucose symporter
Na+/K+ ATPase
GLUT2

Question 28

How does cholera work?

Answer 28

Dehydration due to diarrhea

Cholera toxin targets chloride channel in apical membrane, reverses solute concentrations, efflux of chloride, net efflux of solute out of apical cells, causes osmotic gradient, water leaves cells. Solution of glucose and sodium targets this so you can reverse this while your body targets the toxin

Question 29

Where would you find a Na+ _ Ca2+ antiporter?

Answer 29

Cardiac muscle cell membranes

Question 30

What are Na+ _ Ca2+ antiporters important for?

Answer 30

Clearing Ca2+ after an action potential

Question 31

In a cardiac muscle cell plasma membrane, why do Na+ _ Ca2+ antiporters bring in three Na+ for every Ca2+?

Answer 31

Ca2+ needs to be very low concentration within the cell, (nanomolar conentrations) so has to overcome a large electrochemical gradient (since inside cell is negative as well), needs the energy associated with 3 Na+ down their electrochemical gradient to pump the Ca2+ out

Question 32

In cardiac muscle cells, what maintains the Na+ gradient across the membrane?

Answer 32

Na+/K+ ATPase maintains Na+ gradient, pumping Na+ out

This is to enable Ca2+ efflux via the Na+_Ca2+ antiporter

Question 33

How did heart failure use to be treated?

Answer 33

oubain was used to inhibit the Na+/K+ ATPase, to elevate cytosolic Ca2+ which would increase force of heart muscle contractions

Question 34

In plant roots, how are nutrients like ions taken up?

Answer 34

Symported in due to H+ electrochemical gradient
(how many H+ are required depends on concentration gradient of desired ion, how difficult it is to symport it)