Membrane transport

Question 1

What is passive transport?

Answer 1

Solutes travelling down their concentration gradient, which requires no energy

Question 2

What type of solute can get across the membrane without any help?

Answer 2

Small hydrophobic molecules and gases

Question 3

What is facilitated diffusion?

Answer 3

Solute goes down its concentration gradient but needs a transporter to get across

Question 4

What is active transport?

Answer 4

A solute going against its concentration gradient. Requires energy

Question 5

How do ions get through membranes?

Answer 5

Through ion channels or small molecules called ionophores

Question 6

Why do polar solutes need a transporter to get through the membrane?

Answer 6

The interior of the transporter is polar and forms favourable interactions with the solute. Otherwise the water would have to be removed from the solute for it to go through the membrane, which is very unfavourable

Question 7

What are the 2 classes of transport proteins?

Answer 7

Channels and carriers

Question 8

What are channels?

Answer 8

Holes in the membrane that can be gated. They provide fast transport across the membrane

Question 9

Can channels be saturated?

Answer 9

No

Question 10

How good is the substrate specificity of channels?

Answer 10

Not as good as carriers

Question 11

What is usually the structure of a channel?

Answer 11

A homooligomer with multiple subunits

Question 12

What are carriers?

Answer 12

Proteins that undergo a conformational change to get the solute across the membrane

Question 13

Can carriers be saturated?

Answer 13

Yes

Question 14

How good is the substrate specificity of carriers?

Answer 14

Highly stereoselective

Question 15

What is usually the structure of carriers?

Answer 15

Usually monomers

Question 16

What are GLUT1 transporters?

Answer 16

Carrier proteins that transport glucose across the membrane of red blood cells through facilitated diffusion

Question 17

What are the 2 conformations of GLUT1?

Answer 17

T1: open to the outside
T2: open to the inside

Question 18

What is the structure of GLUT1?

Answer 18

12 transmembrane helices with the polar portions on the inside

Question 19

What does the graph of facilitated diffusion kinetics look like?

Answer 19

Michaelis-Menton

Question 20

What is Kt? What does it measure?

Answer 20

The transport constant. Measures solute affinity

Question 21

What drives transport of glucose into red blood cells through GLUT1?

Answer 21

Glucose gets converted into glucose-6-phosphate, which locks it into the cell

Question 22

What happens if there is a deficiency of GLUT1?

Answer 22

De vivo syndrome. The brain can’t get enough fuel and causes microencephaly and seizures

Question 23

Why can we use radioactive glucose to image cancer?

Answer 23

Cancer cells overexpress GLUT1 and have high rates of glycolysis

Question 24

Where is GLUT2 found?

Answer 24

Liver

Question 25

Why is the high Kt of GLUT2 a good thing?

Answer 25

It lets the muscles get their share of glucose before the liver takes the rest and stores it as glycogen

Question 26

What is GLUT5 used for?

Answer 26

Fructose transport

Question 27

Where is GLUT4 found?

Answer 27

Muscle, heart, fat cells

Question 28

How good is the affinity of GLUT4 for glucose?

Answer 28

High affinity and low Kt

Question 29

What regulates the expression of GLUT4?

Answer 29

Insulin

Question 30

What happens to GLUT4 expression when muscle cells receive an insulin signal?

Answer 30

Insulin signals high glucose in the blood. Vesicles with GLUT4 get brought to the membrane and they start bringing in glucose. When the insulin signal drops off, the transporters are brought back into their vesicles

Question 31

What are 4 ways to get the energy for active transport?

Answer 31

Chemical reactions (ATP hydrolysis), oxidation, absorption of light, flow of another solute down its concentration gradient

Question 32

What are the 3 types of ATPases?

Answer 32

P-type, F-type, V-type

Question 33

How does a P-type ATPase work?

Answer 33

Involves reversible phosphorylation of the transporter to cause a conformation change

Question 34

How does an F-type ATPase work?

Answer 34

Couples the energy from ATP hydrolysis to transport something against its gradient

Question 35

What does a V-type ATPase do?

Answer 35

Acidifies vacuoles

Question 36

What is SERCA?

Answer 36

A P-type ATPase on the ER membrane that pumps one calcium into the ER for 1 ATP

Question 37

How does SERCA work?

Answer 37

Reversible conformation change from phosphorylation

Question 38

What type of ATPase is the sodium-potassium pump?

Answer 38

P-type

Question 39

What is lactose permease?

Answer 39

Symport that brings in one lactose and one proton through secondary active transport

Question 40

What are ionophores?

Answer 40

Small cyclic peptides that coordinate a metal ion with carbonyl groups. They are specific to an ion and have a hydrophobic exterior

Question 41

What do ionophores do?

Answer 41

They dissipate ion gradients and disrupt secondary transport

Question 42

What are aquaporins?

Answer 42

Very narrow channels that allow water across the membrane

Question 43

What are the only 2 molecules that can fit through an aquaporin?

Answer 43

Water and hydronium

Question 44

Why doesn't hydronium go through aquaporins?

Answer 44

The alpha helices of the aquaporin have their partially positive N-terminus pointing into the channel. The electrostatic repulsion keeps the hydronium out

Question 45

What is the structure of a potassium channel?

Answer 45

Cone within a cone. It's a homotetramer with each subunit having 2 long helices and 1 short helix. The C-terminus of the short helix points into the channel

Question 46

How do potassium channels select for their substrate?

Answer 46

Size. Only potassium ions make optimal contacts with the backbone carbonyls, sodium is too small

Question 47

How is the structure of voltage gated potassium channels different from normal potassium channels?

Answer 47

Still has the same cone in a cone structure, but has a few extra helices on the outside. Helix 4 had 4 arginine residues that sense the voltage change