Carriers

Question 1

Valinomycin - a model carrier?

Answer 1

~ ring structure (K+ center)
~ K+ binding by carbonyl Os
~ outer perimeter of ring = lipophilic
~ highly selective for K+ over Na+

Question 2

Valinomycin mechanism

Answer 2

~ hydrophilic pocket sitting in the membrane

~ K+ is dehydrated, passes through membrane then rehydrated

Question 3

Kinetic properties of carriers (4)

Answer 3

1. binding of substrate from outside and formation of carrier-substrate (CS) complex
2. re-orientation of CS to align binding site to inside
3. release of substrate
4. empty carrier re-orientates to align to binding site outside

Question 4

What was found out about valinomycin from chilling experiments?

Answer 4

it diffuses across the membrane and exposes the ion binding site alternatively to each side of the membrane

Question 5

Which model does valinomycin support?

Answer 5

the kinetic model

NOT the molecular mechanism model

Question 6

Method for uniport transport by GLUT1

Answer 6

1. glucose binding site faces outwards, ready to bind glucose
2. glucose binds
3. GLUT1 undergoes conformation change to orientate the binding site to face inwards
4. glucose is released
5. GLUT1 re-orientates

Question 7

Neurotransmitter sodium symporters (NSSs)

Answer 7

~ remove dopamine from receptors and into glia cells

~ require Na+/Cl- to drive up hill transport

Question 8

Neurotransmitter sodium symporters (NSSs) - structure

Answer 8

~ two hairpin loops that form gates and don’t sit in the membrane
~ gates alternate between being open and being closed

Question 9

Alternating access model

Answer 9

through conformational changes, the substrate binding site alternately faces either side of the membrane

Question 10

How does Na+ alter binding affinity

Answer 10

Na+ binding increases affinity of binding site for neurotransmitter

Question 11

GLUT1 facilitator (3)

Answer 11

~ main transporter of glucose to brain
~ deficiency = epilepsy
~ over expression = cancer indicator

Question 12

GLUT2 facilitator (4)

Answer 12

~ main transporter of glucose between liver and blood
~ forms part of ‘glucose sensor’ in pancreatic beta cells
~ insulin recruits more GLUT2 to PM
~ disregulation = diabetes

Question 13

Proof of carriers

Answer 13

glucose transport in RBC examined
~ L-glucose showed diffusion via lipids
~ D-glucsoe showed existence of ‘something’ which facilitated its diffusion

Question 14

What is the model eukaryotic cell most often used?

Answer 14

S. cerevisiae

Question 15

Fungi + glucose uptake

Answer 15

most have it coupled with H+ uptake

not yeast

Question 16

Na+/glucose symporter

secondary active transporter

Answer 16

Glucose + Na+ uptake is coupled in some cells e.g. intestinal cells
~ GLUT 2 then transports glucose into the blood
~ Na+/K+ ATPase transports Na+ out, K+in and ATP–>ADP + pi (inside cell)

Question 17

Na+ - Ca2+ antiporter

secondary active transporter

Answer 17

~ cardiac muscle cell PM
~ maintains low cytosolic Ca2+ conc. = relaxation after contraction
~ Ca2+ efflux and Na+ influx
~ Na+ - K+ channel sorting out Na+

Question 18

Nutrient transport in plants is…

Answer 18

coupled to H+ gradient