Lecture 22 C2: transport across the cellmembrane

Question 1

What is the difference between active and passive transport

Answer 1

Passive transport moves substances down their concentration or electrochemcial gradients with only their kinetic energy
whereas Passive transport uses energy to drive substances against their concentration or electrochemical gradients

Question 2

What is the difference between mediated and non mediated transport

Answer 2

non mediated transport does not directly use a transport protein - eg permeable to the lipid bilayer.
Mediated tranposrt moves materials with the help of a transport protein

Question 3

What is non mediated transport important for

Answer 3

the absorption of nutrients and secretion of wastes.

Question 4

How does the structure of ion channel allow ions to diffuse through and what speed is it ?

Answer 4

The water filled pore is lined by specific hydrophilic amino acids and It has hydrophobic amino acids in the core. Ions going through the pore are shielded from the lipid bilayer. They don’t bind so its fast

Question 5

How do ion channels exhibit selectivity and why is it important

Answer 5

Specific amino acids lining the pore can repel ions. Pores size can discriminate against different sized ions. By being selective to a particular ion the channel can harness the energy stored in the different ion gradients

Question 6

Why is it important for ion channels to have a gating system ie not open all the time

Answer 6

Diffusion is rapid so this would mean that ions could quickly make an equilibrium-> so no longer gradient and-> no longer stored energy.

Question 7

What are the different stimuli that control gate opening and closing

Answer 7

voltage (change in mp), ligand binding (Ach) cell volume (stretch of cytoskeleton), pH and phosphorylation (cell signalling pathways)

Question 8

What does the patch clamp technique measure

Answer 8

The current flowing through an individual channel due to the diffusion of ions

Question 9

What do current fluctuations recorded represent

Answer 9

the opening and closing of single ion channels, the conformational changes in channel protein structure that are associated with channel gating.

Question 10

What is current generated by 1 million ions per second through channel

Answer 10

10^ -12 amp

Question 11

What is the difference between carrier mediated transport & channel transport

Answer 11

Carrier mediated transport has the substrate transported, directly interacting with the transport protein and as it undergoes conformational change transport rates are slower than channels.

Question 12

What features of carrier mediated transport proteins make them similar to enzymes

Answer 12

They have binding specificity, can be inhibited if binding site is messed up, competition- 2 molecules competing for the binding site can slow down the rate of transport and saturation- when increasing conc will increase rate up to a transport maximum

Question 13

How do carrier mediated transport proteins differ from enzymes

Answer 13

They do not catalyse chemical reactions, they mediate transport across the cell membrane at a faster than normal rate.

Question 14

Describe the facilitated diffusion of glucose (3 steps)- passive carrier mediated transport

Answer 14

1. Glucose bind s to tranpsport protein (GLUT)
2. Transport protein changes shape. Glucose moves across the cell membrane down conc gradient
3. Kinase enzyme converts glucose to glucose-6-phosphate therefore reducing intracellular glucose conc and maintaining conc gradient

Question 15

What is the difference between the two forms of Active transport

Answer 15

Primary active transport uses energy directly derived from the hydrolysis of ATP whereas Secondary active transport uses energy stored in the ionic conc gradient set up by primary to drive active transport of molecule against its gradient

Question 16

Give examples of primary active transport

Answer 16

Na/K ATPase. Ca/K ATPase in muscle, H/K ATPase in stomach

Question 17

How does Na/K ATPase work (4 steps)

Answer 17

1. 3 Na+ bind from inside the cell.
2. ATP split/ Na+ pushed out
3. 2K+ bind from outside cell, phosphate is released
4. K+ is pushed in

Question 18

A pump that generates a net current is called what

Answer 18

electrogenic

Question 19

What is difference in ion conc established by active carrier mediated transport important for

Answer 19

maintenance of RMP
electrical excitability 
muscle contraction
maintenance of steady cell volume
Uptake of nutrients & maintenance of intracellular pH  by secondary active transporters

Question 20

What is the ‘leak-pump’ hypothesis

Answer 20

Because there is continual leakage of Na+ back into the cell and K+ out of the cells (down their gradients) the pump works continuously

Question 21

What powers the secondary active transport of molecules when its set up by a Na+ gradient

Answer 21

Na+ pump establishes a gradient by using ATP to pump Na+ out of the cell: more positive outside and this can be used to power the secondary active transport using energy released by Na+ when its allowed to go down its conc gradient (into cell)

Question 22

What are the two types of secondary active transport

Answer 22

Antiporter/ exchanger: eg. Na+ goes in Ca2+ or H+ being pushed out of the cell. 
Or Symporter (co transporter) Glucose or amino acids bind to carrier with Na+ and come in cell.

Question 23

Do ions bind to ion channel

Answer 23

no