Crossing membranes

Question 1

What property allows cells to control what enters and leaves?

Answer 1

Impermeability

Question 2

2 Different types of transport

Answer 2

Passive

Active

Question 3

Types of passive transport

Answer 3

Simple diffusion
Channel mediated
Carrier mediated

Question 4

Simple diffusion

Answer 4

Gases and small uncharged molecules

Diffusion from a high to low concentration

Question 5

What does the rate of simple diffusion depend on?

Answer 5

Diffusion gradient

Hydrophobicity of molecules

Question 6

Rate limiting step in simple diffusion

Answer 6

Transition from aqueous to hydrophobic interior of the membrane

Question 7

Isotonic media

Answer 7

No net flow of water

Question 8

Hypotonic media

Answer 8

Water flows into a cell causing swelling

Question 9

Hypertonic media

Answer 9

Water flows out and the cell shrinks

Question 10

Named example of a water channel

Answer 10

Aquaporin

Question 11

Aquaporin

Answer 11

Tetramer - has 4 channels
Protein has several membrane embedded alpha helices which are connected by loops. Each subunit forms a channel and channels are lined with polar amino acid side chains so hydrogen bonds can form with water.

Question 12

What are the four types of channels/transporters?

Answer 12

Uniporter
Symporter
Antiporter
ATP-powered pump

Question 13

Uniporter

Answer 13

Movement is gradual due to difference in concentration gradient which is accelerated by conformational changes in the protein

Question 14

Symporter

Answer 14

Movement of two molecules in the same direction where one of the molecules is moving against its concentration gradient. Facilitated by the movement of the molecule that is moving with the concentration gradient.

Question 15

Antiporter

Answer 15

Movement of two molecules in opposite directions where one of the molecules is moving against its concentration gradient. This is facilitated by the movement of the molecules that is moving with the concentration gradient.

Question 16

Glucose uptake

Answer 16

Symporter imports 2 Na+ and 1 glucose molecule from the intestinal lumen into the cytosol. An Na+/K+ ATPase pump exports Na+ into the blood and K+ into the cytosol. Glucose is then transported out of the cell into the blood by GLUT2 uniporter.

Question 17

Glut 1

Answer 17

Found in the blood barrier and transports glucose from high to low concentrations.

Question 18

Movement of glucose as facilitated by Glut 1

Answer 18

Glucose binds inducing a conformational change so Glut1 changes from having an outward facing conformation to inward facing. Facilitated diffusion and glucose has moved down its concentration gradient into the cytosol of the cell.

Question 19

How do uniporters (Glut1) exhibit simple enzyme kinetics?

Answer 19

Bind affinity determines the rate of movement Km

Question 20

How is transport controlled in uniporters (Glut1)?

Answer 20

Limited number so can reach a point of full saturation
Blocked by competitive and non-competitive inhibitors
Transport can be controlled by controlling the number of transporters

Question 21

Glucose transport by Glut4

Answer 21

Controlled by insulin-responsive exocytosis
Insulin binds to a receptor to create a signalling cascade that stimulates glucose transporters to fuse with the plasma membrane so glucose can enter a cell

Question 22

Glut 4 glucose and insulin movement explained

Answer 22

In the absence of insulin glucose cannot enter a cell
Insulin present and binds to receptor, signal transduction cascade, GLUT 4 transporters inserted into plasma membrane, glucose enters the cell.

Question 23

Symporters and antiporters compared

Answer 23

Both couple the movement of two molecules where one molecule is moving with its concentration gradient and one molecule is moving against its concentration gradient (energetically unfavourable). The energetically favoured reaction drives the unfavoured reaction.
In symporters both molecules are moving in the same direction, in antiporters the molecules are moving in opposite directions

Question 24

Movement of sodium and glucose into the small intestine

Answer 24

2 Na+ molecules move into a symporter as well as one glucose molecule. The symporter has an occluded conformation. Conformational change is induced so the symporter changes from having an outward to an inward facing conformation. Na+ and glucose released into the cell. Na+ has moved with its concentration gradient and glucose against. The outward facing conformation of the symporter is re-gained