Membrane transport

Question 1

What are the two properties that influence whether a particle can permeate the plasma membrane?

Answer 1

Solubility of the particle in lipid

Size of particle

Question 2

What is requires for movement across a membrane?

Answer 2

Pathway and a driving force

Question 3

What are the driving forces for membrane transport?

Answer 3

Passive

Active (requires ATP)

Question 4

Describe unassisted membrane transport?

Answer 4

> passively driven across the membrane by two forces:

• Diffusion down a concentration gradient
• Movement along an electrical gradient

Question 5

What 5 factors make up Fick’s law of diffusion?

Answer 5

1. The magnitude of the concentration gradient
2. The surface area of the membrane across which diffusion is taking place
3. The lipid solubility of the substance
4. The molecular weight of the substance
5. The distance through which diffusion must take place

Question 6

What are the ion-specific channel proteins?

Answer 6

Leak or ligand-gated

Question 7

What is an electrochemical gradient?

Answer 7

net effect of simultaneous electrical and concentration gradients on this ion is called an electrochemical gradient

Question 8

Define osmosis?

Answer 8

net diffusion of water down its own concentration gradient through a selectively permeable membrane

Question 9

How can water molecules pass through plasma membrane?

Answer 9

Aquaporins (water channels)

Question 10

How does water move via osmosis?

Answer 10

water moves by osmosis to the area of higher solute concentration

Question 11

Define osmolarity?

Answer 11

concentration of osmotically active particles present in a solution
>Can be calculated or measured
>Units: osmoles (Osm) of solute per litre (Osm/l or osmol/l)
>Body fluids: ~300 mOsm/l

Question 12

Define tonicity?

Answer 12

effect a solution has on cell volume
>Iso-, hypo- or hypertonic
>Tonicity has no units

Question 13

What are the passive transport mechanisms?

Answer 13

> Diffusion down concentration gradients (simple diffusion)
Movement along electrical gradients (ion channels)
Osmosis
- All depend on lipid solubility or ability to fit through specific channels

Question 14

What are the two different mechanisms for selective transport?

Answer 14

1. Carrier-mediated transport

2. Vesicular transport

Question 15

Describe carrier mediated transport?

Answer 15

Substance binds onto a specific carrier which undergoes a conformational change (shape change) which transports the substance

Question 16

What is carrier mediated transport specificity?

Answer 16

> Each carrier is specialised to transport a specific substance or a few closely related chemical compounds.
Cystinuria

Question 17

What is carrier mediated transport saturation?

Answer 17

Transport maximum (Tm).  
Renal glucose re-absorption

Question 18

What is carrier mediated transport competition?

Answer 18

An amino acid carrier can transport both Gly and Ala. The presence of both diminishes the rate of transfer for either.

Question 19

What two forms does carrier-mediated transport come in?

Answer 19

1. Facilitated diffusion (not requiring energy)

2. Active tranport (requiring energy)

Question 20

Describe facilitated diffusion?

Answer 20

uses a carrier to facilitate (assist) the transfer of a substance across the membrane ‘downhill’ from high to low concentration.

Question 21

Describe active transport?

Answer 21

requires the carrier to expend energy to transfer a substance ‘uphill’ against a concentration gradient.

Question 22

Describe the model for facilitated diffusion?

Answer 22

1. Carrier protein takes conformation in which solute binding site is exposed to region of higher concentration
2. Solute binds to carier protein
3. Carrier protein changes conformation so that binding site is exposed to region of lower concentration
4. Transported solute is released and carrier protein returns to conformation in 1.

Question 23

What are the two forms of active transport?

Answer 23

1. Primary active transport

2. Secondary active transport

Question 24

Describe primary active transport?

Answer 24

Energy is directly required to move a substance against its concentration gradient.

Question 25

Describe secondary active transport?

Answer 25

> Energy is required, but it is not used directly to produce ‘uphill’ movement.

> The carrier does not split ATP – instead it moves a molecule ‘uphill’ by using secondhand energy stored in the form of an ion concentration gradient (usually a Na+ gradient)

Question 26

Describe the model for simple active transport?

Answer 26

1. carrier protein splits ATP. Pi binds to carrier, increasing affinity of its binding site for ion
2. ion to be transported binds to carrier on low-concentration side
3. carrier changes conformation so that binding site is exposed to opposite side of membrane. This change reduced affinity of site for ion
4. Carrier releases ion to side of higher concentration, Pi is released
5. when binding site is free carrier reverts to original state

Question 27

Where is Na+/K+ ATPase found?

Answer 27

Plasma membrane of all cells

Question 28

What are the three roles of Na+/K+ ATPase?

Answer 28

1. Helps establish Na+ and K+ concentration gradients across the plasma membrane of all cell
2. helps regulate cell volume by controlling concentration of solutes inside the cell
3. the energy used to drive the pump indirectly serves as the energy source for secondary active transport

Question 29

Define secondary active transport?

Answer 29

The transfer of a solute across the membrane is always coupled with the transfer of the ion that supplies the driving force (typically Na+)

Question 30

What are the two mechanisms of secondary active transport?

Answer 30

1. Symport (co-transport)

2. Antiport (exchange or countertransport)

Question 31

Describe the symport?

Answer 31

The solute and Na+ move in the same direction.

Question 32

Describe the anti port?

Answer 32

The solute and Na+ move in opposite directions (Na+ into, solute out of the cell)

Question 33

Describe vesicular transport?

Answer 33

Requires energy for vesicle formation and movement within the cell (active).

Question 34

Describe the steps of vesicular transport

Answer 34

1. Endocytosis
   ‘Pinching off’ of membrane to engulf substance
2. Exocytosis
   - vesicle fuses with the plasma membrane, releasing its contents to the ECF.
   - Secretion of enzymes, protein hormones
   - Way of adding carriers, channels or receptors to the plasma membrane (E.g. GLUT4 & insulin)