WEEK 1: TRANSMEMBRANE TRANSPORT

Question 1

What is meant by that the membrane is selectively permeable?

Answer 1

It allows some particles to pass through and exclude others.

Question 2

What is free movement of particles across cell membrane dependent on?

Answer 2

*Relative solubility of the particle in lipid
*Particle size

Question 3

Describe the concentration of SODIUM, POTASSIUM AND CHLORIDE IN THE ECF and ICF

Answer 3

SODIUM: 15 in, 150 out
POTASSIUM: 150 in 5 out
CHLORIDE: 7 in 110 out

Question 4

Outline the Importance of Concentration Differences.

Answer 4

*Electrical properties of cells
Nerve, muscle, heart, gut etc

*Epithelial Transport
Gut, kidney, brain etc

Question 5

Describe passive and active membrane transport.

Answer 5

*Passive
Down concentration gradient

*Active
May be up concentration gradient

Question 6

Outline categories of Membrane Transport.

Answer 6

1.Aqueous Channels-Passive
2.Solution in Lipid Phase-Passive
3.Carrier Mediated- All active and some passive

Question 7

What is simple diffusion?

Answer 7

It is the random movement of substances from high to low concentration, through a selectively permeable membrane.

Question 8

What is a concentration gradient?

Answer 8

Difference in concentration of a substance between two adjacent areas.

Question 9

What is net diffusion?

Answer 9

The difference between two opposing movements across a selectively permeable membrane.

Question 10

What is dynamic equilibrium?

Answer 10

Balance between the two opposing forces

Question 11

Outline factors that influence diffusion according to Fick’s law.

Answer 11

*Concentration gradient of a substance.
*Surface area of membrane
*Lipid solubility
*Molecular weight of substance
*Distance (thickness)
*Permeability and **diffusion coefficients

Question 12

Ions that can penetrate the membrane also move passively along their electrical gradient.

What is an electrical gradient?

Answer 12

1.A difference of charge between two adjacent areas.

It promotes movement towards the area of opposite charge.
Positive charges move towards the negative charges and vice versa.

2.The net effect of simultaneous electrical and concentration gradient on an ion.

Question 13

What is osmosis?

Answer 13

It is the net diffusion of water down its own concentration gradient.

Question 14

What are aquaporins?

Answer 14

Channels specific for passage of water.

Question 15

What is hydrostatic pressure?
What is osmotic pressure?

Answer 15

A push force that is exerted by a stationary fluid on an object.

A pull force for osmotic flow of water into a solution with low free water concentration.

Question 16

Describe what happens in carrier mediated transport.

Answer 16

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

Question 17

Outline the 3 factors that determine the kid and amount of substance to be transported by carrier mediated transport.

Answer 17

1.Specificity
2.Saturation
3.Competition

Question 18

What is transport maximum?

Answer 18

The amount of a substance carriers can transport across the membrane in a given time.

Question 19

Facilitated diffusion is passive carrier mediated transport.

Define facilitated diffusion.

Answer 19

This is whereby a carrier is used to passively move a substance down its concentration gradient.

Question 20

What is active transport?

Answer 20

Active carrier mediated transport involving moving substances against a concentration gradient.

Question 21

State general Features of Active Transport

Answer 21

*Transport can be against electro-chemical gradient

*Chemical energy source required (ATP)

*There is often movement of more than one substance

*Out of balance movement of ions gives SMALL contribution to membrane potential

*Can be competitively inhibited by substances that combine with the active site

Question 22

Outline the 3 types of active transport.

Answer 22

*Primary Active Transport
Uses the energy source directly.

*Secondary Active Transport
-Indirect use of the energy source

*Tertiary Active Transport

Question 23

Na+ -K+ ATPase is an example of primary active transport.
Describe how it works.

Answer 23

1.The pump has 3 high affinity sites for Sodium ion and 2 low-affinity binding sites for Potassium ion when exposed to the ICF.

2.3 SODIUM OINS BIND TO THE PUMP and split ATP into ADP plus a phosphate group.

-Phosphate group binds to the pump.

3.Phosphorylation occurs and causes pump to change conformation so that the Sodium ion binding sites are exposed to the ECF AND RELEASED.

4.The potassium ion binding sites are exposed to the ECF so the affinity for Potassium ions sites increases.

5.2 Potassium ions bind to the pump and release the phosphate group.

-Dephosphorylation causes the pump to revert to its original position.

6.The 2 potassium ions are released to the ECF as the affinity for Potassium sites decreases.

The affinity for Na ion binding sites increase and the sodium ion bind to the pump.

Note: The affinity for an ion on the binding site is high when at the region where the ion concentration is low and is decreased or low where the ion concentration is high as the pump unloads.

Question 24

Describe Secondary Active Transport.

Answer 24

*Able to move “uphill” without itself breaking down ATP
Something, usually sodium, is running down a concentration gradient.

*The movement of 2 or 3 substances is linked either as co-transport or exchange.

*Sodium ion moves down the concentration gradient and acts as an ion that supplies the driving force for uphill movement of another solute by a secondary active transport carrier.

IT INVOLVES:
*Symport: Sodium and the ion transported move in the same direction
*Antiport: Sodium and the ion transported move in opposite direction.

Sodium moves into the cell and drives the substance out of the cell.

Question 25

Outline the types of vesicular transport.

NOTE: Vesicular transport is active.

Answer 25

1.Endocytosis
2.Exocytosis

Question 26

State the 3 types of endocytosis.

Answer 26

*Pinocytosis
*Phagocytosis
*Carrier-mediated endocytosis

Question 27

1.Unassisted Transport (Passive)
Simple diffusion through lipid bilayer
Simple diffusion through protein channel
Osmosis

2..Assisted Transport (Active or Passive)

Carrier-Mediated Transport
-Passive (Facilitated diffusion)
-Active (Primary and Secondary) & rarely tertiary!

2.Vesicular Transport (All active)
(Endocytosis and Exocytosis)

Question 28

Describe what happens in tertiary active transport.

Question 29

Describe the membrane permeability of an average cell.

Answer 29

*The grand total of aqueous channels results in membrane that is K+ and Cl- permeable but relatively sodium impermeable because of sodium is too large to pass through the channels in the membrane.

*Sodium can only pass through the membrane via opening of sodium channels.

*Selective permeability

*This selective permeability has considerable effect on the concentration of ions because of presence of impermeant negative particles inside cell

Question 30

What is the Donnan Effect?

Answer 30

The Donnan effect is the equilibrium and interaction between a soluble charged molecule and charge existing on a membrane.

Question 31

Describe the intracellular distribution of calcium.

Answer 31

*Very low calcium concentration inside cell

-Free calcium about 10-7M as opposed to about 2x10-3 outside.

-Very important for function e.g., heart, nerve, muscle

*Taken up into organelles of cell at the cell membrane through:

Primary Active Transport and/or sodium exchange

Question 32

Why is it SODIUM Pump?

Answer 32

*It transports both sodium and potassium

*However, will only have significant effect on sodium because of lesser permeability to sodium

*Pump overwhelmed by potassium movement but can cope with sodium

Question 33

Outline factors affecting distribution of ions across cell membrane.

Answer 33

*Selective Membrane Permeability
K+ and Cl- can cross cell membrane relatively readily. Distribution of these ions affected by the negative charged ions trapped inside cell. (The Donnan Effect)

*The Sodium Pump