Membrane transport

Question 1

What are two properties that influence whether a particle can permeate the plasma membrane without assistance?

Answer 1

Solubility of the particle in lipid.

Size of the particle.

Question 2

What is required for movement across a membrane?

Answer 2

Pathway and a driving force is required for movement across a membrane (applies for assisted or unassisted).

Question 3

What are the two different types of driving forces?

Answer 3

Passive.

Active (requires the cell to expend energy (ATP) to produce movement).

Question 4

What is unassisted membrane transport?

Answer 4

Molecules and ions that can penetrate the membrane are passively driven across the membrane by 2 forces:

• Diffusion down a concentration gradient, &/or
• Movement along an electrical gradient.

Question 5

What is net diffusion?

Answer 5

Net diffusion is where diffusion from area A to area B minus diffusion area B to area A.

Question 6

How does diffusion through a membrane occur?

Answer 6

If a substance can permeate the membrane, diffusion occurs.

If the membrane is impermeable to a substance, no diffusion occurs.

Question 7

What is Fick’s Law of Diffusion?

Answer 7

Several factors in addition to concentration gradient influence the rate of net of diffusion across the membrane and collectively make up.
Fick’s Law of Diffusion:
- The magnitude of the concentration gradient
- The surface area of the membrane across which diffusion is taking place
- The lipid solubility of the substance
- The molecular weight of the substance
- The distance through which diffusion must take place.

Question 8

What are the factors influencing the rate of net diffusion of a substance across a membrane (Fick’s Law of Diffusion)?

Answer 8

Increase concentration gradient of substance increases the effect on rate of net diffusion.
Increase surface area of membrane increases effect on rate of net diffusion.
Increase lipid solubility increases effect on rate of net diffusion.
Increase molecular weight of substance decreases effect on rate of net diffusion.
Increase distance (thickness) decreases effect on rate of net diffusion.

Question 9

How do ions that permeate the membrane also move passively along their electrical gradient?

Answer 9

In addition to their concentration gradient, in movement is also affected by their electrical charge
- E.g. ions with like charges repel each other, and ions with opposite charges attract each other.
A difference in charge between 2 adjacent area generate an electrical gradient that promotes the movement of ions toward the area of opposite charge.
When an electrical gradient exists between the ICF and ECF, only ions that can permeate the membrane can move along this gradient.
Ion-specific channel proteins:
- Leak or ligand-gated.

Question 10

What are electrochemical gradients and how do they work?

Answer 10

Both an electrical and a concentration (chemical) gradient may be acting on a particular ion at the same time.
The net effect of simultaneous electrical and concentration gradients on this ion is called an electrochemical gradient.
Electrochemical gradients contribute to the electrical properties of the plasma membrane.

Question 11

What is osmosis and how is it carried out?

Answer 11

Osmosis is the net diffusion of water down it’s own concentration gradient through a selectively permeable membrane.
Water molecules can pass through permeate the plasma membrane more readily than would be expected from solubility in lipid.
Aquaporins (water channels).
Because solutions are always referred to in terms of concentration of solute, water moves by osmosis to the area of higher solute concentration.

Question 12

What are osmolarity and tonicity.

Answer 12

Osmolarity is the concentration of osmotically active particles present in a solution.
- Can be calculated or measured
- Units: osmoles (Osm) of solute per litre (Osml or osmol/l)
- Body fluids: ~300 mOsm/l
Tonicity is the effect a solution has on cell volume
- Iso-, hypo- or hypertonic
- Tonicity has no units. E.g. isotonic saline (0.9% NaCl solution) is used as a vehicle for delivery of drugs intravenously.

Question 13

If the normal cell volume intracellular fluid: 300 mOsm nonpenetrating solutes.
What happens to a red blood cell in 300mOsm non penetrating solutes.
200 mOsm nonpenetrating solutes.
400 mOsm nonpenetrating solutes.?

Answer 13

300 mOsm nonpenetrating solutes. No net movement of water; no change in cell volume. Isotonic conditions.
200 mOsm nonpenetrating solutes.
Water diffuses into cells; cells swell. Hypotonic conditions.
400 mOsm nonpenetrating solutes. water diffuses out of cells; cells shrink. Hypertonic conditions.

Question 14

What are unassisted and assisted membrane transport?

Answer 14

Passive transport mechanisms:
- 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.
What about poorly lipid soluble polar molecules (e.g. glucose or amino acids)?
- Plus, often ins have to be transported across the plasma membrane against their concentration gradient.
Cells use 2 different mechanisms for selective transport:
- Carrier-mediated transport
- Vesicular transport

Question 15

What is carrier mediated transport?

Answer 15

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

Question 16

Three important characteristics determine the kind and amount of material transferred across the membrane. What are these?

Answer 16

Specificity
- Each carrier is specialised to transport a specific substance or a few closely related chemical compounds.
- Cystinuria
Saturation
- Transport maximum (Tm). Renal glucose re-absorption.
Competition
- E.g. An amino acid carrier can transport both Gly and Ala. The presence of both diminishes the rate of transfer for either.

Question 17

What is the comparison of carrier-mediated transport and simple diffusion down a concentration gradient?

Answer 17

As the concentration of transported molecules in ECF increases, the rate of transport of molecule into cell remains the same for carrier-mediated transport down concentration gradient (facilitated diffusion).
As the concentration of transported molecules in ECF increases, the rate of transport of molecule into cell increases steadily for simple diffusion down concentration gradient.

Question 18

Carrier-mediated transport takes two forms, what are these?

Answer 18

Carrier-mediated transport takes two forms:

• Facilitated diffusion (not requiring energy)
• Active transport (requiring energy)

Question 19

What is facilitated diffusion?

Answer 19

Facilitated diffusion uses a carrier to facilitate (assist) the transfer of a substance across the membrane “downhill” from high to low concentration.

Question 20

What is active transport?

Answer 20

Active transport requires the carrier to expend energy to transfer a substance “uphill” against a concentration gradient.

Question 21

What is the process of facilitated diffusion?

Answer 21

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

Question 22

Active transport comes in 2 forms, what are these?

Answer 22

Primary active transport

Secondary active transport

Question 23

What is primary active transport?

Answer 23

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

Question 24

What is secondary active transport?

Answer 24

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 25

What is the process of simple active transport?

Answer 25

• Carrier protein splits ATP into ADP plus phosphate. Phosphate group binds to carrier, increasing affinity of its binding site for ion.
• Ion to be transported binds to carrier on low-concentration side.
• In response to ion binding, carrier changes conformation so that binding site is exposed to opposite side of membrane. The change in shape also reduces affinity of site for ion.
• Carrier releases ion to side of higher concentration. Phosphate group is also released.
• When binding site is free, carrier reverts to it’s original shape.

Question 26

What is the Na+ - K+ ATPase (a.k.a. Na+ - K+ pump)?

Answer 26

More complicated primary active transporter.
Found in the plasma membrane of all cells.
Transports 3X Na+ out of the cell for every 2X K+ in.

Question 27

What are 3 important roles for the Na+ - K+ pump?

Answer 27

• Helps establish Na+ and K+ concentration gradients cross the plasma membrane of all cells (next GRC lecture).
• Helps regulate cell volume by controlling concentration of solutes inside the cell.
• The energy used to drive the pump indirectly serves as the energy source for secondary active transport.

Question 28

What is secondary active transport?

Answer 28

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 29

Secondary active transport occurs by one of two mechanisms, what are these?

Answer 29

Symport (co-transport)

Antiport (exchange or counter transport)

Question 30

What is symport (co-transport)?

Answer 30

The solute and Na+ move in the same direction

- E.g. glucose absorption at the apical membrane of enterocytes.

Question 31

What are antiports (exchange or counter transport)?

Answer 31

The solute and Na+ move in opposite directions (Na+ into, solute out of the cell).
- E.g. cells exchange Na+ and H+ by means of antiport, important in regulation of intracellular pH.

Question 32

What is vesicular transport?

Answer 32

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

Question 33

What is exocytosis?

Answer 33

A secretory vesicle fuses with the plasma membrane, releasing the vesicle contents to the cell exterior. The vesicle membrane becomes part of the plasma membrane.

• A 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 and insulin).

Question 34

What is endocytosis?

Answer 34

Materials from the cell exterior are enclosed in a segment of the plasma membrane that pockets inward and pinches off as an endocytic vesicle.
- “Pinching off” of membrane to engulf substance.

Question 35

What are the 2 mechanisms of carrier-mediated membrane transport?

Answer 35

Facilitated diffusion (not requiring energy).
Active transport (requires energy).

Question 36

Active transport can be one of two things, what are these?

Answer 36

Primary - ATP (e.g. Na+ - K+ ATPase)
Secondary - ion coupled (Na+)
- Secondary transporters can be symporters (co-transporters) or antiporters (exchangers).