Membrane transport

Question 1

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

Answer 1

• Solubility of the particle in lipid

* Size of the particle

Question 2

What two elements are required for any molecule to cross the membrane?

Answer 2

A pathway (assisted or unassisted) and a driving force (passive or active)

Question 3

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

Answer 3

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

Question 4

What two types of gradients can molecule use to cross a membrane unassisted?

Answer 4

• concentration gradient

- electrical gradient

Question 5

What is osmosis?

Answer 5

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

Question 6

What structures quicken the process of osmosis?

Answer 6

Aquaporins (water channels)

Question 7

What does osmolarity mean?

Answer 7

The concentration of osmotically active particles present in a solution. (can be measured or calculated)

Normally around 300mOsm/l in body fluids

Question 8

What does tonicity mean?

Answer 8

The effect a solution has on cell volume. No units.

Question 9

What happens to cells in isotonic, hypotonic and hypertonic conditions?

Answer 9

Isotonic conditions- no net movement so no change in cell volume.

Hypotonic conditions- water diffuses into cells, cells swell.

Hypertonic conditions- water diffuses out of cells, cels shrink.

Question 10

What are the two mechanisms for selective assisted transport?

Answer 10

• Carrier-mediated transport

* Vesicular transport

Question 11

What are the three important characteristics that determine the kind and amount of material transferred across the membrane?

Answer 11

1. Specificity- specialised to transport a certain substance or group of closely related substances.
2. Saturation- what the transport maximum (Tm) is.
3. Competition- e.g. 1 transporter carries two types of molecules, affecting the rate of transfer for either molecule.

Question 12

What happens to the carrier in carrier-mediated transport?

Answer 12

When the substance binds, the specific carrier undergoes a conformational change which transports the substance.

Question 13

What is cystinuria?

Answer 13

A condition in which amino acid cystine cannot be reabsorbed across the membrane in the kidneys. Forms crytals or stones in either the kidney or the urinary tract.

Question 14

What are the two forms of carrier-mediated transport?

Answer 14

• Facilitated diffusion

* Active transport

Question 15

Describe facilitated diffusion.

Answer 15

• no energy

- e.g. GLUT 4 transport

Question 16

Describe active transport.

Answer 16

• requires energy

- works against concentration gradient

Question 17

What are the two forms of Active transport?

Answer 17

1. primary active transport

2. secondary active transport

Question 18

What is primary active transport?

Answer 18

Energy is directly rewired to move a substance against is concentration gradient. e.g. Na/K pump.

Question 19

What is secondary active transport?

Answer 19

Energy is required but not used directly to produce uphill movement.

Doesn’t split ATP to move molecule uphill, it uses secondhand energy stored in the form of an ion concentration gradient e.g. Na+ gradient.

Question 20

What does the Na+/K+ pump transfer (no. of ions)

Answer 20

2K+ into cell

3Na+ out of cell

Question 21

What two mechanisms can secondary active transport use?

Answer 21

1. Symport/co-transport:

The solute moves in same direction as Na+.

2.Antiport (exchange):

The solute moves in an opposite direction to Na+

Question 22

Describe vesicular transport.

Answer 22

• Requires energy for vesicle formation
• Endocytosis- pinching off of membrane to engulf substance
Exocytosis- intracellular vesicle fuses to membrane and releases its contents to ECF e.g. secrete enzymes and proteins and adds carrier or channels to the membrane like insulin & GLUT4