Membrane Transport

Question 1

Diffusion in a solution and through a membrane:

Answer 1

If a substance can permeate a membrane, the substance will move from higher concentration to lower concentration through the membrane

If the membrane is impermeable to the substance, no diffusion will occur

Question 2

Components of Fick’s law of diffusion and how rates of diffusion can be altered

Answer 2

Inc. concentration gradient (𝝙C) = Inc. rate of diffusion (Q)

Inc. surface area of membrane (A) = Inc. rate of diffusion

Inc. lipid solubility = Inc. rate of diffusion

Inc. molecular weight of substance = Dec. rate of diffusion

Inc. distance (thickness) = Dec. rate of diffusion

Question 3

Electrochemical gradient

Answer 3

Electrical and concentration (chemical) gradient acting on an ion simultaneously

Question 4

Explain osmosis, osmolarity and tonicity

Answer 4

Osmosis:
- Net diffusion of water down conc. gradient through a selectively permeable membrane

Osmolarity:
- Conc. of osmotically active particles in a solution (Osm/l)
- E.g. Body fluids ~300 mOsm/l

Tonicity - The effect a solution has on cell volume:
- Isotonic: No net movement of water, no change in cell volume
- Hypotonic: Water diffuses into cells, cells swell
- Hypertonic: Water diffuses out of cells, cells shrink

Question 5

Three important characteristics of carrier mediated transport:

Answer 5

Specificity

Saturation - transport maximum, Tm

Competition - if a carrier can transport two substances, the presence of both diminishes the rate of transfer for either

Question 6

Facilitated diffusion and active (primary and secondary) transport.

Answer 6

Facilitated diffusion: Carrier transfers a substance across the membrane from high to low concentration

Active transport - Requires energy to transfer a substance against a concentration gradient:
- Primary active transport: ATP is directly required to move a substance against its concentration gradient
- Secondary: Uses second hand energy stored as an ion concentration gradient. Symport (moves in same direction as Na+) and Antiport (moves in opposite direction to Na+)

Question 7

Stoichiometry of the Na+-K+ pump and its 3 important roles

Answer 7

Helps establish concentration gradient across the plasma membrane

Helps regulate cell volume

Provides energy for secondary active transport (creates the ion concentration gradient - 3 Na+ out for every 2 K+ in)

Question 8

Exo- and endocytosis:

Answer 8

Both require energy

Exocytosis: Vesicle fuses with plasma membrane, releasing its contents to the ECF

Endocytosis: ‘Pinching off’ of membrane to engulf substance