Membrane transport

Question 1

Passive Transport

Answer 1

• No energy input (diffusion)
• May require carrier protein (facilitated
transport)
• Movement down concentration gradient
• Molecules need to be small and hydrophobic

Question 2

Method of passive transport

Answer 2

• Diffusion
o Passive movement of molecules down
concentration gradient until equilibrium is
reached
• Osmosis
o Diffusion of water across permeable
membrane
o Tonicity: ability for cell to lose or gain
water based on water concentration
o Isotonic solution: solute concentration is
equal in and outside the cell -> water
moves at equal rates in and out of cells
-> no net gain of water
o Hypotonic solution: solute concentration
is low outside cell -> water moves into
cell -> net gain of water
o Hypertonic solution -> solute
concentration is high outside cell ->
water moves out of cell -> net loss of
water

Question 3

Facilitated diffusion

Answer 3

o Polar molecules and ions impeded by
the lipid bilayer, diffuse passively
with the help of transport proteins
o Two types of transport proteins (see
previous page diagram (a))
§ Channel
§ Carrier

Question 4

Active Transport

Answer 4

• Movement of molecules/ions against the
concentration concentration gradient
• Require transport proteins (only carrier)
• Movement occurs from high to low
concentration
• Method allows for cell to maintain internal
concentrations that differ from its environment

Question 5

Method of Active transport

Answer 5

• Requires ATP (energy) to pump solute against
the gradient
o ATP transfers P to carrier protein
o Results in carrier protein changing shape
• Sodium-potassium pump is one example that
uses this method -> Na+ is pumped out and K+
in
o Carrier protein acts as a pump and
transfers the ions to and from the
extracellular fluid

Question 6

Membrane assisted transport (bulk transport)

Answer 6

• Exocytosis: vesicles releasing substances from the cell to the external environment -> eg. the
secretion of substances produced by the cell
o Vesicle attaches to plasma membrane, releases substances, and then forms part of the
membrane

• Endocytosis: involves substances being brought into the cell from the external environment ->
vesicles transport these substances
o Occurs in three ways:
1. Phagocytosis
§ Membrane engulfs food particles -> forms vacuole (larger)
§ Broken down by lysosome
2. Pinocytosis
§ Membrane surrounds the particles -> forms vesicle
§ Vesicles form around liquid or small particles
§ Non-specific particles are ‘gulped’
3. Receptor-mediated endocytosis
§ A form of pinocytosis BUT receptor proteins on the plasma membrane attract specific
molecules
§ Molecules attach to receptors on membrane -> forms vesicle

Question 7

Electrical charge and the sodium-potassium ion pump

Answer 7

• The concentration gradient of Na+ ions across the membrane (higher Na+ concentration
outside) facilitates the diffusion of Na+ into the cell. At the same time, the electrical gradient
across the membrane (excess positive charge outside) drives Na+ into the cell.
• The concentration gradient of K+ ions across the membrane (higher K+ concentration inside)
facilitates the diffusion of K+ out of the cell.
• However, the electrical gradient across the membrane (excess positive charge outside)
impedes the diffusion of K+ out of the cell.
• The electrochemical gradient for an ion is the sum of the concentration (chemical) gradient and
the electrical gradient (charge difference) across the membrane.
• For Na+ ions, diffusion through the Na+ channel is driven by both the concentration gradient
and the electrical gradient.
• But for K+ ions, the electrical gradient opposes the concentration gradient. Therefore, the
electrochemical gradient for Na+ is greater than the electrochemical gradient for K+.