Diffusion, Active Transport & Osmosis

Question 1

Describe the movement across membranes by simple diffusion and factors affecting rate:

Answer 1

1. Net movement of small, non-polar molecules e.g. oxygen or carbon dioxide, across a selectively permeable membrane, down a concentration gradient
2. Passive / no ATP / energy required

Factors affecting rate:
1. Surface area
2. Concentration gradient,
3. Thickness of surface / diffusion distance

Question 2

Describe the movement across membranes by facilitated diffusion and factors affecting rate:

Answer 2

1. Net movement of larger/polar molecules e.g. glucose, across a selectively permeable membrane, down a concentration gradient
2. Through a channel/carrier protein
3. Passive /no ATP/energy required

Factors affecting rate:
1. Surface area
2. Concentration gradients (until the number of proteins is the limiting factor as all are in use / saturated)
3. Number of channel/carrier proteins

Question 3

Describe the role of carrier/channel proteins in facilitated diffusion:

Answer 3

1. Carrier proteins transport large molecules, the protein changes shape when molecule attaches
2. Channel proteins transport charged/polar molecules through its pore (some are gated so can open/close e.g. Voltage-gated sodium ion channels)
3. Different carrier and channel proteins facilitate the diffusion of different specific molecules

Question 4

Describe the movement across membranes by active transport and factors affecting rate.

Answer 4

1. Net movement of molecules/ions against a concentration gradient
2. Using carrier proteins
3. Using energy from the hydrolysis of ATP to change the shape of the tertiary structure and push the substances though
   Factors affecting rate 1. pH/temp (tertiary structure of carrier protein), speed of carrier protein, number of carrier proteins, rate of respiration (ATP production)

Question 5

Describe the movement across membranes by co-transport, illustrated by the absorption of sodium ions and glucose by cells lining the mammalian ileum.

Answer 5

1 . Na+ actively transported from
epithelial cells to blood (by
Na+/K+ pump) .Establishing a conc. gradient of Na+(higher in lumen than epithelial cell)
2 Na+ enters epithelial cell down
its concentration gradient with
glucose against its concentration gradient .Via a co-transporter protein
3. Glucose moves down a conc.
gradient into blood via facilitated diffusion

Question 6

Describe the movement across membranes by osmosis and factors affecting rate.

Answer 6

1. Net movement of water molecules across a selectively permeable membrane down a water potential gradient
2. Water potential is the likelihood (potential) of water molecules to diffuse out of or into a solution; pure water has the highest water potential and adding solutes to a solution lowers the water potential (more negative)
3. Passive
   - Factors affecting rate – surface area, water potential gradient, thickness of exchange surface / diffusion distance

Question 7

Describe how cells might be adapted for transport across their internal or external membranes.

Answer 7

1. By an increase in surface area e.g. membrane folds
2. Increase in number of protein channels / carriers
3. More mitochondria 🡪 more ATP 🡪 higher rate of active transport