Cells - Transport across Cell Membrane

Question 1

Name and describe five ways substances can move across the
cell-surface membrane into a cell.

Answer 1

1. (Simple) diffusion of small/non-polar molecules down a
   concentration gradient;
2. Facilitated diffusion down a concentration gradient via
   protein carrier/channel;
3. Osmosis of water down a water potential gradient;
   4.** Active transport **against a concentration gradient via
   protein carrier using ATP;
4. Co-transport of 2 different substances using a carrier protein;

Question 2

Explain the function of this ATP hydrolase.

Answer 2

1. (ATP to ADP + Pi ) Releases energy;
2. (energy) allows ions to be moved against a concentration gradient
   OR
   (energy) allows active transport of ions;

Question 3

The movement of Na+ out of the cell allows the absorption of glucose into the cell lining the ileum.
Explain how.

Answer 3

1. (Maintains/generates) a concentration/diffusion gradient
   for Na+ (from ileum into cell);
2. Na+ moving (in) by facilitated diffusion, brings glucose with it
   OR
   Na+ moving (in) by co-transport, brings glucose with it;

Question 4

Describe and explain two features you would expect to find in a cell specialised for absorption

Answer 4

1. Folded membrane/microvilli so large surface area (for absorption);
2. Large number of co-transport/carrier/channel proteins so
   fast rate (of absorption)
3. Large number of mitochondria so make (more) ATP (by respiration)
4. Membrane-bound (digestive) enzymes so maintains
   concentration gradient (for fast absorption);

Question 5

The movement of substances across cell membranes is affected by
membrane structure. Describe how.

Answer 5

1. Phospholipid (bilayer) allows movement/diffusion of nonpolar/lipid-soluble substances;
2. Phospholipid (bilayer) prevents movement/diffusion of polar/
   charged/lipid-insoluble substances
3. Carrier proteins allow active transport;
4. Channel/carrier proteins allow facilitated diffusion/co-transport;
5. Shape/charge of channel / carrier determines which
   substances move;
6. Membrane surface area determines how much
   diffusion/movement;
7. Cholesterol affects fluidity/rigidity/permeability;

Question 6

Give two similarities in the movement of substances by diffusion and by
osmosis.

Answer 6

1) moves down a concentration gradient
2) Passive / not active processes;

Question 7

Compare and contrast the processes by which water and inorganic ions
enter cells.

3 marks

Answer 7

comparison: both move down concentration gradient + (protein) channels
Contrast: ions can move against a concentration gradient by active
transport

Question 8

Contrast the processes of facilitated diffusion and active transport.

3 marks

Answer 8

• Facilitated diffusion involves channel or carrier proteins whereas
  active transport only involves carrier proteins
• Facilitated diffusion does not use ATP / is passive whereas
  active transport uses ATP;
• Facilitated diffusion takes place down a concentration gradient
  whereas active transport can occur against a concentration
  gradient.

Question 9

The stomata close when the light is turned off.
Explain the advantage of this to the plant.

3 marks

Answer 9

1) Because) water is lost through stomata
2) (Closure) reduces water loss
3) Maintain water content of cells.

Question 10

High blood pressure leads to an accumulation of tissue fluid. Explain how.

3 marks

Answer 10

1) High blood pressure = high hydrostatic pressure
2) Increases outward pressure from (arterial) end of capillary / reduces
inward pressure at (venule) end of capillary;
3. (So) more tissue fluid formed / less tissue fluid is reabsorbed.

Question 11

The water potential of the blood plasma is more negative at the venule end of the
capillary than at the arteriole end of the capillary. Explain why.

Answer 11

1. Water has left the capillary;
2. Proteins (in blood) too large to leave capillary;
3. Increasing / giving higher concentration of blood proteins