58 WJEC Biology AS Level - Marianne Izen - 2nd Edition (1.3 Cell Membranes And Transport)

Question 1

What are the ways by which exchange of substances between cells and their surroundings can occur?

Answer 1

The exchange of substances between cells and their surroundings can occur in ways that

1. involve metabolic energy (active transport)
2. that do not involve metabolic energy (passive transport).

Question 2

What is the difference between active transport and diffusion?

Answer 2

Unlike diffusion and facilitated diffusion, active transport is an energy-requiring process.

In active transport, ions and molecules are moved across membranes against a concentration gradient.

Question 3

What is the feature of active transport with regards to “concentration gradient”?

Answer 3

lons and molecules are moved from a lower to a higher concentration against the concentration gradient.

Question 4

What is the feature of active transport with regards to “energy required”?

Answer 4

The process requires energy from ATP.

Anything that affects respiration will affect active transport.

Question 5

What is the feature of active transport with regards to “carrier proteins”?

Answer 5

The process occurs through intrinsic carrier proteins spanning the membrane.

Question 6

What is the feature of active transport with regards to
“ rate-limiting”?

Answer 6

The rate is limited by the number and availability of carrier proteins.

Question 7

Name some processes involving active transport?

Answer 7

Processes involving active transport include:

1. muscle contraction
2. nerve impulse transmission
3. reabsorption of glucose in the kidney
4. mineral uptake into plant root hairs.

Question 8

What are the steps in the active uptake of a single molecule or ion?

Answer 8

Active uptake of a single molecule or ion occurs as follows:

1. The molecule or ion combines with a specific carrier protein on the outside of the membrane.
2. ATP transfers a phosphate group to the carrier protein on the inside of the membrane.
3. The carrier protein changes shape and carries the molecule or ion across the membrane, to the inside of the cell.
4. The molecule or ion is released into the cytoplasm.
5. The phosphate ion is released from the carrier molecule back to the cytoplasm and recombines with ADP to form ATP.
6. The carrier protein returns to its original shape.

Question 9

What does this graph show?

Answer 9

At higher concentration differences across a membrane, the rate of uptake increases

It reaches a plateau, when the carrier proteins are saturated, that is, all the solute-binding sites are occupied.

The graph also shows that the rate of uptake is reduced with the addition of a respiratory inhibitor.

This implies that the process requires ATP and so active transport must be taking place.

Question 10

Name a respiratory inhibitor and how it affects active transport.

Answer 10

Cyanide is a respiratory inhibitor.

It will prevent aerobic respiration and the production of ATP
in the mitochondria.

Without ATP, active transport cannot occur, so cyanide reduces active transport.

Question 11

What happens to active transport if more oxygen is available to cells?

Answer 11

Experiments have shown an increase in active transport if more oxygen is available to the cells, when it was previously limiting.

Question 12

If the rate of uptake is increased by more oxygen, what does it indicate?

Answer 12

This also indicates active transport, as the oxygen would have increased the production of ATP by aerobic respiration.

Question 13

Active transport (Key-Term)

Answer 13

The movement of molecules or ions across a membrane against a concentration gradient, using energy from the hydrolysis of ATP made by the cell in respiration.

Question 14

What does up’ or ‘against’ a concentration gradient mean?

Answer 14

‘Up’ or ‘against’ a concentration gradient means going from lower to higher concentration.

Question 15

What does ‘down’ a concentration gradient mean?

Answer 15

‘Down’ a concentration gradient means going from higher to lower concentration.

Question 16

In what directions can active transport occur across a cell membrane?

Answer 16

Some carrier proteins take molecules or ions into the cell and some take them out, so active transport can occur in both directions across a cell membrane.

Question 17

What is “active uptake”?

Answer 17

When molecules or ions are taken up by active transport, the process is sometimes called ‘active uptake’.

Question 18

When is ATP produced and what is its importance?

Answer 18

ATP is produced during respiration and is important in the transfer of energy.

Question 19

What is the feature of cells performing active transport?

Answer 19

Cells performing active transport are packed with mitochondria.

Question 20

Draw a diagram to show how carrier proteins help with active transport.

Answer 20