AQA Bio A Level Unit 3

Question 1

Describe the relationship between size of organisms and volume and surface area (2)

Answer 1

1. As organisms get bigger/their size increases;
2. Their surface area to volume ratio decreases; (and vice versa)

Question 2

Why don’t small organisms such as flatworms have gills or circulatory systems? (2)

Answer 2

1. They are small enough;
2. Their surface area to volume ratio is big enough that they can rely on diffusion through their body surface;

Question 3

Explain three ways in which lungs are adapted to give efficient gas exchange. (3)

Answer 3

1. Large number of alveoli give large surface area;
2. The exchange surface is just 2 flattened cells, reducing diffusion distance;
3. Ventilation and circulation maintain the concentration gradient;

Question 3

The bear species found towards the poles are the largest whereas the bear species found in the tropics are much smaller. Why? (3)

Answer 3

1. Larger bears have a smaller surface area to volume ratio, and lose much less heat (per kg of bear).
2. They are adapted to cold climates.
3. Smaller bears lose too much heat could not survive in the extreme cold

Question 4

Which cells make up the exchange surface in the lung? (2)

Answer 4

1.Alveolar epithelial cell;
2. Capillary endothelial cell;

Question 5

Outline the route taken by air from the atmosphere to alveolus. (5)

Answer 5

1. Mouth/nose
2. Trachea
3. Bronchi
4. Bronchioles
5. Alveolus

Question 6

Outline the processes required for inspiration (breathing in)

Answer 6

1. Contraction of external intercostals moves ribcage upwards and outwards;
2. Diaphragm contracts and moves downwards from domed to flattened shape;
3. Volume of thorax increases;
4. Pressure decreases below atmospheric;

Question 7

Breathing out can be passive (no muscle contraction required). Which feature of lung tissue allows this?

Answer 7

1. Its elasticity/elastic recoil

Question 8

What is pulmonary ventilation a measure of?

Answer 8

The volume of air breathed in one minute.

Question 9

How do you calculate pulmonary ventilation?

Answer 9

Multiply tidal volume by breathing rate.

Question 10

What is tidal volume?

Answer 10

The volume of air breathed in one breath.

Question 11

Explain three ways in which fish gills are adapted to give efficient gas exchange. (3)

Answer 11

1. Large number of lamellae and gill filaments give large surface area; 2. The exchange surface is just 2 flattened cells, reducing diffusion distance;
2. Counter-current (of water and blood) maintains diffusion gradient.

Question 12

Which cells make up the exchange surface in the fish gill? (2)

Answer 12

1. Lamella epithelial cell;
2. Capillary endothelial cell;

Question 13

Outline the route taken by air from the atmosphere to insect cells? (3)

Answer 13

1. Spiracles;
2. Trachea;
3. Tracheoles;

Question 14

Describe and explain the advantage of the counter-current principle in gas exchange across a fish gill. (4)

Answer 14

1. Water and blood flow in opposite directions;
2. Maintains concentration gradient;
3. Equilibrium not reached;
4. Diffusion along length of lamellae;

Question 15

Why might insects close their spiracles? (1)

Answer 15

To reduce water loss.

Question 16

What causes insect spiracles to open? (1)

Answer 16

A build up of carbon dioxide (from cellular respiration).

Question 17

In some insects, their wing muscles compress tracheoles when they contract. What is the effect of this? (2)

Answer 17

1. It has the effect of squeezing air out of tracheoles, and then drawing fresh air in;
2. This maintains the concentration gradient;

Question 18

What is the effect of anaerobic respiration on gas exchange system in insects? (3)

Answer 18

1. Anaerobic respiration produces lactic acid in cells;
2. This lowers the water potential of the cells and some of the water in the ends of the tracheoles moves into the cells by osmosis;
3. The surface area for diffusion is increased;

Question 19

Why might plants close their stomata? (1)

Answer 19

To reduce water loss.

Question 20

Can plants close their stomata and also photosynthesise? Why? (2)

Answer 20

1. No.
2. They can’t photosynthesise without gas exchange.

Question 21

What feature of leaves increases the surface area for diffusion into photosynthesising cells? Explain. (2)

Answer 21

1. Air spaces;
2. Provides short diffusion pathway - the air is in the spaces right outside the cells (rather than outside the whole leaf)