Exchange of materials

Question 1

Why do large multicellular organisms require

specialised exchange surfaces?

Answer 1

● Small SA/V ratio
● Diffusion insufficient to provide all cells with the required
oxygen and nutrients, and to remove all waste products
● Exchange surfaces increase rate of diffusion and shorten
diffusion distance

Question 2

Why do some multicellular organisms (e.g. trees) not

require specialised exchange surfaces?

Answer 2

Trees have a large number of leaves
which provide a large SA/V ratio for
diffusion.

Question 3

How does the size of an organism affect its surface

area to volume ratio?

Answer 3

The larger the organism, the smaller the

SA/V ratio.

Question 4

Calculate the surface area to volume ratio of a cube

that is 1 × 1 × 1 cm

Answer 4

Surface area: (1 × 1) × 6 = 6 cm^2

Volume: 1 × 1 × 1 = 1 cm^3

Surface area : Volume = 6:1

Question 5

Calculate the surface area to volume ratio of a cube

that is 2 × 3 × 4 cm

Answer 5

Surface area:
2(2 × 3) + 2(2 × 4) + 2(4 × 3) = 52 cm^2

Volume: 2 × 3 × 4 = 24 cm^3

Surface area : Volume = 52:24 = 13:6

Question 6

Name some of the substances transported into and

out of the human body

Answer 6

● Oxygen
● Carbon dioxide
● Water
● Dissolved food molecules
● Urea

Question 7

How does oxygen enter and carbon dioxide leave

cells?

Answer 7

They diffuse into and out of cells.

Question 8

How does water enter cells?

Answer 8

It diffuses into cells by osmosis.

Question 9

How do food molecules and mineral ions enter cells?

Answer 9

They are dissolved in water which

diffuses into cells.

Question 10

Why must urea be excreted from the body?

Answer 10

It is a waste product so must be

excreted

Question 11

How is urea excreted from the body?

Answer 11

● Urea diffuses out of cells into the blood plasma
● The kidney filters urea out of the blood
● Urea is excreted in urine

Question 12

Why must the human body exchange oxygen and

carbon dioxide with the environment?

Answer 12

● Oxygen is required for respiration so diffuses
into the body
● Carbon dioxide is a toxic waste product of
respiration so diffuses out of the body

Question 13

How does oxygen enter and carbon dioxide leave

the bloodstream?

Answer 13

● Oxygen diffuses from air in the alveoli (high O2
conc)
into blood in the capillaries (low O2
conc)
● Carbon dioxide diffuses from blood in the capillaries
(high CO2
conc) into air in the alveoli (low CO2
conc)

Question 14

How are alveoli adapted for gaseous exchange? (6)

Answer 14

● Large surface area
● Network of capillaries provide a good blood supply
● Rapid blood flow maintains a steep concentration gradient
● Thin walls-one cell thick- gives a short diffusion distance
● Cell walls have partially permeable membranes enabling gas exchange
● Moist lining, enables gases to dissolve

Question 15

What factors affect the rate of diffusion? (3)

Answer 15

● Diffusion distance
● Concentration gradient
● SA/V

Question 16

How does diffusion distance affect the rate of diffusion?

Answer 16

The greater the diffusion distance, the
further the molecules must travel and the
slower the rate of diffusion.

Question 17

How does concentration gradient affect the rate of

diffusion?

Answer 17

The steeper the concentration gradient,

the faster the rate of diffusion.

Question 18

How does SA/V affect the rate of diffusion?

Answer 18

• The larger the SA/V,
• the greater the number of molecules that can diffuse across in a given time,
• so the faster the rate ofdiffusion.

Question 19

How can the rate of diffusion be calculated?

Answer 19

Using Fick’s law:

rate of diffusion ∝ surface area multiplied by concentration gradient divided by thickness of membrane