Topic 3.2 - Gas Exchange

Question 1

Equation of Fick’s law

Answer 1

Rate of diffusion oc
Surface area x Concentration Gradient/diffusion pathway

Question 2

How does gas exchange take place in single celled organisms and small thin organisms such as flat worms

Answer 2

simple diffusion

Question 3

Small organisms obtain enough oxygen without a specialised gas-exchange system. Why is this possible? Why do larger organisms need a gas-exchange and transport system?

Answer 3

Small organisms have large surface/volume ratio;

All cells are close to the gas exchange surface So diffusion/exchange can take place over the whole body surface/skin;

But cells of larger organisms are a long way from gas exchange surface; and because diffusion is a slow process; gases (and other substances) must be supplied by transport system/circulatory system/blood;

Question 4

Describe how an insect obtains oxygen and reduces water loss

Answer 4

1 Air enters through (open) spiracles;


2 Through tracheae;


3 Diffusion gradient in trachea


4 Tracheae associated with all cells/closely associated with cells;


5 Oxygen diffuses into cells;


6 Ventilation replacing air in tracheae;


7 Body covered with (waterproof) waxy layer/cuticle;


8 Spiracles are able to close

Question 5

How is an insect’s tracheal system adapted for efficient gas exchange?

Answer 5

. Short diffusion pathway due to

a. thin walls of tracheoles

b. lots of tracheoles so no cell is far away from a tracheole

2. large surface area

a. lots of tracheoles which are highly branched

b. fluid from the end of the tracheoles is absorbed into muscles

3. high concentration gradient for carbon dioxide and oxygen maintained by

a. movement of abdomen (ventilation) which brings in oxygen rich and carbon dioxide poor air

4. the fluid from tracheoles is absorbed into muscle tissue so that diffusion can happen faster through the air than it would through liquid

Question 6

Explain how the gills of a fish are adapted for efficient gas exchange

Answer 6

1 Large surface area provided by lamellae/filaments;

2 Increases diffusion/makes diffusion efficient;

3 Thin epithelium/distance between water and blood;

4 Water and blood flow in opposite directions / countercurrent;

5 the countercurrent maintains an oxygen concentration gradient along the entire gill filament so equilibrium is not reached;

6 As water always next to blood with lower concentration
of oxygen;

7 Circulation replaces blood saturated with oxygen;

8 Ventilation replaces water (as oxygen removed);