6.2 Gas exchange in single-celled organisms and insects

Question 1

Describe the gas exchange in single-celled organisms

Answer 1

Single-celled organisms are small and therefore have a large surface area to volume ratio.

Oxygen is absorbed by diffusion across their body surface which is covered by ONLY a cell-surface membrane. Carbon dioxide from respiration diffuses out the same way.

Question 2

What is the internal network of tubes developed in an insect for gas exchange called?

Answer 2

tracheae

Question 3

Describe the structure and function of tracheae.

Answer 3

The tracheae are supported by strengthened rings to prevent them from collapsing.

The tracheae divide into smaller deadens tubes called tracheoles.

The tracheoles extend throughout all the body tissues of the insect.

In this way, atmospheric air is brought directly to the retiring tissues as there is a short diffusion pathway from a tracheal to any body cell.

Question 4

What are the 3 ways respiratory goes move in and out of the tracheal system? (insect)

Answer 4

1. along a diffusion gradient
2. mass transport
3. the ends of the tracheoles are filled with water

Question 5

How does ‘along a diffusion gradient’ exchange work? (insect)

Answer 5

• when cells are retiring, oxygen is used up and so its concentration towards the ends of the tracheoles falls.
• this creates a diffusion gradient that causes gaseous oxygen to diffuse from the atmosphere along the tracheae and the tracheoles to the cells
• carbon dioxide is produced by cells during respiration. This creates a diffusion gradient in the opposite direction.
• thus gaseous carbon dioxide diffuses along the tracheoles and tracheae from the cells to the atmosphere.
• as diffusion in air is much more rapid than in water, respiratory gases are exchanged quickly by this method.

Question 6

How does ‘mass transport’ exchange work? (insect)

Answer 6

• the contractions of muscles in insects can squeeze the trachea enabling mass movements of air in and out
• this further speeds up the exchange of respiratory gases

Question 7

How does ‘ the ends of the tracheoles are filled with water’ exchange work? (insects)

Answer 7

• during periods of major activity the muscle cells around the tracheoles respire carrying out anaerobic respiration
• this produces lactate which is soluble and lowers the water potential o the muscle cells
• water therefore moved into the cells from the tracheoles by osmosis
• The water in the ends of the tracheoles decreases in volume and in doing so draws air further into them
• This means the final diffusion pathway is in a gas rather than in a liquid phase, hence diffusion is more rapid
• This increases the rate at which air is moved in the tracheoles but leads to greater water evaporation

Question 8

What are spiracles?

Answer 8

Tiny pores on the body surface of an insect which are opened and closed by valves.

Gases enter and leave trachea through spiracles.

When spiracles are open, water vapour can evaporate from the insect

Question 9

What are 2 limitations of the tracheal system being an efficient method of gas exchange?

Answer 9

1. it relies mostly on diffusion to exchange gases between the environment and the cells
2. the short diffusion pathway required for efficient gas exchange limits the size that insects can attain