6.2: Gas exchange in singe-celled organisms and insects

Question 1

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 only be a cell-surface membrane

Question 2

Methods of gas exchange in insects

Answer 2

1. Along a diffusion gradient
2. Mass transport
3. Ends of the tracheoles are filled with water

Question 3

Exchange along a diffusion gradient

Answer 3

1. When cells are respiring, oxygen is used up and its concentration toward the ends of the tracheoles falls.
2. This creates a diffusion gradient that causes gaseous oxygen to diffuse from the atmosphere along the trachae and tracheoles
   - Carbon dioxide does the inverse

Question 4

Exchange via mass transport

Answer 4

1. The contraction of muscles in insects can squeeze the trachea enabling mass movements of air in and out. This speeds up exchange

Question 5

Exchange due to water filed ends of the tracheoles

Answer 5

1. During periods of major activity, the muscle cells respire and carry out anaerobic respiration.
2. This produces lactate, which is soluble and lowers the water potential of cells.
3. This causes water to move into the cells from the tracheoles by osmosis.
4. The water in the ends of the tracheoles decreases in volume and in doing so brings air into them.
5. This means the final diffusion pathway is in a gas rather than a liquid phase, and therefore diffusion is more rapid.

Question 6

How does gas enter and leave insect trachae?

Answer 6

Through tiny pores called spiracles. They open and close like a valve.

For most of the time, spiracles are closed to prevent water loss. Periodically spiracles are open to allow gas exchange

Question 7

Limits of the tracheal system

Answer 7

The tracheal system relies on diffusion to exchange gases between the environment and the cells.

For diffusion to be effective, the diffusion pathway needs to be short which is why insects are of small size.

The length of the diffusion pathway limits the size insects can attain.