Ch 9 Nutrition and gas exchange in plants

Question 1

Gas exchange in leaves of terrestrial plants

Answer 1

Gases in:
1. Gases from the environment diffuse into the air space through the stoma
2. Gases dissolve in the wnater film on the surfaces of mesophyll cells and the diffuse into the cells
3. Gases in these cells diffuse to the neighbouring cells

Gases out:
1. Gases produced by the cells diffuse to the neighbouring cells towards the air space
2. Gases diffuse to the water film on the surfaces of mesophyll cells
3. Gases diffuse into the air space and diffuse out through the stoma

Question 2

Adaptations of leaves for gas exchange

Answer 2

External
1. Leaves are broad and flat. They are large in number in a plant. These provide a large surface area for diffusion of gases
2. Leaves are thin. This shortens the distance of diffusion of gases between the plant body and the atmosphere

Internal
1. The spongy mesophyll cells are loosely packed. This provides a large surface area for diffusion of gases
2. There are numerous air spaces among the spongy mesophyll cells. This allows gases to diffuse freely
3. There is a water film on the surfaces of the mesophyll cells. This allows gases to dissolve and diffuse into or out of the cells
4. There are stomata in the epidermis. This allows gases to move into or out of the leaves freely
5. Guard cells are present to control the opening and closing of stomata. The rate of gas exchange can be regulated

Question 3

Effect of light intensity on the exchange of carbon dioxide in a plant
(graph p.15)

Answer 3

A: In complete darkness, only respiration takes place. The plant takes in oxygen and releases carbon dioxide
A-B: Photosynthesis starts to take place when light intensity increases. The rate of photosynthesis is lower than that of respiration. There is a net release of carbon dioxide from the plant
B (compensation point): The rate of photosynthesis equals the rate of respiration. There is no net exchange of gases.
B-C: As the light intensity keeps increasing, the rate of photosynthesis becomes higher than that of respiration. There is a net uptake of carbon dioxide by the plant.
C-D: The net uptake of carbon dioxide reaches a maximum. It does not increase with increasing light intensity. Other factors limit the rate of photosynthesis.

Question 4

Graph: the importance of the area below the line showing the oxygen production rate is greater than the area below the line showing the carbon dioxide production rate

Answer 4

• The area below the line showing oxygen production rate represents the food production
• The area below the line showing carbon dioxide production rate represents the food consumption
• Food production is greater than food consumption, there is a net amount of food produced
• This provides energy for the plant to survive, grow and produce fruits