(g) Gas Exchange (Plants)

Question 1

Define diffusion?

Answer 1

Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient.

Question 2

What is the role of diffusion in gas exchange?

Answer 2

Plants require carbon dioxide for photosynthesis and oxygen for respiration and both of these gases diffuse into (and out of) the leaf through stomata, which is located in the chloroplasts.
They then diffuse through the air spaces in the spongy mesophyll layer to get to all cells that require them.

Question 3

What happends in the gas exchange for Respiration?

Answer 3

Plants need oxygen for aerobic respiration
Aerobic respiration is the breakdown of glucose to release energy in the presence of oxygen
Oxygen is taken in by respiring cells and carbon dioxide is produced as a waste product
Glucose + Oxygen -> Carbon dioxide + Water
C6H12O6 +6O2->6CO2+6H2O

Question 4

What happends in the gas exchange for Photosynthesis?

Answer 4

Plants need carbon dioxide for photosynthesis.
Photosynthesis is the conversion of light energy into chemical energy stored in the bonds of glucose molecules made from atoms from carbon dioxide and water.
Water and carbon dioxide are taken into leaf palisade cells which contain many chloroplasts (the site of photosynthesis), and oxygen is produced as a waste product.
Carbon dioxide + Water -> Glucose + Oxygen
6CO2 + 6H2O -> C6H12O6 + 6O2

Question 5

Adaptations of the Internal air spaces in spongy mesophyll layer?

Answer 5

Increase the surface area to volume ratio for the absorption of carbon dioxide.

Question 6

Adaptations of the guard cells around stomata?

Answer 6

Allows the size of the stomata to be adjusted to allow diffusion of gases.
E.g opens during the day to allow diffusion of carbon dioxide for photosynthesis.

Question 7

Adaptations of the stomata?

Answer 7

Can open to allow diffusion of carbon dioxide into the leaf for photosynthesis, and the diffusion of oxygen and water vapour out of the leaf.

Question 8

Adaptations of the Spongy layer?

Answer 8

Air spaces allow carbon dioxide to diffuse through the leaf.

Question 9

Adapations of the Palisade cell layer at top of leaf?

Answer 9

To absorb more light and increase the rate of photosynthesis.

Question 10

Adapations of the Palisade cells contain many chloroplasts?

Answer 10

To absorb all the available light.

Question 11

Adapations of the Flattened shape of the leaf?

Answer 11

Larger surface area to absorb more light and carbon dioxide for photosynthesis.

Question 12

Adapations of the transparent and thin Epidermis?

Answer 12

Short diffusion distance for carbon dioxide to diffuse into leaf cells and for oxygen to diffuse out of the leaf cells.

Question 13

What are the roles of the stomata in gas exchange?

Answer 13

Stomata are generally located on the lower epidermis of leaves.
They are surrounded by a pair of guard cells that control the opening and closing of the stomata, depending on how turgid or flaccid they are.
(Look for more details on the printed diagram!)

Question 14

What are the factors that can affect the rate of photosynthesis?

Answer 14

1. Light intensity: Without enough light, a plant cannot photosynthesise very quickly - even if there is plenty of water and carbon dioxide.
2. Carbon dioxide concentration: Carbon dioxide is one of the reactants in photosynthesis. If the concentration of carbon dioxide is increased, the rate of photosynthesis will therefore increase.
3. Temperature: Temperature - Increasing the temperature increases the rate of photosynthesis because more energy is provided. However, if the temperature is increased to above 40°C, the enzymes that catalyse the reaction begin to denature.

Question 15

How does the net exchange of CO2 and oxygen depend on the light intensity?

Answer 15

The net exchange of gases depends on the time of day and the light intensity.
Photosynthesis doesn’t occur at night as it requires light energy.
When there is no photosynthesis, there is a net release of carbon dioxide and a net uptake of oxygen (respiration is always occurring as it is not reliant on light)
If there is enough light during the day, then the rate of photosynthesis is higher than the rate of respiration and there is a net release of oxygen and a net uptake of carbon dioxide.
Respiration takes place all the time so oxygen will be taken in by respiring cells, and carbon dioxide will be produced as a waste product.

Question 16

How does the processes of respiration and photosynthesis in terms of CO2 and oxygen happen during the day?

Answer 16

In daytime, light is present which allows photosynthesis to occur, so carbon dioxide will be taken in by photosynthesizing cells in plants and oxygen will be produced as a waste product
Usually during the day the rate of photosynthesis is higher than the rate of respiration
Therefore, more oxygen will be released, and more carbon dioxide will be taken up.

Question 17

How does the processes of respiration and photosynthesis in terms of CO2 and oxygen happen at night?

Answer 17

At night, light is not present so photosynthesis will not occur, so carbon dioxide will not be taken in by photosynthesising cells in plants and oxygen will not be produced as a waste product.
Oxygen will be used by respiration; carbon dioxide will be produced by respiration.
So more carbon dioxide will be released, and more oxygen will be taken up.