6.4 - Gas exchange in the leaf of a plant

Question 1

What is the difference between animals and plants when it comes to gas exchange

Answer 1

• plants need to photosynthesise (which incudes CO2 and oxygen)

Question 2

Describe the balance of gases during photosynthesis

Answer 2

When photosynthesis is taking place
- although some CO2 comes from respiration of cells, most of it is obtained from the external air
- In the same way, some oxygen from photosynthesis is used in respiration but most of it diffuses out of the plant

Question 3

Describe the balance of gases when photosynthesis isn’t occurring

Answer 3

when it isn’t, e.g. in the dark
- oxygen diffuses into the leaf because it is constantly being used by cells during respiration
- In the same way, CO2 produced during respiration diffuses out

Question 4

In what ways is gas exchange in plants similar to that of insects

Answer 4

• no living cell is far from the external air, and therefore a source of oxygen and carbon dioxide
• Diffusion takes place in the gas phase (air), which makes it more rapid than if it were in water

Question 5

Describe how a plant is structured for efficient gas exchange

Answer 5

• short, fast diffusion pathway
• Air spaces inside of leaf have a very large SA compared to volume of living tissue
• There’s no specific transport system for gases —> they simply moving in through diffusion
• Most gas exchange occurs in the leaf (which is specialised for gas exchange)
  The leaf is specialised because it has:
• many pores (stomata) and so no cell is far from a stomata and therefore the diffusion pathway is short
• Numerous interconnecting air-spaces that occur throughout the mesophyll so that gases can readily come in contact with mesophyll cells
• Large SA of mesophyll cells for rapid diffusion

Question 6

What is the structure and function of the stomata

Answer 6

• minute pores that occur mainly (not exclusively) on the leaves, especially on the underside
• Each singular stomata is surrounded by a pair of guard cells (which open and close the stomata pore
• In this way they can control the rate of gas exchange
• This is important because terrestrial organisms lose water by evaporation
• Therefore plants have evolved to close the stomata at times when water loss would be excessive

Question 7

Answer 7

Question 8

Draw a diagram of what an open and closed stomata should look like

Answer 8

Question 9

Name the process which produces CO2 in plants

Answer 9

Respiration

Question 10

Some herbicides cause the stomata of plants to close. Suggest how these herbicides might lead to the death of a plant

Answer 10

• with stomata closed, there is little, if any gas exchange with the environment
• While there will still be some exchange of gases produced by respiration and photosynthesis, neither process can continue indefinitely by relying exclusively on gases produced by the other
• some gases must be obtained from the environment
• in the absence of this supply, both photosynthesis and respiration will ultimately cease and the plant will die

Question 11

Explain why, at point X, CO2 is neither taken up nor given out by the tomato plant

Answer 11

At this light intensity the volume of CO2 taken in during photosynthesis is exactly the same as the volume of CO2 given out during respiration

Question 12

Suggest what information is provided by the point at which the line of the graph meets the y-axis

Answer 12

The rate of respiration (in the dark)