Exercises: Carbon Dioxide and Stomata

Describe the structure of a stoma and name the cells that control its opening and closing.

State what happens to the opening of stomata when the concentration of carbon dioxide in the leaf air spaces rises and when it falls. Explain why this is useful to the plant.

Name two gases that diffuse through stomata and state the direction of net movement of each in a leaf in bright daylight.

Explain how guard cells open a stoma, including the roles of potassium ions, water potential, osmosis and the structure of the guard cell wall.

The mean number of stomata per mm² on the lower surface of leaves of two plants is: ivy 280; water lily upper surface 460, lower surface 0.

1. a) Calculate the difference in lower-surface density between ivy and water lily.

1. b) Explain the position of the water lily stomata in relation to its habitat.

1. c) Explain why most land plants have more stomata on the lower surface.

A potted plant is kept in a sealed, transparent container in bright light and the concentration of carbon dioxide inside is measured over time. It falls from 0.04% to 0.01% over two hours and then remains steady.

1. a) Explain the fall.

1. b) Explain why the level then stays steady, using the term compensation point.

1. c) Predict the effect of covering the container with a dark cloth.

Some crop growers in Irish glasshouses add extra carbon dioxide to the air, raising the concentration from 0.04% to 0.1%.

1. a) Explain the effect on the rate of photosynthesis, identifying the limiting factor.

1. b) Explain why stomata may partially close at high CO₂ and what this means for water loss.

1. c) Discuss why increasing CO₂ alone will not keep increasing growth indefinitely.

A plant is subjected to drought. The hormone abscisic acid (ABA) is released from the roots and leaves.

1. a) Explain how ABA causes stomata to close.

1. b) Explain the conflict between conserving water and obtaining carbon dioxide.

1. c) Describe two structural adaptations of xerophytes that reduce this conflict.

In an experiment, the stomatal conductance of a leaf is measured at different CO₂ concentrations:

1. a) Describe the trend and calculate the percentage change from 400 to 800 ppm.

1. b) Explain the trend in terms of guard cell behaviour.

1. c) Discuss the likely effect of rising atmospheric CO₂ on water use by forests and crops in Ireland.