Abiotic factors: carbon dioxide levels for plants

The idea

Carbon dioxide is an abiotic factor because it is a non-living gas in the environment, and its concentration in air strongly affects plants. Plants absorb carbon dioxide through stomata in their leaves and use it as a reactant in photosynthesis, where it is combined with water to make glucose and oxygen. The normal atmospheric concentration is about 0.04% (around 400 parts per million), which is lower than the optimum for photosynthesis, so increasing carbon dioxide concentration usually increases the rate of photosynthesis, provided light and temperature are not limiting. In enclosed spaces such as greenhouses, growers may raise carbon dioxide levels to boost growth.

What earns the marks

• Carbon dioxide is an abiotic factor because it is a non-living component of the environment, not an organism.
• Plants take in carbon dioxide through stomata in their leaves; it is a reactant in photosynthesis along with water.
• Photosynthesis produces glucose and oxygen: carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll).
• The normal atmospheric carbon dioxide concentration is about 0.04% (approximately 400 parts per million), which is below the optimum for many plants.
• Increasing carbon dioxide concentration increases the rate of photosynthesis, but only until another factor such as light intensity or temperature becomes limiting.
• Carbon dioxide concentration can be measured with a carbon dioxide sensor or data logger, and controlled in a greenhouse or enclosed chamber.
• In an investigation, carbon dioxide level is the independent variable when it is changed, and the rate of photosynthesis (for example oxygen bubbles produced by pondweed) is the dependent variable.
• Plants in dense woodland or near the ground may experience different carbon dioxide levels because of respiration, decomposition and reduced air movement.

Common mistakes and how to put them right

• Saying plants take in carbon dioxide for respiration.

  Put it right: Plants absorb carbon dioxide for photosynthesis; respiration uses oxygen and releases carbon dioxide.

• Writing that carbon dioxide is a biotic factor.

  Put it right: It is a gas, so it is abiotic; biotic factors are living organisms and their interactions.

• Claiming more carbon dioxide always increases growth without limit.

  Put it right: Another factor such as light or temperature will eventually become limiting.

• Confusing the gas exchange surface.

  Put it right: Carbon dioxide enters through stomata, which are mainly in the leaves, not through the roots.

Exam tips

• Quote the approximate atmospheric value, about 0.04% or 400 ppm, to show precise knowledge of normal carbon dioxide levels.
• When explaining limiting factors, state clearly that increasing carbon dioxide raises the rate only until light or temperature becomes limiting.
• Use the word equation for photosynthesis accurately and name glucose and oxygen as the products.
• In practical questions, identify the independent variable (carbon dioxide concentration), dependent variable (rate of photosynthesis) and a control variable such as temperature or light intensity.