Worked example 1
An experiment was set up to investigate the effect of light intensity and carbon dioxide concentration on the rate of photosynthesis in Elodea. The rate of photosynthesis was measured by counting the number of oxygen bubbles produced per minute. The results are shown in the graph below.
[A graph shows Rate of O₂ production vs. Light intensity. Two curves are plotted: 'High CO₂' and 'Low CO₂'. Both start at the origin. The 'Low CO₂' curve increases and then plateaus at a lower rate. The 'High CO₂' curve increases more steeply and plateaus at a much higher rate.]
With reference to the graph:
(a) Describe the effect of increasing light intensity on the rate of photosynthesis at low carbon dioxide concentration. (b) Explain why the rate of photosynthesis at high carbon dioxide concentration is higher than at low carbon dioxide concentration, especially at high light intensities. (c) Identify the limiting factor(s) in the region where the graph for 'High CO₂' concentration plateaus. Justify your answer.
Show solution outline
(a) Description: At low carbon dioxide concentration, as light intensity increases from zero, the rate of photosynthesis initially increases proportionally. However, the rate levels off (plateaus) at a relatively low light intensity. Beyond this point, further increases in light intensity do not cause an increase in the rate of photosynthesis.
(b) Explanation: At high light intensities, light is no longer the limiting factor. Carbon dioxide is required for the light-independent reactions (Calvin cycle) as a substrate for the enzyme RuBisCO. With a high CO₂ concentration, there is more substrate available for RuBisCO to fix. This allows the Calvin cycle to proceed more rapidly, regenerating ADP and NADP for the light-dependent reactions faster, leading to a higher overall rate of photosynthesis compared to when CO₂ is scarce.
(c) Identifying Limiting Factor at Plateau: In the region where the 'High CO₂' curve plateaus, the limiting factor is no longer light intensity (as increasing it has no effect) or carbon dioxide concentration (as it is supplied at a high level). The limiting factor is likely to be temperature or another factor related to the plant itself, such as the concentration of photosynthetic enzymes (e.g., RuBisCO) or the number of chloroplasts. This is because the rate has reached a maximum, indicating that some other component of the photosynthetic machinery is operating at its maximum capacity.