Investigation of limiting factors — common mistakes

For Paper 4, pay close attention to the details of experimental design. Always specify your independent, dependent, and controlled variables. When interpreting graphs, clearly identify the initial linear phase and the plateau phase, and state why a particular factor is limiting in each region. Don't just identify, but explain the biological reason behind the limitation (e.g., 'low light means insufficient energy for the light-dependent stage' or 'low CO₂ means insufficient substrate for RuBisCO').

A limiting factor is a variable that restricts the rate of a biological process when it is in short supply. For Cambridge 9700 Biology, you need to understand how factors like light intensity, CO₂ concentration, and temperature can limit processes like photosynthesis. Our free revision course covers this and provides past paper practice.

To investigate limiting factors for photosynthesis, you design an experiment where you systematically vary one factor (e.g., light intensity) while keeping all other potential limiting factors constant (e.g., temperature, CO₂ concentration). You then measure the rate of photosynthesis (e.g., O₂ production) and plot your results on a graph to identify where the varied factor becomes limiting, shown by a plateau. Check out our free lessons for detailed experimental setups.

The three main limiting factors for photosynthesis are light intensity, carbon dioxide concentration, and temperature. Each of these can independently restrict the rate of photosynthesis if it is not at its optimal level, even if the other two factors are abundant. Practising past paper questions helps solidify your understanding of how each factor operates.