Photosynthesis

In the syllabus and the exam

Photosynthesis anchors the entire nutrition in plants topic, and candidates are expected to know both the word equation and the fact that the process occurs in two linked stages, a light-dependent stage that captures light energy and splits water, and a light-independent stage that uses that captured energy to fix carbon dioxide into glucose. Exam questions frequently ask candidates to state the raw materials (carbon dioxide and water), the products (glucose and oxygen), and the conditions required (light and chlorophyll), and Paper 3 practical questions commonly involve testing a leaf for starch to provide evidence that photosynthesis has occurred. A recurring structured-question format presents a graph of the rate of photosynthesis against light intensity, carbon dioxide concentration, or temperature, and asks candidates to identify the limiting factor at different points on the curve. A frequent error is writing that photosynthesis produces energy, when in fact it stores energy in the chemical bonds of glucose rather than releasing it, which is the opposite of what happens in respiration. Some candidates also incorrectly state that plants only photosynthesise and do not respire, when in fact photosynthesis and respiration occur simultaneously in a green plant during daylight. Candidates should also be able to state the balanced chemical equation alongside the word equation when required, and to explain that the glucose produced is not only used immediately in respiration but may also be converted into starch for storage or into cellulose for building cell walls. This is also why a variegated leaf, with white patches lacking chlorophyll, is a common Paper 3 specimen: testing the green and white regions separately for starch after exposure to light demonstrates directly that chlorophyll, not merely light, is essential for the reaction.

Example

Writing the overall word equation, a candidate states: 'Carbon dioxide and water react in the presence of light and chlorophyll to produce glucose and oxygen,' and adds that the reaction occurs inside chloroplasts.

Related terms

ChlorophyllChloroplastLimiting factor

Source:SRC-DSKP-EN