Exam Tips: Photosynthesis

Always include "light energy" in the equation: The word equation is carbon dioxide + water + light energy → glucose + oxygen. Examiners specifically look for "light energy" as a reactant. Leaving it out costs a mark, even if the rest is correct.

Limiting factors — identify then explain: It is not enough to name the limiting factor. You must explain the mechanism. For light: "Light energy is needed to drive the first stage of photosynthesis; without enough light, the reaction cannot proceed faster." For CO2: "CO2 is the raw material converted into glucose; insufficient CO2 means fewer substrate molecules are available." For temperature: "Enzymes catalyse the reactions; at low temperatures they collide less frequently with substrates."

Temperature above optimum means denaturation, not just slowing: Below the optimum, more heat means faster enzymes (more kinetic energy, more frequent collisions). Above the optimum, enzymes are denatured — the active site changes shape permanently and the substrate cannot bind. These are two different mechanisms with different implications.

Uses of glucose — list all five: Respiration, starch (storage), cellulose (cell walls), lipids (seed oils), proteins (requires nitrate ions from soil). Questions worth 3+ marks expect you to go beyond just "respiration and starch."

RPA5 practical — know why each control is needed: Water bath: controls temperature (so it is not the variable). Black card over sides of beaker: prevents stray light (controls light intensity). Sodium hydrogen carbonate in water: provides dissolved CO2 for the plant (so CO2 is not limiting). Each control variable prevents it from being an unintended limiting factor.

Link photosynthesis to ecology: Exam questions sometimes ask about the environmental importance of photosynthesis. Key points: removes CO2 from the atmosphere (reducing the greenhouse effect), produces oxygen, provides the energy base for food chains, and stores carbon in biomass. These links appear in Unit 4 (Ecology) cross-topic questions.