IGCSE Biology: photosynthesis, limiting factors and the leaf

Key ideas

Chlorophyll is a green pigment found in chloroplasts. It absorbs light and transfers the energy into chemical energy in the carbohydrates made. The glucose is then used in respiration, converted to starch for storage, to cellulose for cell walls, to sucrose for transport in the phloem, and to nectar to attract insects for pollination (Cambridge 6.1.5). Plants also need mineral ions from the soil: magnesium ions to make chlorophyll and nitrate ions to make amino acids, and so proteins. A magnesium-deficient plant has yellowing leaves; a nitrate-deficient plant shows poor growth.

Cambridge splits the content: the word equation, the uses of glucose and the practicals are Core; the balanced symbol equation (6.1.10) and explaining limiting factors (6.1.11) are Supplement. Edexcel 4BI1 asks everyone for the word and balanced equations (2.19) and for how carbon dioxide, light intensity and temperature affect the rate (2.20). Edexcel puts gas exchange in leaves, including the hydrogencarbonate indicator practical (2.45B), in Paper 2 only, while Cambridge lists that practical as Core (6.1.9).

How the leaf is adapted

Limiting factors in mark-scheme language

On a graph of rate against light intensity, the rate rises at first: light is the limiting factor. The line then levels off: something else, usually carbon dioxide concentration or temperature, is now limiting. If a second line at higher carbon dioxide levels off higher, that proves carbon dioxide was limiting on the first line. Temperature acts through the enzymes that control photosynthesis, so the rate rises to an optimum and then falls as enzymes are denatured.

Glasshouse questions apply the same idea: adding paraffin heaters raises temperature and carbon dioxide, and lighting extends the hours of photosynthesis, but extra spending only pays off while that factor is the one limiting the rate.

Worked example: the pondweed practical

A student counts bubbles from a piece of pondweed for 2 minutes at different distances from a lamp. At 10 cm she counts 48 bubbles; at 40 cm she counts 12. Rate at 10 cm = 48 / 2 = 24 bubbles per minute. Rate at 40 cm = 12 / 2 = 6 bubbles per minute.

Conclusion: as the distance increases, light intensity decreases and the rate of photosynthesis decreases. Improvements examiners credit: collect the gas in a syringe or measuring cylinder instead of counting bubbles (bubbles vary in size), let the plant adjust for a few minutes at each distance, use a heat filter or a beaker of water so the lamp does not change the temperature, keep the hydrogencarbonate concentration constant, repeat and take a mean.

Hydrogencarbonate indicator: in a sealed tube it is orange-red at normal air carbon dioxide levels, turns yellow when carbon dioxide rises (leaf in the dark, respiration only) and purple when carbon dioxide falls (leaf in bright light, photosynthesis faster than respiration).

Common mistakes

• Saying plants photosynthesise in the day and respire at night. They respire all the time.
• Saying chlorophyll "makes" energy or "absorbs sunlight to make food" without naming light energy and carbohydrate.
• Testing a leaf for starch without boiling it in water and then warm ethanol to remove chlorophyll. Never heat ethanol over a flame.
• Forgetting to destarch the plant (keep it in the dark for 48 hours) before testing whether light is needed.
• Writing that carbon dioxide enters through the roots.
• Calling the levelled-off part of a light graph "maximum photosynthesis" instead of naming the new limiting factor.

Exam technique

Learn the starch test sequence as a numbered list, because it appears as a six-step method: boil the leaf in water, warm it in ethanol in a water bath, rinse in water, spread on a white tile, add iodine solution, blue-black shows starch. For controls, say what the control is and why: a variegated leaf where the white parts have no chlorophyll, or a plant kept in a bell jar with soda lime to absorb carbon dioxide alongside one without it.

When a question says "explain" a limiting-factor graph, name the factor for each region of the line. When it says "suggest", apply the idea to a context you have not met, such as a crop in a Gulf greenhouse with cooling.

How one-to-one lessons help

Photosynthesis questions are heavy on practical method and graph interpretation, two things students rarely practise enough on their own. A tutor sets past-paper graphs, asks the student to talk through each region and its limiting factor, and corrects wording until it matches the mark scheme. The tutor also links this topic to respiration and ecology, where the same ideas return.

Self-check

1. Write the word equation and the balanced symbol equation.
2. List five uses of the glucose a plant makes.
3. Explain why plants need magnesium and nitrate ions.
4. Label a leaf cross-section and give one adaptation for each tissue.
5. Explain a limiting-factor graph with two carbon dioxide levels.
6. Describe the starch test on a leaf, with safety.
7. Predict hydrogencarbonate indicator colours in light, shade and dark.