Topic 6 of IGCSE Biology (Cambridge 0610) is plant nutrition (photosynthesis). You need to state the word and symbol equations, explain limiting factors (light, temperature, carbon dioxide), label leaf structure and state the function of the palisade layer and stomata, and describe the starch test and why variegated leaves show starch only in green areas. This revision guide walks you through each of these with clear explanations and exam-style tips for full marks.

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The photosynthesis equation

Photosynthesis is the process by which plants make glucose (and then starch and other substances) using light energy, carbon dioxide, and water. It takes place in chloroplasts, which contain the green pigment chlorophyll that absorbs light. The word equation is: carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll). The symbol equation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Oxygen is a product (released); do not write it as a reactant. In exams you may be asked to state the word or symbol equation; always include “light” and “chlorophyll” in the word equation.

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Limiting factors

The rate of photosynthesis is affected by light intensity, temperature, and carbon dioxide concentration. At any moment, only one of these usually limits the rate: increasing that factor increases the rate until another factor becomes limiting. For example, at low light intensity, increasing light increases the rate (more light → more energy for the reaction). At high light intensity, the rate may level off because CO₂ or temperature is now limiting. Similarly, at low CO₂, increasing CO₂ increases the rate; at high CO₂, light or temperature may limit. Temperature affects the rate because the reactions are catalysed by enzymes; there is an optimum temperature; above it, enzymes denature and the rate falls.

You should be able to describe the effect (e.g. “As light intensity increases, the rate of photosynthesis increases up to a point and then levels off.”) and explain why (e.g. “Above that point, another factor such as CO₂ or temperature becomes limiting.”). A typical rate vs light intensity graph rises and then flattens; you should explain the flattening in terms of a limiting factor.

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Leaf structure and the site of photosynthesis

The leaf is the main organ of photosynthesis. The upper epidermis and lower epidermis are thin layers that protect the leaf; the lower epidermis has stomata (pores) surrounded by guard cells, which allow gas exchange (CO₂ in, O₂ out) and control water loss. The palisade mesophyll is the main site of photosynthesis: it is a layer of cells just below the upper epidermis, packed with chloroplasts, so it receives plenty of light. The spongy mesophyll has more air spaces for diffusion of gases. The veins contain xylem (brings water and minerals to the leaf) and phloem (carries sucrose away).

In exams you may need to label a diagram (palisade layer, stomata, xylem, phloem, epidermis) and state that the palisade mesophyll has the most chloroplasts and is the main site of photosynthesis. Do not say “all cells photosynthesise”; the palisade layer is the main one.

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The starch test

To test a leaf for starch, you first kill the leaf (e.g. by boiling in water) to stop further chemical changes, then remove the chlorophyll so that the colour change is visible: heat the leaf in ethanol (in a water bath; ethanol is flammable) until it is decolourised, then wash the leaf in water to soften it. Add iodine solution. If starch is present, the leaf turns blue-black; if not, it stays brown. This test is often used to show that photosynthesis has occurred (e.g. only the green parts of a variegated leaf turn blue-black, because only they have chlorophyll and can photosynthesise).

When you describe the test, state: boil to kill; heat in ethanol to remove chlorophyll; wash; add iodine; blue-black = starch present. When you explain why a variegated leaf shows starch only in green areas, say that only the green parts contain chlorophyll and can carry out photosynthesis, so only those parts make and store starch.

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Exam tips and command words

• State the equation: Word or symbol; include light and chlorophyll for the word equation.
• Describe the effect: e.g. “As light intensity increases, rate increases up to a point then levels off.”
• Explain: Link to limiting factor (e.g. “Above a certain light intensity, CO₂ or temperature becomes limiting.”).
• Label the leaf: Know positions of palisade layer, stomata, xylem, phloem; state function of palisade layer (main site of photosynthesis; most chloroplasts).

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Common mistakes to avoid

• Writing the equation with oxygen as a reactant (oxygen is a product).
• Saying “no light = no photosynthesis” without noting that respiration still occurs (net gas exchange).
• Not stating that palisade mesophyll has the most chloroplasts and is the main site of photosynthesis.
• In the starch test: forgetting to remove chlorophyll (heat in ethanol) so the blue-black colour can be seen.

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Revision checklist

• Write the word and symbol equations for photosynthesis; include light and chlorophyll.
• Explain how light, temperature, and CO₂ affect the rate; draw or interpret a graph of rate vs light intensity.
• Label a diagram of leaf structure and state the function of palisade layer, stomata, and veins.
• Describe the starch test (boil, ethanol, wash, iodine) and explain why a variegated leaf shows starch only in green areas.

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Next steps

Book a free trial with an IGCSE Biology tutor to practise photosynthesis equations and limiting-factor questions, or explore Tutopiya’s learning portal for more revision resources.