Photosynthesis GCSE Biology: The Complete Revision Guide

Photosynthesis is the process plants use to make their own food using light energy, and it sits at the centre of GCSE Biology because so much else — respiration, the carbon cycle, food chains — depends on it. Most students can recite the equation. Far fewer can explain a limiting-factor graph or describe the required practical accurately, which is exactly where the marks are lost.

The equations examiners expect

Word equation: carbon dioxide + water → glucose + oxygen (in the presence of light)

Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Both versions come up. Higher-tier papers sometimes ask you to balance the symbol equation from scratch, so learn it as numbers rather than a shape on the page.

Why leaves are built the way they are

Leaves are adapted for photosynthesis in several specific ways, and "describe how a leaf is adapted" is a recurring 4–6 mark question:

• Broad, flat shape — maximises surface area for light absorption
• Thin — keeps the diffusion path for gases short
• Palisade cells packed with chloroplasts — positioned near the top surface, closest to the light
• Stomata — pores that let CO₂ in and O₂ and water vapour out
• A network of veins — brings water in and carries glucose away

Examiners want the adaptation and the reason it helps — "thin" alone is a description; "thin, which shortens the diffusion distance for gases" is the marking point.

Limiting factors

Photosynthesis can only go as fast as its most restricted resource allows. The three limiting factors are light intensity, carbon dioxide concentration, and temperature, and each produces a graph that behaves the same way: the rate rises, then plateaus once something else becomes limiting.

Factor	Effect on rate
Light intensity	Rises with more light, then plateaus
CO₂ concentration	Rises with more CO₂, then plateaus
Temperature	Rises up to roughly 45°C, then enzymes denature and the rate collapses

If a graph in the exam levels off, that's your cue: something else has become the limiting factor, and naming it is usually worth a mark on its own.

The required practical

The core required practical uses pondweed (commonly Elodea) and counts oxygen bubbles produced per minute:

1. Set up pondweed in water with a lamp at a measured distance
2. Count bubbles produced in a fixed time
3. Move the lamp to change the light intensity
4. Repeat at each distance and calculate a mean

The detail worth remembering: light intensity is inversely proportional to the square of the distance from the light source (the inverse square law), which is a common calculation question attached to this practical.

Common mistakes to avoid

• Treating photosynthesis as only a "daytime" process without being able to explain why light is the limiting factor at night
• Confusing photosynthesis with respiration — one uses CO₂ and releases O₂, the other does the reverse
• Describing leaf adaptations without linking them to function, which caps the mark even with the right list
• Misbalancing the symbol equation, especially the water and oxygen coefficients

Making it stick

Photosynthesis is a small, precise topic — equations, a labelled diagram, a table of factors, a four-step method — which makes it well suited to flashcards rather than re-reading notes. Our guide to spaced repetition covers the review schedule that keeps this kind of factual recall from fading before the exam.

If you've already made notes on this topic, you can upload them and let Cramoo turn them into a flashcard deck automatically, rather than typing out each equation and adaptation by hand.