How do plants make food by photosynthesis, and how is the leaf adapted to do it?
The word and symbol equations for photosynthesis, the role of chlorophyll and chloroplasts, the limiting factors of light, carbon dioxide and temperature, the structure of a leaf and how its tissues and stomata are adapted for photosynthesis and gas exchange.
A focused CCEA GCSE Double Award Science (Biology Unit B1) answer on photosynthesis, covering the word and symbol equations, chlorophyll and chloroplasts, the three limiting factors, the structure of the leaf, and how stomata and guard cells control gas exchange.
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What this dot point is asking
CCEA Double Award wants the word and symbol equations for photosynthesis, the role of chlorophyll, the three limiting factors, and how the leaf is built to carry out photosynthesis and gas exchange. You also need to know how stomata and guard cells control the flow of gases.
The equations
The light energy is absorbed by chlorophyll, the green pigment held inside chloroplasts. The glucose made can be used in respiration for energy, converted to starch for storage, or used to build other molecules such as cellulose for cell walls.
Limiting factors
The three limiting factors of photosynthesis are light intensity, carbon dioxide concentration and temperature. On a rate graph, the line rises as you increase a factor, then levels off (plateaus) when something else becomes limiting. Temperature is slightly different: the rate rises with temperature up to an optimum, then falls as the enzymes start to denature.
The structure of the leaf
A leaf is adapted to capture light and exchange gases:
- It is broad and flat for a large surface area to absorb light.
- It is thin so gases diffuse only a short distance.
- The waxy cuticle on top is transparent to let light through and waterproof to reduce water loss.
- The palisade mesophyll cells are tall, near the top, and packed with chloroplasts to absorb the most light.
- The spongy mesophyll has large air spaces so carbon dioxide can reach the cells.
- Stomata (pores) in the lower epidermis let carbon dioxide diffuse in and oxygen and water vapour diffuse out.
Stomata and guard cells
Each stoma is bordered by two guard cells. When the guard cells take in water they become turgid and curve apart, opening the stoma so gases can move. When they lose water they become flaccid and close the stoma, which reduces water loss. Stomata are usually open in the day, when carbon dioxide is needed for photosynthesis, and mostly closed at night.
Examples in context
Example 1. Greenhouse growing. A grower keeps a greenhouse warm, well lit and rich in carbon dioxide. By removing all three limiting factors, the rate of photosynthesis stays high, so the crop grows faster. This shows the limiting-factor idea applied: whichever factor is in shortest supply caps the rate, so growers raise all three.
Example 2. Pondweed practical. Counting the bubbles of oxygen given off by pondweed at different distances from a lamp measures the effect of light intensity. As the lamp moves closer, the light intensity increases and more bubbles appear per minute, until the rate plateaus when carbon dioxide becomes limiting. This is a standard CCEA practical for the rate of photosynthesis.
Try this
Q1. Write the word equation for photosynthesis. [2 marks]
- Cue. Carbon dioxide + water gives glucose + oxygen.
Q2. Explain why a leaf is thin. [1 mark]
- Cue. So gases diffuse only a short distance to and from the cells.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA-style3 marksName the three factors that can limit the rate of photosynthesis and explain what 'limiting factor' means.Show worked answer →
Three named factors for two or three marks, plus the definition.
The three limiting factors are light intensity, carbon dioxide concentration and temperature.
A limiting factor is the one in shortest supply that holds back the rate. If you increase it, the rate goes up until something else becomes limiting.
For example, on a bright warm day the rate of photosynthesis in a greenhouse is often limited by carbon dioxide, which is why growers add extra carbon dioxide to the air.
CCEA-style4 marksExplain how the structure of a leaf is adapted for efficient photosynthesis.Show worked answer →
Pick adaptations and link each to its job for four marks.
The leaf is broad and flat to give a large surface area to absorb light. It is thin so gases diffuse only a short distance.
The palisade mesophyll cells are near the top and packed with chloroplasts to absorb the most light.
The spongy mesophyll has air spaces so carbon dioxide can diffuse to the cells, and stomata in the lower epidermis let carbon dioxide in and oxygen out.
Markers reward each adaptation tied clearly to its function.
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Sources & how we know this
- CCEA GCSE Science Double Award specification — CCEA (2017)