How do plants make food by photosynthesis, what affects the rate, and how is a leaf adapted for the job?
The word equation for photosynthesis, the role of chlorophyll and chloroplasts, the limiting factors of light, carbon dioxide and temperature, how a leaf is adapted for photosynthesis and gas exchange, and investigating the rate of photosynthesis.
A focused CCEA GCSE Single Award Science answer on photosynthesis, covering the word equation, chlorophyll and chloroplasts, the limiting factors of light, carbon dioxide and temperature, leaf adaptations for photosynthesis and gas exchange, and how to investigate the rate.
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What this dot point is asking
CCEA wants you to give the word equation for photosynthesis, explain the roles of chlorophyll and chloroplasts, describe the three limiting factors, explain how a leaf is adapted for photosynthesis and gas exchange, and design or interpret an experiment that measures the rate of photosynthesis.
The equation
The glucose made can be used in respiration for energy, stored as starch, converted to cellulose for cell walls, or combined with nitrates to make proteins for growth.
Chlorophyll and chloroplasts
Limiting factors
The rate of photosynthesis depends on three factors. A limiting factor is the one in shortest supply that holds back the rate.
- Light intensity. More light means more energy for the reaction, so the rate rises until another factor becomes limiting.
- Carbon dioxide concentration. Carbon dioxide is a raw material, so more of it raises the rate until it is no longer limiting.
- Temperature. Photosynthesis is controlled by enzymes, so a warmer temperature speeds it up until about 40 degrees Celsius, above which the enzymes denature and the rate falls.
How a leaf is adapted
A leaf is built for two jobs: capturing light and exchanging gases.
- Broad and flat for a large surface area to absorb light.
- Palisade mesophyll cells at the top are tall and packed with chloroplasts.
- Thin, so light reaches all the layers and carbon dioxide diffuses in quickly.
- Air spaces in the spongy mesophyll and stomata (with guard cells) in the lower epidermis let carbon dioxide in and oxygen out. The guard cells open and close the stomata.
Examples in context
Example 1. A commercial greenhouse. A tomato grower wants the fastest growth, so they remove every limiting factor. They add carbon dioxide from cylinders to raise the carbon dioxide concentration, use lamps to extend the light hours, and use heaters to keep the temperature near the enzyme optimum. By removing all three limiting factors at once, the plants photosynthesise as fast as possible and grow quickly, which is worth the extra cost in higher yields.
Example 2. The starch test for photosynthesis. To show a leaf has photosynthesised, you test it for starch. The leaf is boiled in water to kill it, then in ethanol to remove the chlorophyll, using a water bath rather than a flame because ethanol is flammable, then dipped in hot water to soften it and spread with iodine solution. A blue-black colour shows starch is present, proving photosynthesis has occurred. A variegated leaf only goes blue-black where it was green, proving chlorophyll is needed.
Try this
Q1. Name the three limiting factors of photosynthesis. [3 marks]
- Cue. Light intensity, carbon dioxide concentration and temperature.
Q2. Why does the rate of photosynthesis fall at very high temperatures? [2 marks]
- Cue. The enzymes controlling photosynthesis denature, so the reactions slow and stop.
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 SAS 20206 marksA student investigates how light intensity affects the rate of photosynthesis in pondweed. Describe a method and explain the expected results.Show worked answer →
A six-mark answer needs a clear method, a measured variable, controlled variables and an explanation of the trend.
Method: place pondweed in water containing sodium hydrogencarbonate, which provides carbon dioxide, and shine a lamp at it. Count the number of oxygen bubbles released per minute, or collect the gas and measure its volume. Move the lamp to set distances to change the light intensity, leaving time to settle each time.
Control variables: keep the temperature constant using a heat shield or water bath, use the same piece of pondweed, and keep the carbon dioxide concentration the same.
Expected results: as light intensity increases, the rate of photosynthesis rises, because light provides the energy for the reaction. Eventually the rate levels off, because another factor such as carbon dioxide concentration or temperature has become the limiting factor.
Markers reward a measurable rate, named control variables, the rising trend, and the idea of a limiting factor causing the plateau.
CCEA SAS 20184 marksExplain how a leaf is adapted for efficient photosynthesis.Show worked answer →
Four marks for four adaptations linked to their purpose.
The leaf is broad and flat, giving a large surface area to absorb as much light as possible.
The palisade cells near the top are tall and packed with chloroplasts, where most photosynthesis happens.
The leaf is thin, so carbon dioxide has only a short distance to diffuse to the cells and light reaches all the layers.
Air spaces in the spongy mesophyll, and stomata in the lower epidermis, let carbon dioxide diffuse in and oxygen diffuse out.
Markers reward each adaptation clearly linked to light capture or gas exchange.
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Sources & how we know this
- CCEA GCSE Science: Single Award specification — CCEA (2017)