How do plants make food by photosynthesis, and what limits how fast they can do it?
Photosynthesis as the process that makes glucose using light energy, its word and symbol equations, the limiting factors of light, carbon dioxide and temperature, and the required practicals.
A focused answer to the WJEC GCSE Biology section 1.5 topic on photosynthesis, covering photosynthesis as the process that uses light energy to make glucose, its word and symbol equations, the limiting factors of light intensity, carbon dioxide and temperature, and the required practicals on starch production and rate.
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What this dot point is asking
WJEC wants you to define photosynthesis, give its word and symbol equations, explain the three limiting factors, and describe the required practicals on starch production and the rate of photosynthesis.
What photosynthesis is
The word equation is:
and the balanced symbol equation is:
Photosynthesis happens in the chloroplasts, which contain the green pigment chlorophyll that absorbs light energy. The glucose made is used for respiration, stored as starch, or used to make other substances the plant needs.
Limiting factors
The rate of photosynthesis depends on the conditions. A limiting factor is the factor in shortest supply that stops the rate increasing.
- Light intensity. More light gives more energy for the reaction, so the rate increases, until another factor becomes limiting.
- Carbon dioxide concentration. More carbon dioxide gives more raw material, so the rate increases, until another factor becomes limiting.
- Temperature. A higher temperature speeds up the enzyme-controlled reactions of photosynthesis, up to an optimum. Too high, and the enzymes denature, so the rate falls.
Reading a limiting-factor graph
On a graph of rate against a factor, the curve usually rises and then levels off. The rising part means the factor is limiting; increasing it speeds up photosynthesis. The flat part (plateau) means that factor is no longer limiting; another factor (or the maximum rate) now caps the rate.
The required practicals
Testing a leaf for starch. Because glucose from photosynthesis is stored as starch, the presence of starch shows photosynthesis has happened. The method is: boil the leaf (to stop reactions and break the cells), then heat it in ethanol (to remove the green chlorophyll), then rinse and add iodine solution. A blue-black colour shows starch is present. By using a destarched plant and varying the conditions, you can show that light, chlorophyll (using a variegated leaf) and carbon dioxide are all needed for photosynthesis.
Measuring the rate of photosynthesis. A common method uses pondweed (such as Elodea) in water; you count the bubbles of oxygen given off, or measure the volume of gas, at different light intensities. Moving a lamp closer increases the light intensity and the rate, until another factor becomes limiting.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style3 marksWrite the word equation for photosynthesis and state where in the plant cell it takes place.Show worked answer →
A 3-mark question. Two marks for the equation, one for the location.
Word equation: carbon dioxide + water produces glucose + oxygen (using light energy and chlorophyll).
It takes place in the chloroplasts, which contain the green pigment chlorophyll that absorbs the light energy.
Markers reward: the correct reactants and products; the chloroplast as the location. Putting the reactants and products on the wrong sides, or naming the chlorophyll rather than the chloroplast as the location, are common slips.
WJEC style6 marksA graph shows the rate of photosynthesis rising as light intensity increases, then levelling off. Explain the shape of the graph using the idea of limiting factors.Show worked answer →
A 6-mark extended question. Mark it for explaining both the rise and the plateau using limiting factors.
At low light intensity, light is the limiting factor, so as light intensity increases the rate of photosynthesis increases, because more light energy is available for the reaction.
At high light intensity the rate levels off, because light is no longer the limiting factor; something else now limits the rate, such as the carbon dioxide concentration or the temperature. Increasing light further makes no difference because the rate is capped by that other factor.
A top answer names light as the limiting factor on the rising part, explains why more light means a faster rate, and explains that the plateau happens because a different factor (carbon dioxide or temperature) has become limiting. Reward the precise idea that a limiting factor is the one in shortest supply that caps the rate.
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Sources & how we know this
- WJEC GCSE Biology specification (from 2016) — WJEC (2016)