How do plants make food by photosynthesis, and what limits the rate of the reaction?
Photosynthesis as the process that makes glucose using light, the word and symbol equations, the factors that limit the rate of photosynthesis, and the related required practical.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on photosynthesis, covering the process and equations, the three limiting factors of light, carbon dioxide and temperature, and the practical investigating the rate of photosynthesis.
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What this dot point is asking
WJEC Double Award Unit 1 wants you to describe photosynthesis and give its equations, explain the factors that limit the rate of photosynthesis, and describe the practical investigating the rate.
What photosynthesis is
Photosynthesis happens in the chloroplasts, which contain the green pigment chlorophyll that absorbs light. It is the opposite of respiration: it builds glucose and uses carbon dioxide, while respiration breaks glucose down and releases carbon dioxide.
The equations
The glucose made is used by the plant for respiration (to release energy), converted to starch for storage, used to make cellulose for cell walls, and combined with nitrate ions to make proteins.
Limiting factors
If any of these is too low, it slows photosynthesis down. Increasing it speeds the reaction up, until another factor becomes limiting and the rate levels off:
- Light intensity: more light gives more energy for photosynthesis, until light is no longer limiting.
- Carbon dioxide concentration: more carbon dioxide provides more raw material, until it is no longer limiting.
- Temperature: a higher temperature speeds up the reactions, but if it is too high (above about 40 degrees Celsius) the enzymes are denatured and the rate falls.
Growers use this knowledge in greenhouses, adding extra light, carbon dioxide and warmth to increase crop yields.
The required practical
The rate of photosynthesis can be measured using pondweed (such as Elodea). The pondweed is placed in water and the number of oxygen bubbles produced per minute is counted, or the volume of gas collected is measured. Changing the distance of a lamp changes the light intensity (light intensity is proportional to ), and a thermometer and sodium hydrogencarbonate solution control temperature and carbon dioxide. The faster the bubbles, the faster the rate of photosynthesis.
Try this
Q1. Name the green pigment that absorbs light for photosynthesis. [1 mark]
- Cue. Chlorophyll.
Q2. State one use the plant makes of the glucose produced. [1 mark]
- Cue. Any one of: respiration, stored as starch, made into cellulose, or made into proteins.
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 style4 marksA student measures the volume of oxygen given off by pondweed at different light intensities. Describe and explain what the results would show as light intensity increases.Show worked answer →
A Unit 1 explain question worth 4 marks. Reward: as light intensity increases, the rate of photosynthesis (oxygen given off) increases (1), because light provides the energy for photosynthesis (1); the rate then levels off (1) when another factor, such as carbon dioxide or temperature, becomes the limiting factor (1). Markers credit the rising rate, the link to light energy, the plateau and the idea of another limiting factor. A common error is to say the rate keeps rising for ever.
WJEC style3 marksWrite the word equation for photosynthesis and state where in a plant cell it takes place.Show worked answer →
A Unit 1 recall question. The word equation is carbon dioxide + water to glucose + oxygen (1 mark for reactants, 1 for products), using light energy. It takes place in the chloroplasts (1 mark), which contain chlorophyll. Markers reward the correct reactants and products and the location. A common error is to reverse the equation (that is respiration) or to forget that light energy and chlorophyll are needed.
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