Photosynthesis Edexcel International GCSE Science (Double Award) revision
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In plain words
Plants don't eat. They make their own food out of air and water, using energy from light. Carbon dioxide comes in through the leaves, water comes up from the roots, and the green chlorophyll in the leaves uses light to turn them into glucose. Oxygen is the leftover.
Three things to know
- Carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- The glucose is used for respiration, stored as starch, made into cellulose for cell walls, or turned into sucrose to be carried round the plant. With nitrate ions it makes amino acids; magnesium ions are needed to make chlorophyll.
- The rate depends on light intensity, carbon dioxide concentration and temperature. Whichever is in shortest supply is the limiting factor.
Tips and tricks
- Chlorophyll absorbs the light. It is not used up and is not a reactant, so it goes over the arrow with light.
- Before testing a leaf for starch, the plant is kept in the dark for a day or two to use up its starch (destarching).
It lands in your notebook with its questions as flashcards.
Photosynthesis: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
Which gas is used in photosynthesis?
It enters the leaf through the stomata.
What absorbs the light energy for photosynthesis?
Chlorophyll is the green pigment in chloroplasts.
What are the products of photosynthesis?
Oxygen is given off as a waste product.
Which mineral ion is needed to make chlorophyll?
Without magnesium, leaves turn yellow.
Which of these can be a limiting factor for photosynthesis?
Light intensity, carbon dioxide and temperature are the three.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 8 marks. Write your answers on paper, then check them.
Photosynthesis
Edexcel International GCSE Science (Double Award) 4SD0 · 8 marks · papermunch.org
Name ______________________________ Date ______________
Write the word equation for photosynthesis.[2]
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Carbon dioxide + water → glucose + oxygen (with light and chlorophyll).
State three things a plant does with the glucose it makes.[3]
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Any three: uses it in respiration; stores it as starch; makes cellulose for cell walls; converts it to sucrose for transport; makes amino acids (with nitrate) and so proteins.
A graph of the rate of photosynthesis against light intensity rises and then levels off. Explain why it levels off.[3]
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At high light intensity, light is no longer the limiting factor. Something else, such as the carbon dioxide concentration or the temperature, is now limiting the rate.
Answers: Photosynthesis
- 1. Carbon dioxide + water → glucose + oxygen (with light and chlorophyll).
- 2. Any three: uses it in respiration; stores it as starch; makes cellulose for cell walls; converts it to sucrose for transport; makes amino acids (with nitrate) and so proteins.
- 3. At high light intensity, light is no longer the limiting factor. Something else, such as the carbon dioxide concentration or the temperature, is now limiting the rate.