In a book published in 1648, after his death, the Flemish physician Jan Baptist van Helmont described an experiment with a willow tree. By his account, he planted a willow weighing 5 pounds in a pot of dried, weighed soil, covered the pot and gave it nothing but water for five years. The tree gained about 164 pounds. The soil, dried and weighed again, had lost only about 2 ounces.
Van Helmont concluded that the tree was made from water. He was half right, and he had shown that the tree wasn't built from the soil. What he missed was the air. Most of the tree's dry mass came from carbon dioxide.
This lesson covers photosynthesis in AQA GCSE Biology Topic 4 (Bioenergetics). It's also useful for Combined Science. The required practical on light intensity has its own deck.
The equation
carbon dioxide + water → glucose + oxygen (with light)
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
You need to recognise the symbols CO₂, H₂O, O₂ and C₆H₁₂O₆. AQA describes photosynthesis as endothermic: energy is transferred from the environment to the chloroplasts by light. That energy ends up stored in glucose.
Where it happens
Photosynthesis happens in chloroplasts, in plant cells and algae. Chloroplasts contain chlorophyll, the green pigment that absorbs light. Carbon dioxide diffuses into the leaf from the air through the stomata. Water comes up from the roots in the xylem. Some of the oxygen made is used by the plant's own cells in respiration; the rest diffuses out of the leaf.
Plants respire all the time, day and night. Photosynthesis happens only when there is light.
Factors that affect the rate
AQA names four factors:
- Light intensity: light supplies the energy, so more light speeds the rate up, until another factor limits it.
- Carbon dioxide concentration: CO₂ is a reactant, so more CO₂ speeds the rate up, until another factor limits it.
- Temperature: the reactions are controlled by enzymes. Warmer speeds them up to an optimum; too hot and the enzymes denature, so the rate falls.
- Amount of chlorophyll: less chlorophyll absorbs less light. Plants need magnesium ions to make chlorophyll, and a shortage turns leaves yellow (Biology only).
Notice the different shapes. Light and CO₂ graphs rise and then level off. A temperature graph rises and then falls.
Two or three factors at once (Higher tier)
These factors interact, and the one in shortest supply is the limiting factor. On a graph with several lines, ask two questions. Where a line is still rising, the factor on the x-axis is limiting. Where lines have levelled off at different heights, the factor that differs between them is limiting.
Greenhouse growers use the same reasoning. Adding heat, light or carbon dioxide costs money, so it is only worth it when that factor is limiting and the extra crop pays for it.
What the plant does with the glucose
The glucose made in photosynthesis may be:
- used for respiration
- turned into insoluble starch for storage
- used to make fat or oil for storage
- used to make cellulose, which strengthens the cell wall
- used to make amino acids for protein synthesis
To make proteins, plants also need nitrate ions absorbed from the soil. Starch is a good store because it is insoluble, so it doesn't affect the movement of water into and out of cells by osmosis.
Common exam mistakes
- "Plants get their food from the soil." They make glucose from carbon dioxide and water. The soil gives water and mineral ions.
- "Plants photosynthesise instead of respiring." They do both. Respiration never stops.
- Calling chlorophyll or light a reactant. Chlorophyll absorbs light; light supplies energy. The reactants are carbon dioxide and water.
- Saying high temperatures "kill" the enzymes. Enzymes aren't alive. Say they are denatured.
- Mixing up starch and cellulose. Both are made from glucose: starch is a store, cellulose builds cell walls.
Frequently asked questions
What is the equation for photosynthesis in GCSE Biology?
carbon dioxide + water → glucose + oxygen, using light. In symbols: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Why is photosynthesis endothermic?
Energy is transferred from the environment to the chloroplasts by light, so the reaction takes in energy rather than giving it out.
What are the uses of glucose in plants?
Respiration, starch for storage, fats or oils for storage, cellulose for cell walls, and amino acids for proteins.
What are the limiting factors of photosynthesis?
Light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll. Whichever is in shortest supply limits the rate.
Try it
Say each answer before you flip the card. Then stand in van Helmont's place: your tree has gained over 160 pounds, the soil has barely changed, and you have only given it water. What would you have tested next?
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