IGCSE Biology / Plants and food
The photosynthesis reaction revision notes
Edexcel International GCSE Biology (4BI1), chapter 10: Plants and food, specification point 2.18, 2.19. Free lesson.
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Photosynthesis uses carbon dioxide and water to make glucose and oxygen, using light energy absorbed by chlorophyll. The glucose supplies energy and materials for growth, while plants respire continuously, day and night.
What the photosynthesis equation shows
Photosynthesis happens in the chloroplasts, mostly in the palisade mesophyll cells near the top of the leaf, where light is strongest. The plant takes in carbon dioxide and water, then makes glucose and oxygen. Light energy is converted into chemical energy and stored in the glucose. Word equation: carbon dioxide + water → glucose + oxygen Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ Chlorophyll absorbs the light energy needed for the reaction. It is not a raw material and is not used up, so it is not included in the equation.
How raw materials enter and products leave
The raw materials reach the leaf by different routes. The products are then used in the plant or leave the leaf. This explains why leaf structures help materials move in and out. Carbon dioxide diffuses from the air through the stomata, mostly on the underside of the leaf. Water is absorbed from the soil by root hair cells by osmosis, then carried up to the leaf in the xylem. Oxygen diffuses out through the stomata. Glucose is used straight away or converted into starch for storage.
What plants do with glucose
Glucose is not only a product of photosynthesis. Plants use it for energy, storage and to build substances they need. Storing glucose as starch is useful because starch is insoluble, so it does not affect the cell’s water balance by osmosis. Respiration: glucose is broken down to release energy in the plant’s cells. Storage: glucose is converted into starch and stored as starch grains. Cellulose: glucose molecules are joined into long chains to build cell walls. Proteins: glucose is used with nitrate ions from the soil to make amino acids, which are then used to make proteins.…
Why plants respire all the time
Plants respire constantly because every living cell needs energy, including at night. Photosynthesis is the process that switches on and off with light. The balance between the two processes affects which gas is released overall. In bright light, photosynthesis is faster than respiration, so oxygen is released overall. In the dark, photosynthesis stops and respiration continues, so carbon dioxide is released.