Everything on the GCSE Biology Photosynthesis poster is written out below, section by section. Use it to search the sheet, copy parts into your own notes, or check a fact quickly.
The equation
Photosynthesis is the process by which plants build glucose from light, CO₂ and water. Everything else in the food chain depends on it.
Word equation: carbon dioxide + water → glucose + oxygen
Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Photosynthesis is endothermic - it takes in energy from the Sun.
Where it happens
Photosynthesis takes place in the chloroplasts of plant cells, mainly in the palisade mesophyll at the top of the leaf. Chloroplasts contain the green pigment chlorophyll, which absorbs light energy.
- Palisade mesophyll - the main site of photosynthesis, packed with chloroplasts
- Spongy mesophyll - air spaces allow gas exchange between cells
- Lower epidermis - the underside of the leaf
- Stomata - pores on the leaf surface where gas exchange (CO₂ in, O₂ out) takes place
Uses of glucose
The glucose made by photosynthesis is used for:
- Respiration - to release energy for the plant's life processes
- Stored as starch - starch is insoluble, so it doesn't affect the cell's water balance
- Cellulose - used to build strong cell walls
- Lipids - stored in seeds as an energy reserve
- Amino acids - combined with nitrates absorbed from the soil to make proteins
Limiting factors
Whichever factor is in shortest supply caps the rate of photosynthesis.
- Light intensity - rate rises with light, then plateaus once something else becomes limiting
- CO₂ concentration - same pattern; rate plateaus once CO₂ is no longer the limiting factor
- Temperature - rate rises to an optimum (around 35-40 °C in most plants), then falls sharply as the enzymes that catalyse photosynthesis denature
- Chlorophyll - less chlorophyll (e.g. from yellowing leaves or mineral deficiency) means less light is absorbed, giving a slower rate