Photosynthesis: Quick Revision
The key facts, equation, limiting factors and common mistakes for photosynthesis.
Key concept
Photosynthesis is the process by which plants, algae and some bacteria use light energy to make glucose from carbon dioxide and water, releasing oxygen. In plants it takes place mainly in the leaves, in organelles called chloroplasts that contain the green pigment chlorophyll.
Equations
Word equation:
carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll)
Balanced symbol equation:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Where the materials come from
- Carbon dioxide enters through stomata, tiny pores in the leaf, mostly on the underside.
- Water is taken up by roots and carried to leaves in the xylem.
- Light is absorbed by chlorophyll.
What happens to the glucose
Plants use glucose for respiration, convert it to starch for storage, make cellulose for cell walls, and use it with nitrate and other minerals to build proteins.
Limiting factors
The rate of photosynthesis can be limited by:
- Light intensity: rate rises with light until another factor limits it.
- Carbon dioxide concentration: rate rises with concentration up to a point.
- Temperature: rate rises with temperature up to an optimum, then falls as enzymes become denatured.
A limiting factor is whichever factor is in shortest supply. Graphs typically show the rate levelling off when a different factor becomes limiting.
Common mistakes
- Saying plants "breathe in carbon dioxide and breathe out oxygen" only. Plants also respire, using oxygen, all the time. Photosynthesis happens only in light.
- Writing that photosynthesis "makes energy". It converts light energy into chemical energy stored in glucose.
- Mixing up photosynthesis and respiration equations. Respiration is roughly the reverse.
Quick recap
Light + CO₂ + water → glucose + oxygen, in chloroplasts. Rate depends on light, CO₂ and temperature.