Photosynthesis · Year 10
Plants make their own food. In their leaves they use energy from light to turn carbon dioxide and water into glucose, giving out oxygen. Almost every food chain starts here, and the oxygen you breathe was made this way. How fast it happens depends on light, carbon dioxide and temperature.
Key facts
- Word equation: carbon dioxide + water → glucose + oxygen, using light energy absorbed by chlorophyll.
- Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Photosynthesis is endothermic: energy is taken in from light.
- It happens in the chloroplasts, mostly in the palisade cells near the top of the leaf. Carbon dioxide gets in through stomata; water comes up from the roots in the xylem.
- Plants use the glucose for respiration, store it as starch, and make cellulose (for cell walls), proteins (with nitrate ions from the soil) and fats and oils.
- Test a leaf for starch: boil it in water, then in ethanol to remove the chlorophyll, rinse, and add iodine. Starch turns the iodine from orange-brown to blue-black.
- Limiting factors: light intensity, carbon dioxide concentration and temperature. Whichever is in shortest supply limits the rate. Too hot, and the enzymes are denatured.
- Light intensity is inversely proportional to the square of the distance from the lamp (light intensity ∝ 1 ÷ d²): double the distance and the light is a quarter as strong.