Photosynthesis
What is photosynthesis?
is the process plants and algae use to make their own food (glucose). It takes place in the , which contain the green pigment . Chlorophyll absorbs light energy from the environment.
The plant takes in carbon dioxide from the air through its leaves, and water from the soil through its roots. Using light energy, these are converted into and oxygen.
Key terms
The photosynthesis equation
Word equation:
carbon dioxide + water → glucose + oxygen
(light energy is needed, and is absorbed by chlorophyll)
Symbol equation:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
The oxygen produced is released into the air or used by the plant for respiration.
An endothermic reaction
Photosynthesis is an reaction: energy is transferred from the environment to the chloroplasts, in the form of light.
This energy is stored in the chemical bonds of glucose. Plants — and the animals that eat them — later release it through respiration.
Key terms
Where the reactants come from
Carbon dioxide diffuses into the leaf from the air through tiny pores called , mostly on the underside of the leaf.
Water is absorbed from the soil by and carried up to the leaves in the .
Light falls on the top of the leaf. The cells just below the upper surface are packed with chloroplasts, so they absorb most of it.
The oxygen made diffuses out of the leaf through the stomata, unless the plant uses it straight away for respiration.
Key terms
Photosynthesis and respiration
Plants carry out both processes. They are almost the reverse of each other:
| Photosynthesis | Aerobic respiration | |
|---|---|---|
| Where | Chloroplasts | Mitochondria |
| When | Only in the light | All the time |
| Takes in | Carbon dioxide and water | Glucose and oxygen |
| Gives out | Glucose and oxygen | Carbon dioxide and water |
| Energy | Endothermic: takes in light energy | Exothermic: releases energy |
Common mistake
Plants respire all the time, not only at night. In bright light they photosynthesise faster than they respire, so overall they take in carbon dioxide and give out oxygen.