Photosynthesis Makes Food Using Light Energy
Photosynthesis
The process in which plants and algae use light energy, absorbed by chlorophyll, to make glucose from carbon dioxide and water, releasing oxygen; it takes place in the chloroplasts.
- Plants and algae are producers: they make their own food by photosynthesis instead of eating.
- In photosynthesis, light energy is used to make glucose from carbon dioxide and water.
- The glucose is used in respiration and to build all the other substances the plant needs.
Photosynthesis captures light energy and stores it as chemical energy in glucose, which feeds almost every food chain on Earth.
Photosynthesis Happens in Chloroplasts
Chloroplast
The green organelle in plant and algal cells where photosynthesis takes place; it contains the pigment chlorophyll.
- Most photosynthesis takes place in the leaves, in mesophyll cells packed with chloroplasts.
- It can happen in any green part of a plant, because green parts contain chloroplasts.
- Algae photosynthesise too, using chloroplasts in the same way.
The leaf is well adapted for photosynthesis; its detailed structure is covered in the plant organisation topic.
Chlorophyll Absorbs the Light
Chlorophyll
The green pigment inside chloroplasts that absorbs the light energy used for photosynthesis.
- Chloroplasts contain the green pigment chlorophyll.
- Chlorophyll absorbs light energy from the environment.
- This transferred energy is what drives the reaction that makes glucose.
A leaf short of chlorophyll, for example a variegated or magnesium-starved leaf, cannot absorb as much light and photosynthesises more slowly.
The Raw Materials and the Equations
- Carbon dioxide comes from the air and diffuses into the leaf through the stomata.
- Water comes from the soil, taken up by the roots and carried to the leaves in the xylem.
- The products are glucose and oxygen; the oxygen is released as a by-product through the stomata.
carbon dioxide + water → glucose + oxygen (light energy needed)
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
You should recognise the symbols CO₂, H₂O, O₂ and C₆H₁₂O₆.
Photosynthesis Is an Endothermic Reaction
Endothermic reaction
A reaction that takes in more energy from the surroundings than it gives out; photosynthesis is endothermic because it needs light energy to happen.
- Photosynthesis is endothermic: it takes in energy from the environment.
- This energy is transferred from the environment to the chloroplasts by light.
- More energy is needed to break the bonds in carbon dioxide and water than is released when the bonds in glucose and oxygen form, and this extra energy comes from light.
- Where in a plant does most photosynthesis take place, and what is the role of chlorophyll?
- Where does a plant get the carbon dioxide and water it needs?
- Write the word equation for photosynthesis.
- Give the balanced symbol equation for photosynthesis.
- Explain why photosynthesis is described as an endothermic reaction.