In Plant Cells and Animal Cells you saw that plant cells have chloroplasts. Here is what they do. (Young learners met the idea in Plants Make Their Own Food.)
Key idea
In chloroplasts, plants use light energy to turn carbon dioxide and water into sugar (glucose), giving off oxygen: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. The sugar stores the energy and is the raw material for the plant's whole body.
Why it works
Where does a tree's mass come from? In the 1600s Jan Baptist van Helmont grew a 5-pound willow in 200 pounds of dried soil, giving it only water. Five years later the tree weighed about 169 pounds, and the soil had lost only about 2 ounces. Today we know most of the missing mass came from the air: the carbon in wood, leaves and roots comes from carbon dioxide, joined with water.
Worked examples
Worked example: Inputs and outputs
Which are the inputs of photosynthesis, and which the outputs?
- Inputs: carbon dioxide, water, and light energy.
- Outputs: sugar (glucose) and oxygen.
Answer: In: CO₂, water, light. Out: sugar, oxygen.
Worked example: Count molecules
How many molecules of carbon dioxide does a leaf use to make 3 molecules of glucose?
- The equation uses 6 CO₂ for each glucose.
- $3 \times 6 = 18$.
Answer: 18 molecules.
Worked example: Where the mass comes from
A tree gains 50 kg while its soil loses 40 g. Did its mass come from the soil?
- 40 g is $0.04$ kg, less than 0.1% of 50 kg.
- The rest came from carbon dioxide and water.
Answer: No, almost all of it came from air and water.
Common mistake
Thinking plants get their food and mass from the soil. Soil gives water and small amounts of minerals. Almost all of a plant's mass comes from carbon dioxide in the air and from water. Light brings energy, not mass.