Learning focus
Develop accurate biological explanations, interpret plant diagrams and data, and connect practical evidence with theory.
Meaning of photosynthesis
Photosynthesis is the process by which green plants manufacture carbohydrates from carbon dioxide and water using energy transferred from light. It is an anabolic process because small raw-material molecules are assembled into a larger energy-rich molecule. Light is the energy source rather than a material reactant.
Raw materials and products
Carbon dioxide enters leaves mainly through stomata and diffuses through air spaces to mesophyll cells. Water is absorbed by root hair cells and transported in xylem. Oxygen is released as a by-product and normally diffuses from the leaf. Glucose is the first carbohydrate named in the equation, but it is quickly used or converted.
Word equation
The word equation is carbon dioxide + water → glucose + oxygen. The arrow should be labelled with light energy and chlorophyll or these conditions should be written above it. Chlorophyll is not consumed by the reaction.
Balanced equation
The balanced chemical equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The numbers balance carbon, hydrogen and oxygen atoms on both sides. The equation summarises many enzyme-controlled steps rather than one single reaction.
Global importance
Photosynthesis introduces chemical energy into most food chains, supplies oxygen for aerobic respiration and removes carbon dioxide from the atmosphere. It also produces biomass used as food, fibre, timber and fuel.
Practical or data skill
Use a leaf, aquatic plant or indicator experiment to identify evidence of photosynthesis. Distinguish a material input from a condition.
Examination tip
Do not write that plants obtain food from soil. Plants absorb water and mineral ions from soil but manufacture carbohydrate by photosynthesis.
Review questions and suggested answers
Question 1
State the raw materials of photosynthesis.
Suggested answer
Carbon dioxide and water.
Question 2
What is the role of light?
Suggested answer
It supplies energy that chlorophyll transfers into chemical energy.
Question 3
Write the balanced equation.
Suggested answer
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
