Question
What is photosynthesis? Write the equation for photosynthesis and explain what each part means.
This is a standard NCERT Class 7 question. CBSE loves asking this in both short-answer and diagram-based formats, so understanding the equation deeply — not just memorising it — is the right move.
Solution — Step by Step
The arrow shows a chemical reaction happening. The substances on the left are reactants (what goes in), and on the right are products (what comes out).
The plant absorbs carbon dioxide (CO₂) from the air through tiny pores on leaves called stomata. It also absorbs water (H₂O) from the soil through its roots.
Both are raw materials — the plant cannot make food without either one.
Sunlight provides the energy needed to break water molecules and combine everything together. Chlorophyll — the green pigment in leaves — traps this sunlight energy.
Think of chlorophyll as a solar panel and sunlight as the electricity supply.
The plant makes glucose (C₆H₁₂O₆) — this is food, stored energy the plant uses to grow, flower, and fruit. As a byproduct, it also releases oxygen (O₂) into the air.
This released oxygen is what we breathe. Every breath you take has a tree to thank.
Photosynthesis is the process by which green plants prepare their own food using carbon dioxide, water, sunlight, and chlorophyll, producing glucose and releasing oxygen.
The balanced equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of sunlight and chlorophyll).
Why This Works
Plants are called autotrophs — they make their own food. They do this because they have chlorophyll, and animals (including us) do not. That single difference is why the entire food chain depends on plants.
The glucose produced isn’t just sitting around. The plant either uses it immediately for energy (through respiration) or converts it to starch for storage. When you eat a potato, you’re eating stored photosynthesis.
The “6” in front of each molecule is about balancing — there must be the same number of each type of atom on both sides of the equation. Six CO₂ molecules provide 6 carbon atoms, and the glucose molecule (C₆H₁₂O₆) contains exactly 6 carbons. The numbers balance perfectly.
Alternative Method
If you’re asked to describe photosynthesis without the chemical equation, use this word equation — it’s equally valid for Class 7 CBSE:
Carbon dioxide + Water → Glucose + Oxygen (in the presence of sunlight and chlorophyll)
NCERT Class 7 Chapter 1 uses this word form first before the chemical equation. In board exams, both forms are accepted for full marks.
For a 3-mark CBSE question, write: (1) the definition, (2) the equation (word or chemical), (3) one line on where each raw material comes from. That structure always fetches full marks.
Common Mistake
Students often write that plants take in oxygen and release carbon dioxide — confusing photosynthesis with respiration. These are opposite processes. In photosynthesis, CO₂ goes in and O₂ comes out. In respiration (which happens 24/7), O₂ goes in and CO₂ comes out. Plants do both, but during daytime, photosynthesis dominates — which is why we’re told not to keep plants in the bedroom at night.
A second trap: writing that sunlight is a reactant in the equation. Sunlight is the energy source, not a substance. It goes above the arrow, not on the left side. Chlorophyll is the catalyst — it speeds up the reaction but isn’t consumed.