Question
What is photosynthesis? Write the chemical equation for it.
This is a standard NCERT Class 7 question and a guaranteed 2-3 marks in board exams. Let’s make sure the answer is tight and complete.
Solution — Step by Step
Photosynthesis is the process by which green plants make their own food using sunlight, carbon dioxide (CO₂), and water (H₂O). The word itself tells you the story: photo = light, synthesis = making.
Plants take in two things from outside:
- Carbon dioxide — absorbed from air through tiny pores called stomata
- Water — absorbed from soil through roots
These are the reactants.
The reaction happens inside chloroplasts, which are present in the green parts of the plant. Chloroplasts contain chlorophyll — the green pigment that actually captures sunlight energy.
Without chlorophyll, no photosynthesis. This is why non-green parts of a plant cannot make food.
In words: Carbon dioxide + Water → Glucose + Oxygen (in the presence of sunlight and chlorophyll)
Two things are produced:
- Glucose (C₆H₁₂O₆) — stored as food, later converted to starch
- Oxygen — released into the atmosphere as a by-product
The oxygen we breathe right now is essentially a by-product of plants making food.
Why This Works
Plants are autotrophs — they make their own food, unlike animals who must eat others. Photosynthesis is the mechanism that makes this possible. Chlorophyll absorbs light energy (mainly red and blue wavelengths) and uses it to break water molecules apart, releasing oxygen.
The carbon dioxide from air gets converted into glucose using that energy. Glucose is essentially the plant’s “sugar” — it’s used for energy, growth, and eventually stored as starch in roots, stems, and seeds. When we eat potatoes or rice, we’re eating stored photosynthesis products.
This process is why plants are called producers in a food chain. Every food chain on Earth ultimately depends on photosynthesis.
Alternative Method
If the exam asks for the word equation instead of the chemical equation, write it exactly like this:
Carbon dioxide + Water → Glucose + Oxygen (in the presence of sunlight and chlorophyll)
Always mention the conditions — sunlight and chlorophyll — above or below the arrow. Many students write the reactants and products correctly but forget the conditions, losing half a mark.
For Class 7 boards, the word equation is often accepted and sometimes preferred. But learning the chemical equation (C₆H₁₂O₆) is worth it since it reappears in Class 10 and beyond.
Common Mistake
The most common error: students write oxygen as a reactant instead of a product — confusing photosynthesis with respiration. Remember: in photosynthesis, oxygen is released. In respiration, oxygen is consumed. The two processes are opposites.
Another slip: forgetting to write chlorophyll as a condition. Sunlight alone is not enough — the equation must show both sunlight and chlorophyll above the arrow.