Question
Write the balanced equation for photosynthesis. Identify the raw materials and products. What conditions are necessary for photosynthesis to occur?
(NCERT Class 7, Chapter 1 — Nutrition in Plants)
Solution — Step by Step
In words: six molecules of carbon dioxide + six molecules of water, in the presence of sunlight and chlorophyll, produce one molecule of glucose + six molecules of oxygen.
- Carbon dioxide () — absorbed from the air through tiny pores on leaves called stomata
- Water () — absorbed by roots from the soil and transported to leaves through xylem vessels
- Glucose () — the food/energy source for the plant; stored as starch
- Oxygen () — released into the atmosphere as a by-product through stomata
Two things are essential but are NOT consumed in the reaction:
- Sunlight — provides the energy to drive the reaction
- Chlorophyll — the green pigment in leaves that captures sunlight energy; acts as a catalyst
Without either of these, photosynthesis simply cannot happen.
Why This Works
Photosynthesis is essentially the reverse of respiration. Plants take low-energy molecules ( and ) and use solar energy to build a high-energy molecule (glucose). The oxygen we breathe is a “waste product” from this process — the plant doesn’t need it.
Chlorophyll absorbs red and blue light (which is why it looks green — it reflects green light). This absorbed energy splits water molecules, releasing oxygen. The hydrogen from water then combines with to form glucose. The entire process happens inside chloroplasts in the leaf cells.
Alternative Method — Think of it as an energy conversion
Think of photosynthesis as a solar-powered food factory: Light energy → Chemical energy (stored in glucose). This is the foundation of almost all food chains on Earth. When the exam asks “why are plants called producers?” — the answer is photosynthesis.
Common Mistake
Many students write chlorophyll and sunlight as “reactants.” They are not. Reactants are substances that get used up in the reaction ( and ). Sunlight is the energy source, and chlorophyll is the catalyst — neither appears in the balanced equation as a reactant. Write them above or below the arrow, not on the left side.