Science · Lesson · Class 5–8

Photosynthesis: how plants make their own food

Q: What is photosynthesis in simple words?

Photosynthesis is the process plants use to make their own food. Using sunlight as energy, leaves take in carbon dioxide from the air and water from the roots, and turn them into glucose (a sugar) while releasing oxygen.

Q: Why are plants green?

Leaves contain chlorophyll, a green pigment that captures sunlight. Chlorophyll absorbs most colors of light but reflects green, which is why leaves look green to our eyes.

Q: What happens to plants at night?

At night there is no sunlight, so photosynthesis stops. Plants then only respire — they take in oxygen and release carbon dioxide, like animals do, but in much smaller amounts than the oxygen they produce by day.

Photosynthesis is the process by which green plants use sunlight to make their own food — glucose — from carbon dioxide and water, releasing oxygen as a bonus. In one line: sunlight + CO₂ + water → glucose + oxygen. It happens in the leaves, inside tiny structures called chloroplasts.

The full equation

carbon dioxide + water —(sunlight, chlorophyll)→ glucose + oxygen

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Chlorophyll — the green pigment inside leaf cells — is the solar panel of the plant. It absorbs sunlight (mostly the red and blue parts) and uses that energy to build glucose from CO₂ and water.

What plants need — and what they give back

Sunlight — the energy source. No light, no food-making.
Carbon dioxide (CO₂) — enters the leaf through tiny pores called stomata.
Water (H₂O) — absorbed from the soil by roots, carried up the stem.
Glucose (C₆H₁₂O₆) — the sugar the plant uses for energy and growth; extra is stored as starch.
Oxygen (O₂) — released back into the air. Every breath you take contains this gift.

See it happen — a real home experiment

Setup: Keep two leafy plants. One in bright sunlight, one in a dark cupboard for 4–5 days.

Test: Add a drop of iodine solution on a leaf from each (carefully, with an adult).

Result: the sunlit leaf turns blue-black (starch present → photosynthesis happened). The dark leaf stays brownish (no starch → no photosynthesis).

Why: plants make glucose and store it as starch. No light → no glucose → no starch.

Day vs night — a common exam question

During the day, plants photosynthesize much faster than they respire, so they release net oxygen. At night, photosynthesis stops (no sunlight); plants only respire — slowly taking in oxygen and releasing CO₂. This is why bedrooms full of plants still fine: the oxygen use of one or two plants at night is tiny.

Try it yourself

1. Which pigment absorbs sunlight during photosynthesis?

2. Which gas do plants take in for photosynthesis?

3. Why do leaves look green?

Remember 3 things

Photosynthesis = sunlight + CO₂ + water → glucose + oxygen.
It happens in chloroplasts; chlorophyll captures the light energy.
At night, photosynthesis stops — plants only respire, releasing a little CO₂.

Frequently asked questions

What is photosynthesis in simple words?

Photosynthesis is how plants make their own food using sunlight, carbon dioxide from the air and water from the soil — releasing oxygen in return.

Why are plants green?

Chlorophyll reflects green light and absorbs the other colours, so leaves look green to us.

What happens to plants at night?

No sunlight means no photosynthesis. Plants switch to respiration only — using oxygen and slowly releasing CO₂.