What is photosynthesis, in simple words?

In one line

The rule

Photosynthesis is the biochemical process by which green plants, algae, and some bacteria convert light energy into chemical energy, using carbon dioxide and water to produce glucose and oxygen. (NCERT)

The main formula is: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂ Where:

• CO₂ = carbon dioxide (from air)
• H₂O = water (from soil)
• C₆H₁₂O₆ = glucose (sugar, food for the plant)
• O₂ = oxygen (released into the air)

Let's understand it simply

Imagine you are making roti in the kitchen. You need flour, water, and heat. Plants also make their own food, but instead of flour, they use carbon dioxide from the air. Instead of a gas stove, they use sunlight. Water comes from the soil. With these three things—sunlight, air, and water—plants make food inside their leaves. This process is called photosynthesis ("photo" means light, "synthesis" means making something new).

Think of a leaf as a tiny kitchen. Sunlight is like the cook, water is brought up from the roots, and air enters through small holes (stomata). The leaf mixes all these and cooks up food (glucose) for the plant.

How it works

1. Sunlight falls on the leaves, which contain a green pigment called chlorophyll (the thing that makes leaves green).
2. Chlorophyll absorbs the sunlight and uses its energy.
3. The plant takes in carbon dioxide (CO₂) from the air through tiny holes called stomata.
4. The roots absorb water (H₂O) from the soil and send it to the leaves.
5. Using the energy from sunlight, the plant combines CO₂ and H₂O to make glucose (a kind of sugar, the plant’s food).
6. Oxygen (O₂) is made as a by-product and goes out into the air.

The story behind it

The idea of photosynthesis was built up over many years. In the 1770s, Joseph Priestley found that plants could "refresh" air that a candle had made bad. Later, Jan Ingenhousz showed that this only happened in sunlight. It took many scientists to figure out that plants use sunlight to make food from air and water. The full chemical equation was worked out in the 1800s and 1900s. The word "photosynthesis" comes from Greek: "photo" (light) and "synthesis" (putting together).

Where you see it in real life

• A potted money plant on your windowsill: It grows towards sunlight, making food by photosynthesis.
• A banyan tree in the park: Its huge leaves use sunlight to make food, helping it grow big and strong.
• A field of wheat in Punjab: Every green stalk is busy making food for itself and, later, for us.
• Algae in a village pond: The green layer you see is algae making food by photosynthesis.
• A pressure cooker whistle in the kitchen: The oxygen you breathe out while cooking was made by plants during photosynthesis.
• Diwali morning: The fresh, clean air after rain is full of oxygen, thanks to trees doing photosynthesis all night and day.

Why it matters

• All our food comes from it: Every grain, fruit, and vegetable is made by photosynthesis.
• Oxygen for breathing: The oxygen we breathe is a gift from plants doing photosynthesis.
• Basis of all life: Even animals eat plants (or eat animals that eat plants), so all food chains start with photosynthesis.
• Controls carbon dioxide: Plants remove carbon dioxide from the air, helping keep the air clean.
• Needed for exams: This is a key topic in biology, and you will see it in many chapters and questions.

Worked example

Let’s say a plant uses 6 molecules of CO₂ and 6 molecules of H₂O.

Given: 6 CO₂, 6 H₂O, sunlight
Step 1: Write the rule        6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂
Step 2: Put in the numbers    6 + 6 + light → 1 glucose + 6 oxygen
Step 3: Answer                The plant makes 1 molecule of glucose and releases 6 molecules of oxygen.

Common mistakes

• Thinking plants only need water: Plants need sunlight, water, and carbon dioxide—all three.
• Believing photosynthesis happens only during the day: The light reactions need sunlight, but some steps (dark reactions) can happen without light, using energy stored earlier.
• Forgetting about oxygen: Plants give out oxygen as a by-product, which is vital for us.
• Mixing up where it happens: Photosynthesis mainly happens in the leaves, not in roots or stems.

Remember it

Plants use sunlight, air, and water to make food and oxygen—photosynthesis is the reason we have life on Earth.