Why are leaves called food factories?

Animals have to find food, but green plants make their own. They do it in their leaves, which work like tiny kitchens open all day long. Just as a factory takes in raw materials and turns them into a product, a leaf takes in sunlight, water and air and turns them into food. That is why leaves are called the food factories of plants.

The parts of a leaf

Pick up any leaf and look closely. The thin stalk that joins it to the stem is the petiole. The flat, green part is the leaf blade or lamina. Running down the middle is a thick line called the midrib, and from it branch out many thin veins. Veins carry water to every part of the leaf and carry food away. On the underside of most leaves there are thousands of tiny openings called stomata, too small to see without a microscope.

How leaves make food

Leaves are green because they contain a green colouring called chlorophyll. Chlorophyll catches energy from sunlight. The roots soak up water from the soil, and the stem carries it up to the leaves. The leaves take in carbon dioxide from the air through their stomata. Using the Sun's energy, the leaf joins water and carbon dioxide to make food, a kind of sugar. This process is called photosynthesis, which means "making with light".

While making food, leaves give out oxygen into the air. This is the oxygen that people and animals breathe. Extra food is stored in different parts of the plant as starch, for example in potatoes, rice grains and fruits.

A factory that works by day

Because photosynthesis needs light, the food factory works only during the day. At night it rests, and the plant uses some of the food it stored. Plants kept in dark corners grow weak and pale, while plants near a sunny window stay green and healthy, often leaning towards the light.

Leaves also breathe and sweat

Like all living things, plants breathe day and night, using oxygen and giving out carbon dioxide. Leaves also lose water vapour through their stomata, a bit like we sweat. This is called transpiration. It helps pull water up from the roots and keeps the plant cool on hot days.

Leaves in all shapes and sizes

Leaves can be tiny like tulsi leaves, huge like banana leaves, thin like pine needles or round like lotus pads. Some have net-like veins that branch out, like the mango and peepal; others have parallel veins that run side by side, like grass and banana. Some leaves have changed to do special jobs: pea plants have curly tendrils to climb, cactus leaves have become spines to save water, and the pitcher plant's leaf is a trap for insects.

Why leaves matter to us

• Every food chain begins with green plants, so all animals depend on leaves.
• Leaves give us oxygen and take in carbon dioxide, keeping the air clean.
• We eat leaves like spinach and methi, use tulsi and neem as medicine, and serve food on banana leaves.
• Fallen leaves rot and enrich the soil.

This Leaves and the Food Factory of Plants worksheet for Class 4 turns these ideas into an island adventure on WitKnowLearn with an online quiz, online activities and interactive games. Flip the cards, run the Food Factory Lab, play the games, do the science labs, take the challenges and then test leaves for starch and water in the Virtual Lab.