Nutrition in Plants: How Plants Make Their Own Food (CBSE Class 7 Science)

Plants are the only living organisms that can make their own food using sunlight, water, and air. This process, called photosynthesis, is the foundation of almost all life on Earth. In this study note, you will understand not just what happens inside a leaf, but why each step matters.

Why Plants Need Nutrition and How They Differ from Animals

Every living cell needs energy to stay alive. In animals, this energy comes from food that is digested and absorbed. In plants, the situation is different: most plants make their own food inside their leaves. Organisms that make their own food are called autotrophs, while organisms that depend on others for food are called heterotrophs.

This difference is the starting point for understanding plant nutrition. A plant does not eat in the way an animal does. Instead, it uses simple raw materials from the environment and converts them into a carbohydrate called glucose. Glucose is not only a source of energy; it is also a building block for making starch, proteins, and fats.

Photosynthesis: The Raw Materials and the Food-Making Process

Photosynthesis is the process by which green plants make glucose from carbon dioxide and water in the presence of sunlight and chlorophyll. The word photosynthesis literally means putting together with light. The overall balanced equation is:

6CO2 + 6H2O → C6H12O6 + 6O2

This equation tells us that six molecules of carbon dioxide combine with six molecules of water to form one molecule of glucose and six molecules of oxygen. The number of atoms of each element is the same on both sides, which is why it is called a balanced equation. Carbon dioxide enters the leaf through tiny pores called stomata, and each stoma is surrounded by two guard cells that control its opening and closing. Water is absorbed by the roots and transported to the leaves.

The Role of Chlorophyll and Sunlight: The Energy Connection

Chlorophyll is the green pigment found in chloroplasts, which are tiny structures inside plant cells. Chlorophyll's job is to absorb sunlight energy. Without chlorophyll, a plant cannot photosynthesise, even if all other raw materials are present.

Sunlight provides the energy needed to split water molecules and combine carbon dioxide into glucose. A common exam misconception is that the oxygen released during photosynthesis comes from carbon dioxide. In fact, the oxygen comes from water. The carbon in glucose comes from carbon dioxide, and the hydrogen comes from water.

Other Modes of Nutrition: Parasitic, Insectivorous, Saprophytic, Symbiotic

Not all plants are completely autotrophic. Some have special ways of getting nutrients, especially nitrogen. These modes are important because they show how plants adapt to different environments.

• Parasitic nutrition: A plant like Cuscuta (amarbel) has no chlorophyll and cannot make food. It twines around a host plant and absorbs ready-made food from it.
• Insectivorous nutrition: Plants like the pitcher plant photosynthesise, but they grow in soil that is poor in nitrogen. To get nitrogen, they trap and digest insects.
• Saprophytic nutrition: Organisms like fungi feed on dead and decaying organic matter. They secrete digestive juices and absorb nutrients from the decaying material.
• Symbiotic nutrition: In a lichen, an alga and a fungus live together. The alga makes food, and the fungus provides water and minerals. Similarly, Rhizobium bacteria live in the root nodules of legumes and help the plant by fixing nitrogen.

Remember: insectivorous plants are still autotrophs because they make their own food by photosynthesis. Insects are an extra source of nitrogen, not a source of energy.

Replenishing Nutrients in Soil: Nitrogen Fixation and Fertilisers

Plants absorb minerals from the soil, and nitrogen is one of the most important. Nitrogen is needed to make proteins, which are essential for growth. When a crop is harvested, the soil loses nitrogen and other minerals. If these are not replaced, the soil becomes less fertile.

Farmers add manure and fertilisers to replenish soil nutrients. But there is also a natural way: nitrogen fixation. Rhizobium bacteria convert atmospheric nitrogen into nitrates that plants can absorb. Legumes such as peas, beans, and gram have Rhizobium in their roots, which is why farmers often grow legumes to improve soil fertility.

Excessive use of chemical fertilisers can harm soil organisms and pollute water. Therefore, a balance of natural and chemical methods is best for sustainable farming.

Key takeaways

• Plants are autotrophs because they make their own food by photosynthesis; animals are heterotrophs because they depend on plants or other animals for food.
• Photosynthesis uses carbon dioxide, water, sunlight, and chlorophyll to produce glucose and oxygen; the balanced equation is 6CO2 + 6H2O → C6H12O6 + 6O2.
• Chlorophyll absorbs sunlight, and the oxygen released during photosynthesis comes from water, not carbon dioxide.
• Some plants show heterotrophic nutrition: parasitic (Cuscuta), insectivorous (pitcher plant), saprophytic (fungi), and symbiotic (lichen, Rhizobium).
• Rhizobium bacteria fix atmospheric nitrogen into nitrates, enriching the soil and helping leguminous plants grow.
• Insectivorous plants photosynthesise but eat insects to obtain nitrogen, not energy.

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