NCERT Explained: Class VII Science, Chapter 1 - Nutrition in Plants

Introduction to the Topic

Welcome to a fascinating journey into the world of plants! In Class VI, you learned that food is essential for all living organisms. You also learnt that carbohydrates, proteins, fats, vitamins and minerals are components of food. These components are called nutrients and are necessary for our body.

Have you ever wondered how plants get their food? Unlike animals and human beings, plants can prepare their food themselves while animals and humans cannot. They get it from plants or animals that eat plants. Therefore, all animals and humans depend directly or indirectly on plants. In this chapter, we will explore the different modes of nutrition in plants, specifically focusing on how they act as food factories.

Key Concepts Explained

Nutrition is the mode of taking food by an organism and its utilization by the body. The mode of nutrition in which organisms make food themselves from simple substances is called autotrophic (auto = self; trophos = nourishment) nutrition. Therefore, plants are called autotrophs. Animals and most other organisms take in food prepared by plants and are called heterotrophs (heteros = other).

Photosynthesis: Food Making Process in Plants

Leaves are the food factories of plants. Therefore, all the raw materials must reach the leaf. Water and minerals absorbed by the roots are transported to the leaves. Carbon dioxide from air is taken in through tiny pores present on the surface of leaves. These pores are surrounded by 'guard cells' and are called stomata.

Water and minerals are transported to the leaves by the vessels which run like pipes throughout the root, the stem, the branches and the leaves. They form a continuous path or passage for the nutrients to reach the leaf. They are called xylem vessels. The leaves have a green pigment called chlorophyll. It helps leaves to capture the energy of the sunlight. This energy is used to synthesize (prepare) food from carbon dioxide and water. Since the synthesis of food occurs in the presence of sunlight, it is called photosynthesis (Photo: light; synthesis: to combine).

We can represent the process of photosynthesis with the following equation:

$$\text{Carbon Dioxide} + \text{Water} \xrightarrow[\text{Chlorophyll}]{\text{Sunlight}} \text{Carbohydrate} + \text{Oxygen}$$

During this process, oxygen is released. The presence of starch in leaves indicates the occurrence of photosynthesis. Starch is also a carbohydrate.

Other Modes of Nutrition in Plants

There are some plants which do not have chlorophyll. They cannot synthesize food. How do they survive and where do they get nutrition from? Like humans and animals, such plants depend on the food produced by other plants. They use the heterotrophic mode of nutrition.

• Parasitic Plants: Look at a plant like Cuscuta (Amar-bell). It does not have chlorophyll. It takes food from the plant on which it is climbing. The plant on which it climbs is called the host. Since it deprives the host of valuable nutrients, it is called a parasite.
• Insectivorous Plants: There are a few plants which can trap insects and digest them. Are they green and carry out photosynthesis? Such plants may be green or of some other color. Look at the pitcher-like plant (pitcher plant). The apex of the leaf forms a lid which can open and close the mouth of the pitcher. Inside the pitcher, there are hairs which are directed downwards. When an insect lands in the pitcher, the lid closes and the trapped insect gets entangled into the hair. The insect is digested by the digestive juices secreted in the pitcher. Such insect-eating plants are called insectivorous plants.
• Saprotrophs: You might have seen fungus packets sold in the vegetable market, or fluffy umbrella-like patches growing on rotting wood during the rainy season. These organisms are called fungi. They have a different mode of nutrition. They secrete digestive juices on the dead and decaying matter and convert it into a solution and then absorb the nutrients. This mode of nutrition in which organisms take in nutrients from dead and decaying matter is called saprotrophic nutrition. Plants which use saprotrophic nutrition are called saprotrophs.
• Symbiosis: Some organisms live together and share both shelter and nutrients. This relationship is called symbiosis. For example, certain fungi live inside the roots of trees. The tree provides nutrients to the fungus and, in return, receives water and certain nutrients. In organisms called lichens, a chlorophyll-containing partner, which is an alga, and a fungus live together. The fungus provides shelter, water and minerals to the alga and, in return, the alga prepares and provides food to the fungus.

Replenishing Nutrients in the Soil

Have you ever seen farmers spreading manures or fertilizers in the fields, or gardeners using them in lawns or in pots? Do you know why they are added to the soil? You learned that plants absorb minerals and nutrients from the soil. So, their amounts in the soil keep on declining. Fertilizers and manures contain plant nutrients such as nitrogen, potassium, phosphorus, etc. These need to be added from time to time to enrich the soil.

Normally, crop plants absorb a lot of nitrogen and the soil gets depleted in nitrogen. Although nitrogen gas is available in abundance in the air, plants cannot use it in the manner they can use carbon dioxide. They need nitrogen in a soluble form. The bacterium called Rhizobium can take atmospheric nitrogen and convert it into a usable form. But Rhizobium cannot make its own food. So it often lives in the roots of gram, peas, moong, beans and other legumes and provides them with nitrogen. In return, the plants provide food and shelter to bacteria. They have a symbiotic relationship. This association is of great significance for the farmers. They can not add nitrogen fertilizer when leguminous plants are grown.

Summary & Key Takeaways

• All animals and plants require food for their growth and functioning of their body.
• Green plants synthesize their food themselves by the process of photosynthesis. They are autotrophs.
• Plants like Cuscuta are parasites. They take food from the host plant.
• Plants use simple chemical substances like carbon dioxide, water and minerals for the synthesis of food.
• Chlorophyll, sunlight, carbon dioxide and water are essential requirements for photosynthesis.
• Complex chemical substances such as carbohydrates are the products of photosynthesis.
• Solar energy is captured by the chlorophyll present in leaves and plants.
• Oxygen is produced during photosynthesis and is released into the atmosphere.
• Oxygen released in photosynthesis is utilized by all living organisms for their survival.
• Fungi derive nutrition from dead, decaying matter. They are saprotrophs.
• A few plants and all animals are dependent on others for their nutrition and are called heterotrophs.