CBSE Class 7 Science Modes of Nutrition Notes Set 07

Why living things need food

Every living thing needs energy to grow, repair its body and carry out life processes. This energy comes from food. Food contains carbohydrates, proteins, fats, vitamins and minerals. The way an organism obtains and uses its food is called its mode of nutrition. Nutrition is the first of the life processes, and nearly all other processes depend on it.

Two broad modes of nutrition

Organisms are placed in two groups according to how they get food: those that prepare their own food and those that depend on others.

Autotrophic nutrition

The word autotroph comes from auto (self) and trophos (nourishment). An autotroph makes its own food from simple raw materials. All green plants, algae and some bacteria are autotrophs. Green plants use carbon dioxide from the air, water from the soil and energy from sunlight, and they build glucose in their leaves. This process is called photosynthesis. Because plants start the food chain, every other organism depends on them directly or indirectly.

Heterotrophic nutrition

Hetero means other. Heterotrophs cannot make their own food and must take ready-made food from other organisms. All animals, fungi and many bacteria are heterotrophs. Heterotrophic nutrition has several types:

1. Holozoic nutrition: the organism takes in solid food through the mouth and digests it inside the body. Humans, cows and lions feed this way.

2. Parasitic nutrition: the organism lives on or inside another living organism, called the host, and takes food from it. The host is usually harmed. Cuscuta (amarbel), a yellow thread-like plant without chlorophyll, grows on a host plant and sucks food from it. Lice, ticks and tapeworms are animal parasites.

3. Saprotrophic nutrition: the organism feeds on dead and decaying matter. It releases digestive juices on the material and absorbs the dissolved food. Mushrooms, moulds and yeast are saprotrophs. They also recycle nutrients back into the soil.

Plants with special feeding habits

A few green plants live in soil poor in nitrogen. The pitcher plant and sundew trap and digest insects to get extra nitrogen, although they still make food by photosynthesis. Cuscuta, in contrast, has no chlorophyll and is a true parasite.

Key points to remember

1. Plants are autotrophs; animals and fungi are heterotrophs.

2. Chlorophyll, the green pigment, is needed for autotrophic nutrition in plants.

3. A parasite harms its host, whereas a saprotroph feeds on dead matter and harms no living organism.

4. Animals depend on plants for food, either directly (herbivores) or indirectly (carnivores).

Worked examples

Example 1. Classify as autotroph or heterotroph: mango tree, mushroom, goat, algae.
Mango tree and algae contain chlorophyll and make their own food, so they are autotrophs. Mushroom and goat depend on other organisms for food, so they are heterotrophs (the mushroom is a saprotroph and the goat is holozoic).

Example 2. Why is Cuscuta called a parasite although it is a plant?
Cuscuta has no chlorophyll, so it cannot make food. It twines around a host plant and absorbs ready-made food from it, harming the host. An organism that lives on another living organism and feeds on it is a parasite.

Example 3. A bread slice kept in a damp place develops fuzzy growth. What is the mode of nutrition of this growth?
The growth is a mould, which is a fungus. It releases digestive juices on the bread and absorbs the dissolved food, so its nutrition is saprotrophic.

Quick Check Questions

Answer in one or two lines.