CBSE Class 7 Science Nutrition in Plants Notes Set 03

Why Living Things Need Food

Every living organism needs food. Food supplies the materials for growth, the substances needed to repair worn-out or damaged parts, and the energy for all life activities. Nutrition is the process of taking in food and using it in the body. The components of food that the body needs, such as carbohydrates, proteins, fats, vitamins and minerals, are called nutrients.

Autotrophs and Heterotrophs

Organisms that make their own food from simple substances are called autotrophs. Green plants and algae are autotrophs. Organisms that cannot make food and depend on other organisms for ready-made food are called heterotrophs. All animals, including humans, are heterotrophs, and so are fungi and some non-green plants.

Photosynthesis: The Food Factory of Green Plants

Photosynthesis is the process by which green plants prepare food (glucose) using carbon dioxide and water in the presence of sunlight and chlorophyll. Oxygen is released as a by-product.

Word equation: carbon dioxide + water, in sunlight and chlorophyll, gives glucose + oxygen.

Chemical equation: \( 6CO_2 + 6H_2O \xrightarrow{\text{sunlight, chlorophyll}} C_6H_{12}O_6 + 6O_2 \)

What is needed for photosynthesis, and where it comes from:

1. Carbon dioxide enters the leaf from the air through tiny pores called stomata.

2. Water and dissolved minerals are absorbed by the roots and carried to the leaves through the stem.

3. Sunlight provides the energy.

4. Chlorophyll, the green pigment inside chloroplasts, captures the light energy.

Chloroplasts are small structures (organelles) found in the cytoplasm of leaf cells. Because most chlorophyll is in the leaves, leaves are called the food factories of the plant. The glucose made is used for energy and growth, and the extra is stored as starch.

Special Case: Desert Plants

In a cactus the leaves are reduced to spines so that very little water is lost through transpiration, which is the loss of water vapour from the leaf surface. The stem is green and contains chlorophyll, so the stem carries out photosynthesis instead of the leaves.

Replenishing Nutrients in Soil

Plants take mineral nutrients such as nitrogen, phosphorus and potassium from the soil. Soil must be enriched from time to time with manure or fertilisers, otherwise crops grow poorly. Nutrients needed in large quantities are called macronutrients, and those needed in tiny quantities are called micronutrients.

Air is about four-fifths nitrogen, but plants cannot use nitrogen gas directly. Rhizobium bacteria live in swellings called root nodules on the roots of leguminous plants such as pea, gram and moong. They convert nitrogen gas into soluble compounds that the plant can absorb. This conversion is called nitrogen fixation. The plant gives the bacteria shelter and food in return.

Other Modes of Nutrition in Plants

1. Parasitic nutrition: A parasite depends entirely on another living organism, the host, for food and harms it. Cuscuta (amarbel) is a yellow, leafless climber with no chlorophyll that draws ready-made food from its host plant.

2. Saprotrophic nutrition: Saprotrophs feed on dead and decaying matter. Fungi such as mushrooms and yeast release digestive juices on the dead matter, turn it into a solution and absorb it.

3. Insectivorous plants: Pitcher plant, Venus flytrap and sundew are green and do photosynthesis, but they grow in soil poor in nitrogen. They trap and digest insects to obtain the nitrogen they lack. In the pitcher plant the leaf is modified into a pitcher with a lid, and insects that slip inside are digested.

4. Symbiosis: In a symbiotic relationship two organisms live together and both gain. Lichens are made of an alga and a fungus. The alga makes food by photosynthesis, while the fungus gives shelter, water and minerals. Rhizobium and leguminous plants are another example.

Worked Examples

Example 1. A plant has no chlorophyll, is yellow and wound around a hedge. Name its mode of nutrition. Answer: It is Cuscuta. It takes food from the hedge plant, so its mode of nutrition is parasitic.

Example 2. A potted plant is kept in a dark cupboard for a week. Why does it become weak? Answer: Without sunlight, photosynthesis cannot take place, so no food is made and the plant weakens.

Example 3. Why does a pitcher plant trap insects although it can make its own food? Answer: Its soil lacks nitrogen. The insects supply the nitrogen it needs.

Quick Check Questions