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We have learnt that food is essential for all living organisms. We also learnt that carbohydrates, proteins, fats, vitamins and minerals are components of food. These components of food are called nutrients and are necessary for our body.
Plants are the only organisms that can prepare food for themselves by using water, carbon dioxide and minerals. The raw materials are present in their surroundings.
The nutrients enable living organisms to build their bodies, to grow, to repair damaged parts of their bodies and provide the energy to carry out life processes. Nutrition is the mode of taking food by an organism and its utilisation 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. They are called heterotrophs (heteros = other).
Leaves are the food factories of plants. Therefore, all the raw materials must reach the leaf. Water and minerals present in the soil are absorbed by the roots and transported to the leaves. These pores are surrounded by ‘guard cells’. Such pores are called stomata.
The leaves have a green pigment called chlorophyll.
The synthesis of food occurs in the presence of sunlight, it is called photosynthesis Photo: light; synthesis : to combine). Sun is the ultimate source of energy for all living organisms.
During photosynthesis, chlorophyll containing cells of leaves, in the presence of sunlight, use carbon dioxide and water to synthesise carbohydrates. The process can be represented in an equation:
Carbon dioxide + water SubLight/Chlorophyll → Carbohydrate oxygen
During the process of photosynthesis, oxygen is released. The presence of starch in leaves indicates the occurrence of photosynthesis. Starch is also a carbohydrate.
The leaves other than green also have chlorophyll. The large amount of red, brown and other pigments mask the green colour. Photosynthesis takes place in these leaves also.
There are some plants which do not have chlorophyll. They cannot synthesise food.
Like humans and animals such plants depend on the food produced by other plants. They use the heterotrophic mode of nutrition.
When an insect lands in the pitcher, the lid closes and the trapped insect gets entangled into the hair. The lid closes and the insect is trapped. The insect is digested by the digestive juices secreted in the pitcher and its nutrients are absorbed. Such insect-eating plants are called insectivorous plants.
The mode of nutrition in which organisms take in nutrients from dead and decaying matter is called saprotrophic nutrition. Such organisms with saprotrophic mode of nutrition are called saprotrophs.
Some organisms live together and share both shelter and nutrients. This relationship is called symbiosis.
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.
We have learnt that plants absorb minerals and nutrients from the soil. So, their amounts in the soil keep on declining. Fertilizers and manures contain nutrients such as nitrogen, potassium, phosphorous, etc.
Usually crop plants absorb a lot of nitrogen and the soil becomes deficient in nitrogen. Nitrogen gas is available in plenty in the air, plants cannot use it in the manner they can use carbon dioxide.
The bacterium called Rhizobium can take atmospheric nitrogen and convert it into a usable form. But Rhizobium cannot make its own food.