Let us understand the nutrition in plants and learn about different modes of nutrition such as autotrophic and heterotrophic nutrition. We know that all living organisms require food for the growth, repair, and functioning of the body.
What is Nutrition?
The process by which the organisms obtain and utilize nutrients in various biological activities is called Nutrition. We feed on a variety of food ranging from vegetables, fruits, or meat. You must have noticed that most of the animals that we consume for meat depend on plants for their nourishment.
We can therefore conclude that we directly or indirectly depend on plants for our nutrition. You must be wondering where the plants obtain their nutrition from. The plants obtain their nutrition by various different modes which we shall learn in this module.
Modes of Nutrition
The plants can obtain their food broadly by two modes namely:
Autotrophic: In the autotrophic mode of nutrition, green plants synthesize their own food by the process of photosynthesis. Such plants are termed autotrophs.
Heterotrophic: On the other hand, the organisms which depend on plants or other organisms for their food are termed heterotrophs and the mode of nutrition is heterotrophic.
The heterotrophic mode of nutrition can be further divided into:
In this mode of nutrition, the organisms take in nutrients in solution form from dead and decaying matter. The plants which use this mode of nutrition are termed saprotrophs. Fungi growing on bread are a well-known example of saprotrophs.
On the other hand, there are some organisms that live together and share shelter and nutrients. This is called a symbiotic relationship and the mode of nutrition is termed Symbiotic. Lichens are the best examples exhibiting this mode of nutrition.
In another mode of nutrition called Parasitic mode, there is a relationship between two organisms where one is benefited while the other known as the host is deprived or harmed. The organisms that are benefitted generally live in or on the host for food and protection and are called parasites.
The organisms that live on or inside other living organisms to derive their food are called Parasites and such a mode of nutrition is known as the Parasitic mode of Nutrition. Some plants, like the Cuscuta, snatch food from other plants by climbing onto them.
How Nutrients are Replenished in the Soil?
Have you seen farmers spreading manure 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 have learned that plants absorb mineral nutrients from the soil. So, their amounts in the soil keep on declining.
Fertilizers and manures contain plant nutrients such as nitrogen, potassium, phosphorous, etc. These nutrients need to be added from time to time to enrich the soil. We can grow plants and keep them healthy if we can fulfill the nutrient requirement of plants. Usually, crops require a lot of nitrogen to make proteins.
After the harvest, the soil becomes deficient in nitrogen. You have learned that though nitrogen gas is available in plenty 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 soluble form. But Rhizobium cannot make its own food. So it lives in the roots of a gram, peas, moong, beans, and other legumes and provides them with nitrogen.
Most of the pulses (dals) are obtained from leguminous plants. In return, the plants provide food and shelter to the bacteria. They, thus, have a symbiotic relationship. This association is of great significance for the farmers.
They do not need to add nitrogen fertilizer to the soil in which leguminous plants are grown. You have learnt that most of the plants are autotrophs. Only a few plants adopt other modes of nutrition like parasitic and saprotrophic. They derive nutrition from other organisms. All animals are categorized as heterotrophs since they depend on plants and other animals for food
Autotrophic Mode of Nutrition
The method by which organisms obtain food is referred to as a mode of nutrition. One of the major modes of nutrition is autotrophic. “Auto”- means self and ‘Trophe’ refers to nutrition. So, the organisms which synthesize their own food are called Autotrophs and the process is called ‘Autotrophic Nutrition”. Therefore, all green plants are called autotrophs.
Autotrophic nutrition is a mode of nutrition in which organisms synthesize their own food from the simple inorganic materials like carbon dioxide and water present in the surroundings. You might be thinking that whether food is made in all parts of a plant or only in certain parts.
Actually, leaves are the food factories of plants. It means the synthesis of food occurs in leaves. So all the raw materials must reach the leaves. The raw material for photosynthesis is carbon dioxide and water.
Water and minerals present in the soil are absorbed by the roots and transported to the leaves. Carbon dioxide from air is taken in through the tiny pores called stomata present on the surface of the leaves. The transportation of water and minerals takes place with the help of vessels that 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. The leaves have a green pigment called chlorophyll. It helps leaves to capture the energy of the sunlight. Using sunlight as the source of energy plants convert simple substances i.e. carbon dioxide and water into complex substances i.e. glucose.
This process of formation of food by the plants using sunlight is called Photosynthesis. The word Photosynthesis is derived from the Greek word Phot, which means light, and Synthesis, which means to obtain.
Process of Photosynthesis
You have learned that green plants are called autotrophs because they make their own food. The process of food formation takes place in leaves. That’s why they are called food- factories. However, other parts of the plant also play an important role. Roots absorb water and minerals from the soil. Stem or trunk conduct them to the leaves.
If you cut a section of the leaf and observe it under a microscope, you will find a complex structure divided into many layers. The uppermost and lowermost layers are called the Upper and Lower epidermis. The epidermis is covered with a thin waxy layer called a “cuticle” that prevents water loss. The lower epidermis has opened called stomata, surrounded by the guard cells.
Carbon dioxide needed for photosynthesis enters from the air into the leaf through the stomata. The vein is the area situated above the lower epidermis. The vein contains vessels that bring water and minerals to the leaves. The middle layer i.e. between the upper and lower epidermis is divided into two-layer – The palisade layer and the spongy layer.
The zone of Palisade cells is present below the upper epidermis. These cells contain a green pigment called “chlorophyll”. You have studied that chlorophyll helps leaves to capture the energy of the sunlight. During photosynthesis, chlorophyll-containing cells of leaves use carbon dioxide and water in presence of sunlight to synthesize carbohydrates and evolve oxygen.
The by-products are oxygen and water which are released through stomata during the daytime. The prepared food is then sent to all parts of the plant through veins. So, we conclude that during photosynthesis, chlorophyll-containing cells of leaves use carbon dioxide and water in presence of sunlight to synthesize carbohydrates.
You often see slimy green patches in water bodies. These are formed by the growth of organisms called “Algae”. You might be thinking why algae are green in color? Actually, they contain chlorophyll. “Algae can also prepare their own food by photosynthesis.