Nutrition is the process by which living organisms obtain and utilise food for growth, maintenance and repair of their body. Every living organism needs food to obtain energy for its life processes. The substances that provide nourishment to organisms are called nutrients, and the process of taking in and using food is called nutrition. Plants, although they do not move about like animals, are living organisms and they too require nutrition to stay alive, grow and reproduce.
In this chapter we will study how plants obtain their food. Interestingly, plants are quite different from animals when it comes to food. Most plants are autotrophs, meaning they prepare their own food. They use simple inorganic substances like carbon dioxide and water to make carbohydrates in the presence of sunlight. This remarkable process is called photosynthesis. However, not all plants prepare their own food; some depend on other organisms, and these are called heterotrophs.
Nutrition can be broadly classified into two main modes. In the autotrophic mode, organisms synthesise their own food from simple inorganic substances like carbon dioxide and water. Green plants are the best examples of autotrophs. In the heterotrophic mode, organisms depend on other organisms for their food since they cannot synthesise their own food. All animals, fungi and many bacteria, as well as some plants, are heterotrophs.
Heterotrophic nutrition is further divided into different types. Parasitic nutrition involves one organism (the parasite) living on or inside another organism (the host) and obtaining food from it, for example, the Cuscuta (amarbel) plant which has no chlorophyll. Saprotrophic nutrition involves organisms that take in soluble organic nutrients from dead and decaying matter; such organisms are called saprotrophs, and examples include fungi like bread mould, yeast and mushrooms. Insectivorous plants are a special category that trap and digest insects to obtain nitrogen, even though they can photosynthesise.
Photosynthesis is the process by which green plants synthesise their own food (glucose) from carbon dioxide and water in the presence of sunlight and chlorophyll. The process takes place mainly in the leaves, which contain the green pigment chlorophyll in structures called chloroplasts. The equation for photosynthesis is given below.
Carbon dioxide is absorbed from the atmosphere through tiny pores on the leaf surface called stomata, which are present mainly on the lower surface of leaves. Water is absorbed by the roots from the soil and transported to the leaves through the stem. The raw materials are brought together in the leaf, where chlorophyll traps the energy from sunlight. This light energy is converted into chemical energy stored in the food. The green colour of leaves shows the presence of chlorophyll. A simple experiment to test whether chlorophyll is necessary for photosynthesis uses a variegated leaf (a leaf with both green and non-green parts) which is boiled in alcohol and then tested with iodine solution; only the green parts turn blue-black, showing starch is formed only there.
The carbohydrates produced are ultimately stored in the plant as starch. To test for starch, a leaf is boiled in water, then in alcohol to remove chlorophyll, and finally iodine solution is added. A blue-black colour confirms the presence of starch. Plants that grow in shady, dry or saline conditions, like desert plants and plants growing on the sides of rivers, have some special adaptations in their leaves. In desert plants, stomata are fewer and the leaf surface is reduced or converted into spines to conserve water, so photosynthesis takes place through the stems. The oxygen released during photosynthesis maintains the balance of oxygen and carbon dioxide in the atmosphere, which is essential for all living beings.
Not all plants are autotrophs. Some plants obtain their nutrition by other means, and these are grouped under heterotrophic nutrition. The main types seen in plants are parasitic, saprotrophic and insectivorous nutrition, as well as a mutually beneficial relationship called symbiosis.
Cuscuta (amarbel) is a parasitic plant that has yellow, wiry, twining stems. It does not have chlorophyll and hence cannot photosynthesise. It takes up ready-made food from the host plant on which it climbs, by penetrating the host's stem with special structures called haustoria. Fungi, such as bread mould and mushrooms, are saprotrophs that grow on dead and decaying organic matter. They secrete digestive juices on the food and absorb the soluble nutrients directly, since they cannot ingest food as animals do. Insectivorous plants, such as the pitcher plant and Venus flytrap, are green plants that can photosynthesise, yet they trap and digest insects. They do this because they grow in soils poor in nitrogen; by digesting insects they obtain the nitrogen compounds they need. The pitcher plant has a special pitcher-shaped leaf with a lid that closes when an insect enters, and digestive juices then break down the insect.
Some organisms live together and benefit from each other; this is called symbiosis. Lichens are a classic example in which an alga and a fungus live together. The fungus provides water and minerals to the alga, while the alga prepares food for both through photosynthesis. Each partner benefits in this relationship. Another important example involves the nitrogen-fixing bacteria called Rhizobium, which live in the root nodules of leguminous plants like peas and beans. The plant provides food and shelter to the bacteria, and in return the bacteria convert atmospheric nitrogen into nitrogenous compounds that the plant can use. This improves soil fertility naturally.
Plants continually absorb nutrients from the soil for their growth. If this is not compensated, the soil will gradually lose its fertility. Farmers therefore need to add nutrients back to the soil. The nutrients required in large amounts by plants are called macronutrients, such as nitrogen, phosphorus and potassium. Nitrogen is especially important because it is an essential part of proteins, nucleic acids and chlorophyll.
Two common ways to replenish soil nutrients are through fertilisers and manure. Fertilisers are chemical substances that provide specific nutrients, while manure is organic matter from decomposed plants and animals that enriches the soil with humus. A very important natural mechanism for replenishing nitrogen is the action of nitrogen-fixing bacteria. Rhizobium bacteria live in the root nodules of leguminous plants and convert atmospheric nitrogen into usable nitrogenous compounds. These bacteria cannot make their own food, so they obtain ready-made food from the host plant; in return they give the plant nitrogen compounds. This mutual relationship not only feeds the plant but also enriches the soil, which is why farmers often grow leguminous crops like pulses in rotation with other crops to restore soil fertility.
| Mode of Nutrition | Organisms | Source of Food | Examples |
|---|---|---|---|
| Autotrophic | Green plants | Self-prepared food via photosynthesis | Mango, grass, banyan |
| Parasitic | Heterotrophs living on hosts | Food from living host | Cuscuta, dodder |
| Saprotrophic | Heterotrophs feeding on dead matter | Soluble nutrients from dead/decaying matter | Fungi, bread mould, mushrooms |
| Insectivorous | Green plants trapping insects | Photosynthesis + digested insects | Pitcher plant, Venus flytrap |
| Symbiotic | Two organisms sharing benefit | Mutual exchange | Lichens, Rhizobium in root nodules |
| Raw Material | Source | Use in Photosynthesis |
|---|---|---|
| Carbon dioxide | Atmosphere through stomata | Combined with water to form glucose |
| Water | Soil through roots and stem | Provides hydrogen for carbohydrates |
| Sunlight | Sun | Energy to drive the reaction |
| Chlorophyll | Chloroplasts in leaves | Traps solar energy |
Nutrition is fundamental to all life, and plants show fascinating diversity in how they obtain food. Most plants are autotrophs that carry out photosynthesis using sunlight, water and carbon dioxide, releasing oxygen in the process. Others have evolved parasitic, saprotrophic, insectivorous or symbiotic modes to survive in different conditions. The study of nutrition in plants also teaches us about the nitrogen cycle and soil fertility, reminding us how organisms are interconnected. Understanding this chapter builds the foundation for further study of food chains, ecosystems and plant physiology in higher classes.