Chapter 1 – Nutrition in Plants
Study Notes
Nutrition is the mode of taking food by an organism and its utilisation by the body.
The plants make food themselves from simple substances, hence they are called autotrophs (auto = self; trophos = nourishment).
Animals and most other organisms take in food prepared by plants. They are called heterotrophs (heteros = other).
Photosynthesis — Food Making Process in Plants
All the raw materials must reach the leaves because leaves are the food factories of plants. Water and minerals present in the soil are absorbed by the roots and transported to the leaves.
Carbon dioxide (CO2) from air is taken in through the tiny pores present on the surface of leaves. These pores are surrounded by ‘guard cells’. Such pores are called stomata
Water and minerals are transported to the leaves by the vessels which 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. They are called vessels.
The leaves have a green pigment called chlorophyll. It helps leaves to capture the energy of the sunlight. This energy is used to synthesise (prepare) food from carbon dioxide and water.
Since the synthesis of food occurs in the presence of sunlight, it is called photosynthesis (Photo: light; synthesis: to combine). So, we find that chlorophyll, sunlight, carbon dioxide and water are necessary to carry out the process of photosynthesis. It is a unique process on the earth. The solar energy is captured by the leaves and stored in the plant in the form of food. Thus, 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. During this process, oxygen which is essential for the survival of all organisms is produced. The process can be represented in an equation:
Carbon dioxide + water 
Carbohydrate + oxygen
The presence of starch in leaves indicates the occurrence of photosynthesis. Starch is a carbohydrate.
In the absence of photosynthesis there would not be any food. The survival of almost all living organisms directly or indirectly depends upon the food made by the plants. In the absence of photosynthesis, life would be impossible on the earth.
Activity
Take two potted plants of the same kind.
Keep one in the dark (or in a black box) for 3 days and the other in sunlight.
Take a leaf from the plant kept in dark. Place it in a petri-dish. Pour dilute iodine solution over the leaf with the help of a dropper. Nothing happens. It shows the absence of starch in the leaf.
Take a leaf from the plant kept in sunlight and place it in a petri-dish. Pour dilute iodine solution over the leaf with the help of a dropper. The leaf turns blue-black showing the presence of starch in it. This starch is the food which has been prepared by the leaf by photosynthesis.
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 slimy, green patches in ponds or stagnant water bodies. These are generally formed by the growth of organisms called algae. Algae are green in colour because they contain chlorophyll. Algae can prepare their own food by photosynthesis.
Synthesis of plant food other than carbohydrates
The carbohydrates are made of carbon, hydrogen and oxygen. These are used to synthesise other components of food such as proteins and fats. But proteins are nitrogenous substances which contain nitrogen.
Nitrogen is present in abundance in gaseous form in the air.
However, plants cannot absorb nitrogen directly.
Soil has certain bacteria that convert gaseous nitrogen into a usable form and release it into the soil. These are absorbed by the plants along with water. Also, fertilisers rich in nitrogen, are added to the soil. In this way the plants fulfil their requirements of nitrogen along with the other constituents. Plants can then synthesise proteins and vitamins.
Other Modes of Nutrition in Plants
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, i.e., they are heterotrophs.
A tree with a yellow wiry branched structure twining around the stem and branches, is a plant called Cuscuta (Amarbel). It does not have chlorophyll.
It takes readymade food from the plant on which it climbs. The plant on which it climbs is called the host. Since it deprives the host of valuable nutrients, Cuscuta is called the parasite.
Insectivores
There are a few plants which can trap insects and digest them. Such plants may be green or of some other colour.
The pitcher-like or jug-like structure is the modified part of leaf. The apex of the leaf forms a lid which can open and close the mouth of the pitcher. Inside the pitcher there are hair which are directed downwards.
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.
Mushrooms are examples of Saprotrophs.
Activity
Take a piece of bread and moisten it with water. Leave it in a moist warm place for 2–3 days or until fluffy patches appear on them.
Observe the patches under a microscope or a magnifying glass.
We will see cotton-like threads spread on the piece of bread.
These organisms are called fungi.
They have a different mode of nutrition.
They absorb the nutrients from the bread.
Fungi also grow on pickles, leather, clothes and other articles that are left in hot and humid weather for long time.
During the rainy season they spoil many things.
The fungal spores are generally present in the air. When they land on wet and warm things they germinate and grow.
Symbiosis
Some organisms live together and share both shelter and nutrients. This relationship is called symbiosis. For example, certain fungi live inside the roots of plants.
The plants provide nutrients to the fungus and, in return, the fungus provides water and certain nutrients.
In organisms called 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.
Replenishment of Nutrients in the Soil
Plants absorb minerals and nutrients from the soil.
So, their amounts in the soil keep on declining.
Usually crop plants absorb a lot of nitrogen and the soil becomes deficient in nitrogen.
These nutrients need to be added from time to time to enrich the soil.
Fertilisers and manures contain nutrients such as nitrogen, phosphorous, potassium (NPK), etc.
Nitrogen gas is available in plenty in the air, but plants cannot use nitrogen directly in the manner they can use carbon dioxide.
Plants need nitrogen in a soluble form, which could be transported from roots to other parts with water.
The bacterium called Rhizobium can take atmospheric nitrogen and convert it into a usable form.
Rhizobium cannot make its own food. So, it often lives in the roots of leguminous plants like gram, peas, moong, and beans, and provides them with nitrogen.
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 because they can reduce the use of nitrogenous fertiliser where leguminous plants are grown. Most of the pulses (dals) are obtained from leguminous plants.
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