Sources of Food

The sources of food are broadly classified into two groups - plants and animals. Humans consume everything from fruits, flowers and even the stem of some plants. Leaves and stem like lettuce, celery, etc; roots of some plants like carrots, beetroot, etc. and seeds like wheat, rice, etc. All food come from plants as even the animals depend on plants. Hence, the source of all food in this world are plants, directly or indirectly.

Animal products are used as food directly or indirectly. You might consume the muscles or organs of the animals which are the direct method or rely on its products like egg, milk and dairy products, honey and so on.


All living organisms require food. Nutrition is defined as a process by which living beings obtain food, change food into simple absorbable forms and use it to make substances needed by the body. There are two main modes of nutrition:

  1. Autotrophic Nutrition
  2. Heterotrophic Nutrition

Autotrophic Nutrition

The green plants, algae and certain bacteria manufacture their own food through photosynthesis. They are termed autotrophs and their mode of nutrition autotrophic nutrition. They are the producers of the food chain as all organisms depend for food on them.

Heterotrophic Nutrition

The organisms, which depend on other organisms for their food, are called heterotrophs and their mode of nutrition is heterotrophic nutrition. Heterotrophic nutrition is of various types:

Holozoic Nutrition: Holozoic nutrition includes ingestion, digestion and absorption of food as in Amoeba, frogs and human beings.

Parasitic Nutrition: Those organisms that live on or inside other living organisms, and derive their food from them are called parasites and the nutrition is called parasitic nutrition. Cuscuta or Dodder plant (Amar bel) is a parasite on green plants.

Saprotrophic Nutrition: Organisms that derive their food from dead and decaying organisms are called saprotrophs. Saprotrophs help in cleaning the environment by decomposing the dead and decaying organic matter. For example, Fungi derive nutrition from dead and decaying matter.

Nutrition in Plants - Photosynthesis

Green plants synthesize their food themselves by the process of photosynthesis. Photosynthesis is a biochemical process by which green plants manufacture their own food using carbon dioxide and water as raw materials in the presence of sunlight and chlorophyll. Oxygen is released as a by-product in this process. 

Photosynthesis is the only process by which solar (sun’s) energy is converted into chemical energy.

Carbon dioxide + Water → Glucose + Water + Oxygen

6 CO2 + 12 H2O → C6H12O6 + 6 H2O + 6 O2 ↑


To carry out photosynthesis, plants require as raw materials, carbon dioxide (CO2), water (H2O), light and chlorophyll. Light gives energy for photosynthesis. Photosynthesis takes place in chloroplasts in the cells of leaves. The green colour of plants is due to chlorophyll. Chlorophyll is in the chloroplasts. It can trap light.


Sunlight is absorbed by chlorophyll as solar energy.

Carbon dioxide and water

Carbon dioxide and water are combined in the chloroplast with the help of a number of enzymes to yield sugar which is converted into starch. Oxygen formed during photosynthesis diffuses out into the atmosphere through the stomata.


Photosynthesis occurs in two steps: (i) the light reaction and (ii) the dark reaction. In the light reaction, light is captured by chloroplast. The reaction occurs in the chloroplasts. In the dark reaction glucose is formed. Dark reaction occurs in chloroplasts. The dark reaction and light reaction occur simultaneously.

Glucose is formed in photosynthesis. It is either used up by the cells or is converted and stored in the form of starch. The other end product oxygen is released into the atmosphere. Energy is released during photosynthesis.

Symbiotic Relationship

Some organisms live together and share shelter and nutrients. This is called symbiotic relationship symbiotic relationship. For example, certain fungi live in the roots of trees. The tree provides nutrients to the fungus and, in return, receives help from it to take up water and nutrients from the soil.

In organisms called lichens lichens, a chlorophyll-containing partner, which is an alga, and a fungus live together.

Insectivorous Plants

There are a few plants which can trap insects and digest them.  Such insect-eating plants are called insectivorous plants.

Nutrition in Humans

For healthy growth and development of the body, you need to eat food that provides enough of all essential nutrients. Nutrients are the chemical substances present in the food which nourish the body. Nutrients are broadly divided into three groups:

  1. Energy-yielding nutrients: carbohydrates and fats
  2. Body-building nutrients: proteins
  3. Growth-regulating nutrients: vitamins and minerals

Nutrition in Animals

Herbivores, Carnivores & Omnivores

Animals which eat only plants are called herbivores. Animals which eat only animals are called carnivores. Animals which eat both plants as well as other animals are called omnivores.


The components of food such as carbohydrates are complex substances. These complex substances cannot be utilized as such. So they are broken down into simpler substances. The breakdown of complex components of food into simpler substances is called digestion digestion.

Components of Food

The components of food are called nutrients. The major nutrients in food are carbohydrates, proteins, fats, vitamins and minerals. In addition, food contains dietary fibres and water which are also needed by the body. 


Carbohydrates are the main source of energy. Carbohydrates may be in the form of sugars, starch or cellulose.

  • Sugar: Fruits, milk, sugarcane
  • Starch: Potato, wheat, rice, sweet potato
  • Cellulose (Roughage): Salads and raw vegetables


fats keep the body warm, help in the transport of fat-soluble vitamins. Some common sources of fats are edible oil, ghee, butter, meat and nuts like groundnuts. 

One gram of fat on oxidation given about 37 kilo joules (9 kilocalorie) of energy.


Growth of body tissues is the main function of proteins.


Vitamins are necessary for normal growth, and maintenance of the body, and are required in relatively small amounts. Deficiency of a particular vitamin causes disease. Overdose of certain vitamins, such as vitamins A and D, is harmful. Vitamins may be water-soluble or fat-soluble.

  • Water-soluble: Vitamins B-complex (B1, B2, B4, B12) and C
  • Fat-soluble: Vitamins A, D, E and K


Minerals such as iron, calcium, sodium, potassium, iodine etc.are required by the body in small quantities.

Dietary Fibres

Dietary fibres are also known as roughage. Roughage is mainly provided by plant products. Whole grains and pulses, potatoes, fresh fruits and vegetables are main sources of roughage. Roughage does not provide any nutrient to our body, but is an essential component of our food and adds to its bulk. This helps our body get rid of undigested food.


Water is an important part of diet. It makes 65-70% of the body weight. Water regulates the body temperature, and provides is a medium for biochemical reactions taking place in the body. It also helps in throwing out some wastes from body as urine and sweat.

Deficiency Diseases

Diseases that occur due to lack of nutrients over a long period are called deficiency diseases.

Nutrient Deficiency Disease / Disorder
Vitamin A Loss of Vision, Night Blindness
Vitamin B1 Beri-beri (disease which affects nervous system)
Vitamin B12 Anaemia (deficiency of red blood corpuscles)
Vitamin C Scurvy (disease in which gums swell & bleed)
Vitamin D Rickets (disease which affects bones)
Vitamin E Affects fertility
Vitamin K Excessive bleeding from wounds
Calcium Bone & Tooth Decay
Iodine Goiter
Iron Anaemia