Sources of food

Our food provides us various nutrients that are essential for our growth and development. Food we consume may predominantly give us one or more types of nutrients. Thus we need to know sources of food and their associated nutrition value to plan our daily meals.

Oil, ghee, sweets, etc provide us fats.

Milk and other dairy products, meat provides us proteins.

Vegetables and fruits provide us vitamins and minerals.

Cereals, grains and other starchy foods provide us carbohydrates.

Autotrophic Nutrition

In autotrophic mode of nutrition, the organism is able to prepare their own food from simple raw materials like water, carbon dioxide and mineral salts in the presence of sunlight. Chlorophyll present in the chloroplast of green plants are the site of food production. Accordingly all green plants are the examples of this category.

The process by which they synthesize food is known as photosynthesis. Some non green bacteria like sulphur bacteria can use energy which they derive from some chemical reactions occurring in them. With this energy they manufacture their food. This process is called chemosynthesis. Thus the autotrophs include both the photosynthetic and chemosynthetic organisms.

Heterotrophic Nutrition

The organisms which derive their food from others are known as heterotrophic organisms. The ones who depend on other organisms for their food are called consumers.

All animals & human beings are heterotrophic organisms. They consume complex organic foods prepared by autotrophs or producers and break it into simple form to get their nutrients.

Thus the difference between heterotrophs and autotrophs is basically in the mode of production of food. Due to lack of chlorophyll, heterotrophs cannot synthesize their food while autotrophs can perform photosynthesis.


Some organisms absorb nutrients from dead plants and animal materials in the environment. They are known as saprophytes. They secrete extracellular enzymes and degrade organic materials into simple compounds. Hence, they are very important for removing or recycling dead materials in the environment. Saprophytes play an important role in almost all the biogeochemical cycles such as nitrogen cycle, carbon cycle, hydrogen cycle, and mineral cycles.

Most fungi, some bacteria species are saprophytes. They grow on dead organic materials and absorb necessary nutrients while decomposing it. Surprisingly there are plants that are also saprophytes. When saprophytes decompose organic materials, many nutrients are returned back to the soil for the usage of plants and other living organisms.


Some organisms live on or within another living organism and obtain nutrients from them. They are known as parasites and the organism which supplies nutrients is known as the host organism. Parasites feed only on living organisms for their nourishment. Hence, the host organism is affected by the parasitic organism. There are parasitic plants and animals which depend on other living organisms.

Dodder plants are popular as parasites as they do not contain chlorophyll to carry out photosynthesis. These plants grown on other plants and absorb nutrients through the penetrations in their stems via suckers.

Many human diseases are caused by parasites. For example, Malaria is caused by a parasite called Plasmodium


Plants need food but they make it themselves. They do not depend on any other organism for food. This is done using light and the process is called photosynthesis.

Thus photosynthesis is the process by which plants make their own food. “Photo” means light & “synthesis” means putting together.

For a plant to make food, it needs:

  • Carbon dioxide
  • Water
  • Sunlight

Let’s take a look at how these are collected by plants.

  • Carbon dioxide from the air passes through small pores (holes) in the leaves. These pores are called stomata.
  • Water is absorbed by the roots and passes through vessels in the stem on its way to the leaves.
  • Sunlight is absorbed by a green chemical in the leaves.

The Process of Photosynthesis

Photosynthesis takes place in the leaves of plants. The leaves are made up of very small cells. Inside these cells are tiny structures called chloroplasts. Each chloroplast contains a green chemical called chlorophyll which gives leaves their green color.

  • Chlorophyll absorbs the sun’s energy.
  • It is this energy that is used to split water molecules into hydrogen and oxygen.
  • Oxygen is released from the leaves into the atmosphere.
  • Hydrogen and carbon dioxide are used to form glucose or food for plants.

Some of the glucose is used to provide energy for the growth and development of plants while the rest is stored in leaves, roots or fruits for later use by plants.