Anything which we eat to live is called food. Food contains energy. The food (or energy) can be transferred from one organism to the other through food chains. The starting point of a food chain is a category of organisms called producers. Producers are, in fact, plants. So, we can say that all the food chains begin with a green plant (or grass) which is the original source of all food. Let us take an example to understand the meaning of food chain.
Suppose there is a field having a lot of green plants (or producers of food). Now, plants can be eaten up by a rat. The rat, in turn, can be eaten up by a cat. And finally, the cat can be eaten up by a dog.
So, we find that there is a sequence (or order) in which one organism eats up the other organism (or consumes the other organism) to fill its belly. The sequence of living organisms in a community, in which one organism consumes another organism to transfer food energy, is called a food chain.
In simple words, a list of organisms (living beings) showing "who eats whom" is called a food chain. Let us make this point more clearly by taking the example of a simple food chain operating in a grassland or forest. In a grassland or forest, there is a lot of grass (which are green plants). This grass is eaten up by animals like deer. And this deer is then consumed (eaten up) by a lion. This simple food chain operating in a grassland or forest can be represented as:
Grass --> Deer -->Lion (Producer) (Herbivore) (Carnivore)
This food chain tells us that grass is the starting point of this food chain. The grass is eaten up by deer and the deer is then eaten up by a lion. In this food chain, grass is the producer organism which uses sunlight energy to prepare food like carbohydrates by the process of photosynthesis.
This grass is then consumed by a herbivore called deer. And the deer is consumed by a carnivore called