Food is very essential for survival for all living organisms. We take food because it provides us with the energy to perform all the necessary activities. Without energy, a person will feel weak. It also repairs damaged parts, helps in the formation of new cells and helps in the overall growth and development of the body. Plants and animals are the main source of food for all the organisms on earth.

The change in the surrounding which affects an organism or individual is called stimulus. The reaction that an organism such as a plant or animal shows to the stimulus is called a response. Touch, light, smell, heat are some examples of a stimulus. For example, the sunflower plant shows movement in direction of the sun. Here, sunlight is the stimulus and the movement of the sunflower is a response.

The ability of organisms which move from one place to another is called locomotion. These movements include walking, running, jumping, etc. Animals show locomotion in search of food and water. Plants do not show the type of movements that animals exhibit. Only the body parts move. The face of a sunflower turning towards the sun is also an example of plant movement.

The process of removing harmful nitrogenous substances from the body is called excretion. Animals excrete wastes in the form of urine, tears and Carbon dioxide (while respiring). Plants excrete in the form of gums, resins, etc. Kidney helps in removal of waste from the body in the form of urine.