All living organisms need energy in its usable form (ATP) by their cells to perform their life processes. Organisms harness this energy by breaking down organic molecules (glucose), which store energy in the chemical bonds between them, to yield ATP (energy), water and C02. The freed energy after breaking down glucose is used to power the organism's movements and physiological functioning. It can occur in presence of oxygen (aerobic respiration) or without oxygen (anaerobic respiration).
The function described in (2) above is peculiar to a set of organisms called AUTOTROPHS. They are so called because they have the ability to make their own food in form of organic molecules e.g. glucose. This process is called PHOTOSYNTHESIS. Autotrophs including green plants, are capable of performing this process due to the possession of green pigments called CHLOROPHYLL in their leaves, which they use to capture light energy from the sun. They use this captured energy to power the synthesis of glucose.
