People sometimes call the process of breathing 'respiration' but that is inaccurate. Breathing in animals just gets oxygen air into lungs, gas exchange gets oxygen into the blood.
Respiration also occurs in plants, which don't 'breathe'. So what is respiration?
Respiration is the reason that we need oxygen.
Respiration is the process of releasing energy from food. You could think of it as burning the fuel, glucose.
Glucose is the key molecule. It is made by plants in photosynthesis. In animals, although lots of different molecules are absorbed after digestion they are usually turned into glucose. In animals glucose is especially important as two organs can only use glucose as a fuel.
Respiration can be summarized with this equation:
The series of reactions are controlled carefully by a number of enzymes so that the energy released doesn't damage the cells. The energy is stored forlater use by making a special 'energy-rich' molecule called Adenosine TriPhosphate (ATP).
The equation shown above involves oxygen. This form of respiration is known as aerobic respiration. Remember that you breathe when you do your aerobics - a lot usually!
However sometimes plants or animals need to get energy from their food when there is not enough oxygen. Then they use another form of respiration, called anaerobic respiration. (Here 'an-' means 'without'). We will look at that later.