Under normal conditions, oxygen is a colourless gas, constituting almost 21 per cent of the Earth’s atmosphere. Oxygen is an element that is found in a vast number of molecules including water and organic compounds such as amino acids and fats. After we breathe it in, oxygen is used in our bodies for respiration, to release energy from glucose. However, even though oxygen is essential for life, breathing pure oxygen can actually kill you. Atmospheric oxygen is replenished by plants by photosynthesis.