Cellular respiration

Releasing energy

Respiration releases energy – it is an exothermic process.

Photosynthesis is the process by which plants, and some bacteria, synthesise food molecules – which they then use, in addition to other things, for respiration. The process of photosynthesis requires energy – it is endothermic.

Don't confuse respiration with breathing, which is ventilation. Respiration happens in cells.

Why organisms need energy

All organisms need energy to live. This energy is used:

• to drive the chemical reactions needed to keep organisms alive – the reactions to build complex carbohydrates, proteins and lipids from the products of photosynthesis in plants, and the products of digestion in animals, require energy
• movement – in animals, energy is needed to make muscles contract, while in plants, it is needed for transport of substances in the phloem

Respiration is only around 40 per cent efficient. As animals respire, heat is also released. In birds and mammals, this heat is distributed around the body by the blood. It keeps these animals warm and helps to keep a constant internal temperature.

Energy is also used:

• for cell division
• to maintain constant conditions in cells and the body – homeostasis
• to move molecules against concentration gradients in active transport
• for the transmission of nerve impulses