Respiration
Respiration releases energy - it is an exothermic process.
Why organisms need energy
All organisms need energy to carry out metabolic processes. This energy is used:
To drive the chemical reactions needed to keep organisms alive. An example is the reactions used to build complex carbohydrates, proteins and lipids from the products of photosynthesis in plants, and the products of digestion in animals.
Movement - in animals, energy is needed to make muscles contract, while in plants, it is needed for phloem transport.
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.
As animals respire, heat is also released. In birds and mammals, this heat is distributed around the body by the blood and helps to maintain a constant internal temperature.
Aerobic respiration
Respiration using oxygen to break down food molecules is called aerobic respiration. Glucose is the molecule normally used for respiration - it is the main respiratory substrate. Glucose is oxidised to release its energy.
The word equation for aerobic respiration is:
glucose + oxygen → carbon dioxide + water (+ energy released)
C6H12O6 + 6O2 → 6CO2 + 6H2O
Respiration is a series of reactions, but this summarises the overall process.
The first stages of respiration occur in the cytoplasm of cells, but most of the energy released is in the mitochondria.
Anaerobic respiration
During anaerobic respiration, the oxidation of glucose is incomplete. The reaction therefore releases much less energy - around a nineteenth of the energy released during aerobic respiration.
In animals
Most organisms cannot respire without oxygen. But some organisms and tissues can continue to respire if the oxygen runs out. These organisms and tissues use the process of anaerobic respiration.
Animal muscles can respire anaerobically for short periods of time - even though the process is relatively inefficient, it's better to continue respiring and be able to run away from danger - or run a race.
The glucose in muscle is converted to lactic acid:
glucose → lactic acid (+ energy released)
In plants and microorganisms
Glucose in yeast cells is converted to carbon dioxide and ethanol, which we refer to simply as 'alcohol'. This reaction is also called fermentation:
glucose → ethanol + carbon dioxide (+ energy released)
Some plants, and some fungi such as yeast can respire anaerobically - it's preferable to release less energy but remain alive.
Anaerobic respiration occurs only in the cytoplasm of cells.