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)
You need to be able to recognise the chemical symbols:
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.
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.
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)
Some plants, and some fungi such as yeast can respire anaerobically - it's preferable to release less energy but remain alive.
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)
Anaerobic respiration occurs only in the cytoplasm of cells.