Respiration is an essential process by which all living organisms release energy . Organisms respire aerobically when oxygen is available and anaerobically when it is lacking.
Respiration is the process by which all living cells continuously release energy from glucose .
Animals get their glucose from digested food, plants make their own glucose in photosynthesis.
Respiration is an exothermic reaction because it releases energy .
Energy is required for many living processes :
The type of respiration a cell does is dependent on the amount of oxygen it receives.
GCSE Combined Science Foundation Combined Science Higher Biology Foundation Biology Higher AQAWhen cells receive sufficient oxygen , they will respire aerobically .
Glucose and oxygen are converted into carbon dioxide and water , which releases energy .
Glucose + Oxygen ————> Carbon dioxide + Water (+ energy)
Aerobic respiration is the most efficient way of releasing energy and takes place in the mitochondria.
GCSE Combined Science Foundation Combined Science Higher Biology Foundation Biology Higher AQA 
When cells do not receive enough oxygen , they respire anaerobically ( without oxygen ).
The glucose is not fully oxidised so it is much less efficient than aerobic respiration and makes different products .
Animals
In animals, glucose is broken down into lactic acid if there is no oxygen .
Glucose ————> Lactic acid (+ energy)
This often occurs in muscle cells during vigorous exercise as the body cannot supply them with enough oxygen .
Plants and Yeast
In plants and yeast cells, glucose is broken down into ethanol and carbon dioxide .
Glucose ————> Ethanol + Carbon dioxide (+ energy)
Anaerobic respiration in yeast cells is called fermentation and is a very important process in the production of food and drinks:
