Why are lipids not used a the main source of energy?
1 Answer
Lipids are not used as the main source of energy as these can't be converted easily into substrate for cellular respiration that releases energy.
Explanation:
Glucose is the basic substrate for cellular respiration, that releases energy in the form of ATPs to be used for all metabolic activities.
The carbohydrates are stored as reserved food in the form of starch in plants and glycogen in animals.
Both starch and glycogen can easily be converted into glucose by simple metabolic enzymatic reactions reactions For example starch can easily be hydrolyzed into glucose in presence of water and enzyme starch phosphorylase.
Lipids are much richer source of energy. But still these are not used as the main source of energy as these are not converted into glucose directly, the basic respiratory substrate. The entry of lipid into respiratory metabolism occurs at different levels and involves lengthy metabolic reactions.
A lipid molecule is triglyceride, composed of one glycerol and 3 molecules of long chain fatty acids. Firstly, a lipid molecule is hydrolyzed into glycerol and 3 molecules of fatty acids.
Glycerol enters into glycolysis as glyceraldehyde-3-phosphate.
Each fatty acid molecule is converted into many molecules of
acetyl co-enzyme A, step-wise during beta-oxidation.
Acetyl co-enzyme A enters at the starting level of citric acid cycle, the second step in aerobic respiration.
Thus carbohydrates are used as main source of energy, because these are much easily made available as respiratory substrate in spite of the fact that lipids are much richer source of energy as compared to the carbohydrates. Lipids are used in respiration only when carbohydrates are not available.
