Respiratory Substrates (DP IB Biology)
Revision Note
Respiratory Substrates: Lipids & Carbohydrates
Lipids are an excellent source of energy
When oxidised during respiration, they transfer more than twice the amount of energy per gram as carbohydrates
This is because lipids have less oxygen atoms per molecule than carbohydrates, which makes the hydrogen and carbon atoms in lipid molecules more oxidisable
Lipids are also very good at their role as an energy storage molecule
This is mainly due to the fact that they are insoluble and will not cause a decrease in the water potential of a cell
A decrease in the water potential will cause water from nearby cells to move into the cell by osmosis
Fat stores will allow animals to survive unfavourable conditions, while plants may store oil reserves in seeds and fruits
Lipids can also be used as a source of metabolic water for desert animals
This is because oxidation of lipids will produce much more water than the same amount of carbohydrates
Glycolysis and anaerobic respiration can only occur if carbohydrate is the substrate
In order to enter respiratory pathways, lipids must first be broken down into fatty acids
The fatty acids are then further broken down into 2C acetyl groups
These can then combine with coenzyme A to form acetyl coenzyme A which can enter the Krebs cycle
Comparing Lipids and Carbohydrates Table
Function | Lipids | Carbohydrates |
Energy storage | Higher energy content per gram | Lower energy content per gram |
Source of metabolic water | Oxidation produces higher volume of metabolic water | Oxidation produces lower volume of metabolic water |
Solubility in cells | Insoluble thereby not affecting the osmotic properties of cells | Soluble thereby affecting the osmotic properties of cells |
Ability to be broken down | Hydrolysed less easily so energy is transferred more slowly | Hydrolysed more easily with energy transferred more quickly |
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