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
- This is mainly due to the fact that they are insoluble and will not cause a decrease in the water potential of a cell
- 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
- In order to enter respiratory pathways, lipids must first be broken down into fatty acids
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 |