Respiratory Substrates (HL) (HL IB Biology)

• 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