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First teaching 2023

First exams 2025

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Respiratory Substrates (HL) (HL IB Biology)

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Marlene

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Marlene

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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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Marlene

Author: Marlene

Expertise: Biology

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.