CIE A Level Biology (9700) 2019-2021

Revision Notes

12.1.5 Energy Values of Respiratory Substrates

Energy Values of Respiratory Substrates

  • Glucose is the main respiratory substrate for aerobic respiration in most cells
  • When the supply of glucose in a cell has been used up a cell may continue respiration using other substrates
  • These may be:
    • Other carbohydrates
    • Lipids
    • Proteins
  • Amino acids from proteins are only respired aerobically when all other substrates have been used up
    • This is because they often have essential functions elsewhere in the cell
    • Amino acids are required to make proteins which have structural (eg. in the cytoskeleton) and functional (eg. enzymatic) roles
  • When these different substrates are broken down in respiration, they release different amounts of energy

Respiratory substrate table

Respiratory Substrate Table, downloadable AS & A Level Biology revision notes

Explaining the differences in energy values

  • Lipids have the highest energy value (39.4 kJ g-1) followed by proteins (17.0 kJ g-1) and then carbohydrates (15.8 kJ g-1)
  • The differences in the energy values of substrates can be explained by their molecular composition
    • Specifically how many hydrogen atoms become available when the substrate molecules are broken down
  • During respiration hydrogen atoms play a vital role:
    • The substrate molecules are broken down and the hydrogen atoms become available
    • Hydrogen carrier molecules called NAD and FAD pick them up (become reduced) and transfer them to the inner mitochondrial membrane
    • Reduced NAD and FAD release the hydrogen atoms which split into protons and electrons
    • The protons are pumped across the inner mitochondrial membrane into the intermembrane space – forming a proton / chemiosmotic gradient
    • This proton gradient is used in chemiosmosis to produce ATP
    • After the protons have flowed back into the matrix of the mitochondria via ATP synthase they are oxidised to form water
  • This means that a molecule with a higher hydrogen content will result in a greater proton gradient across the mitochondrial membrane which allows for the formation of more ATP via chemiosmosis
  • Fatty acids in lipids are made up of long hydrocarbon chains with lots of hydrogen atoms. These hydrogen atoms are released when the lipid is broken down


Exam Tip

You may be expected to explain why different respiratory substrates have different energy values. Here’s an example question:

Explain why carbohydrates, lipids and proteins have different relative energy values as substrates in respiration in aerobic conditions. (6 marks)

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Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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