12.1.3 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

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

Substrate molecules with a greater hydrogen content result in a greater energy release through respiration

Structure of a lipid (triglyceride)