Energy Values of Respiratory Substrates
- Glucose is the principal respiratory substrate for aerobic respiration in most cells
- When the supply of glucose in a cell has been used up, that 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 exhausted
- This is because they often have essential functions elsewhere in the cell
- Amino acids are required to make proteins which have structural (e.g. in the cytoskeleton) and functional (e.g. enzymatic) roles
- When these different substrates are broken down in respiration, they release different amounts of energy
Respiratory substrate table
| Respiratory substrate | Energy value / kJ g-1 |
| Carbohydrate | 15.8 |
| Lipid | 39.4 |
| Protein | 17.0 |
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 a high proportion of hydrogen atoms
- These hydrogen atoms are released when the lipid is broken down
Hydrogen Atom Content and Energy Release Diagram
Substrate molecules with a greater hydrogen content result in a greater energy release through respiration
Lipid Structure Diagram
Structure of a lipid (triglyceride)
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]
Your answer will need to relate to the differing hydrogen concentrations held within those substrates, and the consequent effect on chemiosmosis and energy release.
