Glucose is an essential fuel for some cells, e.g. brain cells, red blood cells
and lymphocytes, but some cells, e.g. liver cells, also oxidise lipids and excess
amino acids.
The fatty acid components of lipids are important: carbon atoms are
detached in pairs as ACoA and fed into the Krebs cycle.
Amino acids are deaminated and their carbon–hydrogen skeletons
converted into pyruvate or into ACoA.
The energy values of these different substrates are not the same.
Energy values of respiratory substrates
Most of the energy released in respiration comes from the oxidation of hydrogen
to water. The more hydrogens there are (in comparison with carbon or oxygen
atoms) in the structure of a molecule, the greater the energy value. It is
hydrogen atoms that are used to generate ATP via the electron transport chain.
Fatty acids have more hydrogens per unit mass than carbohydrates, so lipids
have a greater energy value per unit mass (lipid provides more than twice as
much energy per gram as carbohydrate or protein). Energy values in kJ g–1 are
determined by burning a known mass of the substance in oxygen in a bomb
calorimeter. Typical energy values are:
