Respiratory substrates

Alternative respiratory substrates

• substances other than glucose

• eg lipids or proteins

• can be oxidised by cells to release energy

Lipids as respiratory substrates

• lipids are hydrolysed into glycerol and fatty acids

• glycerol is converted into triose phosphate and enters the glycolysis pathway and this thus converted to pyruvate

• fatty acids are broken down into two carbon fragments via beta oxidation and converted into acetyl coenzyme A, which enters the Krebs cycle

Proteins as respiratory substrates

• proteins are hydrolysed into amino acids

• number of ATP molecules produced depends on R groups of amino acids produced by hydrolysis of the protein

• the amino group is deaminated

  • deamination requires ATP so reduces yield from respiration

• 3C compounds are converted into pyruvate

• 4C and 5C compounds are converted into intermediates in the Krebs cycle

Respiratory quotient (RQ)

• the ratio of the volume of CO2 produced to the volume of oxygen used in respiration

• 

Interpreting RQ

• indicates different metabolic conditions

• an RQ of more than 1 indicates that anaerobic respiration is occurring

• plants - low RQ values might suggest that photosynthesis is occurring as the CO2 released during respiration is being used

• use stoichiometry