5.7.8- energy values of different respiratory substrates

What are respiratory substrates?

An organic substance that can be oxidised by respiration, releasing energy to make molecules of ATP

Carbohydrates

Glucose is the chief respiratory substrate. Some mammalian cells such as brain cells and red blood cells can use only glucose for respiration. Animals and some bacteria store carbohydrate as glycogen, which can be hydrolysed to glucose for respiration. Plant cells store carbohydrate as starch, and this can also be hydrolysed to glucose for respiration.

• disaccharides can be digested to monosaccharides

• Monosaccharides such as fructose and galactose can be changed by isomerase enzymes to glucose for respiration

Lipids

Triglycerides are hydrolysed by lipase to glycerol and fatty acids. Glycerol can then be converted to triose phosphate and respired.

1. With the aid of some energy from the hydrolysis of one molecule of ATP to AMP, each fatty acid is combined with coenzyme A

2. The fatty acid-CoA complex is transported into the matrix, where it is broken down into two-carbon acetyl groups, each attached to CoA

3. This beta-oxidation pathway generates reduced NAD and FADa

4. The acetyl groups are released from CoA and enter the Krebs cycle by combining with the four-carbon oxaloacetate

   • for every acetyl group oxidised in the Krebs cycle, three molecules of reduced NAD, one molecule of reduced FAD, and one molecule of ATP, by substrate-level phosphorylation are made

Proteins

• excess amino acids are deaminated in the liver

• Deamination of an amino acid involves removal of the amine group and its subsequent conversion to urea

• The rest of the amino acid molecule, keto acid, enters the respiratory pathway as pyruvate, acetyl COA or a Krebs cycle acid such as oxaloacetic acid

• During fasting, starvation or prolonged exercise, when insufficient glucose or lipid are available for respiration, protein from muscle can be hydrolysed to amino acids which are then respired

Energy values of different respiratory substrates

Carbohydrate- 15.8

Lipid- 39.4

Protein- 17.0

Respiratory quotient

RQ = CO2 produced / O2 consumed

RQ values for glucose, fatty acids and amino acids

Glucose- 1

Fatty acids- 0.7

Amino acids- 0.8

What does a respiratory quotient greater than one mean?

Indicates that some anaerobic respiration is taking place because it shows that more carbon dioxide is produced than oxygen consumed