5.7.8

• Respiratory substrates

  besides carbohydrates, lipids + proteins can also provide respiratory substrates

  • they can be oxidised in the presence of oxygen to produce molecules of ATP, carbon dioxide + water
  • they have different relative energy values
• Carbohydrates

  glucose is the chief respiratory substrate

  some mammalian cells can only use glucose for respiration; animals + some bacteria store carbohydrate as glycogen, which can be hydrolysed to glucose for respiration

  plant cells store carbohydrates as starch which can be hydrolysed to glucose for respiration:

  • disaccharide can be digested to monosaccharides for respiration
  • monosaccharides such as fructose + galactose can be changed, by isomerase enzymes, to glucose for respiration
• Lipids

  important respiratory substrate

  triglycerides are hydrolysed by lipase to glycerol + fatty acids ; glycerol is then converted to triose phosphate

  fatty acids are long-chain hydrocarbons with a carboxylic group so there are more carbons and hydrogen atoms than oxygen atoms

  • these molecules are a source of protons for oxidative phosphorylation, so faits produce much more ATP than an equivalent mass of carbohydrate

  
• Lipids 2

  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. this fatty acid-CoA complex is transported into mitochondrial matrix, where it is broken down into two-carbon acetyl groups, each attached to CoA
  3. this beta-oxidation pathway generates reduced NAD + reduced FAD
  4. the acetyl groups are released from CoA + enter the Krebs cycle by combining with the four-carbon oxaloacetate
• Lipids 3

  for every acetyl group oxidised in the Krebs cycle, 3 molecules of reduced NAD, 1 molecule of reduced FAD + 1 molecule of ATP, by substrate-level phosphorylation, are made

  heart muscles can respire fatty acids
• Proteins

  excess amino acids, released after digestion of proteins, are deaminated in the liver

  • deamination involves an amino group removal + its subsequent conversion to urea that is removed via the kidney
  • the rest of the amino acid molecule, a keto acid, enters the respiratory pathway pyruvate, acetyl CoA or a Krebs cycle acid such as oxaloacetic acid

  
• Proteins 2

  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

  → these amino acids may be converted to pyruvate or acetate + enter Krebs cycle

  • fatty acids + amino acids can only be aerobically respired, as they cannot undergo glycolysis; glycerol can be converted to TP, so can enter glycolysis pathway
• Energy values of different respiratory substrates

  • the greater the availability of protons for chemiosmosis, the more ATP can be produced

  -> therefore the more hydrogen atoms there are in a molecule of respiratory substrate, the more ATP can be generated per molecule of substrate

  
• as the protons ((hydrogen ions) ultimately combine with oxygen atoms to form water, the greater the proportion of hydrogen atoms in a molecule, the more oxygen will be needed for its respiration

  Respiratory quotient

  RQ = CO2 produced/O2 consumed

  (it’s a ratio so no units)

  • for carbohydrate, lipid + protein (proteins have values of 0.8-0.9 for the various amino acids)
  • if the RQ value is greater than 1, this indicates that some anaerobic respiration is taking place, because it shows that more carbon dioxide is being produced than oxygen being consumed