Respiratory substrates

Respiratory substrates are organic molecules that living cells break down in a series of enzyme-controlled steps to release chemical potential energy, which is then used to synthesize ATP (adenosine triphosphate). This process is known as respiration. While glucose is the primary or "essential" respiratory substrate for most cells, other complex organic molecules can also be used.

Types of Respiratory Substrates and Their Entry into Respiration: All carbohydrates, lipids, and proteins can serve as respiratory substrates.

• Carbohydrates (e.g., Glucose):

  • Glucose is generally the main organic molecule used.

  • It undergoes glycolysis in the cytoplasm, where it is split into two molecules of pyruvate. Glycolysis produces a net gain of 2 ATP and reduced NAD per glucose molecule.

  • If oxygen is available (aerobic respiration), pyruvate is then actively transported into the mitochondrial matrix.

• Lipids (Fats):

  • Lipids are commonly used as respiratory substrates.

  • They are first broken down into fatty acids and glycerol.

  • Glycerol can be converted into triose phosphate, which enters glycolysis.

  • Fatty acids are broken down (cut up by enzyme action) into 2-carbon fragments (acetyl units). These fragments are then fed into the Krebs cycle via coenzyme A (acetyl CoA).

  • Vertebrate muscle, including heart muscle, is well adapted to respire fatty acids.

• Proteins:

  • Proteins can also be used as respiratory substrates, especially by carnivorous animals.

  • They are first hydrolysed into amino acids.

  • Individual amino acids undergo deamination, where the amino group (–NH2) is split off and excreted (as ammonia or urea).

  • The remaining carbon compound (organic acid) then enters the respiratory pathway as pyruvic acid (pyruvate), acetyl coenzyme A, or as a Krebs cycle acid.

  • Plants rarely respire proteins.

Energy Values of Respiratory Substrates: The energy value of respiratory substrates is typically expressed in kilojoules (kJ) per 100g or per gram.

• Lipids have a significantly higher energy density (kJ/g) than carbohydrates or proteins.

  • Typical values: 3700–4000 kJ per 100g or 39.4 kJ/g.

• Carbohydrates are intermediate.

  • Typical values: 1600–1760 kJ per 100g or 15.8 kJ/g.

• Proteins are also intermediate.

  • Typical values: 1700–1720 kJ per 100g or 17.0 kJ/g.

This difference in energy value is because most of the energy released in aerobic respiration comes from the oxidation of hydrogen to water when reduced NAD and FAD pass electrons to the electron transport chain. Therefore, the greater the proportion of hydrogen atoms in a substrate molecule, the more energy it can provide. Lipids have a greater proportion of hydrogen relative to oxygen, making them more highly reduced than sugars.

Respiratory Quotient (RQ): The Respiratory Quotient (RQ) is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed in a given time during respiration. Calculating the RQ can indicate which substrate is being used.

• Carbohydrates: RQ = 1.0 (e.g., C6H12O6 + 6O2 → 6CO2 + 6H2O).

• Lipids: RQ = 0.7 (e.g., 2C57H110O6 + 163O2 → 114CO2 + 110H2O). More oxygen is required for fat respiration because fats are more highly reduced.

• Proteins: RQ = 0.9.

• Anaerobic Respiration:

  • Alcoholic fermentation (in yeast and plants): RQ is theoretically infinity (∞) because carbon dioxide is produced but no oxygen is consumed. However, if some aerobic respiration occurs, the RQ will be very high (well above 1).

  • Lactate fermentation (in muscle cells): No RQ can be calculated because no carbon dioxide is produced.

In practice, an organism rarely respires a single substrate. Many organisms have an RQ between 0.8–0.9, indicating a mix of carbohydrate and fatty acid respiration.

Specific Substrate Usage:

• While most cells can use various substrates, brain cells (neurones) can primarily use only glucose.

• Heart muscle preferentially uses fatty acids.

• Amino acids are generally used as a last resort, as they have more specialized functions.

• In starvation, proteins become the major respiratory substrate in mammals.

• For germinating seeds, the RQ can change over time, indicating a shift in the primary respiratory substrate (e.g., from lipids to carbohydrates).Respiratory substrates are organic molecules that living cells break down in a series of enzyme-controlled steps to release chemical potential energy, which is then used to synthesize ATP (adenosine triphosphate). This process is known as respiration. While glucose is the primary or "essential" respiratory substrate for most cells, other complex organic molecules can also be used.

Types of Respiratory Substrates and Their Entry into Respiration: All carbohydrates, lipids, and proteins can serve as respiratory substrates.

• Carbohydrates (e.g., Glucose):

  • Glucose is generally the main organic molecule used.

  • It undergoes glycolysis in the cytoplasm, where it is split into two molecules of pyruvate. Glycolysis produces a net gain of 2 ATP and reduced NAD per glucose molecule.

  • If oxygen is available (aerobic respiration), pyruvate is then actively transported into the mitochondrial matrix.

• Lipids (Fats):

  • Lipids are commonly used as respiratory substrates.

  • They are first broken down into fatty acids and glycerol.

  • Glycerol can be converted into triose phosphate, which enters glycolysis.

  • Fatty acids are broken down (cut up by enzyme action) into 2-carbon fragments (acetyl units). These fragments are then fed into the Krebs cycle via coenzyme A (acetyl CoA).

  • Vertebrate muscle, including heart muscle, is well adapted to respire fatty acids.

• Proteins:

  • Proteins can also be used as respiratory substrates, especially by carnivorous animals.

  • They are first hydrolysed into amino acids.

  • Individual amino acids undergo deamination, where the amino group (–NH2) is split off and excreted (as ammonia or urea).

  • The remaining carbon compound (organic acid) then enters the respiratory pathway as pyruvic acid (pyruvate), acetyl coenzyme A, or as a Krebs cycle acid.

  • Plants rarely respire proteins.

Energy Values of Respiratory Substrates: The energy value of respiratory substrates is typically expressed in kilojoules (kJ) per 100g or per gram.

• Lipids have a significantly higher energy density (kJ/g) than carbohydrates or proteins.

  • Typical values: 3700–4000 kJ per 100g or 39.4 kJ/g.

• Carbohydrates are intermediate.

  • Typical values: 1600–1760 kJ per 100g or 15.8 kJ/g.

• Proteins are also intermediate.

  • Typical values: 1700–1720 kJ per 100g or 17.0 kJ/g.

This difference in energy value is because most of the energy released in aerobic respiration comes from the oxidation of hydrogen to water when reduced NAD and FAD pass electrons to the electron transport chain. Therefore, the greater the proportion of hydrogen atoms in a substrate molecule, the more energy it can provide. Lipids have a greater proportion of hydrogen relative to oxygen, making them more highly reduced than sugars.

Respiratory Quotient (RQ): The Respiratory Quotient (RQ) is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed in a given time during respiration. Calculating the RQ can indicate which substrate is being used.

• Carbohydrates: RQ = 1.0 (e.g., C6H12O6 + 6O2 → 6CO2 + 6H2O).

• Lipids: RQ = 0.7 (e.g., 2C57H110O6 + 163O2 → 114CO2 + 110H2O). More oxygen is required for fat respiration because fats are more highly reduced.

• Proteins: RQ = 0.9.

• Anaerobic Respiration:

  • Alcoholic fermentation (in yeast and plants): RQ is theoretically infinity (∞) because carbon dioxide is produced but no oxygen is consumed. However, if some aerobic respiration occurs, the RQ will be very high (well above 1).

  • Lactate fermentation (in muscle cells): No RQ can be calculated because no carbon dioxide is produced.

In practice, an organism rarely respires a single substrate. Many organisms have an RQ between 0.8–0.9, indicating a mix of carbohydrate and fatty acid respiration.

Specific Substrate Usage:

• While most cells can use various substrates, brain cells (neurones) can primarily use only glucose.

• Heart muscle preferentially uses fatty acids.

• Amino acids are generally used as a last resort, as they have more specialized functions.

• In starvation, proteins become the major respiratory substrate in mammals.

• For germinating seeds, the RQ can change over time, indicating a shift in the primary respiratory substrate (e.g., from lipids to carbohydrates).