Respiratory Substrates & Quotient
Biology: Cambridge International A Level, topic 12.1.2
Respiratory Substrates and Quotients
A respiratory substrate is a molecule from which energy can be released in order to synthesise ATP. Glucose is the most common substrate for respiration, however, carbohydrates, lipids and proteins can all be used as respiratory substrates.
Different respiratory substrates
- Some cells, such as liver cells, often use lipids and proteins as reactants for respiration.
- Carbon atoms from lipids are detached to form acetyl coenzyme A and are fed directly into the Krebs cycle.
- Proteins are broken down into amino acids, which are deaminated to remove nitrogen before being converted into pyruvate or acetyl coenzyme A.
Energy values of different substrates
- The energy value of a substrate is related to how many hydrogens there are in the molecule, as the hydrogen becomes oxidised and forms water and this is where the majority of energy is released.
- Lipids have more hydrogen by mass than both proteins and carbohydrates and so have higher energy values:
- Lipids - 39 KJ/g.
- Carbohydrates - 16 KJ/g.
- Proteins - 17 KJ/g.
Respiratory quotient
- Respiratory quotient is the ratio of the amount of carbon dioxide produced to oxygen consumed.
- It is calculated by dividing carbon dioxide production by oxygen consumption.
- The respiratory quotient can be used to identify the respiratory substrates that the organism is using when undergoing aerobic respiration:
- Carbohydrates have a respiratory quotient of 1.0.
- Lipids have a respiratory quotient of 0.7.
- Proteins have a respiratory quotient of 0.9.
Test yourself on Respiratory Substrates & Quotient
Which of these statements about respiratory substrates are true?
Which substrate has the highest energy value (39 KJ/g)?