Question

The most common respiratory substrate is
(a) Glucose
(b) Sucrose
(c) Maltose
(d) Glycogen

Answer 1

Hint: A respiratory substrate is an organic molecule that is broken down in the body to release energy for the production of ATP. The most common substrate should be a simple molecule and easily available. It should also be a component of multiple metabolic reactions. It could be a molecule that aids in the formation of carbohydrates as they are first broken down to provide energy in the body.

Complete step by step answer:
- A respiratory substrate is a molecule that gets oxidized during respiration. The most common respiratory substrate in the body is glucose.
- One molecule of glucose gives 38 molecules of ATP, hence it is an instant source of energy.
- It is also abundantly available and is stored easily in the body in the form of glycogen. It is also stored in plants in the form of starch or complex sugars.
- Glucose is broken down to pyruvate in glycolysis, one of the first steps in the formation of ATP.
- Other common respiratory substrates include carbohydrates, lipids, proteins, and organic acids.
- Some cells, such as neurons, rely only on glucose for respiration. Other cells in the body can utilize lipids or amino acids.
- Lipids are broken down into fatty acids which are then converted to Acetyl-CoA that enters the Krebs cycle.
- Proteins are hydrolyzed to amino acids that further undergo deamination that removes the amino group or transamination which transfers the amino group to another molecule. The result is converted to Acetyl-CoA which enters the Krebs cycle.
So, the correct answer is ‘(a) Glucose’.

Note:
- The respiratory quotient of a substrate is defined as the ratio of ${ CO }_{ 2 }$ to ${ O }_{ 2 }$ produced/consumed to break down that molecule.
- A respirometer is an instrument to measure the respiratory quotient.
- Energy of a substance is also measured using a calorimeter.