Glucose is the chief respiratory substrate which fields maximum number of ATP molecules. Glucose is the most common substate in glycolysis. Any other carbohydrate is first converted into glucose. During glycolysis it changes to pyruvic acid and net gain is of 2 ATP and 2 NADH2 molecules. And later on during Krebs cycle 30 molecules of ATP are produced. So a total of 38 ATP molecules are produced from 1 mol of glucose during aerobic respiration.

Introduction
The respiratory substrate that yields the maximum number of ATP molecules is crucial for understanding energy metabolism in living organisms.
Respiratory Substrates Overview
- Glycogen: A polysaccharide that serves as a form of energy storage in animals. It is broken down into glucose, which then enters the glycolysis pathway.
- Amylase: This is an enzyme that breaks down starch into sugars but is not a respiratory substrate itself.
- Ketogenic Amino Acids: These amino acids can be converted into ketone bodies and used for energy. However, they yield less ATP compared to carbohydrates.
- Glucose: The primary energy source for cells, it undergoes glycolysis, the Krebs cycle, and oxidative phosphorylation to produce ATP.
ATP Yield Comparison
- Glycogen:
- Yields approximately 36-38 ATP molecules per glucose unit after conversion.
- Amylase:
- Not applicable as it is not a substrate.
- Ketogenic Amino Acids:
- Produce fewer ATP molecules, typically around 16 ATP per acetyl-CoA.
- Glucose:
- Yields about 36-38 ATP molecules through complete oxidation.
Conclusion
Based on the ATP yield:
- Glucose and glycogen yield the maximum number of ATP molecules (36-38 ATP). However, since glycogen must first be converted to glucose, glucose is the most efficient respiratory substrate overall.
- Therefore, the answer is (4) glucose, as it is the most efficient substrate for ATP production in cellular respiration.

4)glucose