Biology · Respiration in Plants · NEET
It is the organic food molecule that a cell oxidises (breaks the C-C bonds of) to release energy. NCERT defines cellular respiration as the breaking of C-C bonds of complex organic molecules by oxidation to release energy. That organic molecule being broken down is the respiratory substrate. It is the input fuel, not the product (CO2, water, ATP are products).
Glucose. NCERT states clearly: 'Glucose is the favoured substrate for respiration.' A common NEET question (NCERT exercise 2) literally asks 'Name the most common respiratory substrate' and the answer is glucose. All carbohydrates are usually first converted into glucose before they are used for respiration.
No. Glucose is only the favoured (default) one. Fats and proteins can also be respired to yield energy. The difference is where they enter the pathway: glucose enters at the very first step (glycolysis), while fats and proteins enter at later intermediary stages, not at the first step.
They do not enter at step one like glucose. Fats are first broken into glycerol and fatty acids. Fatty acids are degraded to acetyl CoA and enter there; glycerol enters after being converted to a glycolysis intermediate. Proteins are broken into amino acids, which enter at different points depending on which amino acid. Study NCERT Figure 12.6 for the exact entry points.
No. NCERT states: 'in living organisms, respiratory substrates are often more than one; pure proteins or fats are never used as respiratory substrates.' A cell uses a mixture. This single line is a favourite NEET fact and an easy statement-based question.
The respiratory substrate is the fuel molecule itself. The Respiratory Quotient (RQ) is a number (CO2 released / O2 used) that tells you which type of substrate is being burned: RQ = 1 for carbohydrates, less than 1 (about 0.7) for fats, about 0.9 for proteins. So RQ is a clue to the substrate, not the substrate itself.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A respiratory substrate is the organic food molecule (like glucose, fat or protein) that a cell oxidises to release energy for ATP synthesis.
Glucose. All carbohydrates are usually first converted to glucose before being respired.
Yes. Proteins are broken into amino acids that enter the respiratory pathway at various intermediate stages, giving an RQ of about 0.9. But pure protein is never used alone.
Fats must first be split into glycerol and fatty acids. Fatty acids become acetyl CoA and glycerol becomes a glycolysis intermediate, so they join the pathway midway, not at glucose's starting step.
No. Other carbohydrates are first converted into glucose, and then that glucose enters glycolysis. Glucose is the common entry point.
