Question

How many molecules of pyruvic acid are produced at the end of glycolysis from 206 molecules of glucose?

• A. 412
• B. 206
• C. 309
• D. 103

Hint:

To avoid confusion during respiratory stoichiometry calculations, always rely on conservation of carbon atoms: \[ \text{Total Initial Carbon Atoms} = \text{Total Final Carbon Atoms} \] \[ \text{Number of Glucose molecules} \times 6 = \text{Number of Pyruvate molecules} \times 3 \] This basic arithmetic rule prevents errors under exam pressure!

Answer 1

The Correct Option is A

Concept: Glycolysis (originating from the Greek words glykys meaning sweet and lysis meaning splitting) is also known as the EMP pathway (Embden-Meyerhof-Parnas pathway). It is the foundational sequence of reactions in cellular respiration that occurs universally in the cytoplasm of all living cells. During this anaerobic process, a single molecule of hexose sugar (glucose) undergoes a structured series of ten enzyme-catalyzed steps to break down into a simpler 3-carbon organic acid.

Step 1: Determine the standard stoichiometric ratio of glycolysis
Let us look at the chemical composition and carbon count of the primary reactant and the final product:

• Glucose molecule: Contains a chain of

6 carbon atoms ($\text{C}_6\text{H}_{12}\text{O}_6$).

• Pyruvic acid (Pyruvate) molecule: Contains a chain of

3 carbon atoms ($\text{CH}_3\text{COCOOH}$).
Since no carbon atoms are lost as carbon dioxide ($\text{CO}_2$) during glycolysis, the total carbon pool is conserved from start to finish. Splitting a 6-carbon backbone down into 3-carbon structures yields a definitive stoichiometric output: \[ 1 \text{ molecule of Glucose (6C)} \xrightarrow{\text{Glycolysis}} 2 \text{ molecules of Pyruvic Acid (3C)} \] This gives us a fixed conversion factor of

2 molecules of pyruvic acid per glucose molecule.

Step 2: Calculate pyruvic acid output for 206 glucose molecules
The problem presents a starting pool consisting of exactly 206 molecules of glucose. Using the fixed conversion ratio established in Step 1, we find: \[ \text{Total Pyruvic Acid produced} = (\text{Molecules of Glucose}) \times 2 \] Substituting the value: \[ \text{Total Pyruvic Acid produced} = 206 \times 2 \] Let us carry out the multiplication step-by-step: \[ 200 \times 2 = 400 \] \[ 6 \times 2 = 12 \] \[ 400 + 12 = 412 \] \[ \text{Total Pyruvic Acid produced} = 412 \text{ molecules} \] Thus, the total number of pyruvic acid molecules formed at the end of the glycolytic pathway is exactly 412, corresponding to option (1).