Question

which substance below is not formed during the krebs cycle? image with atp (a), h₂o (b), nadh (c) options: atp, water, nadh, both a and b

Explanation:

1. Recall the products of the Krebs Cycle (Citric Acid Cycle):

• During the Krebs Cycle, ATP (via substrate - level phosphorylation), NADH (and FADH₂) are produced.
• Water ($\ce{H_2O}$) is not a direct product of the Krebs Cycle. The majority of water in cellular respiration is produced in the electron transport chain when oxygen accepts electrons and protons.

1. Analyze each option:

• Option A (ATP): ATP is formed during the Krebs Cycle (e.g., when succinyl - CoA is converted to succinate, GTP (which is equivalent to ATP in terms of energy transfer in the cell) is produced and can be converted to ATP).
• Option B (Water): As explained, water is not formed during the Krebs Cycle.
• Option C (NADH): NADH is formed during the Krebs Cycle as NAD⁺ is reduced when isocitrate is oxidized to α - ketoglutarate, and other steps.
• Option “Both A and B”: Since ATP is formed in the Krebs Cycle, this option is incorrect.

Answer:

Water (the option corresponding to $\ce{H_2O}$, which is the option labeled as "Water" or the option with $\ce{H_2O}$ in the diagram, i.e., the option related to B in the diagram)