A student argues that the Krebs cycle is the main ATP-producing stage of cellular respiration because it runs in the mitochondria along with the electron transport chain. What is wrong with this reasoning?
ANothing — the Krebs cycle does produce most ATP through substrate-level phosphorylation
BThe Krebs cycle produces only 1 GTP per turn; its primary role is generating NADH and FADH₂ that the electron transport chain uses to produce the bulk of ATP
CThe Krebs cycle does not occur in the mitochondria — it occurs in the cytoplasm like glycolysis
DThe Krebs cycle does not produce any energy-storing molecules; all energy is released as CO₂
Per turn, the Krebs cycle produces 1 GTP (equivalent to ATP), 3 NADH, and 1 FADH₂. The single GTP is modest. The real payoff is the 3 NADH and 1 FADH₂ — electron carriers that donate their electrons to the electron transport chain, which uses them to pump protons and drive ATP synthase to produce ~34 of the ~36–38 ATP per glucose. The Krebs cycle is best understood as an electron harvesting machine, not an ATP factory. The CO₂ released is a byproduct of carbon oxidation, not a sign of energy waste — the electrons stripped off with the CO₂ go straight into NADH.
Question 2 Multiple Choice
A cell's oxaloacetate supply drops sharply because oxaloacetate is being redirected to gluconeogenesis. Acetyl-CoA remains plentiful. What happens to the Krebs cycle?
AThe cycle speeds up to compensate, since more acetyl-CoA drives faster turnover
BThe cycle slows or stalls, because oxaloacetate is required at the entry point to accept the acetyl group
CThe cycle continues normally by substituting another 4-carbon acid for oxaloacetate
DAcetyl-CoA accumulates and is directly converted to ATP without entering the cycle
The cycle begins when acetyl-CoA condenses with oxaloacetate to form citrate. Oxaloacetate is not consumed net — it is regenerated at the end of each turn — but if it is siphoned off faster than it is regenerated (e.g., for gluconeogenesis), the cycle loses its acceptor molecule and stalls. No other metabolite can substitute at that step. This regulatory sensitivity connects the Krebs cycle to the broader metabolic network: the cell can slow the cycle when it needs oxaloacetate for biosynthesis, linking energy metabolism to anabolic demands.
Question 3 True / False
The Krebs cycle is the primary site of ATP production in aerobic cellular respiration.
TTrue
FFalse
Answer: False
The Krebs cycle produces only 1 GTP per turn (2 per glucose). The electron transport chain and oxidative phosphorylation produce approximately 34 ATP per glucose — the vast majority of aerobic ATP yield. The Krebs cycle's role is to strip electrons from the acetyl group and load them onto NAD⁺ and FAD, producing NADH and FADH₂. These carriers ferry electrons to the ETC, which generates the proton gradient that drives ATP synthase. The Krebs cycle enables the ETC; it is not itself the primary ATP source.
Question 4 True / False
Oxaloacetate is consumed in each turn of the Krebs cycle and should be replenished from other metabolic sources to keep the cycle running.
TTrue
FFalse
Answer: False
Oxaloacetate is regenerated at the end of each turn when malate is oxidized — it is the last product of the cycle and also the first acceptor in the next turn. In this sense, oxaloacetate acts as a catalyst: it picks up the acetyl group, facilitates its oxidation through the cycle, and is released unchanged in quantity at the end. The cycle is a true cycle precisely because oxaloacetate is not consumed net. What the cycle consumes per turn is one acetyl group (from acetyl-CoA), and what it releases is two CO₂, 3 NADH, 1 FADH₂, and 1 GTP.
Question 5 Short Answer
Why is the Krebs cycle better described as an 'electron harvesting' process than an 'ATP production' process?
Think about your answer, then reveal below.
Model answer: The Krebs cycle completely oxidizes the two-carbon acetyl group, stripping electrons at multiple steps and transferring them to NAD⁺ (forming NADH) and FAD (forming FADH₂). These electron carriers are then oxidized by the electron transport chain, which uses the released energy to pump protons and drive ATP synthase. The cycle itself yields only 1 GTP per turn by substrate-level phosphorylation. The 3 NADH and 1 FADH₂ per turn carry far more energy — they are the primary output. Without the ETC to reoxidize them, the cycle would halt because NAD⁺ and FAD would be depleted. The cycle's function is to load electrons onto carriers, not to make ATP directly.
This framing clarifies why the Krebs cycle is essential even though it produces little ATP directly: it is the upstream stage that captures most of the chemical energy from the acetyl group in a form (reduced electron carriers) that the ETC can convert to ATP with high efficiency. Understanding this prevents the common misconception that CO₂ release represents energy loss — the carbon leaves but the electrons (and their energy) stay in NADH and FADH₂.