Cellular Respiration
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AP Biology › Cellular Respiration
In an experiment, isolated mitochondria are supplied with NADH and ADP + Pi. When oxygen is removed from the chamber, electrons can no longer be transferred to the final electron acceptor in the electron transport chain. As a result, proton pumping across the inner mitochondrial membrane decreases, reducing the proton-motive force that drives ATP synthase. The citric acid cycle enzymes remain present, but their ability to regenerate NAD+ depends on electron flow through the chain. No inhibitors of ATP synthase are added. Which outcome is most likely when oxygen is absent?
ATP production is maintained because oxygen directly phosphorylates ADP at the inner mitochondrial membrane
ATP production by oxidative phosphorylation decreases because the proton gradient cannot be maintained without electron flow to oxygen
ATP production is unchanged because substrate-level phosphorylation in the citric acid cycle replaces ATP synthase activity
ATP production increases because oxygen removal shifts mitochondria to fermentation to regenerate NAD+
ATP production increases because electrons accumulate in the electron transport chain and drive additional proton pumping
Explanation
This question assesses the skill of analyzing cellular respiration, specifically the dependence of oxidative phosphorylation on oxygen as the terminal electron acceptor. When oxygen is removed, electrons from NADH cannot be transferred to the electron transport chain's final acceptor, halting electron flow and proton pumping across the inner mitochondrial membrane. This disruption prevents the maintenance of the proton gradient necessary for ATP synthase to produce ATP, leading to decreased ATP production as described in choice A. The stimulus notes that proton pumping decreases and the proton-motive force is reduced, directly supporting that oxidative phosphorylation ceases without electron flow to oxygen. A tempting distractor is choice C, which is wrong because substrate-level phosphorylation in the citric acid cycle cannot fully replace the ATP yield from oxidative phosphorylation and requires NAD+ regeneration dependent on the electron transport chain, reflecting the misconception that the citric acid cycle operates independently of mitochondrial electron transport. For respiration questions, always trace how interruptions in electron flow affect the proton gradient and ATP synthesis to predict outcomes accurately.
Two populations of cells are given equal amounts of glucose. Population X has normal mitochondria; Population Y has mitochondria lacking a functional electron transport chain, but glycolysis enzymes are normal. In both populations, glycolysis produces pyruvate and NADH in the cytosol. In Population X, NADH-derived electrons enter the electron transport chain, supporting proton pumping and ATP synthase activity in the mitochondria. In Population Y, mitochondrial electron transport does not occur, so NADH is not efficiently oxidized by the chain. Which outcome is most likely in Population Y compared with Population X?
Higher ATP yield per glucose because oxygen is no longer needed as the terminal electron acceptor
Unchanged ATP yield per glucose because ATP synthase functions independently of a proton gradient
Higher total ATP yield per glucose because glycolysis produces more ATP than oxidative phosphorylation
Lower total ATP yield per glucose because oxidative phosphorylation cannot occur in mitochondria
Unchanged ATP yield per glucose because oxygen-independent citric acid cycling generates most ATP
Explanation
This question assesses the skill of analyzing cellular respiration, comparing ATP yields with and without functional mitochondrial electron transport. Population Y lacks electron transport, preventing oxidative phosphorylation and limiting ATP to glycolytic output, resulting in a lower total yield per glucose compared to Population X as in choice A. The stimulus highlights NADH supporting proton pumping and ATP synthase in X but not Y, emphasizing aerobic efficiency. Both perform glycolysis, but Y misses mitochondrial ATP. A tempting distractor is choice B, which is incorrect because glycolysis yields only 2 net ATP versus up to 36 aerobically, stemming from the misconception that anaerobic processes are more productive. For respiration questions, compare total ATP by summing contributions from each stage and noting oxygen's role in maximizing yield.
A lab measures oxygen consumption and ATP synthesis in mitochondria supplied with ADP + Pi and a steady source of NADH. When ADP becomes depleted, oxygen consumption drops sharply even though oxygen is still present. Electron transport proteins remain intact, and no inhibitors are added. The inner membrane remains impermeable to protons except through ATP synthase. Because ATP synthase lacks its substrate, protons accumulate outside the matrix, strengthening the gradient and opposing further proton pumping by the electron transport chain. Which explanation best accounts for the decreased oxygen consumption when ADP is depleted?
Without ADP, fermentation begins in mitochondria and replaces oxygen as the terminal electron acceptor
Without ADP, oxygen cannot diffuse into mitochondria because ATP synthase normally transports oxygen
Without ADP, the proton gradient builds up and slows electron transport, reducing oxygen reduction at the end of the chain
Without ADP, the citric acid cycle stops producing CO2, and oxygen consumption requires CO2 release
Without ADP, glycolysis stops in the cytosol and no longer supplies oxygen to mitochondria
Explanation
This question assesses the skill of analyzing cellular respiration, specifically respiratory control by ADP availability. Without ADP, ATP synthase cannot phosphorylate it, causing protons to accumulate in the intermembrane space and build a steep gradient that opposes further pumping, slowing electron transport and oxygen consumption as in choice B. The stimulus indicates the membrane remains impermeable except through ATP synthase, leading to gradient buildup and reduced electron flow. Oxygen is present, but consumption drops due to inhibited chain activity. A tempting distractor is choice A, which is incorrect because glycolysis supplies pyruvate and NADH, not oxygen, to mitochondria, stemming from the misconception that ADP directly influences cytosolic oxygen delivery. For respiration questions, consider how feedback from proton gradients regulates electron transport and oxygen use in response to energy demand.
In a sealed chamber, cells are supplied with glucose and a limited amount of oxygen. As oxygen is consumed, the electron transport chain gradually slows because electrons cannot be transferred to the final electron acceptor. The oxidation of NADH to NAD+ in mitochondria decreases, affecting earlier steps that require NAD+. Assume the cells can use fermentation pathways to regenerate NAD+ in the cytosol when needed. Which change is most likely as oxygen becomes depleted?
Oxygen depletion causes NADH to be converted into ATP directly without a membrane or proton gradient
Total ATP production remains unchanged because oxygen is only required for glycolysis, not mitochondria
ATP from oxidative phosphorylation decreases, while ATP from glycolysis can continue if NAD+ is regenerated by fermentation
ATP from glycolysis increases because oxygen directly inhibits the enzymes of glycolysis under aerobic conditions
ATP from the citric acid cycle increases because the lack of oxygen provides more electron acceptors
Explanation
This question assesses the skill of analyzing cellular respiration, exploring shifts in ATP production under oxygen depletion. As oxygen depletes, the electron transport chain slows, reducing NADH oxidation and thus decreasing ATP from oxidative phosphorylation. However, glycolysis can persist by regenerating NAD+ through fermentation, maintaining some ATP via substrate-level phosphorylation. The stimulus notes that fermentation pathways are available, supporting continued cytosolic ATP production despite mitochondrial decline. A tempting distractor is choice B, which wrongly claims increased glycolytic ATP because oxygen inhibits glycolysis, based on the misconception that aerobic conditions suppress rather than enable efficient glycolysis. A transferable strategy for respiration questions is to distinguish between aerobic and anaerobic pathways, noting how cells adapt NAD+ regeneration to sustain ATP output.
A researcher adds oligomycin, a drug that blocks the proton channel of ATP synthase, to mitochondria actively consuming oxygen. Electrons normally flow from NADH through the electron transport chain to oxygen, and the associated proton pumping builds a gradient. With ATP synthase blocked, protons cannot return to the matrix through the enzyme. Over a short time, the proton gradient becomes very steep, making it harder for the electron transport chain to pump additional protons outward. Which outcome is most likely after oligomycin addition?
Oxygen consumption increases because ATP synthase normally competes with oxygen for electrons
NAD+ levels increase because NADH is oxidized faster when ATP synthase is inhibited
The citric acid cycle stops because acetyl-CoA cannot be produced without ATP synthase activity
ATP production increases because blocking ATP synthase causes more ADP to be phosphorylated elsewhere
Oxygen consumption decreases because electron transport slows when proton pumping becomes energetically unfavorable
Explanation
This question assesses the skill of analyzing cellular respiration, investigating how blocking ATP synthase affects electron transport. Oligomycin prevents protons from flowing through ATP synthase, causing the proton gradient to build excessively in the intermembrane space. This steep gradient makes further proton pumping difficult, slowing electron transport and reducing oxygen consumption as electron flow to oxygen decreases. The stimulus describes the gradient becoming very steep, which supports the backpressure effect on the chain. A tempting distractor is choice B, which incorrectly suggests increased oxygen consumption due to reduced competition, arising from the misconception that ATP synthase diverts electrons from oxygen. A transferable strategy for respiration questions is to consider feedback mechanisms, such as how gradient accumulation regulates electron transport chain activity.
A culture of aerobic cells is shifted from normal oxygen conditions to very low oxygen. Glycolysis in the cytosol converts glucose to pyruvate and produces a small amount of ATP by substrate-level phosphorylation while reducing NAD+ to NADH. Under normal oxygen, NADH is oxidized by the mitochondrial electron transport chain, allowing continued glycolysis. When oxygen becomes limiting, electrons cannot be passed efficiently to the final electron acceptor, and NAD+ regeneration by the electron transport chain slows. Which outcome is most likely in the cells shortly after oxygen drops?
Glycolysis stops immediately because oxygen is a required reactant for the enzymes that split glucose
Oxygen is produced as a byproduct of glycolysis, partially restoring aerobic respiration
Pyruvate enters the citric acid cycle at a higher rate because NAD+ becomes more abundant
ATP production increases because low oxygen directly activates ATP synthase to spin faster
NADH accumulates and ATP production from oxidative phosphorylation decreases due to reduced electron transport activity
Explanation
This question assesses the skill of analyzing cellular respiration, focusing on the effects of oxygen limitation on glycolysis and oxidative phosphorylation. Under low oxygen, the electron transport chain slows because oxygen cannot efficiently accept electrons, leading to NADH accumulation as it is not oxidized back to NAD+. This reduces ATP production from oxidative phosphorylation, as the proton gradient diminishes without sustained electron flow and proton pumping. The stimulus explains that NAD+ regeneration slows, which supports decreased mitochondrial ATP while glycolysis might continue if NAD+ is available. A tempting distractor is choice D, which wrongly states pyruvate enters the citric acid cycle faster due to more NAD+, based on the misconception that low oxygen increases NAD+ when it actually decreases it. A transferable strategy for respiration questions is to consider how oxygen availability impacts electron acceptors and the regeneration of cofactors like NAD+ across pathways.
In an isolated mitochondrion preparation supplied with abundant NADH and ADP + Pi, researchers add cyanide, which blocks electron transfer to oxygen at the end of the electron transport chain. Protons had been pumped into the intermembrane space as electrons moved through complexes, creating a gradient used by ATP synthase. After cyanide addition, electron flow through the chain stops and NADH can no longer be oxidized to NAD+. Which outcome is most likely to occur immediately in this preparation?
ATP production by ATP synthase decreases because the proton gradient is no longer maintained by electron transport
Oxygen consumption increases because cyanide causes oxygen to accept electrons more rapidly at the final complex
ATP production increases because electrons accumulate and drive additional substrate-level phosphorylation in mitochondria
The citric acid cycle speeds up because NAD+ becomes more available when the chain is inhibited
NADH levels decrease because blocked electron flow forces NADH to donate electrons directly to ATP synthase
Explanation
This question assesses the skill of analyzing cellular respiration, specifically how inhibitors disrupt the electron transport chain and ATP synthesis. Cyanide blocks the final step of electron transfer to oxygen, halting electron flow through the chain and preventing proton pumping into the intermembrane space. Without ongoing proton pumping, the existing gradient dissipates as protons leak back or are used without replenishment, leading to decreased ATP production by ATP synthase. The stimulus notes that NADH can no longer be oxidized, which aligns with stopped electron flow, further supporting that oxidative phosphorylation ceases. A tempting distractor is choice B, which incorrectly suggests increased ATP via substrate-level phosphorylation due to electron accumulation, stemming from the misconception that blocked chains shift to alternative ATP pathways in mitochondria. A transferable strategy for respiration questions is to trace the flow of electrons and protons, identifying how disruptions affect the proton gradient and ATP yield.
A cell is placed in an environment with very low oxygen. Glycolysis in the cytosol continues to convert glucose to pyruvate, producing a small amount of ATP by substrate-level phosphorylation and reducing NAD+ to NADH. However, electron transport in mitochondria slows because oxygen is scarce as the final electron acceptor. Without sufficient electron flow, NADH is not efficiently oxidized back to NAD+. The cell begins converting pyruvate to lactate in the cytosol. Which explanation best accounts for lactate production under low oxygen?
Lactate production regenerates NAD+ from NADH, allowing glycolysis to continue producing ATP
Lactate production increases ATP yield by replacing oxidative phosphorylation with extra substrate-level phosphorylation
Lactate production directly transfers electrons to oxygen, restoring electron transport chain activity
Lactate production occurs because pyruvate cannot be formed during glycolysis when oxygen is limited
Lactate production provides oxygen atoms needed to keep the citric acid cycle running in mitochondria
Explanation
This question assesses the skill of analyzing cellular respiration, particularly the shift to fermentation under low oxygen conditions. With scarce oxygen, mitochondrial electron transport slows, limiting NADH oxidation and NAD+ regeneration, but converting pyruvate to lactate in the cytosol reoxidizes NADH to NAD+, enabling glycolysis to continue ATP production as in choice B. The stimulus describes glycolysis producing NADH and the need for NAD+ regeneration, which lactate fermentation provides without relying on oxygen. This logic highlights how anaerobic conditions favor cytosolic pathways to sustain limited ATP yield. A tempting distractor is choice C, which is wrong because lactate production does not increase ATP yield but merely sustains glycolysis' net 2 ATP per glucose, reflecting the misconception that fermentation is more efficient than oxidative phosphorylation. For respiration questions, always identify how cells regenerate NAD+ to maintain glycolytic flux when oxygen is limited.
Cells are grown in a medium containing either glucose or a fatty acid as the primary fuel source, with oxygen available in both treatments. Both fuels can be oxidized to generate NADH and FADH2, which donate electrons to the electron transport chain, where oxygen acts as the terminal electron acceptor. Oxidation of fatty acids generates a larger number of reduced electron carriers per molecule than oxidation of a single glucose molecule. Which outcome is most likely when cells oxidize fatty acids instead of glucose under these conditions?
More ATP is produced because more electrons enter the electron transport chain, increasing proton pumping and oxidative phosphorylation
Less ATP is produced because fatty acids bypass the electron transport chain and rely only on glycolysis
Less ATP is produced because fatty acids cannot be oxidized when oxygen is present in the environment
More ATP is produced only if fermentation occurs, because fatty acids require lactate production to enter mitochondria
Equal ATP is produced because oxygen determines ATP yield, not the number of reduced carriers generated
Explanation
This question assesses the skill of analyzing cellular respiration, comparing ATP yields from different fuel sources under aerobic conditions. Fatty acids produce more NADH and FADH2 per molecule than glucose, leading to more electrons entering the transport chain and increased proton pumping. This enhances the proton gradient, driving greater ATP production via oxidative phosphorylation when oxygen is available. The stimulus highlights that fatty acids generate more reduced carriers, directly correlating with higher ATP output. A tempting distractor is choice B, which falsely states less ATP because fatty acids bypass the chain, based on the misconception that fats rely solely on glycolysis without mitochondrial oxidation. A transferable strategy for respiration questions is to calculate relative yields of reduced cofactors from substrates and link them to electron transport and ATP synthesis efficiency.
During aerobic respiration, electrons removed from organic molecules reduce NAD+ to NADH during glycolysis and the citric acid cycle. NADH then transfers electrons to the electron transport chain in the inner mitochondrial membrane. As electrons move through the chain, energy is used to pump protons from the matrix to the intermembrane space, creating an electrochemical gradient. Oxygen is present and accepts electrons at the end of the chain, forming water. ATP synthase uses the proton gradient to phosphorylate ADP. Which statement best describes oxygen’s role in ATP production?
Oxygen provides the phosphate group that is added to ADP during oxidative phosphorylation
Oxygen binds to ATP synthase and supplies the activation energy needed to form ATP
Oxygen is reduced during glycolysis, which directly generates most of the cell’s ATP
Oxygen accepts electrons at the end of the electron transport chain, allowing continued electron flow and proton pumping
Oxygen is converted to carbon dioxide in the citric acid cycle, releasing energy that makes ATP
Explanation
This question assesses the skill of analyzing cellular respiration, clarifying oxygen's function in the process. Oxygen acts as the terminal electron acceptor in the electron transport chain, enabling continuous electron flow from NADH, proton pumping, and the gradient that powers ATP synthase, as described in choice B. The stimulus outlines electrons moving through the chain to oxygen, forming water and sustaining the process. Without this acceptance, electrons back up, halting respiration. A tempting distractor is choice A, which is incorrect because phosphate comes from Pi, not oxygen, arising from the misconception that oxygen directly participates in phosphorylation rather than facilitating electron flow. For respiration questions, pinpoint the roles of key molecules like oxygen in maintaining the chain's redox reactions and energy transfer.