BIOL0200 Lecture 15 - Aerobic Respiration

83 lecture-focused cards on pyruvate oxidation, the citric acid cycle, electron transport, chemiosmosis, and regulation, with labeled textbook supplements.

What connects glycolysis to the citric acid cycle?

Pyruvate oxidation, the bridge reaction, converts pyruvate to acetyl-CoA in the mitochondrial matrix.

What are bridge-reaction products per glucose?

Two acetyl-CoA, two NADH, and two CO2, with no ATP made directly.

How many carbons does pyruvate have, and how many remain in its acetyl group after oxidation?

Pyruvate has three carbons; one leaves as CO2 and two remain in the acetyl group attached to CoA.

What are the other names of the citric acid cycle?

The Krebs cycle and tricarboxylic acid (TCA) cycle.

Where do most citric acid cycle reactions occur in eukaryotes?

In the mitochondrial matrix; succinate dehydrogenase is associated with the inner mitochondrial membrane.

What four stages organize the citric acid cycle in the lecture?

Citrate generation; redox decarboxylation; substrate-level phosphorylation; redox regeneration of oxaloacetate.

What combines to make citrate at the start of a turn?

A two-carbon acetyl group from acetyl-CoA joins four-carbon oxaloacetate, producing six-carbon citrate and releasing CoA.

Why is this pathway a cycle rather than a linear sequence?

Oxaloacetate is regenerated at the end and can accept another acetyl group.

What does oxidative (redox) decarboxylation mean?

A reaction releases a carbon as CO2 while oxidizing the remaining substrate and reducing an electron carrier.

How does carbon number change during the CO2-releasing reactions of one cycle turn?

A six-carbon intermediate becomes five-carbon, then four-carbon, releasing two CO2 in total.

What is generated by substrate-level phosphorylation in one TCA turn?

One GTP or ATP equivalent, depending on the enzyme form and cell type.

How many NADH, FADH2, GTP equivalents, and CO2 are produced per turn?

Three NADH, one FADH2, one GTP/ATP equivalent, and two CO2.

How many cycle turns occur per glucose, and why?

Two, because one glucose produces two acetyl-CoA.

What are the citric acid cycle totals per glucose?

Six NADH, two FADH2, two GTP/ATP equivalents, and four CO2.

Which step numbers in the eight-step cycle diagram reduce NAD+?

Steps 3, 4, and 8 generate NADH.

Which step in the cycle diagram reduces FAD, and which makes GTP?

Step 6 reduces FAD to FADH2; step 5 makes GTP/ATP by substrate-level phosphorylation.

Which cycle step regenerates oxaloacetate while producing NADH?

Step 8, oxidation of malate to oxaloacetate.

How many CO2 molecules arise per completely oxidized glucose, and where are they released?

Six total: two in pyruvate oxidation and four during the citric acid cycle; none in glycolysis.

After the citric acid cycle, where is most captured usable energy from glucose found?

In reduced electron carriers, especially ten NADH and two FADH2 per glucose, rather than in the four ATP equivalents made directly.

Why have only four ATP equivalents been made directly by the end of glycolysis, pyruvate oxidation, and the cycle?

Glycolysis yields two net ATP and the cycle yields two GTP/ATP equivalents; the bridge reaction yields none. Most energy capture is in electron carriers.

How many NADH per glucose come from each pre-ETC stage?

Glycolysis: 2; pyruvate oxidation: 2; citric acid cycle: 6; total: 10.

How many FADH2 per glucose are generated, and where?

Two, both from the citric acid cycle.

Do glycolysis, pyruvate oxidation, or the TCA cycle directly use O2 as a reactant?

No. Their sustained aerobic operation depends indirectly on O2 allowing electron-carrier regeneration by the ETC.

[Textbook supplement] List the eight main cycle intermediates in order.

Citrate -> isocitrate -> alpha-ketoglutarate -> succinyl-CoA -> succinate -> fumarate -> malate -> oxaloacetate.

[Textbook supplement] Which cycle enzyme is also electron transport complex II?

Succinate dehydrogenase, which oxidizes succinate to fumarate and transfers electrons through FAD toward ubiquinone.

Should carbon accounting imply that an entering acetyl group's labeled carbons necessarily leave as CO2 on its very first turn?

No. Per-turn accounting releases two CO2 while accepting two acetyl carbons, but particular labeled acetyl carbons can persist through later turns.

What two coupled processes make up oxidative phosphorylation?

Electron transport establishes a proton electrochemical gradient; chemiosmosis through ATP synthase uses that gradient to make ATP.

Where are the respiratory ETC and ATP synthase located in mitochondria?

In the inner mitochondrial membrane, including the cristae.

What is the electron transport chain?

A series of carriers that transfer electrons through redox reactions, releasing energy that drives proton transport across the inner membrane.

What supplies electrons to the mitochondrial electron transport chain?

Reduced carriers NADH and FADH2, which are oxidized back to NAD+ and FAD.

What is the final electron acceptor in aerobic respiration?

Oxygen, which accepts electrons and combines with H+ to form water.

Which way does the ETC move protons?

From the mitochondrial matrix into the intermembrane space, against their electrochemical gradient.

Which side of the inner membrane has higher H+ concentration during active electron transport?

The intermembrane space has higher H+ concentration and is relatively positive compared with the matrix.

Which mitochondrial compartment is relatively alkaline during active electron transport?

The matrix, because H+ is pumped out, lowering its H+ concentration relative to the intermembrane space.

What two components give the proton gradient stored potential energy?

A chemical H+ concentration difference and an electrical potential difference across the inner membrane.

Which direction do protons flow through ATP synthase during ATP production?

From the intermembrane space back into the matrix, down their electrochemical gradient.

What directly drives ATP synthase to make ATP?

Energy released by downhill H+ movement through the enzyme, not direct phosphate transfer from NADH.

What are the roles of ATP synthase's F0 and F1 units?

F0 is the membrane-embedded proton-driven rotor; rotation transmitted through the shaft changes F1's catalytic conformations to form ATP.

Is ATP synthase static while making ATP?

No. Proton flow drives rotation and repeated conformational changes that couple gradient energy to ATP synthesis.

Why is an intact, proton-restricting inner membrane necessary for efficient ATP synthesis?

It separates the two compartments so a useful H+ gradient can persist; unrestricted proton leakage dissipates that energy.

Trace energy transfer from glucose to most cellular ATP.

Glucose oxidation -> reduced carriers -> ETC redox reactions -> proton gradient -> ATP synthase -> ATP.

[Textbook supplement] Where do NADH and FADH2 electrons enter the ETC?

NADH electrons enter through complex I; FAD-linked electrons from succinate dehydrogenase enter through complex II and then ubiquinone.

[Textbook supplement] Which ETC complexes contribute to net proton translocation?

Complexes I, III, and IV; complex II does not pump protons.

[Textbook supplement] What are the roles of ubiquinone (Q) and cytochrome c?

Q transfers electrons within the membrane from complexes I/II to III; cytochrome c transfers electrons from III to IV on the intermembrane-space side.

[Textbook clarification] Does the slide's 'NADH transports 3 H+' mean exactly three protons are pumped per NADH?

No. Treat it as a simplified pumping-site comparison. Typical net ETC translocation is about 10 H+ per NADH electron pair and 6 per FADH2 electron pair; NADH uses three proton-translocating complexes and FAD-linked entry bypasses I.

Using the lecture's accounting, how much ATP does one NADH support?

Approximately 2.5 ATP through oxidative phosphorylation.

Using the lecture's accounting, how much ATP does one FADH2 support?

Approximately 1.5 ATP through oxidative phosphorylation.

Why does FADH2 yield less ATP than NADH?

Its electrons enter downstream of complex I, driving less proton translocation and storing less energy in the gradient.

Using lecture yields, how much oxidative ATP comes from ten NADH and two FADH2?

10 x 2.5 + 2 x 1.5 = 25 + 3 = 28 ATP.

What total ATP per glucose should you use for this lecture's respiration calculations?

Approximately 32: 2 net glycolytic ATP + 2 cycle GTP/ATP + 28 oxidative ATP.

How much of the lecture's approximately 32 ATP total is made directly by substrate-level phosphorylation?

Four ATP equivalents: two net from glycolysis and two from the citric acid cycle.

Including later oxidation of its NADH, what is glycolysis's contribution to the lecture's 32-ATP total?

Two net ATP plus 2 NADH x 2.5 = approximately 7 ATP equivalents, using the lecture's accounting.

Including its NADH, what ATP-equivalent contribution does pyruvate oxidation make per glucose?

2 NADH x 2.5 = approximately 5 ATP equivalents; none is made directly in the bridge reaction.

Including its reduced carriers, what ATP-equivalent contribution does the cycle make per glucose?

2 direct + 6 NADH x 2.5 + 2 FADH2 x 1.5 = approximately 20 ATP equivalents.

Using the lecture's convention, how much total ATP is expected from two fully oxidized glucose?

Approximately 64 ATP, assuming the same yield per glucose.

What is the ATP-equivalent yield of one cycle turn after its carriers are oxidized?

1 GTP/ATP + 3 NADH x 2.5 + 1 FADH2 x 1.5 = approximately 10 ATP equivalents.

Calculate oxidative ATP from four NADH and three FADH2 using lecture yields.

4 x 2.5 + 3 x 1.5 = 14.5 ATP equivalents; these are average yield estimates.

[Textbook clarification] Why do the lecture and textbook give different total ATP yields?

Lecture 15 uses approximately 32, while this textbook presents approximately 29 under its accounting. Actual yield varies with transport costs, proton use, leakage, and conditions; use the lecture's stated assumptions for its questions.

What happens first to aerobic electron transport if O2 is removed?

Electron transfer at the end of the chain stops; upstream carriers become reduced, and sustained NADH oxidation and proton pumping cease.

After O2 removal, what happens to the proton gradient and oxidative ATP production?

Existing gradients may support a brief period of ATP production, but they dissipate without continued pumping, so oxidative ATP synthesis falls sharply.

How does removing O2 affect NADH, NAD+, and the citric acid cycle?

NADH tends to accumulate and NAD+ becomes scarce; cycle and pyruvate oxidation rates fall as carrier regeneration fails.

Can glycolysis continue after oxygen removal, and what is required?

Yes, if fermentation or another appropriate pathway regenerates NAD+; glycolysis plus fermentation yields two net ATP per glucose.

Why does fermentation yield only two ATP while the lecture's aerobic pathway yields approximately 32?

Fermentation supports glycolysis but lacks the ETC-driven oxidative ATP production that provides most aerobic ATP.

Does oxygen removal directly convert a TCA enzyme into a fermentation enzyme?

No. Changed carrier and energy conditions alter pathway flux; existing fermentation enzymes can regenerate NAD+ when needed.

[Textbook application] What happens if an uncoupler lets H+ return to the matrix without ATP synthase?

The gradient decreases, ATP production per fuel falls, and more energy is released as heat; electron transport can continue if fuel and O2 are available.

[Textbook supplement] How do uncoupling proteins help brown adipose tissue generate heat?

They allow proton return that bypasses ATP synthesis, dissipating gradient energy as heat.

Predict the initial effects of blocking ATP synthase while the ETC remains intact.

Proton return through ATP synthase stops, ATP synthesis falls, and the gradient initially builds; its backpressure then slows electron transport.

Why can an ETC inhibitor and an uncoupler both reduce ATP while affecting oxygen use differently?

An ETC inhibitor blocks electron flow and reduces O2 consumption; an uncoupler dissipates the gradient and can permit faster ETC flow and O2 use under suitable conditions.

If oxygen is present but complex IV cannot transfer electrons to it, will aerobic respiration remain normal?

No. Electron flow backs up, NADH oxidation and proton pumping decline, and oxidative ATP synthesis fails despite available O2.

[Textbook supplement] What did artificial vesicles containing ATP synthase and a light-driven proton pump demonstrate?

Illumination established a proton gradient that powered ATP synthesis without an ETC, supporting chemiosmosis.

[Textbook application] Could an artificial proton gradient drive ATP synthesis temporarily without oxygen?

Yes, if ATP synthase, ADP, Pi, the proper orientation, and an intact membrane are present; O2 is needed to sustain the normal aerobic ETC, not for ATP synthase's chemical reaction itself.

[Textbook supplement] Where do respiration-capable bacteria and archaea place their ETC and ATP synthase?

In their plasma membrane, since they lack mitochondria.

[Textbook supplement] Is all respiration necessarily aerobic?

No. Some prokaryotes use non-O2 terminal acceptors such as nitrate or sulfate; this is anaerobic respiration and still uses an ETC.

Which metabolites are energy-rich signals in the lecture?

High ATP, NADH, and citrate, which generally reduce fuel oxidation and ATP-production pathway flux.

Which metabolites are energy-poor signals in the lecture?

High ADP, AMP, and NAD+, which generally favor increased fuel oxidation and ATP production when substrates and conditions permit.

How does high ATP generally affect glycolysis and the citric acid cycle?

It favors inhibition at regulatory steps, slowing further ATP production.

How do high ADP and AMP generally affect glycolysis?

They signal ATP demand and promote increased glycolytic activity, including PFK regulation.

How does citrate link mitochondrial metabolism to glycolysis regulation?

Accumulated citrate signals abundant carbon and energy and helps inhibit PFK, reducing further glycolytic input.

Why does high NADH slow oxidative fuel breakdown?

It signals abundant reduced carriers and often inhibits dehydrogenase reactions; a shortage of NAD+ can also limit oxidation.

[Textbook clarification] Does every feedback signal regulate every enzyme through an allosteric site?

No. Allostery is important, but mechanisms differ; the textbook also describes competitive inhibition by NADH and covalent regulation of pyruvate processing.

Predict pathway behavior when exercise raises ATP consumption and O2 remains adequate.

ADP/AMP rise, ATP inhibition weakens, and fuel oxidation and oxidative phosphorylation accelerate to meet demand.

If ATP falls but O2 is absent, can low-energy activation restore normal aerobic ATP output?

No. Regulatory activation cannot replace the missing terminal acceptor; fermentation can support glycolytic ATP production instead.

What happens when oxygen returns to a cell relying heavily on fermentation?

The ETC can reoxidize NADH, carrier availability improves, and pyruvate can be directed toward mitochondrial oxidation as conditions and regulation permit.