Difficulty: Intermediate | Prerequisites: Part 1 (Glycolysis) study notes, understanding of ATP, NADH, pyruvate, and the difference between aerobic and anaerobic processes.
Glycolysis (covered in Part 1) produces only 2 ATP per glucose, which is a small fraction of the energy stored in the molecule. The Krebs cycle and the electron transport chain (ETC) are where the cell extracts the remaining energy, and they take place inside the mitochondria. These two stages are aerobic, meaning they require oxygen. Together with glycolysis, they form the complete pathway of aerobic cellular respiration, which yields up to 36 to 38 ATP per glucose molecule.
The Krebs cycle finishes breaking down the carbon skeleton of glucose, releasing CO₂ and loading up electron carriers (NADH and FADH₂). The electron transport chain then uses those carriers to generate the bulk of the cell's ATP through a process called oxidative phosphorylation. If glycolysis is the opening act, the ETC is the main event.
You should already be comfortable with glycolysis, ATP, NADH, and the idea that pyruvate is the bridge molecule between glycolysis and the Krebs cycle.
The Krebs cycle breaks down pyruvate's carbon skeleton in the mitochondrial matrix, releasing CO₂ and producing NADH, FADH₂, and a small amount of ATP. The electron transport chain, embedded in the inner mitochondrial membrane, then uses those electron carriers to pump protons and drive ATP synthase, producing the vast majority of ATP (up to 34 of the 36 to 38 total). Oxygen is the final electron acceptor, combining with hydrogen to form water.
Krebs cycle (citric acid cycle, TCA cycle)
A series of chemical reactions in the mitochondrial matrix that completes the oxidation of acetyl-CoA (derived from pyruvate). Each turn of the cycle releases 2 CO₂, produces 3 NADH, 1 FADH₂, and 1 ATP (or GTP).
In simple terms, this is where the cell finishes pulling apart what is left of glucose and loads up electron carriers for the next stage.
Acetyl-CoA
The 2-carbon molecule formed when pyruvate (3C) loses one carbon as CO₂ and binds to coenzyme A. It is the entry point into the Krebs cycle.
Think of acetyl-CoA as the ticket that lets pyruvate's remaining carbons into the Krebs cycle.
FADH₂
The reduced form of FAD (flavin adenine dinucleotide). Like NADH, it is an electron carrier that delivers high-energy electrons to the electron transport chain. It produces slightly fewer ATP than NADH (about 1.5 ATP per FADH₂ vs. 2.5 per NADH).
Electron transport chain (ETC)
A series of protein complexes and electron carriers embedded in the inner mitochondrial membrane. Electrons from NADH and FADH₂ are passed along the chain, and the energy released is used to pump H⁺ ions across the membrane, creating a proton gradient.
In simple terms, this is the cell's assembly line for mass-producing ATP.
Oxidative phosphorylation
The process by which ATP is produced using the energy of the proton gradient created by the electron transport chain. It combines the ETC with chemiosmosis.
Chemiosmosis
The movement of H⁺ ions (protons) down their concentration gradient through ATP synthase, which drives the synthesis of ATP from ADP + Pᵢ.
Think of it as water flowing through a turbine: the protons flow through ATP synthase like water through a dam, and the spinning generates ATP.
ATP synthase
The enzyme complex in the inner mitochondrial membrane through which protons flow, driving the conversion of ADP + Pᵢ into ATP. It literally rotates as protons pass through it.
Proton gradient (electrochemical gradient)
The difference in H⁺ concentration across the inner mitochondrial membrane. The intermembrane space has a higher concentration of H⁺ than the matrix. This stored energy (called the proton-motive force) drives ATP synthesis.
Inner mitochondrial membrane
The highly folded membrane (the folds are called cristae) where the electron transport chain and ATP synthase are located. Its large surface area maximises ATP production.
Mitochondrial matrix
The innermost compartment of the mitochondrion, enclosed by the inner membrane. This is where the Krebs cycle takes place and where pyruvate is converted to acetyl-CoA.
Before entering the Krebs cycle, each pyruvate (3C) is converted to acetyl-CoA (2C) in the mitochondrial matrix.
This reaction removes one carbon as CO₂ and transfers electrons to NAD⁺, producing 1 NADH per pyruvate.
Because glycolysis produces 2 pyruvates per glucose, this bridge reaction happens twice per glucose: 2 CO₂ released, 2 NADH produced.
Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). This is why it is also called the citric acid cycle.
Through a series of reactions, 2 carbons are removed as CO₂ (restoring the 4-carbon oxaloacetate for the next turn).
Each turn produces: 3 NADH, 1 FADH₂, 1 ATP (via GTP in some cells).
The cycle regenerates oxaloacetate, so it can run continuously as long as acetyl-CoA is supplied.
Since each glucose yields 2 acetyl-CoA, the Krebs cycle turns twice per glucose molecule.
Total Krebs cycle output per glucose: 6 NADH, 2 FADH₂, 2 ATP, 4 CO₂.
Adding the bridge reaction: 2 NADH (bridge) + 6 NADH (Krebs) = 8 NADH; 2 FADH₂; 2 ATP; 6 CO₂.
Glucose started with 6 carbons.
2 CO₂ released in the bridge reaction + 4 CO₂ released in the Krebs cycle = 6 CO₂ total.
All 6 carbons of glucose have now been fully released as carbon dioxide. The carbon skeleton is completely broken down by the end of the Krebs cycle.
The Krebs cycle itself produces only 2 ATP directly. Its primary role is to load up electron carriers (NADH and FADH₂).
These carriers hold the energy that will be converted into large amounts of ATP in the next stage: the electron transport chain.
The ETC is located in the inner mitochondrial membrane.
The inner membrane is folded into cristae, which increase the surface area available for the ETC and ATP synthase.
NADH and FADH₂ donate their high-energy electrons to the first protein complex of the chain.
Electrons are passed from one complex to the next (Complexes I, II, III, IV), releasing energy at each step.
That energy is used to pump H⁺ ions (protons) from the mitochondrial matrix into the intermembrane space, building up a proton gradient.
At the end of the chain, the electrons are accepted by oxygen (O₂), which combines with H⁺ to form water (H₂O). This is why you need to breathe: oxygen is the final electron acceptor.
The proton gradient across the inner membrane stores potential energy (like water behind a dam).
H⁺ ions flow back into the matrix through ATP synthase, a channel protein that acts like a molecular turbine.
As protons flow through, ATP synthase rotates and catalyses the reaction: ADP + Pᵢ → ATP.
This process is called chemiosmosis, and it is responsible for producing the vast majority of ATP during cellular respiration.
Each NADH produces approximately 2.5 ATP.
Each FADH₂ produces approximately 1.5 ATP (it enters the chain at Complex II, so fewer protons are pumped).
From one glucose molecule, the ETC receives 10 NADH and 2 FADH₂ (totalled from glycolysis, the bridge reaction, and the Krebs cycle). This yields roughly 25 ATP (from NADH) + 3 ATP (from FADH₂) = 28 ATP from the ETC alone.
Oxygen is the final electron acceptor in the ETC. Without it, electrons have nowhere to go, the chain backs up, proton pumping stops, and ATP production via oxidative phosphorylation ceases.
This is why aerobic organisms die without oxygen: the ETC produces the vast majority of their ATP, and it cannot run without a final electron acceptor.
The water you exhale is a by-product of oxygen accepting electrons and combining with H⁺ at the end of the chain.
Overall equation for aerobic cellular respiration:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~36 to 38 ATP
Complete ATP accounting per glucose molecule:
Stage | ATP (direct) | NADH | FADH₂ | CO₂ |
|---|---|---|---|---|
Glycolysis | 2 | 2 | 0 | 0 |
Bridge reaction (x2) | 0 | 2 | 0 | 2 |
Krebs cycle (x2) | 2 | 6 | 2 | 4 |
Totals before ETC | 4 | 10 | 2 | 6 |
ATP from electron carriers (at the ETC):
Carrier | Count | ATP per carrier | ATP subtotal |
|---|---|---|---|
NADH | 10 | ~2.5 | ~25 |
FADH₂ | 2 | ~1.5 | ~3 |
ETC total | ~28 |
Grand total: 4 (direct) + ~28 (ETC) = ~32 ATP. The commonly cited figure of 36 to 38 ATP uses older estimates of 3 ATP per NADH and 2 ATP per FADH₂. Exam answers vary by textbook, so check which figure your course uses.
The reason you breathe is to supply oxygen to the electron transport chain. Oxygen is the final electron acceptor. Without it, the ETC stops, ATP production plummets, and cells begin to die within minutes (brain cells are especially vulnerable).
Cyanide poisoning works by blocking Complex IV of the electron transport chain. Electrons cannot pass to oxygen, the proton gradient collapses, and ATP synthase stops. This is why cyanide is so rapidly lethal: it shuts down the cell's primary energy source.
Students often think the Krebs cycle produces most of the ATP. It does not. The Krebs cycle produces only 2 ATP per glucose directly. Its main job is to generate NADH and FADH₂, which carry energy to the ETC.
Students often confuse where the Krebs cycle and the ETC happen. The Krebs cycle occurs in the mitochondrial matrix. The ETC is in the inner mitochondrial membrane. These are two distinct locations within the same organelle.
Students sometimes think oxygen is used in the Krebs cycle. It is not used directly there. Oxygen is consumed only at the end of the electron transport chain, as the final electron acceptor. However, the Krebs cycle is considered aerobic because it cannot run without the ETC (which needs oxygen) to recycle NADH back to NAD⁺.
Students often forget that CO₂ is released during the Krebs cycle (and the bridge reaction), not during the ETC. The carbon atoms of glucose leave as CO₂ before the ETC even begins.
⚠️ Know the total ATP yield from all three stages combined. Be able to break it down by stage (glycolysis, bridge reaction, Krebs cycle, ETC) and state which stage produces the most ATP.
⚠️ Be able to trace the path of carbon through the entire process: glucose (6C) → 2 pyruvate (3C each) → 2 acetyl-CoA (2C each) + 2 CO₂ → 4 CO₂ from the Krebs cycle. Total: 6 CO₂ = all 6 original carbons accounted for.
⚠️ Know the role of oxygen as the final electron acceptor. Exams love to ask what happens if oxygen is removed (the ETC stops, ATP production drops to only 2 ATP from glycolysis, the cell switches to fermentation).
⚠️ Understand the difference between substrate-level phosphorylation (glycolysis and Krebs cycle) and oxidative phosphorylation (ETC + chemiosmosis).
⚠️ Be able to explain the proton gradient and how ATP synthase uses it. Diagram questions commonly ask you to label the inner membrane, matrix, intermembrane space, and the direction of proton flow.
True or False: The Krebs cycle takes place in the inner mitochondrial membrane.
Fill in the blank: The final electron acceptor in the electron transport chain is ______.
True or False: FADH₂ produces the same amount of ATP as NADH in the electron transport chain.
Fill in the blank: All 6 carbon atoms of glucose are released as ______ by the end of the Krebs cycle.
True or False: ATP synthase uses the proton gradient to convert ADP into ATP.
Answers: 1. False (it takes place in the mitochondrial matrix), 2. Oxygen (O₂), 3. False (FADH₂ produces ~1.5 ATP vs. ~2.5 for NADH), 4. CO₂ (carbon dioxide), 5. True
Q: According to the Krebs cycle diagram, what happens to pyruvic acid during the Krebs cycle?
A: Pyruvic acid (3C) first loses one carbon as CO₂ and is converted to acetyl-CoA (2C) in the bridge reaction. Acetyl-CoA then enters the Krebs cycle, where the remaining 2 carbons are released as CO₂. The energy from these reactions is captured in NADH, FADH₂, and ATP.
Q: Where does the Krebs cycle occur, and what structures are involved?
A: The Krebs cycle occurs in the mitochondrial matrix, the innermost compartment of the mitochondrion (enclosed by the inner mitochondrial membrane). The diagram shows the outer mitochondrial membrane, inner mitochondrial membrane, and the matrix where NAD⁺, FAD, and ADP are converted to NADH, FADH₂, and ATP.
Q: Why is the electron transport chain considered the most important stage for ATP production?
A: The ETC produces roughly 28 of the total ~32 ATP per glucose (or 34 of 36 to 38 using older textbook estimates). Glycolysis and the Krebs cycle together produce only 4 ATP directly. The ETC converts the energy stored in 10 NADH and 2 FADH₂ into ATP via oxidative phosphorylation.
Q: What would happen to a cell if oxygen were suddenly removed?
A: The electron transport chain would stop because oxygen is the final electron acceptor. Without the ETC, NADH and FADH₂ cannot be recycled to NAD⁺ and FAD, so the Krebs cycle would also stop. The cell would be left with only glycolysis (2 ATP per glucose) and would need to use fermentation to regenerate NAD⁺.
Q: Explain the relationship between the proton gradient and ATP synthesis.
A: The ETC uses energy from electron transfers to pump H⁺ ions from the matrix into the intermembrane space, creating a high concentration of H⁺ outside the matrix. These protons flow back through ATP synthase (down the gradient), and the energy of this flow drives the enzyme to phosphorylate ADP into ATP. This coupling of the proton gradient to ATP production is called chemiosmosis.
The Krebs cycle and ETC connect back to glycolysis: without glycolysis producing pyruvate, the Krebs cycle has no fuel. And without the ETC recycling NAD⁺, both glycolysis and the Krebs cycle stall. These three stages are deeply interdependent.
This topic also connects to photosynthesis. The CO₂ released by cellular respiration is the same CO₂ that plants fix during the Calvin cycle. The O₂ released by photosynthesis is the same O₂ consumed by the ETC. Together, photosynthesis and cellular respiration form the global carbon and energy cycle.
The electron transport chain and chemiosmosis also appear in photosynthesis (the light reactions use a similar proton gradient across the thylakoid membrane). Understanding one makes the other far easier to learn.
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