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Electron Transport Chain
Components & Carriers

Master the protein complexes and electron carriers that power oxidative phosphorylation: how \(\text{NADH}\) and \(\text{FADH}_2\) drive the production of ATP.

Biochemistry

What Is the Electron Transport Chain?

The Electron Transport Chain (ETC) is a series of protein complexes and mobile electron carriers embedded in the inner mitochondrial membrane. Electrons flow through these complexes from high-energy donors to the final acceptor, \(\text{O}_2\), releasing energy that is used to pump \(\text{H}^+\) ions and ultimately synthesize ATP.

Location: The Inner Mitochondrial Membrane

Outer Membrane Inner Membrane (Cristae) ETC Complexes I-IV embedded here Intermembrane Space (IMS) Mitochondrial Matrix H+ H+

Meet the Electron Carriers

\(\text{NADH}\)
Delivers 2 electrons to Complex I. Produced in glycolysis, pyruvate oxidation, and the citric acid cycle. Yields ~2.5 ATP per molecule.
\(\text{FADH}_2\)
Delivers 2 electrons to Complex II. Produced in the citric acid cycle. Yields ~1.5 ATP per molecule (bypasses Complex I).
Coenzyme Q
A mobile lipid-soluble carrier (ubiquinone). Shuttles electrons from Complexes I & II to Complex III within the membrane.
Cytochrome c
A small water-soluble protein in the IMS. Ferries electrons from Complex III to Complex IV.

The Four Protein Complexes

Inner Mitochondrial Membrane Intermembrane Space Matrix Complex I NADH Dehydrogenase
\(4\,\text{H}^+\) pumped
 Complex II Succinate DH
\(0\,\text{H}^+\) pumped
 CoQ Complex III Cytochrome bc1
\(4\,\text{H}^+\) pumped
 Cyt c Complex IV Cytochrome c Oxidase
\(2\,\text{H}^+\) pumped
\(\text{O}_2 \rightarrow \text{H}_2\text{O}\)
 NADH
\(\text{FADH}_2\)
 → Direction of electron flow →

Complex I: NADH Dehydrogenase

Complex I is the largest complex in the ETC. It accepts two electrons from \(\text{NADH}\), oxidizing it to \(\text{NAD}^+\). The electrons pass through a flavin mononucleotide (FMN) prosthetic group and a series of iron-sulfur (Fe-S) clusters before being transferred to Coenzyme Q (ubiquinone).

Complex II: Succinate Dehydrogenase

Complex II is unique: it is the only complex that participates in both the citric acid cycle and the ETC. It accepts electrons from \(\text{FADH}_2\) (generated when succinate is oxidized to fumarate) and passes them through Fe-S clusters to Coenzyme Q.

No Proton Pumping
Complex II does not pump \(\text{H}^+\). This is why \(\text{FADH}_2\) produces fewer ATP (~1.5) than \(\text{NADH}\) (~2.5).
Dual Role
Acts as succinate dehydrogenase in the citric acid cycle and as a membrane-bound ETC entry point simultaneously.

Complex III & Cytochrome c

Complex III (cytochrome bc1 complex) accepts electrons from reduced CoQH\(_2\) and transfers them to cytochrome c, a small mobile protein in the intermembrane space. This process involves the Q cycle, which effectively moves protons across the membrane.

1
\(\text{CoQH}_2\) docks at Complex III and donates its 2 electrons.
2
One electron goes through the Rieske Fe-S protein to cytochrome c\(_1\), then to cytochrome c.
3
The other electron is recycled via cytochrome b in the Q cycle to reduce another CoQ.
4
4 \(\text{H}^+\) are pumped per pair of electrons processed through Complex III.

Complex IV: Cytochrome c Oxidase

Complex IV is the terminal oxidase of the ETC. It accepts electrons from cytochrome c and transfers them to the final electron acceptor: molecular oxygen (\(\text{O}_2\)). This is why we breathe: oxygen is required to keep the chain flowing.

\(\text{NADH}\) vs \(\text{FADH}_2\): Key Differences

\(\text{NADH}\) \(\text{FADH}_2\)
Entry Point Complex I Complex II
Electrons Donated 2 2
\(\text{H}^+\) Pumped 10 (4+4+2) 6 (0+4+2)
ATP Yield ~2.5 ATP ~1.5 ATP
Reduction Potential \(E^\circ\prime = -0.32\text{ V}\) \(E^\circ\prime = -0.22\text{ V}\)
Source Pathways Glycolysis, Pyruvate Ox., TCA TCA Cycle (Step 6)

Energy Flow: Redox Potential Drives Electrons

Reduction Potential (V) -0.32 -0.22 +0.07 +0.22 +0.82 NADH
\(\text{FADH}_2\)
 CoQ Cyt c
\(\text{O}_2\)
 Energy released

The Proton Motive Force & ATP Synthase

The \(\text{H}^+\) gradient created by the ETC is called the proton motive force (PMF). It has two components: a chemical gradient (\(\Delta\text{pH}\)) and an electrical gradient (\(\Delta\psi\)). Protons flow back into the matrix through ATP synthase (Complex V), driving the synthesis of ATP.

1
ETC pumps \(\text{H}^+\): Complexes I, III, and IV pump a total of ~10 \(\text{H}^+\) per NADH into the IMS.
2
Gradient builds: The IMS becomes acidic (low pH, high \([\text{H}^+]\)) and positively charged relative to the matrix.
3
ATP synthase: \(\text{H}^+\) flows through the \(F_0\) channel, rotating the \(F_1\) head, which catalyzes: \(\text{ADP} + P_i \rightarrow \text{ATP}\).
4
Stoichiometry: Approximately 4 \(\text{H}^+\) must pass through ATP synthase to produce 1 ATP.

How to Trace Electrons Through the ETC

1
Identify the electron donor: Is it \(\text{NADH}\) (enters at Complex I) or \(\text{FADH}_2\) (enters at Complex II)?
2
Follow to Coenzyme Q: Both paths converge at CoQ, which becomes reduced to \(\text{CoQH}_2\).
3
Complex III receives electrons from CoQH\(_2\) and passes them to cytochrome c via the Q cycle.
4
Complex IV receives electrons from cytochrome c and transfers them to \(\text{O}_2\), forming \(\text{H}_2\text{O}\).
5
Count \(\text{H}^+\) pumped: Add up protons from each complex the electron passes through. More pumping = more ATP.

Example 1: Tracing \(\text{NADH}\) Electrons

Problem: How many \(\text{H}^+\) ions are pumped per \(\text{NADH}\), and how much ATP does this yield?

1
Complex I: \(\text{NADH} \rightarrow \text{NAD}^+ + 2e^-\). Pumps 4 \(\text{H}^+\).
2
CoQ shuttle: Electrons travel via \(\text{CoQH}_2\) to Complex III.
3
Complex III: Q cycle transfers electrons to Cyt c. Pumps 4 \(\text{H}^+\).
4
Complex IV: Electrons to \(\text{O}_2 \rightarrow \text{H}_2\text{O}\). Pumps 2 \(\text{H}^+\).

Example 2: Tracing \(\text{FADH}_2\) Electrons

Problem: Why does \(\text{FADH}_2\) yield less ATP than \(\text{NADH}\)?

1
Complex II: \(\text{FADH}_2 \rightarrow \text{FAD} + 2e^-\). 0 \(\text{H}^+\) pumped (no proton pumping here!).
2
CoQ shuttle: Electrons move to Complex III (same as NADH path from here).
3
Complex III: Pumps 4 \(\text{H}^+\).
4
Complex IV: Pumps 2 \(\text{H}^+\). Electrons to \(\text{O}_2\).

Example 3: Total ATP from One Glucose

Problem: One glucose molecule produces 10 NADH and 2 FADH\(_2\) through aerobic respiration. How much ATP does the ETC generate?

Carrier Quantity ATP Each Subtotal
\(\text{NADH}\) 10 ~2.5 ~25 ATP
\(\text{FADH}_2\) 2 ~1.5 ~3 ATP
ETC Total ~28 ATP

Common Mistakes to Avoid

✕ Confusing Entry Points
NADH enters at Complex I, FADH\(_2\) enters at Complex II. Mixing them up changes the entire \(\text{H}^+\) count.
✕ Thinking O\(_2\) is at Complex I
Oxygen is the final electron acceptor at Complex IV, not at the beginning of the chain.
✕ Complex II Pumps \(\text{H}^+\)
Complex II does not pump protons. This is the key reason FADH\(_2\) yields less ATP than NADH.

ETC Summary at a Glance

Component Electron Source Electron Acceptor \(\text{H}^+\) Pumped
Complex I NADH CoQ 4
Complex II FADH\(_2\) CoQ 0
CoQ Complex I/II Complex III --
Complex III CoQH\(_2\) Cyt c 4
Cyt c Complex III Complex IV --
Complex IV Cyt c O\(_2\) 2
ATP Synthase H+ gradient ADP + P\(_i\) Uses ~4 \(\text{H}^+\)/\(\text{ATP}\)

Question 1 of 5

Which complex in the ETC accepts electrons directly from \(\text{NADH}\)?

Question 2 of 5

Why does \(\text{FADH}_2\) yield fewer ATP than \(\text{NADH}\)?

Question 3 of 5

What is the final electron acceptor in the electron transport chain?

Question 4 of 5

Which mobile electron carrier shuttles electrons from Complex I and Complex II to Complex III?

Question 5 of 5

Approximately how many \(\text{H}^+\) ions are pumped into the intermembrane space per \(\text{NADH}\) molecule oxidized?