Table of Contents
Overview of the Electron Transport Chain
The electron transport chain (ETC) is the final stage of cellular respiration that occurs after glycolysis, pyruvate transport, and the citric acid cycle. Its main roles are:
- to use high‑energy electrons from NADH and FADH$_2$ to pump protons (H$^+$) across a membrane, creating an electrochemical gradient
- to transfer electrons stepwise to molecular oxygen (O$_2$), forming water
- to drive ATP synthesis via oxidative phosphorylation (covered elsewhere)
In eukaryotic cells, the ETC is located in the inner mitochondrial membrane; in prokaryotes, it is in the plasma membrane.
Location and Structural Organization
Mitochondrial Location
In mitochondria, the ETC components are:
- embedded in the inner mitochondrial membrane
- facing:
- the matrix (inside) where NADH and FADH$_2$ are produced by the citric acid cycle
- the intermembrane space (between inner and outer membrane), where protons are pumped
The orientation is crucial: protons are pumped from the matrix into the intermembrane space, building up a gradient that can later be used by ATP synthase.
Main Components
The classical mitochondrial ETC consists of:
- Complex I – NADH:ubiquinone oxidoreductase (NADH dehydrogenase)
- Complex II – Succinate dehydrogenase
- Coenzyme Q (ubiquinone) – mobile lipid electron carrier
- Complex III – Cytochrome bc$_1$ complex
- Cytochrome c – small mobile protein electron carrier
- Complex IV – Cytochrome c oxidase
ATP synthase is functionally coupled to the ETC but is not itself part of the chain; it is dealt with in detail in the context of ATP regeneration.
Redox Principles in the ETC
Electrons move from lower to higher redox potential, that is, from better electron donors to better electron acceptors. This flow is stepwise, not in a single jump:
- NADH and FADH$_2$ donate electrons at high energy levels
- oxygen is the terminal electron acceptor with very high affinity for electrons
- each redox step releases a bit of free energy $\Delta G$, which can be harnessed for proton pumping
Conceptually, this resembles a staircase: each step down in energy is coupled to work (pumping protons) rather than losing all energy as heat at once.
Electron Flow Through the Complexes
Complex I – NADH:Ubiquinone Oxidoreductase
Input and function:
- Substrates: NADH + H$^+$ + oxidized coenzyme Q (Q)
- Products: NAD$^+$ + reduced coenzyme Q (QH$_2$)
Key features:
- oxidizes NADH, transferring two electrons to FMN (flavin mononucleotide) and then through a series of iron–sulfur (Fe–S) centers
- passes electrons finally to ubiquinone (Q), reducing it to ubiquinol (QH$_2$)
- uses the released energy to pump 4 H$^+$ from the matrix to the intermembrane space per NADH oxidized (number may vary slightly between sources, but 4 is the standard value)
Overall reaction (simplified):
$$
\text{NADH} + \text{H}^+ + \text{Q} + 4\ \text{H}^+_{\text{(matrix)}}
\rightarrow \text{NAD}^+ + \text{QH}_2 + 4\ \text{H}^+_{\text{(intermembrane)}}
$$
Complex I therefore links NADH oxidation directly to proton pumping.
Complex II – Succinate Dehydrogenase
Complex II is part of both the citric acid cycle and the ETC.
Input and function:
- Substrates: succinate + FAD (inside the enzyme)
- Products: fumarate + FADH$_2$, then electrons transferred to Q
Key features:
- oxidizes succinate to fumarate (a citric acid cycle reaction)
- reduces FAD to FADH$_2$
- transfers electrons via Fe–S centers to coenzyme Q, forming QH$_2$
- does not pump protons
Because complex II does not contribute directly to the proton gradient, electrons from FADH$_2$ yield less ATP than electrons from NADH.
Coenzyme Q (Ubiquinone)
Coenzyme Q (Q):
- is a small hydrophobic molecule that moves freely in the inner mitochondrial membrane
- accepts 2 electrons and 2 protons, being reduced to ubiquinol (QH$_2$)
- receives electrons from:
- complex I (NADH)
- complex II (succinate)
- and, in some cells, other dehydrogenases (e.g., glycerol‑3‑phosphate dehydrogenase, fatty acyl-CoA dehydrogenase)
- delivers electrons to complex III
Q can exist in three forms:
- oxidized: Q
- semi‑reduced radical: Q$^\cdot{}^-$ (semiquinone)
- fully reduced: QH$_2$
These different forms are central to the Q cycle in complex III.
Complex III – Cytochrome bc$_1$ Complex
Input and function:
- Substrates: QH$_2$ + 2 oxidized cytochrome c (cyt c$_{\text{ox}}$)
- Products: Q + 2 reduced cytochrome c (cyt c$_{\text{red}}$)
Key features:
- contains:
- cytochrome b (with two heme groups)
- cytochrome c$_1$
- an Fe–S center
- catalyzes a mechanism known as the Q cycle that:
- transfers 2 electrons from QH$_2$ to two separate cytochrome c molecules (each carries 1 electron)
- releases 2 H$^+$ from QH$_2$ into the intermembrane space
- takes up 2 H$^+$ from the matrix to regenerate QH$_2$ in part of the cycle
- results effectively in the translocation of 4 H$^+$ per QH$_2$ oxidized (2 pumped + 2 released)
Overall simplified effect:
- 4 H$^+$ moved from the matrix to the intermembrane space per pair of electrons that traverse complex III
- 2 cytochrome c molecules reduced, each carrying 1 electron to complex IV
Cytochrome c
Cytochrome c:
- is a small, soluble heme protein located on the outer surface of the inner mitochondrial membrane (in the intermembrane space)
- carries 1 electron at a time
- shuttles electrons from complex III to complex IV
Because it is mobile, cytochrome c plays a role analogous to coenzyme Q, but in the aqueous intermembrane space rather than in the membrane lipid phase.
Complex IV – Cytochrome c Oxidase
Input and function:
- Substrates: electrons from reduced cytochrome c, O$_2$, and matrix H$^+$
- Product: water (H$_2$O)
Key features:
- accepts electrons from cytochrome c via multiple redox centers:
- cytochromes a and a$_3$
- copper centers (Cu$_A$, Cu$_B$)
- four electrons are required to fully reduce one O$_2$ molecule to two H$_2$O:
$$
\text{O}_2 + 4\ \text{e}^- + 4\ \text{H}^+_{\text{(matrix)}} \rightarrow 2\ \text{H}_2\text{O}
$$
- pumps 2 H$^+$ across the membrane per 2 electrons transferred (numbers in textbooks vary: often 2 H$^+$ per pair of electrons, so 4 H$^+$ per O$_2$ reduced)
- also consumes matrix protons when forming water, which effectively contributes to the proton gradient (protons removed from the matrix side)
Complex IV is where O$_2$ is reduced, making it the place where oxygen is actually used in aerobic cellular respiration.
Proton Gradient and Proton-Motive Force
As electrons move through the chain:
- Complex I pumps 4 H$^+$ per NADH
- Complex III effectively moves about 4 H$^+$ per pair of electrons via the Q cycle
- Complex IV pumps ~2 H$^+$ per pair of electrons (plus consumes matrix protons)
For electrons from:
- NADH: total of about 10 H$^+$ pumped per NADH oxidized (4 + 4 + 2)
- FADH$_2$ (entering via complex II): about 6 H$^+$ pumped (0 + 4 + 2)
The result is a proton gradient across the inner mitochondrial membrane composed of:
- a chemical gradient (difference in [H$^+$], $\Delta \text{pH}$)
- an electrical gradient (matrix is more negative than intermembrane space, $\Delta \psi$)
Together, these form the proton‑motive force, often expressed as:
$$
\Delta p = \Delta \psi - \frac{2.3 RT}{F} \Delta \text{pH}
$$
where:
- $\Delta \psi$ = membrane potential
- $\Delta \text{pH}$ = pH difference across the membrane
- $R$ = gas constant
- $T$ = absolute temperature
- $F$ = Faraday constant
This stored energy is then used by ATP synthase to produce ATP from ADP and P$_i$ (detailed under ATP regeneration/oxidative phosphorylation).
ATP Yield Linked to the ETC
Because of the proton gradient generated by the ETC:
- experimentally, about 4 H$^+$ flowing back through ATP synthase (and other transport processes) are associated with synthesis of 1 ATP
- from NADH (10 H$^+$ pumped) this yields approximately:
- $\approx 10 / 4 \approx 2.5$ ATP per NADH
- from FADH$_2$ (6 H$^+$ pumped) this yields approximately:
- $\approx 6 / 4 \approx 1.5$ ATP per FADH$_2$
These values are P/O ratios (phosphate transferred to ADP per oxygen atom reduced), often rounded in introductory treatments.
Regulation and Control Points
The ETC is not “on” at full speed all the time. Its activity depends strongly on:
- availability of substrates:
- NADH, FADH$_2$
- ADP and inorganic phosphate (P$_i$) indirectly, via ATP synthase
- O$_2$ as the terminal electron acceptor
- energy demand of the cell:
- high ATP levels slow down oxidative phosphorylation (due to lack of ADP)
- when ATP is consumed and ADP rises, proton flow through ATP synthase increases, which:
- reduces the proton gradient
- stimulates the ETC to pump more protons and oxidize more NADH/FADH$_2$
This coupling between electron transport and ATP synthesis is known as respiratory control.
Inhibitors and Uncouplers of the ETC
Certain substances interfere with the ETC or its coupling to ATP synthesis. These are important both biologically (toxins) and experimentally.
Classic Inhibitors
- Complex I inhibitors:
- e.g. rotenone, amytal
- block electron transfer from Fe–S centers to Q
- Complex III inhibitors:
- e.g. antimycin A
- block electron transfer within the bc$_1$ complex
- Complex IV inhibitors:
- e.g. cyanide (CN$^-$), carbon monoxide (CO), azide (N$_3^-$)
- prevent reduction of O$_2$ by binding to cytochrome a$_3$–Cu$_B$
- halt electron flow and quickly stop ATP production; highly toxic
Because electron flow ceases, NADH and FADH$_2$ accumulate, oxidative metabolism stops, and cells die if inhibition persists.
Uncouplers
Uncouplers do not stop electron flow; instead, they dissipate the proton gradient.
- Lipid‑soluble weak acids (e.g. 2,4‑dinitrophenol, DNP) shuttle protons across the membrane:
- protons re‑enter the matrix without going through ATP synthase
- electron transport speeds up (as gradient is reduced)
- energy is released as heat instead of being stored in ATP
In mammals, a physiological uncoupling system exists:
- uncoupling protein 1 (UCP1, thermogenin) in brown adipose tissue allows protons to flow back into the matrix
- generates heat, important for non‑shivering thermogenesis in newborns and hibernating animals
Variants in Prokaryotes and Adaptations
In prokaryotes (bacteria, archaea):
- ETCs are located in the plasma membrane
- components can be more diverse:
- different dehydrogenases feeding electrons into the chain
- alternative terminal oxidases with varying affinity for O$_2$
- sometimes different terminal electron acceptors (e.g. nitrate, sulfate) in anaerobic respiration
- proton gradients can also be used for:
- ATP synthesis
- active transport of nutrients and ions
- rotation of flagella for motility
Some organisms have shorter or branched chains, adjusting ATP yield and efficiency to environmental conditions (e.g. low vs. high O$_2$ availability).
Biological Significance
The ETC:
- is the main generator of ATP in aerobic organisms via oxidative phosphorylation
- allows highly efficient extraction of energy from NADH and FADH$_2$
- maintains redox balance by reoxidizing NADH to NAD$^+$ and FADH$_2$ to FAD, enabling glycolysis and the citric acid cycle to continue
- illustrates how cells couple redox reactions, ion gradients, and mechanical work (ATP synthase rotation) into one coherent energy‑conversion system