Course 10.2Lesson 2 of 3

The electron transport system

The electron transport system lies in the inner membrane. NADH and FADH2 give electrons to complexes I and II, and the electrons pass through ubiquinone, complex III, cytochrome c and complex IV to oxygen, which becomes water. One NADH gives 3 ATP and one FADH2 gives 2 ATP.

about 25 min1 simulation1 practice question

Lesson 2 of 3Show all lessons

Try the code and the simulations beside the lesson.

A relay of electrons

The NADH and FADH $_2$ made in glycolysis, the link reaction and the Krebs cycle hold electrons. The cell must hand these over, so that NAD $^+$ and FAD are free for the next round. The hand-over runs along a chain of carriers on the inner mitochondrial membrane. This chain is the electron transport system (ETS).

The electrons go downhill, from carrier to carrier, like a runner passing a baton. At the end, the electrons are taken by oxygen.

A diagram of the electron transport system in the inner mitochondrial membrane, drawn as a horizontal band with the intermembrane space above and the matrix below. In the band from left to right: complex I, NADH dehydrogenase; complex II, succinate dehydrogenase; a round carrier Q, ubiquinone; complex III, cytochrome bc1; a round carrier cytochrome c lying just above the band; complex IV, cytochrome c oxidase; and complex V, ATP synthase, with a round F1 knob hanging into the matrix. Blue dashed arrows show electrons: NADH enters complex I from the matrix, succinate enters complex II, both pass electrons to Q, then to complex III, to cytochrome c, to complex IV, and to half an oxygen molecule with two protons, which becomes water. Red arrows show protons pumped up into the intermembrane space at complexes I, III and IV, and two protons flowing back down through ATP synthase for each ATP made from ADP and phosphate.

The five complexes and two carriers

The carriers are grouped into protein complexes in the membrane.

Name	What it does
Complex I, NADH dehydrogenase	takes electrons from NADH and passes them to ubiquinone
Complex II, succinate dehydrogenase	takes electrons from FADH $_2$ (made from succinate) and passes them to ubiquinone
Ubiquinone (Q)	a small carrier that moves inside the membrane, from I and II to III
Complex III, cytochrome bc $_1$	passes electrons from reduced ubiquinone to cytochrome c
Cytochrome c	a small protein on the outer surface of the inner membrane, which moves between III and IV
Complex IV, cytochrome c oxidase	has cytochromes a and a $_3$ and two copper centres, and passes electrons to oxygen
Complex V, ATP synthase	makes ATP. The next lesson is about it.

The order of the electrons is this.

From NADH: complex I, ubiquinone, complex III, cytochrome c, complex IV, oxygen.
From FADH $_2$ : complex II, ubiquinone, complex III, cytochrome c, complex IV, oxygen.

So FADH $_2$ joins the chain after complex I. That is why it gives less ATP.

Oxygen at the end

At complex IV, oxygen takes the electrons and some protons, and becomes water.

\displaystyle \tfrac{1}{2}\,\text{O}_2 + 2\,\text{H}^+ + 2\,e^- \longrightarrow \text{H}_2\text{O}

Oxygen is used only at the last step. Yet its presence is vital, because it drives the whole process. By removing the electrons and hydrogen at the end, it lets every carrier pass its load on, so the NADH and FADH $_2$ can be turned back into NAD $^+$ and FAD. Oxygen is the final hydrogen acceptor.

How much ATP?

The number of ATP depends on the donor of the electrons.

One NADH gives 3 ATP.
One FADH $_2$ gives 2 ATP.

Each carrier gives two electrons, and two electrons reduce half an oxygen molecule to one water. So the ETS makes one water for every NADH and every FADH $_2$ .

You should find that eight NADH and two FADH $_2$ , which come from the link reaction and the Krebs cycle, give 28 ATP and 10 water, and use 5 oxygen molecules.

Try it in Python

The program works out the ATP, the water and the oxygen from the carriers that reach the ETS.

Practise with new numbers

Common mistakes

Putting the ETS in the matrix. It is on the inner membrane.
Giving oxygen a job at the start. It acts only at the end, but without it the whole chain stops.
Saying that FADH $_2$ enters at complex I. It enters at complex II.
Giving FADH $_2$ the larger yield. One NADH gives 3 ATP and one FADH $_2$ gives 2.
Saying that cytochrome c lies in the matrix. It is a small mobile protein on the outer surface of the inner membrane.

Remember these

The ETS is on the inner mitochondrial membrane. It passes electrons from NADH and FADH $_2$ to oxygen.
I: NADH dehydrogenase. II: succinate dehydrogenase. III: cytochrome bc $_1$ . IV: cytochrome c oxidase, with cytochromes a and a $_3$ and two copper centres. V: ATP synthase.
Ubiquinone and cytochrome c carry electrons between the complexes.
Oxygen is the final acceptor and makes water. One NADH gives 3 ATP. One FADH $_2$ gives 2 ATP.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

Where is the electron transport system located?

At which complex do the electrons of FADH $_2$ enter, and to which carrier do they go?

Which complex passes electrons to oxygen, and what does it contain?

What does oxygen become at the end of the chain?

Why does one FADH $_2$ give less ATP than one NADH?

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Problems (3)

Put these in the order in which electrons from NADH pass through them…Easy · High School (11–12) · by StemPy Lab

For one glucose, the link reaction and the Krebs cycle send 8 NADH and 2 FADH₂ to the electron…Medium · High School (11–12) · by StemPy Lab

A poison blocks cytochrome c oxidase (complex IV) in the mitochondria of a cell…Hard · High School (11–12) · by StemPy Lab

Next: lesson 3Oxidative phosphorylation

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