Electron Transport System and Oxidative Phosphorylation
After the citric acid cycle, the cell still holds its greatest energy reserve in the form of NADH + H⁺ and FADH₂. This quiz explores how those reducing equivalents are oxidised along the electron transport system embedded in the inner mitochondrial membrane, how electrons travel through complexes I to IV to reach oxygen, and how ATP synthase converts that flow into ATP.
What you will practise
- The location of the ETS and the role of each complex and mobile carrier.
- ATP yields from NADH and FADH₂ oxidation.
- Why oxygen is the final hydrogen acceptor.
- How oxidative phosphorylation differs from photophosphorylation.
Work carefully through each question — the numbers and terminology here form the backbone of the respiratory balance sheet later in the course.
Q1. Where is the electron transport system located in the mitochondrion?
The ETS is present in the inner mitochondrial membrane, where its carriers are embedded and arranged in sequence.
Q2. Electrons from NADH produced in the mitochondrial matrix during the citric acid cycle are oxidised by which complex?
NADH is oxidised by an NADH dehydrogenase, which is complex I of the electron transport system.
Q3. Ubiquinone receives reducing equivalents from FADH2 generated during the oxidation of which substrate?
Complex II passes reducing equivalents to ubiquinone via FADH2 generated during the oxidation of succinate in the citric acid cycle.
Q4. Which complex transfers electrons from reduced ubiquinone (ubiquinol) to cytochrome c?
The reduced ubiquinone is oxidised with transfer of electrons to cytochrome c via the cytochrome bc1 complex, which is complex III.
Q5. Which carrier acts as a mobile electron carrier between complex III and complex IV?
Cytochrome c is a small protein attached to the outer surface of the inner membrane and shuttles electrons between complex III and complex IV.
Q6. Complex IV of the electron transport system is best described as:
Complex IV refers to the cytochrome c oxidase complex, which contains cytochromes a and a3 along with two copper centres.
Q7. Oxidation of one molecule of NADH through the ETS gives rise to how many ATP molecules?
The number of ATP molecules synthesised depends on the electron donor; oxidation of one NADH gives 3 ATP.
Q8. Oxidation of one molecule of FADH2 through the ETS produces how many ATP molecules?
Oxidation of one molecule of FADH2 produces 2 molecules of ATP, fewer than NADH because it enters at complex II.
Q9. What is the role of oxygen in aerobic respiration as described in this lesson?
Oxygen acts as the final hydrogen acceptor, removing hydrogen from the system and driving the whole process.
Q10. Electrons passing through complexes I to IV are coupled to which complex for ATP production?
Electron flow through complexes I to IV is coupled to ATP synthase, complex V, which makes ATP from ADP and inorganic phosphate.
Q11. In respiration, what provides the energy for creating the proton gradient required for phosphorylation?
In respiration the energy of oxidation-reduction is used for the proton gradient, unlike photophosphorylation where light energy is used.
Q12. Why is the process in respiration called oxidative phosphorylation?
The process is named oxidative phosphorylation because the energy of oxidation-reduction, not light, drives the proton gradient needed for phosphorylation.
Q13. How does photophosphorylation differ from oxidative phosphorylation according to the lesson?
In photophosphorylation light energy produces the proton gradient, whereas in respiration the energy of oxidation-reduction serves the same purpose.
Q14. Why must NAD+ and FAD+ be regenerated from NADH and FADH2 for the TCA cycle to continue?
The continued oxidation of acetyl CoA via the TCA cycle requires regeneration of NAD+ and FAD+ from NADH and FADH2 respectively.
Q15. What happens to electrons after they are passed along the carriers of the ETS to oxygen?
When ##NADH + H+ and## FADH2 are oxidised through the ETS, electrons are passed on to ##\ce{O2}## resulting in the formation of ##\ce{H2O}##.