Aerobic Respiration: ETC and Oxidative Photophosphorylation and The Respiratory Balance Sheet, Practice problems and FAQs

Have you ever been to a bank to withdraw money? In the bank if you give a cheque on one counter, you get cash of the same amount in return for the cheque. Why do we need the cash? We need the cash to buy commodities that help us to live a comfortable life. Did you know that something similar happens in our cells too?

We know that the energy currency of our cell is ATP, right? Almost all the activities that our body performs requires energy in the form of ATP. Here ATP can be compared with the cash money as we are spending ATP to get the work done. We are aware that ATP is produced in the cells by the process of respiration which is a multi step process. At some steps, ATP is generated directly by removing phosphate from the substrates. ATP can also be generated by oxidising the reduced form of some electron acceptors such as NAD and FAD. 

During glycolysis and Krebs cycle of respiration, NAD⁺ is reduced to NADH ⁺ H⁺ and FAD is reduced to FADH₂. These reduced compounds can be used by the cell as cheques to encash ATP molecules. As these reduced compounds release electrons to become oxidised, their electrons pass through multiple electron carriers (similar to bank counters) and in the process ATP is generated.

Interesting right? Let us now learn how this amazing process is carried out within the simplest unit of our body, that is, the cell.

Table of contents

• NADH and FADH₂ as electron donors
• Electron Transport Chain and its Complexes 
• Oxidative Phosphorylation
• Respiratory Balance Sheet
• Practice problems
• FAQs

Electron acceptors

NAD (Nicotinamide adenine dinucleotide) and FAD (Flavin adenine dinucleotide) are the electron acceptor coenzymes present in the mitochondrial matrix which take up H⁺ ions and electrons released due to the oxidation of substrates in the various steps of respiration. Coenzymes are organic molecules that are required for the activity of an enzyme. 

Accepting hydrogen and electrons reduces NAD and FAD to NADH + H⁺ and FADH₂ . In order to return back to their original forms, these compounds release their protons (H+ ions) and electrons. 

NADH + H⁺ ⇋ NAD⁺ + 2H⁺ + 2e^-

FADH2 ⇋ FAD + 2H⁺ + 2e^-

These protons and electrons are passed down an electron transport chain (ETC) consisting of many electron carrier complexes and finally combine with molecular oxygen to form water. 

2H+ + 2e^- + ½ O2 → H2O

Electron Transport Chain and its Complexes

To release and utilise the energy stored in NADH+H⁺ and FADH₂, they are oxidised with the help of the electron transport system. The electron transport chain or mitochondrial respiratory chain is a series of coenzymes and cytochromes that are located in the inner mitochondrial membrane and act as electron carriers. 

The electrons move from a higher energy level to a lower energy level, thereby releasing energy. Some of the energy is used to move the electrons from the matrix to the intermembrane space. Thus, an electrochemical gradient is established. The electrons are then transferred to the oxygen molecule which splits into half and uptakes H⁺ to form water.

Fig: The electron transport system in mitochondria

Complexes of ETC

All the components of the electron transport chain are arranged in four complexes - Complex I, II, III and IV. Complex I and II are flavoproteins and Complex III and IV are cytochromes. There is a fifth complex, named ATP synthase complex which is involved in ATP generation.

Fig: Complexes of the electron transport chain

Complex-1

This complex is named as NADH dehydrogenase complex and contains two prosthetic groups - flavin mononucleotide (FMN) and Fe-S. It is the first complex through which the electrons of NADH enter the ETC. In this step, electrons from NADH (produced in the mitochondrial matrix during citric acid cycle and glycolysis) are oxidised by an NADH dehydrogenase and electrons are then transferred to the FMN and then to the Fe-S complex in which the Fe3⁺ is reduced to Fe2⁺. Complex I being a proton pump, for every pair of electrons that pass through it, it pumps 2 H⁺ into the intermembrane space from the matrix.

Complex I then transfers the electrons to a mobile electron carrier called ubiquinone (Q) which is located within the inner mitochondrial membrane. Reduced ubiquinone or ubiquinol then transfers its electrons to cytochrome b-c₁ complex (complex III).

Fig: Electron transport through Complex I

Complex-II

It is also called succinate dehydrogenase complex and has FAD and a Fe-S complex as the prosthetic group. During the oxidation of succinate to fumarate, FAD is reduced to FADH₂. FADH₂ transfers two electrons to Ubiquinone which also accepts to H⁺ ions from the matrix and is reduced to ubiquinol. Thus ubiquinone is reduced by receiving electrons both from NADH through complex-I and FADH₂ through complex-II. This complex does not account for the pumping out of protons in the peri mitochondrial space.

Fig: Electron transport through Complex II

Complex-III

It is also called Cytochrome b-c1 complex. It consists of cytochromes b, c1 and a Fe-S complex. It receives electrons from reduced ubiquinone (ubiquinol) and one electron at a time is passed through cytochrome b, then the Fe-S complex and then through cytochrome c1 to another carrier protein Cytochrome c. For every two electrons that are transferred, 2H⁺ ions are pumped into the intermembrane space. Cytochrome c is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for transfer of electrons between complex III and IV. 

Fig: Electron transport through Complex III

Complex-IV

Complex IV refers to a Cytochrome c oxidase complex containing cytochromes a and a3 , and two copper centres. Reduced Cytochrome c ( a mobile carrier) moves along the outer surface of the inner mitochondrial membrane and transfers electrons to cytochrome a and a3. Finally these electrons are transferred to O₂. in this process the enzyme cytochrome c oxidase catalyses the reduction of O₂ to H₂O.

Fig: Electron transport through Complex IV

Thus, we can say that there are two routes for the entry of electrons into the ETC. Electrons released from NADH enter the ETC through FMN (Complex I) and the ones released from FADH2 enter directly through the complex II.

The electron transferring reactions are oxidation-reduction reactions or redox reactions in which each electron donor and acceptor pair is acting as a redox pair. Electrons flow from higher electronegative components to lower electronegative components. 

Transport of protons across the inner mitochondrial membrane

When electrons pass from one electron carrier to another via complex I to IV in the ETC, protons are pumped from the matrix to the intermembrane space which creates a proton gradient.

For oxidation of NADH + H⁺ to NAD⁺, two electrons and two protons are released. Two protons or H+ ions are transported from the matrix to the intermembrane space for every electron pair that is transferred from-

• NADH to FMN (Complex I)
• Cytochrome b to Cytochrome c
• Cytochrome a to Cytochrome a3

For oxidation of FADH₂ to FAD, two electrons and two protons are released. Two protons or H+ ions are transported from the matrix to the intermembrane space for every electron pair that is transferred from -

• Cytochrome b to Cytochrome c
• Cytochrome a to Cytochrome a₃

Oxidative phosphorylation

The electron transport down the energy gradient through the electron transport system leads to the formation of ATP from ADP and inorganic phosphate. This ATP generation is called oxidative phosphorylation. The process involves Complex V or the ATP synthase complex. The number of ATP molecules synthesised depends on the nature of the electron donor.

Complex-V

It is referred to as ‘ATP synthase complex’ or ATPase. This complex consists of two major components, F₁ and F₀. The F1 is called the headpiece and is a peripheral membrane protein complex and contains the site for synthesis of ATP from ADP and inorganic phosphate. F0 is an integral membrane protein complex that forms the channel through which protons cross the inner membrane.

Fig: Complex V

Mechanism of oxidative phosphorylation

In respiration, the energy of oxidation-reduction is utilised for the production of proton gradient required for phosphorylation. 

Transport of two electrons from one molecule of NADH+H⁺ over the ETC helps in the transport of three pairs of protons from the matrix to the inter membrane space. To break this gradient, three pairs of protons are transported by the F0 channel of Complex V from the inter membrane space into the matrix and this generates three ATP molecules.

Transport of two electrons from one molecule of FADH₂ over the ETC helps in the transport of two pairs of protons from the matrix to the inter membrane space. To break this gradient, two pairs of protons are transported by the F₀ channel of Complex V from the inter membrane space into the matrix and this generates two ATP molecules.

Fig: Mechanism of ATP synthesis by Complex V

Practice problems

Q1. Write down the correct sequence of electron acceptors in ETS for production of ATP?

(a) Cyt b, c, a, a3
(b) Cyt a, a, b, c
(c) Cyt c, b, a, a3
(d) Cyt b, c, a3, a

Solution: In ETC, the electrons from NADH and FADH₂ are carried to O2 through a series of electron carriers,which are located on the inner mitochondrial membrane and together termed as electron transport system or electron transport chain. Four complexes transfer the electrons from NADH and FADH₂ to the electron acceptor. The electrons from NADH are first passed on to the FMN component of Complex I which gets reduced and to be oxidised again, releases its electrons to ubiquinone. Similarly, electrons from succinate in the Krebs cycle are taken up by the FAD component of Complex II and it is reduced to FADH₂. FADH₂ releases its electrons to ubiquinone. Ubiquinone is a mobile electron transporter which transfers electrons from Complex I and II to the Cytochrome b component of Complex III. Cytochrome b passes it over to another mobile electron carrier named Cytochrome c which further transfers the electrons first to Cytochrome a and then to Cytochrome a₃ of Complex IV. Complex IV passes over the electrons to oxygen.

The fifth complex called the complex V or ATP synthase generates the ATP.

Fig: The electron transport system in mitochondria

Thus, the correct option is a.

Q2. Which complex of ETC is not involved in pumping out of protons from the mitochondrial matrix?

(a) Complex I
(b) Complex III
(c) Complex II
(d) Complex IV

Solution: Complex II is also called Succinate dehydrogenase complex. FADH2 that is generated during oxidation of succinate in the citric acid cycle, enters through complex II directly by transferring 2 electrons to Ubiquinone. Thus ubiquinone is reduced by receiving electrons both from NADH through complex-I and FADH2 through complex-II. This complex does not account for the pumping out of protons in the peri mitochondrial space.

Thus, the correct option is c.

Q3. Which products of glucose metabolism are necessary for oxidative phosphorylation?

(a) Pyruvate
(b) NADH and FADH2
(c) Acetyl CoA
(d) NADPH and ATP

Solution: To release and utilise the energy stored in NADH+H⁺ and FADH₂, they are oxidised through the electron transport system. Since NADH and FADH₂ cannot directly reduce O₂ to form H₂O,the electrons from both are passed on to O₂ resulting in the formation of H₂O. The electrons from NADH and FADH₂ are carried to O₂ through a series of electron carriers,which are located on the inner mitochondrial membrane and together termed as ‘electron transport system’ or electron transport chain. 

Thus, the correct option is b.

Q4. Which of the following is true for the cytochrome c oxidase complex?

(a) It donates electrons to O₂
(b) It accepts electrons from Cyt c
(c) It pumps two protons out of the mitochondrial matrix
(d) All of the above

Solution: Complex IV refers to a Cytochrome c oxidase complex containing cytochromes a and a3 , and two copper centres. Reduced Cytochrome c ( a mobile carrier) transfers electrons to cytochrome a and a3. Finally these electrons are transferred to O₂. in this process the enzyme cytochrome c oxidase catalyses the reduction of O₂ to H₂O. For every pair of electron that it transfers, it pumps a pair of protons (H+ ions) from the mitochondrial matrix to the intermembrane space.

Thus, the correct option is d.

FAQs

Question 1. How are oxidative phosphorylation and electron transport chain related to each other?
Solution: The electrons from NADH and FADH₂ are carried to O₂ through a series of electron carriers,which are located on the inner mitochondrial membrane and together termed as ‘electron transport system’ or electron transport chain. The electron transport down the energy gradient through the electron transport system leads to the formation of ATP from ADP and inorganic phosphate. This ATP generation is called oxidative phosphorylation. 

Question 2. What will be the result if the electron transport chain is interrupted?
Solution: If the electron transport chain is interrupted, protons (H⁺) will not be pumped into the intermembrane space. This will decrease its H+ concentration but increase the pH.

Question 3. What is the mechanism of regulation of oxidative phosphorylation ?
Solution: The rate of Oxidative phosphorylation is mainly determined by the energy needs of a cell, and also by the ratio of ADP to ATP.In respiration, the energy of oxidation-reduction is utilised for the production of proton gradient required for phosphorylation. For each ATP produced, 4H⁺ passes through F0 from the intermembrane space to the matrix down the electrochemical proton gradient. Rotation of F₁ forms ATP.

Question 4. What is the need of oxygen in oxidative phosphorylation?
Solution: In oxidative phosphorylation, oxygen receives the electrons from the protein complexes. In this process the enzyme cytochrome c oxidase catalyses the reduction of O₂ to H₂O.

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