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Electron Transport, Practice Problems and FAQs

Electron Transport, Practice Problems and FAQs

You all know about photosynthesis. It is the process by which the light energy is fixed as chemical energy. It is a combination of light and dark reactions. We know that the energy fixed during the light reactions are used for the synthesis of sugar in dark reactions. 

CO2



GIF: Photosynthesis

But how the energy is generated during the light reaction? Is there any structure responsible for this? What do you think, for generating this energy, plants need some other sources of energy? What are the conditions provided by the plants to generate this energy? 

All these questions will be there in your mind while you think about the energy which is fixed during the light reaction. We know that the electrons are transferring from one molecule to another during the cyclic and non cyclic photophosphorylation. This really creates many changes in the thylakoid. Now we are going to discuss more about this transport of electrons in the thylakoid and the different mechanisms associated with it in this article.

Table of contents

  • Electron transport system
  • Chemiosmotic hypothesis
  • Practice Problems
  • FAQs

Electron transport system 

During the light reaction of photosynthesis, the electrons which get excited are accepted by the electron acceptors and then it is passed onto an electron transporting system consisting of cytochromes. The movement of electrons is always downhill in terms of redox potential scale. So electrons move from lower redox potential to higher.

The main function of the electron transport system is to transfer the electrons from one molecule to another. When the electrons get excited in the reaction centre of PS II, it is picked by an electron acceptor. Then it moves through the electron transport system of PS II. But the electrons are not used by the PS II. They later passed on to the PS I. The electrons at PS I are also passed through another electron transport system.

Finally the electrons are used for the production of NADPH and ATP. The electron transport is present in both cyclic and non cyclic photophosphorylation. 



                                                      Fig: Light reactions

The electrons which are lost in the PS II will get replaced through the photolysis of water in the lumen. The protons formed during the splitting of water will be then moved into the stroma of the chloroplast. 



                                                       Fig: Photolysis of water

The protons are passed through the ATP synthase present in the thylakoid membrane. ATP is formed by the ATP synthase when protons pass through it. This mechanism is based on the chemiosmotic hypothesis.

Chemiosmotic hypothesis

It was proposed by Peter Mitchell in 1961. It is the process by which ATP molecules are produced by the action of ATP synthase. This hypothesis states that the synthesis of ATP is linked to the development of a proton gradient across a membrane (thylakoid membrane, in case of photosynthesis). The facilitated diffusion of protons or H + ions (due to the proton gradient) through the ATP synthase across a membrane leads to the formation of ATP with the breakdown of the gradient.



                                              Fig: Peter Mitchell

Requirements for ATP formation

ATP formation requires the following: 

  • A semipermeable membrane
  • A proton pump
  • A proton gradient
  • ATPase 

Semipermeable membrane 

A semipermeable membrane will allow the passage of certain molecules while blocking the passage of other molecules. The thylakoid membrane acts as the semipermeable membrane here. It possesses electron transport proteins. 

Proton pump 

It transports hydrogen ions from areas of low concentration to areas with high concentration. But the FoF1 ATP synthase of mitochondria in the cell normally conducts protons from high to low concentration across the membrane. By drawing the required energy from this flow to synthesise ATP.

Proton gradient

A high concentration of H + ions or protons is created in the thylakoid lumen due to the influx of H+ ions during the electron transport by PQ and Release of H + ions due to the splitting of water catalysed by PS II. A higher H+ ion concentration at the thylakoid lumen means that there is a lower H+ ion concentration in the stroma. This creates a H+ ion gradient or a proton gradient across the thylakoid membrane.

ATP synthase

In addition to the photosystems and the cytochrome, the thylakoid membrane has another important protein, the ATP synthase. It is an enzyme consisting of the following two parts such as CF0 and CF1 .

CF

It is embedded in the thylakoid membrane. It forms a transmembrane channel that carries out facilitated diffusion of H+ ions or protons across the membrane.

CF

It protrudes on the outer surface of the thylakoid membrane. It faces the stroma. F1 is composed of three copies of subunits α and β, and one copy of subunits γ, δ and ε.



                                    Fig: ATP synthase

Working of ATP synthase

There is a higher concentration of protons in the thylakoid lumen and a lower concentration of protons in the stroma. So, the protons should be able to diffuse from the lumen to the stroma. However, the thylakoid membrane is impermeable to H+ ions or protons. This is why the protons undergo facilitated diffusion instead. This is done through the ATP synthase enzyme. When the protons from the lumen diffuse through the ATP synthase, they cause the enzyme to churn and rotate. During this rotation, there is a conformational change in the CF1. Adenosine diphosphate (ADP) molecules get hit with free phosphate molecules to form adenosine triphosphate (ATP). ATP is the energy currency of the cell. This ATP will be used immediately in the biosynthetic reactions taking place at stroma.



                                                Fig: Working of ATP Synthase

Practice Problems

Q 1. Which of the following can donate electrons according to the chemiosmotic hypothesis?

a. NADH and FADH2
b. NADH and ATP
c. ATP and FADH2
d. NADH and FMN

Answer: Electron carriers like NADH and FADH provide electrons to the electron transport chain during chemiosmosis. To pump H+ through a selectively permeable cell membrane, the electrons produce conformational changes in the morphologies of the proteins. Because of the positive charge of hydrogen ions and their aggregation on one side of the membrane, the uneven distribution of H+ ions across the membrane creates both concentration and electrical gradients (hence an electrochemical gradient). Hence the correct option is a.

Q 2. The accumulation of protons inside the lumen of thylakoids is due to ___________________.

a. splitting of water
b. formation of NADPH
c. both a and b
d. formation of ATP

Answer: Water splitting causes protons to collect inside the lumen of thylakoids. PS II is connected with a water splitting complex that is found on the inner side of the thylakoid membrane (facing the lumen of thylakoids). It aids in the splitting of water. Protons, electrons, and oxygen are released as a result of water splitting (photolysis). Protons are used in the stroma of chloroplasts, and NADP + is converted to NADPH. A proton gradient is formed normally when thylakoid lumen has a high concentration of protons and the stroma of the chloroplast has a low concentration of protons. The synthesis is aided by this proton gradient.

Q 3. Which of the following statements are correct according to the electron transport chain?

1. The electrons which get excited are accepted by the electron acceptors and then it is passed onto an electron transporting system consisting of cytochromes. 
2. The movement of electrons is always downhill in terms of redox potential scale. 
3. The electrons are used for the production of NADPH and ATP. 
4. The electron transport is present in both cyclic and non cyclic photophosphorylation. 
5. The protons are passed through the ATP synthase present in the thylakoid lumen. 

a. A, B, C, D, E
b. A, B, C and D
c. A and C
d. All of the above

Answer: During the light reaction of photosynthesis, the electrons which get excited are accepted by the electron acceptors and then it is passed onto an electron transporting system consisting of cytochromes. The movement of electrons is always downhill in terms of redox potential scale. So electrons move from lower redox potential to higher. The main function of the electron transport system is to transfer the electrons from one molecule to another. Finally the electrons are used for the production of NADPH and ATP. The electron transport is present in both cyclic and non cyclic photophosphorylation. The protons are passed through the ATP synthase present in the thylakoid membrane. Hence the correct option is b.

Q 4. How the proton gradient is formed in the electron transport system?

Answer: A high concentration of H + ions or protons is created in the thylakoid lumen due to the influx of H+ ions during the electron transport by PQ and Release of H + ions due to the splitting of water catalysed by PS II. A higher H+ ion concentration at the thylakoid lumen means that there is a lower H+ ion concentration in the stroma. This creates a H+ ion gradient or a proton gradient across the thylakoid membrane.

FAQs

Q 1. What is the difference between mitochondrial and chloroplast chemiosmosis?
Answer: The source of energy is one of the fundamental variations between chemiosmosis in mitochondria and chloroplasts. The high-energy electrons in mitochondria are retrieved from the food molecule (through a redox reaction), whereas the source in chloroplast is photons acquired from the light source.

Q 2. Is chemiosmosis oxygen-dependent?
Answer: Oxygen is the final electron acceptor in the electron transport chain, without it, the electron transport system or ETS will stop working and chemiosmosis will not produce ATP. Hence we can say that chemiosmosis is oxygen dependent.

Q 3. Is chemiosmosis possible without a closed membrane system?
Answer: Yes, chemiosmosis is possible without a closed membrane system. Chemiosmosis does not require a closed membrane system. The creation of a proton gradient across the membrane of the thylakoid requires only a membrane, and proton buildup occurs on the inside of the membrane.

Q 4. Which of Peter Mitchell's discoveries won him the Nobel Prize?
Answer: Peter Dennis Mitchell, a British biochemist, won the Nobel Prize in Chemistry in 1978 for discovering the chemiosmotic mechanism of ATP creation. The energy currency of life, ATP, was recognised in the 1960s, but the method by which it was made in the mitochondria was considered to be substrate-level phosphorylation. Mitchell's chemiosmotic hypothesis laid the groundwork for understanding the oxidative phosphorylation process. The discovery of ATP synthase and André Jagendorf's finding that a pH difference across the thylakoid membrane in the chloroplast results in ATP synthesis confirmed his idea.

YOUTUBE LINK:  https://www.youtube.com/watch?v=ddzqKugHy38&feature=emb_logo

Related Topics

Photosynthesis : Location (Chloroplast), Resonance transfer, Practice Problems and FAQs 

Light reactions - Cyclic photophosphorylation, Practice Problems and FAQs 

Light reactions - Non-cyclic photophosphorylation, Practice Problems and FAQs 

ATP synthase, Practice problems and FAQs

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