Introduction

ATP is an essential requirement for the various metabolic processes taking place in living organisms. The continuous supply of ATP is necessary for the continuity of life. Any interruption in the synthesis of ATP can result in harmful and life-threatening events.

In most living systems, ATP is made by phosphorylation of already existing ADP molecules. This phosphorylation process is an endothermic process requiring some chemical energy. This energy is provided either by breaking the complex compounds obtained from food as happens in heterotrophic organisms or by capturing and utilizing solar energy in the form of light as happens in photosynthesis.

In both cases, ATP synthesis occurs by the process of chemiosmosis. Chemiosmosis is defined as the movement of ions down their concentration gradient through a semipermeable membrane i.e. osmosis of the ions. In this article, we will discuss in detail the chemiosmotic theory, and the mechanisms by which it helps in making ATP, the energy currency of the cell.

Chemiosmotic Theory

The chemiosmotic theory was first presented by Peter D. Mitchell in 1961. He suggested that most of the ATP in the metabolic cells is synthesized by utilizing the energy stored in the electrochemical gradient across the inner mitochondrial membrane. This electrochemical gradient was first established by using the high energy molecules, NADH, and FADH₂. These compounds were formed during the metabolism of food molecules like glucose etc.

During oxygen metabolism, it is metabolized to form acetyl CoA which is further metabolized in the mitochondrial matrix. The molecules of acetyl  CoA are subject to oxidation in a process called the citric acid cycle. This cycle is coupled with the reduction of intermediates like NAD and FAD. The high energy intermediates (NADH and FADH₂)  formed as a result of reduction are carried to the electron transport chain (ETC).

These high energy intermediates are in fact the carriers of electrons. The electrons of NADH and FADH­₂ are donated to the electron transport chain. As the electrons move down the ETC, a large amount of energy is released that is used to produce the electrochemical gradient across the inner mitochondrial membrane. The energy of this gradient is later used to drive a proton pump, which eventually phosphorylates ADP to ATP.

This chemiosmotic theory was not accepted immediately as it was against the views of scientists at that time. It was believed that the energy of the electron flow was stored in the form of some high energy intermediates that were directly used to make ATP. However, with time, scientific evidence began to prove the chemiosmotic hypothesis. The theory was accepted, and Mitchell was awarded Nobel Prize in Chemistry in 1978.

The chemiosmotic theory now explains the ATP synthesis in mitochondria, chloroplasts, and many bacteria. The applications of chemiosmotic theory in all these organelles are discussed in detail in the subsequent parts of this article.

Chemiosmosis in Mitochondria

Chemiosmosis is the major source of ATP during cellular respiration in the prokaryotes. This process takes place in the mitochondria of the living cells. Let us understand the structure of mitochondria before diving into the process of chemiosmosis.

Structure of Mitochondria

Mitochondria are double membrane-bound organelles present in all the eukaryotic cells with some exceptions. The outer membrane is smooth while the inner membrane shows various infoldings. The electron transport chain is located on the inner mitochondrial membrane.

Electron Transport Chain

The electron transport chain is composed of four protein complexes embedded in the inner mitochondrial membrane.

• Complex I: It is composed of NADH dehydrogenase, FMN, and an iron-sulphur protein.
• Complex II: This complex has enzyme succinate dehydrogenase, FAD, and an iron-sulphur protein just like Complex I.
• Complex II: It is cytochrome complex having cytochrome b and cytochrome c₁. Cytochromes are the heme proteins that act as electron carriers.
• Complex IV: It is another cytochrome complex containing cytochrome a and cytochrome a₃. The cytochrome a₃ is copper-containing cytochrome. In addition, another copper-containing protein Cu_A is also present in this complex.

Coenzyme Q is also a member of the electron transport chain. It is a quinine derivative having a long isoprenoid tail embedded in the inner mitochondrial membrane. It is ubiquitous in nature and is also called ubiquinone. Because of its lipid solubility and isoprenoid structure, Coenzyme Q can move freely along the inner mitochondrial membrane. Therefore, it is also regarded as a free or mobile electron carrier.

Cytochrome c, a cytochrome present in the intramembranous space, is also a component of the electron transport chain.

ATP Synthase

In addition to the electron transport chain, another complex is present in the inner mitochondrial membrane called Complex V. This complex acts as a proton channel and has an intrinsic ability to phosphorylate ADP to ATP. Thus, it is also known as ATP synthase.

The proton channel in ATP synthase is linked with a ring. As the protons pass through the channel, they rotate the ring and energy is generated that is used to phosphorylate ADP.

Intermembranous Space

It is a space between the outer and inner mitochondrial membranes. The concentration of different ions in this space is different from the mitochondrial matrix. The protons from the mitochondrial matrix are pumped and stored in this space for chemiosmosis.

Process

The chemiosmotic process in mitochondria involves the following steps;

• Electrons are provided to the electron transport chain via the high energy electrons carriers like NADH and FADH₂. NADH provides electrons to Complex I of the ETC while FADH₂ provides electrons to Complex II.
• The electrons then move down the electron transport chain liberating a considerable amount of energy. The flow of electrons in ETC can be represented by the following equation:

Complex I   ->   Complex II  ->  Coenzyme Q  ->  Complex III  ->  Cytochrome c  ->  complex IV  ->  Oxygen

Oxygen acts as the final acceptor of electrons in the electron transport chain.

• The hydrogen ions or protons are already in lower concentration within the mitochondrial matrix. The energy liberated by electrons is used to pump these protons into the intermembranous space against their concentration gradient. In this way, the energy of electrons is stored in the form of an electrochemical gradient.
• As the protons gather in the intermembranous space to a particular concentration, they start moving down their concentration gradient through the proton channel in the ATP synthase. During this process, they rotate the proton ring and liberate energy.
• This energy is used by ATP synthase to phosphorylate ADP to ATP on the stromal side of the inner mitochondrial membrane.

Importance

The chemiosmotic process in mitochondria is the source of obtaining energy via cellular respiration. Any hindrance in this process will make it impossible to obtain energy via cellular respiration.

Inhibition

This process can be inhibited by an inhibitor of the electron transport chain or uncoupler proteins. Uncoupler protein channels provide an alternate path to protons for entering mitochondrial stroma without passing through the ATP synthase. The energy of the electrochemical gradient is wasted in the form of heat and no ATP is made. Some drugs also act as uncoupler proteins like Asprin.

Read more about Electrochemical Gradients

Chemiosmosis in Chloroplasts

Chloroplasts are the organelles present in photosynthetic autotrophs. Chemiosmosis in the organelles takes place during light-dependent reactions of photosynthesis when the energy of photoexcited electrons is used to make ATP for dark reactions.

Let us first understand the structure of chloroplasts.

Structure

Just like mitochondria, chloroplasts are also double-membrane organelles. However, both the membranes of chloroplasts are smooth without any infoldings. The stroma of chloroplasts filled most of the space of organelles.

Thylakoids are coin-shaped structures present inside the chloroplasts that are piled on one another to form grana. Thylakoids are the site for light-dependent reactions and chemiosmosis. They are composed of a lumen bound by a membrane called the thylakoid membrane.

Photosystems of chlorophyll molecules and the electron transport chain are located on the thylakoid membrane.

Electron Transport Chain

The electron transport chain on thylakoid membranes is different than that present in the mitochondria. It is coupled with the photosystems present on the thylakoid membranes.

Photosystems are the clusters of chlorophyll molecules that gather the light energy, use it to excite the electrons of chlorophyll molecules and transfers it to the electron transport chain.

An electron carrier called plastoquinone (Pq) is present in close association with the photosystem II.

A cytochrome complex consisting of two cytochromes are present next to the photosystem II.

Next in the series is the photosystem I. A copper-containing protein called plastocyanin (Pc)  and an iron-containing protein called ferredoxin (Fd) are present in close contact with photosystem I. Both these proteins are the electron carriers.

ATP Synthase

Next to the photosystem I is ATP synthase. It has a structure similar to the ATP synthase present in the inner mitochondrial membrane. The only difference is that the proton channel is located towards the lumen of thylakoid while the F₀ domain having phosphorylation ability is located towards the stroma of chloroplast.

Process

The chemiosmosis on thylakoid membranes takes place during the light-dependent reactions. It occurs in case of both cyclic and non-cyclic electron flow.

Non-cyclic Electron Flow

During this process, photoexcited electrons move through the both photosystems. It involves the following steps;

• The photons of light fall on the photosystems and excite the electrons.
• The photoexcited electrons move through the electron transport chain. The path of these electrons can be represented by the following equation:

Photosystem I  ->  Plastoquinone  ->  Cytochrome Complex  ->  Plastocyanin  ->  Photosystem II  -> Ferredoxin  ->  NADP

NADP is the final acceptor of electrons.

• As the electrons move down the electron transport chain, energy is liberated that is used to pump hydrogen ions from the stroma of chloroplasts into the lumen of thylakoids.
• The energy of electrons is stored in the form of electrochemical gradient of protons across the thylakoid membrane.
• These protons move down the concentration gradient back into the stroma while passing through the proton channel of ATP synthase. During their journey, the protons rotate the ring and liberate energy.
• This energy is used to phosphorylate ADP to ATP in the stroma of chloroplasts.

Cyclic Electron Flow

In the cyclic flow, the photoexcited electrons pass through the electron transport chain and return to photosystem I after every cycle. The flow of electrons is represented as follows:

Photosystem II  ->  Ferredoxin  ->  Cytochrome Complex  ->  Plastocyanin  -> Photosystem II

As the electron pass through the electron transport chain, their energy is used to pump protons into the thylakoid lumen. ATP is made when these protons diffuse back into the stroma just like the non-cyclic flow of electrons.

Importance

Chemiosmosis in chloroplasts is the source of ATP molecules for dark reactions of photosynthesis. If the chemiosmotic process fails to make ATP molecules, the dark reactions cannot proceed, and the organisms fail to manufacture glucose. This chemiosmotic process holds the primary importance in the process of photosynthesis. It is the process by which light energy is converted into chemical energy and stored as high energy bonds in the molecules of ATP.

Summary

Chemiosmosis is the movement of protons down the concertation gradient coupled with the ATP synthesis in cellular respiration as well as photosynthesis.

Peter D. Mitchell first proposed this hypothesis in 1961. At first, it was not accepted. However, after few years, it was widely accepted based on the experimental evidence.

Chemiosmosis involves the electron transport chains located in the mitochondria and chloroplasts.

The chemiosmotic process in mitochondria occurs during cellular respiration.

• NADH and FADH₂ provide electrons to the ETC on the inner mitochondrial membrane.
• As the electrons move down the ETC, protons are pumped against the concentration gradient.
• The proton move back into the matrix by passing through the ATP synthase.
• The protons release energy that is used to make ATP.

The chemiosmotic process in chloroplasts stakes place during photosynthesis.

• The photoexcited electrons move down the ETC on thylakoid membrane.
• The energy of electron is used to pump proton from stroma into the thylakoid lumen.
• When the protons move back to stroma, they pass through ATP synthase.
• The energy or protons is used to make ATP by ATP synthase.

This process occurs during both cyclic and non-cyclic flow of electrons in the light-dependent reactions.

Frequently Asked Questions

What is chemiosmosis?

Chemiosmosis is a process of movement of protons or hydrogen ions down the concentration gradient across the mitochondrial membrane. This movement of protons is coupled with the synthesis of ATP. It is the coupling of the electron transport chain with the generation of ATP.

How is ATP formed in chemiosmosis?

As the electrons move down the electron transport chain on the inner mitochondrial membrane, energy is released which is used to pump protons from the mitochondrial matrix to the intermembranous space, against their concentration gradient. In this way, energy is stored in the proton gradient. This energy is later used to generate ATP when protons move back into the matrix by passing through ATP synthase. 

Where does chemiosmosis take place?

Chemiosmosis takes place in plants as well as animal cells. It happens in the mitochondria of an animal cell. In plant cells, it takes place in chloroplasts in addition to mitochondria. 

What is the role of chemiosmosis in photosynthesis?

During light-dependent reactions of photosynthesis, chemiosmosis is the process by which light energy is converted to chemical energy in the form of ATP to be used in dark reactions.

References

1. Peter Mitchell (1961). “Coupling of phosphorylation to electron and hydrogen transfer by a chemi-osmotic type of mechanism”. Nature. 191 (4784): 144–148. Bibcode:1961Natur.191..144M. doi:10.1038/191144a0. PMID 13771349.
2. Cooper, Geoffrey M. (2000). “Figure 10.22: Electron transport and ATP synthesis during photosynthesis”. The Cell: A Molecular Approach (2nd ed.). Sinauer Associates, Inc. ISBN 0-87893-119-8.
3. Alberts, Bruce; Alexander Johnson; Julian Lewis; Martin Raff; Keith Roberts; Peter Walter (2002). “Figure 14-32: The importance of H⁺-driven transport in bacteria”. Molecular Biology of the Cell. Garland. ISBN 0-8153-4072-9.
4. Nicholls D. G.; Ferguson S. J. (1992). Bioenergetics 2 (2nd ed.). San Diego: Academic Press. ISBN 9780125181242.
5. Stryer, Lubert (1995). Biochemistry (fourth ed.). New York – Basingstoke: W. H. Freeman and Company. ISBN 978-0716720096

Image sources

1. https://en.wikipedia.org/wiki/File:Mitochondrial_chemiosmosis_(annotated_diagram).svg
2. https://en.wikipedia.org/wiki/File:Chloroplast_diagram.svg