Chemiosmosis is the movement of ions down an electrochemical gradient through a semipermeable membrane. For instance, hydrogen ions (H+) move across a membrane during photosynthesis or in cellular respiration to form adenosine triphosphate (ATP). In a channel (red), the ions can pass through an ion gradient that has potential energy. In order to make ATP, a concentration gradient of hydrogen ions, or protons, across a membrane is exploited, which diffuses from areas of high proton concentration to areas of lower proton concentration. Because it involves the diffusion of water across a membrane, chemiosmosis is related to osmosis.
Chemiosmosis is the process by which ATP is made. By allowing protons to pass through the membrane, ATP is made by phosphorylating adenosine diphosphate (ADP). By using chemiosmosis to produce ATP, mitochondria, and chloroplasts, as well as bacteria and archaea, produce hydrogen ions (protons) via thylakoid membranes to the stroma (fluid). By reducing the free energy difference between the electron and proton, ATP synthase can allow proton to pass through them and in turn, make ATP by photophosphorylation of ADP.
What is Chemiosmosis?
An electrochemical gradient can be used to drive ATP synthesis in living cells when chemiosmosis occurs. Chemiosmosis refers to the process of moving ions (e.g. protons) across membranes. With the help of the proteins embedded in the membrane, the gradient also causes the ions to passively return to the plasma. In passive motion, the ions move from areas with higher concentrations of the atoms to areas with lower concentrations.
Water molecules passively move in this process, similar to osmosis. Chemiosmosis, however, involves the movement of ions across the membrane, whereas osmosis involves the movement of water molecules. In either case, a gradient is necessary. This is known as an osmotic gradient in osmosis.
In osmosis, pressure differences between opposing membrane sides produce the reaction. A proton gradient is an electrochemical gradient that drives chemiosmosis. Osmosis and chemiosmosis are not the only similarities. As well as paralleling passive transport, facilitated diffusion is another similar method. The principle is the same.
It is downhill for the ions. Membrane proteins are also responsible for carrying molecules across the membrane. Basically, the bilipid structure of the membrane prevents ions from permeating it readily, so membrane proteins allow them to move across. Various membrane proteins serve as temporary shuttles, channels, or passageways for the movement of these particles. A membrane protein transports an ion during chemiosmosis.
Additionally, these mechanisms require no chemical energy (e.g. ATP), unlike active transport. A gradient of ions forms in chemiosmosis, which generates potential energy sufficient to drive the process. Chemiosmosis occurs where? During the respiration process as well as in chloroplasts, it occurs in eukaryotes during photosynthesis. Prokaryotes lack these organelles and therefore chemiosmosis will occur in their cell membrane.
The process of chemiosmosis allows living things to produce energy by coupling energy with ATP. As part of cellular respiration, it is one of the major processes. The diagram below illustrates how chemiosmosis contributes to cellular respiration and further explains how it occurs.
An illustration of the mitochondrion is shown above. Because most ATPs are produced in this area, it is known as the cellular powerhouse. ATP synthesis is its primary function. There are two membranes on the organelle. Membranes on both sides of the mitochondrial ring make up the mitochondrial membrane. Both layers consist of lipid layers, which preventions from passing easily. The intermembrane space is situated between two membranes.
There are many infoldings in the membrane. In the inner membrane of the mitochondria, there is a space called the mitochondrial matrix. The matrix is home to the citric acid cycle, a cyclic metabolic reaction where food molecules churn to generate phosphate compounds with high energy. The pyruvate that is generated during glycolysis is converted to acetyl CoA, which is then oxidized and broken down into carbon dioxide in the mitochondrion.
Through substrate phosphorylation, the citric acid cycle produces one ATP for every pyruvate molecule. The electron transport chain (ETC) and ATP synthase are embedded in the mitochondrial membrane where most of the ATP comes from oxidative phosphorylation.
Most of the high-energy electrons are transferred to NAD+ and FAD to produce NADH (and H+) and FADH2, respectively, through redox reactions. By transporting electrons to the ETC for oxidative phosphorylation, these molecules shuttle electrons to the ETC.
Each member of the ETC undergoes a redox reaction as electrons are passed along the chain. As electrons are passed from one electron acceptor to another, the molecular oxygen will receive all the electrons. Water is formed as a result of the reaction:
2 H+ + ½ O2 → H2O
ATP is not produced by ETC. Instead, H+ (protons) are pumped into the intermembrane space as electrons are passed along. (See the diagram above) While protons are being pumped across the membrane, they accumulate on one side. Gradients of proton-ion (H+) are created in this way. Proton-motive force is the name given to it by scientists. A proton (or electron) can be converted to energy by transferring it across a membrane transmitting energy.
Bypassing through the ATP synthase channel, protons will move into their gradient, which is between the intermembrane space and the matrix. ATP is synthesized by the movement of hydrogen ions across the ATP synthase, which releases the energy. The energy causes the rotor and the rod of the enzyme to rotate. The enzyme is, then, activated to harness this force so as to build the high-energy bond between the ADP molecule and the inorganic phosphate (Pi) to produce an ATP molecule. The reaction: ADP + Pi → ATP.
Function of Chemiosmosis
The process of chemiosmosis involves energy coupling. It is believed that chemiosmosis promotes ATP synthesis by generating a proton motive force. By oxidative phosphorylation, chemiosmosis drives cellular respiration by driving ATP synthesis. To shuttle electrons to the ETC, electrons from the citric acid cycle (where pyruvate-turned-acetyl coenzyme A is broken down to carbon dioxide) are transferred to electron carriers.
By building ATP from ADP and inorganic phosphate, the proton motive force that develops from the accumulation of protons on one side of the membrane will be used to transfer energy. Therefore, the ATP synthase cannot be driven by proton motion without chemiosmosis. As a result, fewer ATP end products will result without the need for chemiosmosis. A similar impact can be expected in photosynthesis, where chemiosmosis plays a crucial role in ATP synthesis.
- Chemiosmosis in chloroplasts
The process of chemiosmosis takes place in the mitochondria of eukaryotes. Photosynthesis occurs in eukaryotes, including plants, in addition to the mitochondria – the chloroplast.
Chloroplasts are organelles primarily responsible for photosynthesis. It harvests light through its thylakoid system. As such, it governs the reactions initiated by light (or processes triggered by light). The chloroplast matrix is known as the stroma. The dark reactions (or light-independent processes) are carried out in the thick liquid that contains enzymes, molecules, and other substrates.
Thylakoids in chloroplasts undergo chemiosmosis. ATP synthases and a transport chain are part of this membrane system. In chloroplasts chemiosmosis is energy-dependent; in mitochondria, it is not. Unlike mitochondria, chloroplasts capture photons directly from the light source, rather than obtaining electrons from food molecules (derived from redox reactions).
H+ ions accumulate in the thylakoid compartment (i.e., the space inside the thylakoid) to form the proton gradient (H+). A stromal H+ ion can be formed by (1) splitting water during the light reactions; (2) translocating protons via the transport chain; or (3) picking up H+ ions by NADP+ during the light reactions. ATP synthases embedded in the thylakoid membrane let the H+ ions across the membrane and diffuse to the stroma as they are greater in number inside this compartment (lumen).
- Chemiosmosis in prokaryotic cells
Chemiosmosis occurs in the cell membrane of prokaryotes like bacteria and archaea, which lack mitochondria and chloroplasts.
Diagram of chemistryosmosis in photosynthetic bacterium’s cell membrane. In the case of a proton gradient forming on the other side of the membrane, hydrogen ions (protons) are transported across the biological membrane by ATP synthase (a transport protein).
In electron transport and redox reactions, the hydrogen ions are forced to accumulate on the other side in order to form a proton gradient. ATP synthase allows the hydrogen ions to cross the membrane to get back into the cell as they move further away from it on the side where more hydrogen ions are. Through phosphorylation, energy is released to convert ADP to ATP.
Chemiosmosis vs Oxidative Phosphorylation
ATP is produced in the ETC by oxidative phosphorylation, a metabolic pathway that uses energy generated by redox reactions. In addition to electron transport-linked phosphorylation, this is also called phospholipid phosphatase. Due to the final electron acceptor being molecular oxygen, this is an aerobic process. This distinguishes it from the other form of phosphorylation, namely the substrate-level phosphorylation, where ATP is generated directly from an intermediate substrate. In contrast, oxidative phosphorylation is an indirect method of synthesizing ATP. Protons are moved across the membrane with the help of chemiosmosis.
Oxidative phosphorylation directly makes ATP through chemiosmosis. The ATP synthase, however, will not be able to do so without the proton motive force that results from the electron transfer chain that moves protons (H+) to the other side of the membrane as the electrons are passed along.
Important Points to Remember
- Chemical reactions in the electron transport chain generate free energy that is used to pump hydrogen ions across the membrane during chemiosmosis, establishing an electrochemical gradient.
- The inner mitochondrial membrane only allows ions of hydrogen to pass through it thanks to a membrane protein called ATP synthase.
- ADP is turned into ATP by the ATP synthase while protons move through it.
- In mitochondria, oxidative phosphorylation uses chemiosmosis to produce ATP.
Important Terms to Remember
- ATP synthase: An important enzyme that provides energy for the cell to use through the synthesis of adenosine triphosphate (ATP).
- Oxidative phosphorylation: A metabolic pathway that uses energy released by the oxidation of nutrients to produce adenosine triphosphate (ATP).
- Chemiosmosis: The movement of ions across a selectively permeable membrane, down their electrochemical gradient.