Complex I can pump four hydrogen ions across the membrane from the matrix into the intermembrane space; it is in this way that the hydrogen ion gradient is established and maintained between the two compartments separated by the inner mitochondrial membrane. The compound connecting the first and second complexes to the third is ubiquinone Q. The Q molecule is lipid soluble and freely moves through the hydrophobic core of the membrane. Once it is reduced to QH 2 , ubiquinone delivers its electrons to the next complex in the electron transport chain.
This enzyme and FADH 2 form a small complex that delivers electrons directly to the electron transport chain, bypassing the first complex. Since these electrons bypass, and thus do not energize, the proton pump in the first complex, fewer ATP molecules are made from the FADH 2 electrons. The number of ATP molecules ultimately obtained is directly proportional to the number of protons pumped across the inner mitochondrial membrane.
The third complex is composed of cytochrome b, another Fe-S protein, Rieske center 2Fe-2S center , and cytochrome c proteins; this complex is also called cytochrome oxidoreductase. Cytochrome proteins have a prosthetic heme group.
The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen. The heme molecules in the cytochromes have slightly different characteristics due to the effects of the different proteins binding them, which makes each complex.
Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes. Cytochrome c is the acceptor of electrons from Q; however, whereas Q carries pairs of electrons, cytochrome c can accept only one at a time. The fourth complex is composed of cytochrome proteins c, a, and a 3. This complex contains two heme groups one in each of the cytochromes a and a 3 and three copper ions a pair of Cu A and one Cu B in cytochrome a 3.
The cytochromes hold an oxygen molecule very tightly between the iron and copper ions until the oxygen is completely reduced. The reduced oxygen then picks up two hydrogen ions from the surrounding medium to produce water H 2 O. The removal of the hydrogen ions from the system also contributes to the ion gradient used in the process of chemiosmosis.
Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient. Describe how the energy obtained from the electron transport chain powers chemiosmosis and discuss the role of hydrogen ions in the synthesis of ATP.
Chemiosmosis : In oxidative phosphorylation, the hydrogen ion gradient formed by the electron transport chain is used by ATP synthase to form ATP. If the membrane were open to diffusion by the hydrogen ions, the ions would tend to spontaneously diffuse back across into the matrix, driven by their electrochemical gradient.
However, many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels. Note that the ATP synthase is not part of the electron transport chain, but is shown here because it uses the proton gradient to power ATP synthesis.
This proton gradient is analogous to water stored in an elevated reservoir. The higher the water level in the reservoir, the more potential energy is available to accomplish mechanical work like turning a water wheel to grind grain. In the same way, the greater the difference in proton concentrations across the membrane, the more energy is available for ATP synthase to make ATP.
Indeed, the ATP synthase complex even resembles a water wheel, in that the flow of protons down their concentration gradient, through ATP synthase, causes a part of ATP synthase to rotate.
F1ATP Synthase — watch the video below and know this! The ATP synthase enzyme complex is located in the membrane, and is a remarkable rotor-stator molecular machine Stock et al. The proton motive force drives protons through a channel in the ATP synthase, and turns the rotor at approx rpm.
Each degree turn of the rotor results in synthesis of 3 ATP molecules. The ATP synthases in mitochondria, chloroplasts, and Bacteria are all structurally similar, and their amino acid sequence similarities are consistent with a common evolutionary origin Watt et al. Lesser degrees of similarity, and more distant evolutionary relationships, exist with Archaeal ATP synthases and with vacuolar membrane ATPases.
Indeed, Bacterial and mitochondrial ATP synthases can work in reverse to hydrolyze ATP and pump protons across the membrane to increase the membrane proton gradient see end of video above.
What creates the proton gradient across the membrane? Chemiosmosis — this is really important! We have seen how ATP synthase acts like a proton-powered turbine, and uses the energy released from the down-gradient flow of protons to synthesize ATP. The process of pumping protons across the membrane to generate the proton gradient is called chemiosmosis.
Chemiosmosis is driven by the flow of electrons down the electron transport chain , a series of protein complexes in the membrane that forms an electron bucket brigade. Each of these protein complexes accepts and passes on electrons down the chain, and pumps a proton across the membrane for each electron it passes on.
Ultimately, the last complex in the electron transport chain passes the electrons to molecular oxygen O2 to make water, in the case of aerobic respiration. We define respiration as the passage of electrons down the electron transport chain. We breathe respire oxygen because oxygen is the terminal electron acceptor , the end of the line for our mitochondrial electron transport chain.
The video below shows the details of the electron transfer reactions, and how they are coupled to pumping protons across the membrane. This is a form of active transport, because the electron transfers release free energy that is used to pump protons against their concentration gradient. What is chemiosmosis? In biology, chemiosmosis refers to the process of moving ions e.
The gradient also incites the ions to return passively with the help of the proteins embedded in the membrane. By passively, it means that the ions will move from an area of higher concentration to an area of lower concentration. This process is similar to osmosis where water molecules move passively.
In the case of chemiosmosis, though, it involves the ions moving across the membrane; in osmosis, it is the water molecules. Nevertheless, both processes require a gradient. In osmosis, this is referred to as an osmotic gradient. The differences in the pressures between the two sides of the membrane drive osmosis. As for chemiosmosis, the movement of ions is driven by an electrochemical gradient, such as a proton gradient. Not only is chemiosmosis similar to osmosis.
It is also similar to other forms of passive transport, such as facilitated diffusion. It employs a similar principle. The ions move downhill. Also, the molecules are transferred to the other side of the membrane with the help of membrane proteins.
Membrane proteins help the ions to move across since the membrane is not readily permeable to ions, basically because of its bilipid feature. These proteins in the membrane facilitate their movement by acting as a temporary shuttle or by serving as a channel or a passageway.
Chemiosmosis uses membrane proteins to transport specific ions. Furthermore, it does not require chemical energy e. ATP as opposed to an active transport system that does. In chemiosmosis, the formation of an ion gradient leads to the generation of potential energy that is sufficient to drive the process. Where does chemiosmosis occur? In eukaryotes, it occurs in the mitochondria during cellular respiration and in the chloroplasts during photosynthesis.
Prokaryotes lack these organelles and therefore chemiosmosis will occur in their cell membrane. Variant: chemosmosis. Chapter Ecosystems. Chapter Population and Community Ecology. Chapter Biodiversity and Conservation. Chapter Speciation and Diversity. Chapter Natural Selection. Chapter Population Genetics.
Chapter Evolutionary History. Chapter Plant Structure, Growth, and Nutrition. Chapter Plant Reproduction. Chapter Plant Responses to the Environment. Full Table of Contents.
This is a sample clip. Sign in or start your free trial. JoVE Core Biology. Previous Video Next Video. Next Video 8. Embed Share. During the electron transport chain, hydrogen ions are pumped into the inter-membrane space to create a proton gradient.
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