But passage between complexes also requires some electron carriers. These carriers are respectively Ubiquinone Coenzyme Q, shortly just Q , Iron sulphur proteins and cytochromes.
By the pumping electrons to IMS, proton motive force is generated. For oxidative phosphorylation, oxidation and phosphorylation are said to be coupled. Oxidation of electron carriers provides proton passage form matrix to IMS, thus promotes the activity of ATP synthase. In photosynthesis, when light is absorbed in photosystem 2, electrons are energized. They are transferred to the reaction center. From the reaction center, the electrons enter the electron transport chain and pass the etransport chain molecules.
Protons go down proton pumps and a concentration gradient forms as protons move from the stroma into the thylakoid space. The ETC takes electrons from these compounds and returns the products back to the citric acid cycle. The folds of the inner membrane give it a large surface area with lots of room for electron transport chain reactions.
Most single cell organisms are prokaryotes, which means the cells lack a nucleus. These prokaryotic cells have a simple structure with a cell wall and cell membranes surrounding the cell and controlling what goes into and out of the cell.
Prokaryotic cells lack mitochondria and other membrane-bound organelles. Instead, cell energy production takes place throughout the cell. Some prokaryotic cells such as green algae can produce glucose from photosynthesis , while others ingest substances that contain glucose.
The glucose is then used as food for cell energy production via cell respiration. Because these cells don't have mitochondria, the ETC reaction at the end of cell respiration has to take place on and across the cell membranes located just inside the cell wall. The ETC uses high energy electrons from chemicals produced by the citric acid cycle and takes them through four steps to a low energy level.
The energy from these chemical reactions is used to pump protons across a membrane. These protons then diffuse back through the membrane. For prokaryotic cells, proteins are pumped across the cell membranes surrounding the cell.
For eukaryotic cells with mitochondria, the protons are pumped across the inner mitochondrial membrane from the matrix into the intermembrane space. The chemicals NAD and FAD are given back to the citric acid cycle while the oxygen combines with hydrogen to form water.
The protons pumped across the membranes create a proton gradient. The gradient produces a proton-motive force that allows the protons to move back through the membranes. The overall chemical process is called oxidative phosphorylation.
At the end of this process, the proton gradient is produced by each complex pumping protons across the membranes. The resulting proton-motive force draws the protons through the membranes via the ATP synthase molecules. As they cross into the mitochondrial matrix or the interior of the prokaryotic cell, the action of the protons allows the ATP synthase molecule to add a phosphate group to an ADP or adenosine diphosphate molecule.
Each of the three cellular respiration phases incorporates important cell processes, but the ETC produces by far the most ATP. Since energy production is one of the key functions of cell respiration, ATP is the most important phase from that point of view.
Where the ETC produces up to 34 molecules of ATP from the products of one glucose molecule, the citric acid cycle produces two, and glycolysis produces four ATP molecules but uses up two of them. As a result, the citric acid cycle is dependent on the ETC. Since the ETC can only take place in the presence of oxygen, which acts as the final electron acceptor, the cell respiration cycle can only operate fully when the organism takes in oxygen.
All advanced organisms need oxygen to survive. Some animals breathe in oxygen from the air while aquatic animals may have gills or absorb oxygen through their skins. In higher animals, the red blood cells absorb oxygen in the lungs and carry it out into the body. Arteries and then tiny capillaries distribute the oxygen throughout the body's tissues.
As mitochondria use up oxygen to form water, oxygen diffuses out of the red blood cells. Oxygen molecules travel across cell membranes and into the cell interior.
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, 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 group of heme. 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, giving slightly different characteristics to 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 two 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 make water H 2 O.
The removal of the hydrogen ions from the system contributes to the ion gradient used in the process of chemiosmosis. In chemiosmosis, the free energy from the series of redox reactions just described is used to pump hydrogen ions protons across the membrane. If the membrane were open to diffusion by the hydrogen ions, the ions would tend to diffuse back across into the matrix, driven by their electrochemical gradient.
Recall that many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels. Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane through an integral membrane protein called ATP synthase Figure 2. This complex protein acts as a tiny generator, turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient. The turning of parts of this molecular machine facilitates the addition of a phosphate to ADP, forming ATP, using the potential energy of the hydrogen ion gradient.
Figure 2. Credit: modification of work by Klaus Hoffmeier. Dinitrophenol DNP is an uncoupler that makes the inner mitochondrial membrane leaky to protons.
It was used until as a weight-loss drug. What effect would you expect DNP to have on the change in pH across the inner mitochondrial membrane? Why do you think this might be an effective weight-loss drug?
Chemiosmosis Figure 3 is used to generate 90 percent of the ATP made during aerobic glucose catabolism; it is also the method used in the light reactions of photosynthesis to harness the energy of sunlight in the process of photophosphorylation.
Recall that the production of ATP using the process of chemiosmosis in mitochondria is called oxidative phosphorylation. The overall result of these reactions is the production of ATP from the energy of the electrons removed from hydrogen atoms.
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