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Alberts B, Johnson A, Lewis J, et al. Molecular Biology the the Cell. Fourth edition. Brand-new York: Garland Science; 2002.


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Through a collection of reaction that take place in the cytosol, energy acquired from the partial oxidation that energy-rich carbohydrate molecule is offered to type ATP, the chemical energy money of cells (discussed in chapter 2). But a much an ext efficient technique of energy generation appeared very early in the history of life. This process is based upon membranes, and also it allows cells come acquire power from a wide range of sources. For example, that is main to the counter of light energy into chemistry bond energy in photosynthesis, as well as to the aerobic respiration that permits us to usage oxygen to produce big amounts the ATP indigenous food molecules.

The membrane the is provided to produce ATP in procaryotes is the plasma membrane. However in eucaryotic cells, the plasma membrane is reserved for the transfer processes described in thing 11. Instead, the specialized membranes within energy-converting organelles space employed because that the production of ATP. The membrane-enclosed organelles are mitochondria, which are existing in the cells of practically all eucaryotic biology (including fungi, animals, and also plants), and also plastids—most significantly chloroplasts—which happen only in plants. In electron micrographs the most striking morphological function of mitochondria and chloroplasts is the big amount of internal membrane castle contain. This interior membrane gives the framework for one elaborate set of electron-transport processes that develop most that the cell"s ATP.

The common pathway used by mitochondria, chloroplasts, and also procaryotes to harness power for organic purposes operates through a process known together chemiosmotic coupling—reflecting a link in between the chemical bond-forming reactions that create ATP (“chemi”) and membrane-transport processes (“osmotic”). The coupling process occurs in two connected stages, both of which are performed through protein complexes embedded in a membrane:


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Figure 14-1

Harnessing power for life. (A) The essential requirements because that chemiosmosis room a membrane—in which are installed a pump protein and an ATP synthase, plus a source of high-energy electrons (e-). The proton (H+) displayed are freely easily accessible from (more...)


The electrochemical proton gradient is additionally used to drive other membrane-embedded protein machines (Figure 14-2). In eucaryotes, special proteins couple the “downhill” H+ circulation to the deliver of particular metabolites into and out of the organelles. In bacteria, the electrochemical proton gradient drives an ext than ATP synthesis and also transport processes; as a keep of straight usable energy, it additionally drives the fast rotation the the bacterial flagellum, which allows the bacterium to swim.


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Figure 14-2

Chemiosmotic coupling. Energy from sunshine or the oxidation of foodstuffs is very first used to produce an electrochemical proton gradient across a membrane. This gradient serves as a versatile energy store and is offered to journey a range of energy-requiring (more...)


It is advantageous to to compare the electron-transport procedures in mitochondria, which transform energy from chemistry fuels, through those in chloroplasts, which convert energy from sunshine (Figure 14-3). In the mitochondrion, electrons—which have actually been exit from a carbohydrate food molecule in the food of its degradation to CO2—are transferred through the membrane by a chain the electron carriers, finally reducing oxygen gas (O2) to kind water. The cost-free energy released together the electrons circulation down this path from a high-energy state to a low-energy state is offered to journey a series of 3 H+ pumps in the within mitochondrial membrane, and it is the 3rd H+ pump in the collection that catalyzes the transport of the electron to O2 (see figure 14-3A).


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Figure 14-3

Electron transfer processes. (A) The mitochondrion converts power from chemical fuels. (B) The chloroplasts converts power from sunlight. Inputs room light green, products are blue, and also the course of electron circulation is suggested by red arrows. Each of the (more...)


The system of electron transport can be compared to an electrical cell driving a current through a collection of electrical motors. However, in biological systems, electrons are carried between one site and also another no by conducting wires, however by diffusible molecule that deserve to pick up electrons at one location and also deliver them come another. For mitochondria, the first of this electron carrier is NAD+, i beg your pardon takes up 2 electrons (plus one H+) to come to be NADH, a water-soluble tiny molecule the ferries electron from the sites where food molecules are degraded come the inner mitochondrial membrane. The entire collection of protein in the membrane, together with the little molecules involved in the orderly sequence of electron transfers, is referred to as an electron-transport chain.

Although the chloroplast deserve to be described in similar terms, and several of its main contents are comparable to those that the mitochondrion, the chloroplast membrane has some critical components not uncovered in the mitochondrial membrane. Foremost amongst these are the photosystems, wherein light energy is recorded by the green pigment chlorophyll and harnessed to journey the deliver of electrons, lot as fabricated photocells in solar panels absorb light energy and use it to journey an electrical current. The electron-motive pressure generated by the chloroplasts photosystems cd driver electron transfer in the direction opposite to the in mitochondria: electrons are taken native water to create O2, and they space donated (via NADPH, a compound closely related to NADH) to CO2 to synthesize carbohydrate. Thus, the chloroplastic generates O2 and carbohydrate, vice versa, the mitochondrion consumes them (see figure 14-3B).

It is generally thought that the energy-converting oribel of eucaryotes progressed from procaryotes the were engulfed by primitive eucaryotic cells and also developed a symbiotic relationship with them. This would define why mitochondria and also chloroplasts contain their own DNA, which password for several of their proteins. Because their initial absorb by a host cell, this organelles have lost much of their very own genomes and have become heavily dependency on proteins that are encoded by genes in the nucleus, synthesized in the cytosol, and then imported into the organelle. Conversely, the organize cells have end up being dependent on these organelles for lot of the ATP they need for biosyntheses, ion pumping, and also movement; they have likewise become dependence on selected biosynthetic reactions that take place inside this organelles.

See more: When An Electric Current Is Passed Through Water During The Process Of Electrolysis


The MitochondrionElectron-Transport Chains and also Their Proton PumpsChloroplasts and PhotosynthesisThe genetic Systems the Mitochondria and also PlastidsThe evolution of Electron-Transport ChainsReferences

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