Cell membranes citric acid cycle

The surface metabolism hypothesis can also be used to deal with questions on the formation of the first cell structures for example, molecules required for the construction of cell membranes could have been formed via the so-called reductive Krebs cycle (citric acid cycle). 

Neurons oxidize glucose by glycolysis and the citric acid cycle, and the flow of electrons from these oxidations through the respiratory chain provides almost all the ATP used by these cells. Energy is required to create and maintain an electrical potential across the neuronal plasma membrane. The membrane contains an electrogenic ATP-driven antiporter, the Na+K+ ATPase, which simultaneously pumps 2 K+ ions into and 3 Na+ ions out of the neuron (see Fig. 11-37). The resulting... 

In eukaryotes, electron transport and oxidative phosphorylation occur in the inner membrane of mitochondria. These processes re-oxidize the NADH and FADH2 that arise from the citric acid cycle (located in the mitochondrial matrix Topic L2), glycolysis (located in the cytoplasm Topic J3) and fatty acid oxidation (located in the mitochondrial matrix Topic K2) and trap the energy released as ATP. Oxidative phosphorylation is by far the major source of ATP in the cell. In prokaryotes, the components of electron transport and oxidative phosphorylation are located in the plasma membrane (see Topic Al). 

Mitochondria arc membranous organelles (Fig. 1-9) of great importance in the energy metabolism of the cell they are the source of most of the ATP (Chap. 14) and the site of many metabolic reactions. Specifically, they contain the enzymes of the citric acid cycle (Chap. 12) and the electron-transport chain (Chap. 14), which includes the main oxygen-utilizing reaction of the cell. A mammalian liver cell contains about 1.000 of these organelles about 20 percent of the cytoplasmic volume is mitochondrial. 

Inside the inner membrane of a mitochondrion is a viscous region known as the matrix (Fig. 1-9). Enzymes of the tricarboxylic acid (TCA) cycle (also known as the citric acid cycle and the Krebs cycle), as well as others, are located there. For substrates to be catabolized by the TCA cycle, they must cross two membranes to pass from the cytosol to the inside of a mitochondrion. Often the slowest or rate-limiting step in the oxidation of such substrates is their entry into the mitochondrial matrix. Because the inner mitochondrial membrane is highly impermeable to most molecules, transport across the membrane using a carrier or transporter (Chapter 3, Section 3.4A) is generally invoked to explain how various substances get into the matrix. These carriers, situated in the inner membrane, might shuttle important substrates from the lumen between the outer and the inner mitochondrial membranes to the matrix. Because of the inner membrane, important ions and substrates in the mitochondrial matrix do not leak out. Such permeability barriers between various subcellular compartments improve the overall efficiency of a cell. 

Mitochondria generate most of the ATP required by aerobic cells by a joint endeavor of the reactions of citric acid cycle, which take place in the mitochondrial matrix, and oxidative phosphorylation, which takes place in the inner mitochondrial membrane. Mitochondria are descendents of a free-living bacterium that established a symbiotic relation with another cell. 

Various tiny structures, so-called organelles, are embedded in the cytoplasm where they make numerous cell functions possible, (s. fig. 2.9) (s. tab. 2.1) The enzyme-rich mitochondria have an outer and an inner membrane, with the latter forming creases (cristae). The outer membrane is relatively permeable for small molecules. However, the inner membrane (which surrounds the matrix) must use specific transport proteins to enable protons, calcium, phosphate and so on to pass. Energy-rich substrates are transformed into ATP in the mitochondria. The enzymes which are responsible for fatty-acid degradation and the citric-acid cycle can be found in the matrix. The inner membrane also contains the enzymes of the so-called respiratory cycle. An enormous number of energy-providing reactions and metabolic processes take effect at this site. They have a round-to-oval shape with a diameter of about 1 im. There are 1,400-2,200 mitochondria per liver cell (18-22% of the liver cell volume). They generally lie in... 

In eukaryotic cells aerobic metabolism occurs within the mitochondrion. Acetyl-CoA, the oxidation product of pyruvate, fatty acids, and certain amino acids (not shown), is oxidized by the reactions of the citric acid cycle within the mitochondrial matrix. The principal products of the cycle are the reduced coenzymes NADH and FADH, and C02. The high-energy electrons of NADH and FADH2 are subsequently donated to the electron transport chain (ETC), a series of electron carriers in the inner membrane. The terminal electron acceptor for the ETC is 02. The energy derived from the electron transport mechanism drives ATP synthesis by creating a proton gradient across the inner membrane. The large folded surface of the inner membrane is studded with ETC complexes, numerous types of transport proteins, and ATP synthase, the enzyme complex responsible for ATP synthesis. 

It is important for the cell to convert the potential energy in reduced coenzymes formed in glycolysis, and particularly the citric-acid cycle, into utilizable energy in the form of ATP. This is accomplished by a mitochondrial system that is part of the inner membrane and is referred to as the electron-transport system (Fig. 

Mitochondria are football-shaped organelles that are roughly the size of a bacterial cell. They are surrounded by an outer mitochondrial membrane and an inner mitochondrial membrane (Figure 22.1). The space between the two membranes is the intermembrane space, and the space inside of the inner membrane is the matrix space. The enzymes of the citric acid cycle, of the (3-oxidation pathway for the breakdown of fatty acids, and for the degradation of amino acids are all found in the mitochondrial matrix space. 

The mitochondria are aerobic cell organelles that are responsible for most of the ATP production in eukaryotic cells. They are enclosed by a double membrane. The outer membrane permits low-molecular-weight molecules to pass through. The inner mitochondrial membrane, by contrast, is almost completely impermeable to most molecules. The inner mitochondrial membrane is the site where oxidative phosphorylation occurs. The enzymes of the citric acid cycle, of amino acid catabolism, and of fatty acid oxidation are located in the matrix space of the mitochondrion. 

Processes occurring inside the mitochondrial matrix include pyruvate oxidation, fatty acid oxidation, amino acid metabolism, and the citric acid cycle. Furthermore, respiratory proteins are bound to the inner membrane, so the density of cristae corresponds to the respiratory activity of a cell. For example, mitochondria in heart muscle cells (high rates of respiration) are densely packed with cristae, whereas mitochondria in liver cells (low rates of respiration) have more sparsely distributed cristae. 

L-malate is an intermediate in the citric acid cycle, urea cycle, amino acid metabolism, the glyoxylate cycle, and shuttles across membranes of the cell (Figure 18.31). 

Aerobic metabolism is a highly efficient way for an organism to extract energy from nutrients. In eukaryotic cells, the aerobic processes (including conversion of pyruvate to acetyl-GoA, the citric acid cycle, and electron transport) all occur in the mitochondria, while the anaerobic process, glycolysis, takes place outside the mitochondria in the cytosol. We have not yet seen any reactions in which oxygen plays a part, but in this chapter we shall discuss the role of oxygen in metabolism as the final acceptor of electrons in the electron transport chain. The reactions of the electron transport chain take place in the inner mitochondrial membrane. 

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