Membrane in eukaryotes

The processes of electron transport and oxidative phosphorylation are membrane-associated. Bacteria are the simplest life form, and bacterial cells typically consist of a single cellular compartment surrounded by a plasma membrane and a more rigid cell wall. In such a system, the conversion of energy from NADH and [FADHg] to the energy of ATP via electron transport and oxidative phosphorylation is carried out at (and across) the plasma membrane. In eukaryotic cells, electron transport and oxidative phosphorylation are localized in mitochondria, which are also the sites of TCA cycle activity and (as we shall see in Chapter 24) fatty acid oxidation. Mammalian cells contain from 800 to 2500 mitochondria other types of cells may have as few as one or two or as many as half a million mitochondria. Human erythrocytes, whose purpose is simply to transport oxygen to tissues, contain no mitochondria at all. The typical mitochondrion is about 0.5 0.3 microns in diameter and from 0.5 micron to several microns long its overall shape is sensitive to metabolic conditions in the cell. 

Transport of Solutes Across Biological Membranes in Eukaryotes an Environmental Perspective... 

Handy, R. D. and Eddy, F. B. (2004). Transport of solutes across biological membranes in eukaryotes sodium and copper homeostasis in gill epithelial cells, In Physicochemical Kinetics and Transport at Biointerfaces, eds. van Leeuwen, H. P. and Koster, W., Yol. 9, IUPAC Series on Analytical and Physical Chemistry of Environmental Systems, Series eds. Buffle, J. and van Leeuwen, H. P., John Wiley Sons, Ltd, Chichester, pp. 337-356. 

The citric acid cycle operates in the mitochondria of eukaryotes and in the cytosol of prokaryotes. Succinate dehydrogenase, the only membrane-bound enzyme in the citric acid cycle, is embedded in the inner mitochondrial membrane in eukaryotes and in the plasma membrane in prokaryotes. 

The most well understood pathway is the one that delivers secretory and membrane proteins to the endoplasmic reticulum (ER) membrane in eukaryotic cells and to the inner membrane in bacteria. In both kinds of cells, the pivotal role is played by the so-called Sec61 (in eukaryotes) or SecYEG (in prokaryotes) translocon, a multisubunit translocation channel that provides a conduit for soluble proteins to cross the membrane. The same translocon also serves to integrate membrane proteins into the lipid bilayer. 

This enzyme is embedded in the mitochondrial inner membrane in eukaryotic cells and in the cytoplasmic membrane in prokaryotic cells (Ferguson-Miller and Babcock 1995, Malmstrom, 1990). Cytochrome c oxidase reduces dioxygen (O2) to water with electrons from cytochrome c... 

An extensive array of internal membranes in eukaryotes creates compartments within a cell for distinct biochemical functions. For instance, a double membrane surrounds the nucleus, the location of most of the cell s genetic material, and the mitochondria, the location of most ATP synthesis. A single membrane defines the other internal compartments, such as the endoplasmic reticulum. Some compartments can exchange material by the process of membrane budding and fusion. As with all membranes, the proteins associated with these membranes determine the specific biochemical function. Specific amino acid sequences in the proteins direct these molecules to the appropriate compartment. 

The nuclear membrane. In eukaryotes, transcription and translatio i take place in different cellular compartments transcription takes place in the membrane-bounded nucleus, whereas translation takes place outside the nucleus in the cytoplasm. In prokaryotes, the two processes are closely coupled Figure 29.15). Indeed, the translation of bacterial mRNA begins while... 

The electron transport chain is a series of complexes consisting of electron carriers located in the inner mitochondrial membrane in eukaryotic cells. 

Cytochrome oxidases are transmembrane protein complexes, which are localized at the inner mitochondrial membrane in eukaryotes or at the plasma membrane in bacteria. In addition to the reduction of oxygen, all cytochrome oxidases studied so far function as proton pumps as well, maintaining the proton gradient for the production of ATP [283 - 286]. While all cytochrome oxidases oxidize oxygen, they vary in their electron donors. Those receiving electrons from cytochrome c are called cytochrome c oxidases and those from ubiquinone ubiquinone oxidases [287]. 

In contrast to the electron carriers in the early stages of electron transport, such as NADH, FMN, and GoQ, the cytochromes are macromolecules. These proteins are found in all types of organisms and are typically located in membranes. In eukaryotes, the usual site is the inner mitochondrial membrane, but cytochromes can also occur in the endoplasmic reticulum. 

The energy source for the motility is, again, more diverse in prokaryotes proton-motive force, sodium-motive force, or ATP. In eukaryotes, the energy source appears to be solely ATP. This may simply be due to the fact that, in prokaryotes only, the proton- or sodium-motive force is readily available for the motility organelle. There, the ion gradient is maintained across the cytoplasmic membrane. In eukaryotic cells, the ion gradient is maintained across the mitochondrial membrane and is, therefore, not directly available for motility. The outcome is that the molecular mechanisms underlying motility of eukaryotic cells are less diverse than those of prokaryotic cells. 

A biological membranes system is typically formed by the combination of lipids and proteins. In eukaryotic cells, the plasma membrane, also referred to as the cell membrane, is a protective barrier which regulates what enters and leaves the cell. The endomembrane system is composed of different kinds of membranes which divide the cell into structural and functional compartments within a eukaryotic cell, such as the endoplasmic reticulum, Golgi apparatus, mitochondria, endosome and lysosome. Covalent modification of proteins with lipid anchors (protein lipidation) facilitates association of the lipidated proteins with particular membranes in eukaryotic cells. Protein lipidation is one of the most important protein post-translational modifications (PTMs). Studying lipidated protein function in vitro or in vivo is of vital importance in biological research. 

These data indicate that chromosomes are attached to the nuclear membrane in eukaryotes. Do interphase chromosomes possess specific regions which bind with certain areas on the inner nuclear membrane Comings (1968) has suggested that there exists an orderly arrangement of chromatin in the interphase nucleus and that this order is maintained by the nonrandom attachment of chromosomes to the inner nuclear membrane. Evidence for such order includes the apparent nomandom arrangement of chromosomes in metaphase and irradiation studies which suggest that chromosomes in somatic cells occur in polarized and fixed positions (Comings, 1968). 

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