Cytoplasm biochemical function

The major innovation of the fluid mosaic model, was to propose that the integral proteins were not scattered at random in the lipid bilayer, but formed specific, floating islands , which will here be called tesserae by analogy with a mosaic. The particular combination of proteins, including glycoproteins, in any tessera will determine its functional capacities and a given protein may form a part of several different tesserae. By this means, a relatively modest number of polypeptides could generate a wide diversity of biochemical functions. Individual tesserae can be linked to the cytoskeleton at their inner cytoplasmic face, by way of internal peripheral proteins and, probably, a hierarchy of filamentous proteins. At their outer faces they can be similarly... 

Another aspect of these problems is the effect of ions existing in the electrolytic solutions that constitute the fluid component of the cytoplasm. These ions can affect the conformation, interactions and biochemical functions of molecules in the cell. The Hofmeister series, which was first noted in 1888 [95], is invoked in this more modem context. It ranks the relative influence of ions on the physical behaviour of a wide variety of aqueous processes ranging from colloidal assembly to protein folding. The influence of an ion on the properties of macromolecules was initially thought to arise, at least in part, from its capacity of modifying bulk water stmcture. However, recent time-resolved and thermodynamic studies of water molecules in salt solutions show that bulk water structure is not central to the Hofmeister effect. Models are now being developed that take into account direct interactions between ion and macromolecule, and the interactions with water molecules that are operative in the first hydration shell of the macromolecule. 

The cytochrome subunit is exported to the periplasm via the Sec preprotein secretion apparatus. On the other hand, the small subunit is exported via the Tat system. Because the dehydrogenase subunit has no signal sequence for translocation across the cytoplasmic membrane, after completion of protein folding as well as FAD incorporation, it is exported by piggybacking on the small subunit being exported. The small subunit is required not only for translocation but also for active enzyme formation (Yamaoka et al. 2004) the biochemical function of the small subunit remains to be clarified. The subcomplex and the cytochrome subunit exported now assemble into the complex in the periplasm. 

Biochemically, most quaternary ammonium compounds function as receptor-specific mediators. Because of their hydrophilic nature, small molecule quaternaries caimot penetrate the alkyl region of bdayer membranes and must activate receptors located at the cell surface. Quaternary ammonium compounds also function biochemically as messengers, which are generated at the inner surface of a plasma membrane or in a cytoplasm in response to a signal. They may also be transferred through the membrane by an active transport system. 

Nonmuscle cells perform mechanical work, including self-propulsion, morphogenesis, cleavage, endocytosis, exocytosis, intracellular transport, and changing cell shape. These cellular functions are carried out by an extensive intracellular network of filamentous structures constimting the cytoskeleton. The cell cytoplasm is not a sac of fluid, as once thought. Essentially all eukaryotic cells contain three types of filamentous struc-mres actin filaments (7-9.5 nm in diameter also known as microfilaments), microtubules (25 nm), and intermediate filaments (10-12 nm). Each type of filament can be distinguished biochemically and by the electron microscope. 

Combining structural and biochemical information, MacLennan and his colleagues [8,11,42,45,48,87] constructed a hypothetical model of the tertiary structure of Ca " -ATPase that has interesting mechanistic implications (Fig. 2). The structure was divided into three major parts, designated as the cytoplasmic headpiece, the stalk domain and the transmembrane domain each was assigned distinct functional... 

Elliott, C., Muller, J., Miklis, M., Bhat, R. A., Schulze-Lefert, P. and Panstruga, R. (2005). Conserved extracellular cysteine residues and cytoplasmic loop-loop interplay are required for functionality of the heptahe-lical MLO protein. Biochem J. 385, 243-54. 

Specific receptors for siderophores and vitamin B12 have been identified in the OM of Gram-negative bacteria. The translocation of these ligands across the outer membrane follows an energy-dependent mechanism and also involves the TonB, ExbB, ExbD proteins anchored in the cytoplasmic membrane. Biochemical and genetic data indicate that these proteins form a functional unit (the Ton complex), which couples the outer membrane receptor-mediated... 

Grinstein, S., Swallow, C. J., Rotstein, O. D. (1991). Regulation of cytoplasmic pH in phagocytic cell function and dysfunction. Clin. Biochem. 24, 241-7. 

Passive transcellular transport across the intestinal epithelium involves three discrete steps (1) uptake across the apical membrane, (2) diffusion through the cytoplasm, and (3) efflux across the basolateral membrane. Occasionally, drug molecules without favorable physicochemical properties traverse the intestinal epithelium using endogenous membrane transporters.6-8 In addition, the intestinal mucosa, with its numerous drug-metabolizing enzymes and efflux transporters, such as P-glycoprotein (Pgp), functions as a biochemical barrier.9... 

Within the cell membrane is the cytoplasm. This is where many biochemical reactions take place. The most important structure within the human cell is the nucleus. It is bounded by a nuclear membrane and is separated from other organelles (noncellular structures in a cell that serve specific functions) in the cytoplasm. 

Sodium and potassium are not the only ions which can participate in pumps and channels. Calcium is also pumped, channeled, exhanged,and stored. See Figure 23. Calcium concentration within the cell cytoplasm is very low. This allows the calcium to play a pivotal role in cellular activity. The cytoplasmic protein calmodulin binds and stores calcium ion. Various intracellular structures and organelles such as the mitochondria and sarcoplasmic reticulum also store calcium. Calcium is vital to such functions as the release of neurotransmitters from nerve cells. There are at least seven known modes of biochemical action for this ion, one of the most important of which involves stimulation of cardiac muscle protein (actin-myosin). Certain types of angina (heart pain) are believed to be caused by abnormal stimulation of cardiac arteries and muscle (coronary spasm) A relatively new class of drugs, known as the calcium channel blockers, has brought relief from pain and arrhythmias (irregular heart beats). 

Biochemical reactions are organized so that different reactions occur in different parts of the cell. This organization is most apparent in eukaryotes, where membrane-bounded structures are visible proof for the localization of different biochemical processes. For example, the synthesis of DNA and RNA takes place in the nucleus of a eukaryotic cell. The RNA is subsequently transported across the nuclear membrane to the cytoplasm, where it takes part in protein synthesis. Proteins made in the cytoplasm are used in all parts of the cell. A limited amount of protein synthesis also occurs in chloroplasts and mitochondria. Proteins made in these organelles are used exclusively in organelle-related functions. Most ATP synthesis occurs in chloroplasts and mitochondria. A host of reactions that transport nutrients and metabolites occur in the plasma membrane and the membranes of various organelles. The localization of functionally related reactions in different parts of the cell concentrates reactants and products at sites where they can be most efficiently utilized. 

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