Primary transporters

The primary transporter of cholesterol in the blood is low density Hpoprotein (LDL). Once transported intraceUularly, cholesterol homeostasis is controlled primarily by suppressing cholesterol synthesis through inhibition of P-hydroxy-P-methyl gluterate-coenzyme A (HMG—CoA) reductase, acyl CoA—acyl transferase (ACAT), and down-regulation of LDL receptors. An important dmg in the regulation of cholesterol metaboHsm is lovastatin, also known as mevinolin, MK-803, and Mevacor, which is an HMG—CoA reductase inhibitor (Table 5). 

In addition to biogeochemical cycles (discussed in Section 6.5), the hydrosphere is a major component of many physical cycles, with climate among the most prominent. Water affects the solar radiation budget through albedo (primarily clouds and ice/snow), the terrestrial radiation budget as a strong absorber of terrestrial emissions, and global temperature distribution as the primary transporter of heat in the ocean and atmosphere. 

Magnetic Resonance Microscopy of Bioflm and Bioreactor Transport primary transport mechanism is diffusive. The residence time distribution inte-... 

The primary transporters discussed in this chapter belong to three distinct genomic superfamilies that differ markedly in structure and reaction mechanism 74... 

Packaging neurotransmitters into presynaptic vesicles is mediated by proton-coupled antiporters. As discussed above, Golgi-derived membranes, including presynaptic vesicle membranes, contain V-type primary transporters that pump protons into the lumen of these membranes and vesicles. 

Figure 1. Solute transfer across an idealised eukaryote epithelium. The solute must move from the bulk solution (e.g. the external environment, or a body fluid) into an unstirred layer comprising water/mucus secretions, prior to binding to membrane-spanning carrier proteins (and the glycocalyx) which enable solute import. Solutes may then move across the cell by diffusion, or via specific cytosolic carriers, prior to export from the cell. Thus the overall process involves 1. Adsorption 2. Import 3. Solute transfer 4. Export. Some electrolytes may move between the cells (paracellular) by diffusion. The driving force for transport is often an energy-requiring pump (primary transport) located on the basolateral or serosal membrane (blood side), such as an ATPase. Outward electrochemical gradients for other solutes (X+) may drive import of the required solute (M+, metal ion) at the mucosal membrane by an antiporter (AP). Alternatively, the movement of X+ down its electrochemical gradient could enable M+ transport in the same direction across the membrane on a symporter (SP). A, diffusive anion such as chloride. Kl-6 refers to the equilibrium constants for each step in the metal transfer process, Kn indicates that there may be more than one intracellular compartment involved in storage. See the text for details...

EXPORT FROM THE CELL TO THE BLOOD BY PRIMARY TRANSPORT ATPASES... 

Primary transport the active transport of a solute against the electrochemical gradient by carrier proteins which derive energy for transport directly from the hydrolysis of ATP. 

Secondary transport the transport of solutes against the electrochemical gradient using energy derived from the co- or counter-transport of another solute down its electrochemical gradient. The latter gradient ultimately being derived from other primary transporters. 

Johnson ME, Blankschtein D, Langer R (1997) Evaluation of solute permeation through the stratum corneum lateral bilayer diffusion as the primary transport mechanism. J Pharm Sci 86 1162-1172. 

Ion transport pathways across the luminal and basolateral membranes of the thick ascending limb cell. The lumen positive electrical potential created by K+ back diffusion drives divalent (and monovalent) cation reabsorption via the paracellular pathway. NKCC2 is the primary transporter in the luminal membrane. 

The ion gradients formed by primary transport of Na+ or H+ can in turn provide the driving force for cotransport of other solutes. Many cell types contain transport... 

FIGURE 11-42 Lactose uptake in E. coli. (a) The primary transport... 

Trehalose, or "mushroom sugar," is found not only in fungi but also in many other organisms.39 41. It serves as the primary transport sugar in the hemolymph of insects and also acts as an "antifreeze" in many species. 

In addition to the Na+,K+- ATPases there is a very active Ca2+-ATPase which transports two Ca2+ from the inside of cells to the outside while returning two H+ from outside per ATP.510 543a This is the primary transporter by which cells maintain a low internal [Ca2+]. During its action it becomes phosphorylated on Asp 351. However, in neurons, in which the membrane potential is maintained at a high negative value by the sodium pump, an Na+/ Ca2+ ion exchange plays an even more important role.540... 

The water mould Blastocladiella emersonii generates a transcellular electrical current. It appears that K+ is actively accumulated in a primary transport process that may exchange K+ for H+, and that K+ leaks passively outwards through the K+ channel.84... 

In this equation, the diffusion coefficient D is related to air viscosity r A and particle diameter dp, with k being the Boltzmann constant and T the absolute temperature. It is clear from this description that diffusion is a rather slow deposition mechanism compared with impaction and sedimentation processes because it depends on the thermal velocity of the particles and not on airflow. It is the primary transport mechanism for small particles and is important when the transport distance becomes small, as in the deep lung. Efficiency of this deposition mechanism can be increased significantly by breath-holding because a portion of the ultrafine particles that are not deposited will be exhaled by the patient. 

We have already seen in Section 2.2 how the transport of both anions and cations is a vital part of biochemistry. We will examine supramolecular models of biological ion channels in detail in Chapter 12 but here we focus on some simple ion transport systems (ionophores) relevant to simultaneous anion and cation binding. Because of the need to maintain overall and local charge neutrality during any transport process the transport of individual ions across a biological or artificial membrane never occurs in isolation. There are two kinds of primary transport processes. Ion exchange or antiport, occurs when chemically different ions of like charge such as Na+ and K+ are simultaneously transported in... 

Can methanol achieve the necessary support from businesses and consumers to be viable as the nation s primary transportation fuel ... 

Carnitine supplementation is also critical in these patients. In acute severe metabolic crises, carnitine can be administered intravenously. Oral carnitine supplementation can be used for chronic maintenance therapy. Note that as the primary transport defect will still be present, carnitine supplementation cannot normalize the patient s carnitine homeostasis. 

Rutter AR, Fradley RL, Garrett EM, Chapman KL, Lawrence JM, et al. 2007. Evidence from gene knockout studies implicates ASC-1 as the primary transporter mediating D-serine reuptake in the mouse CNS. Eur J Neurosci 25 ... 

Hindered diffusion, the primary transport mechanism in porous solids, can be qualitatively described as a series of hops by the analyte, via gas-phase diffusion, from one surface site to the next. Thus, hindered diffusion is composed of two main components a pure diffusion-related term, often Fickian in nature, associated with movement of the analyte in the gas phase and a term describing the noninstantaneous equilibration between gas-phase analyte and the solid surface at each point where the analyte touches down (adsorbs). In extended porous solids (e.g., a chromatographic column tightly packed with porous beads), transport is often more complex, requiring the consideration of such factors as eddy diffusion and Knudsen effusion. This is important if there is a significant pressure drop along the path of the analyte [109]. Finally, the presence of any external fields (thermal, electric, etc.) must be considered as well. 

Trap-Controlled Hopping. Physical defects that are associated with intermolecular conformations that deviate in some specific way from the normal statistical distribution of molecular arrangements and extrinsic chemical species can generate additional localized states that lie outside of the distribution of bulk states. The bulk states control the primary transport channel, the microscopic mobility, whereas the additional extrinsic states... 

Active transport may be subdivided into primary transport/ which is directly coupled to substrate oxidation or high-energy phosphate hydrolysis/ or secondary transport/ which is coupled to cotransport of another molecule or ion down its thermodynamic gradient. We will first consider the thermodynamics of membrane transport and then review the four basic mechanisms. 

During the weathering process, elements can disperse from source mineralisation by a variety of chemical processes. For reasons discussed below, electrochemical processes are increasingly thought to be the primary transport mechanism in environments of thick, young, exotic (i.e., transported) overburden. They are also likely to operate in other environments but their dominance as a transport mechanism is less certain. This chapter presents the principles behind electrochemical masj transport and discusses the role of natural geoelectrochemical processes in the formation of selective leach and conventional geochemical soil anomalies. 

In the parts per billion water vapor range, the primary transporting gaseous species are A1 and AlH. However, in the parts per million water vapor range, the partial pressures of the volatile species diminishes, and an oxide skin forms . 

The United States, contrary to California, has never really had an energy policy it continues to sustain petroleum as the primary transportation fuel without effective market intervention. Except for the few "energy crises" that spurred interest in energy policy needs and created a few programs—crises that so far have been quickly forgotten—air quality concerns have carried the banner for fuel policy program needs. 

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