Active transport defined

Active transport. The definition of active transport has been a subject of discussion for a number of years. Here, active transport is defined as a membrane transport process with a source of energy other than the electrochemical potential gradient of the transported substance. This source of energy can be either a metabolic reaction (primary active transport) or an electrochemical potential gradient of a substance different from that which is actively transported (secondary active transport). 

At a more molecular level, the influences of the composition of the membrane domains, which are characteristic of a polarized cell, on diffusion are not specifically defined. These compositional effects include the differential distribution of molecular charges in the membrane domains and between the leaflets of the membrane lipid bilayer (Fig. 3). The membrane domains often have physical differences in surface area, especially in the surface area that is accessible for participation in transport. For example, the surface area in some cells is increased by the presence of membrane folds such as microvilli (see Figs. 2 and 6). The membrane domains also have differences in metabolic selectivity and capacity as well as in active transport due to the asymmetrical distribution of receptors and transporters. 

Acetylcholine synthesis and neurotransmission requires normal functioning of two active transport mechanisms. Choline acetyltransferase (ChAT) is the enzyme responsible for ACh synthesis from the precursor molecules acetyl coenzyme A and choline. ChAT is the neurochemical phenotype used to define cholinergic neurons although ChAT is present in cell bodies, it is concentrated in cholinergic terminals. The ability of ChAT to produce ACh is critically dependent on an adequate level of choline. Cholinergic neurons possess a high-affinity choline uptake mechanism referred to as the choline transporter (ChT in Fig. 5.1). The choline transporter can be blocked by the molecule hemicholinium-3. Blockade of the choline transporter by hemicholinium-3 decreases ACh release,... 

It is important to establish an in vitro system which will allow in vivo transport across the bile canalicular membrane to be predicted quantitatively. By comparing the transport activity between in vivo and in vitro situations in isolated bile canalicular membrane vesicles, it has been shown that there is a significant correlation for nine types of substrates [90]. Here, in vivo transport activity was defined as the biliary excretion rate, divided by the unbound hepatic concentration at steady-state, whereas in vitro transport activity was defined as the initial velocity for the transport into the isolated bile canalicular membrane vesicles divided by the medium concentration [90]. Collectively, it is possible to predict in vivo canalicular transport from in vitro experiments with the isolated bile canalicular membrane vesicles. 

Active transport is defined as transport against a concentration gradient and is accomplished by pumps that must be coupled to energy expenditure to make the process spontaneous. 

Oxidation of fatty acids occurs in three well-defined steps namely, activation, transport into mitochondria, and oxidation to acetyl-CoA. 

Secondary active transport Secondary active transport is more complex. It involves the permeation of two different substances (A and B) across the membrane. The transport of A is active - it is an uphill process driven by the chemical reaction X—>Y. The transport of B is passive, but facilitated by a carrier C, which co-transports A (Equation 3). Co-transport is defined above in the section on passive transport. 

Bile formation occurs by processes that are not hilly defined. It takes place in canaliculi, minute passages lined by specialized modihcations of the hepatocyte membrane, that ultimately unite to form bile ductules. Hepatic bile contains 5% to 15% total solids, the major component of which is bile acids. The increase in biliary water and electrolyte excretion caused by this osmotic effect represents the bile acid-dependent fraction of bile flow. Even with severe depletion of the circulating bile acid pool, as is seen with bile duct diversion, some bile flow continues. The active transport of sodium and of glutathione and bicarbonate is mediated by Na-K-ATPase, which is responsible for the bile acid-independent flow of bile (up to 40% of total flow). Hormones such as secretin increase bile flow by stimulating secretion of sodium, bicarbonate, and chloride. Hormone-dependent flow accounts for 20% to 25% of the total. 

Provided that metabolism and active transport systems are not the dominating factors for the drug absorption process, there are - in general terms - two factors that fairly well characterize the oral absorption the solubility in combination with the permeability throngh membranes. Consequently, four classes of substances have been defined by Amidon G. et al. ... 

Extensive studies (57, 58, 59) defined the controlling processes for activity transport in the power reactors. These are oxide solubility, particle deposition, difiusion through oxide films, and rates of crystallization. Detailed models for activity production in-core and surface activation out-core have been developed (60) that successfully predict the growth of corrosion product fields in each of the CANDU reactors. 

Heavy fuel deposits were expected in boiling systems, and therefore the initial studies of deposition and activity transport for power reactors concentrated on the CANDU-BLW concept until the fields at Douglas Point became a concern. The deposit thickness was proportional to iron concentration in the coolant and to the square of the heat flux (69) deposition was reversible and quickly reached a steady value set by the local conditions. The corrosion products initially deposit by hydrodynamic and electrostatic effects then boiling accelerates deposition by drawing water and its contained iron into the deposit to replace the steam that leaves. Local alkalinity gradients within the deposit determine whether iron crystallizes to cement the deposit or dissolves to weaken it, and erosion processes then define the equilibrium thickness (70), This model works well in explaining deposition under boiling conditions. 

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