Secondary transport

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). 

One interesting reference standard may be the methane stabilized helium neon laser at 3.39 pm. Its infrared frequency can be compared directly with the microwave cesium frequency standard with the help of a relatively short frequency chain [32], An accuracy of 1 part in 1012 or better appears feasible for a transportable secondary standard. 

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. 

Southam, E., Thomas, P.K., King, R.H.M., Goss-Sampson, M.A. and Muller, D.P.R. (1991) Experimental vitamin E deficiency in rats, morphological and functional evidence of abnormal axonal transport secondary to free radical damage. Brain 114 915-936. 

Thyroid hormones are taken into cells by facilitated diffusion or by active transport secondary to a sodium gradient (11). Once in the cell, thyroid hormones bind to cytosolic binding proteins and are not readily available for exchange with plasma hormones. Both T3 and T4 are not evenly distributed in body cells A great part of T4 is... 

Primary Transport Secondary Secondary Transport Tertiary... 

Primary active transport occurs when the transport of a substrate is coupled to an energy-yielding metabolic reaction. The energy required may come from several different sources (a) the high-energy compound ATP used by a specific ATPase (ATPase pump) (b) energy from the electron transport system released as electrons that flow down the cytochrome chain (redox-pump) and (c) the electric field produced by free radicals. Implicit in these three theories is the participation of ions and ion transport. Secondary active transport is a term often used to denote the transport of one substrate linked to the flow of a second substrate. Wilbrandt (1975) refers to this as flow-coupled active transport it may be this form of transport that is most often involved in the active uptake of sugars and amino acids. A review of some models of carrier-mediated active transport transport has recently been presented by Crane (1977). 

Air pollution can be considered to have three components sources, transport and transformations in the atmosphere, and receptors. The source emits airborne substances that, when released, are transported through the atmosphere. Some of the substances interact with sunlight or chemical species in the atmosphere and are transformed. Pollutants that are emitted directiy to the atmosphere are called primary pollutants pollutants that are formed in the atmosphere as a result of transformations are called secondary pollutants. The reactants that undergo transformation are referred to as precursors. An example of a secondary pollutant is O, and its precursors are NMHC and nitrogen oxides, NO, a combination of nitric oxide [10102-43-9] NO, and NO2. The receptor is the person, animal, plant, material, or ecosystem affected by the emissions. 

Commercially pure (< 99.997%) helium is shipped directiy from helium-purification plants located near the natural-gas supply to bulk users and secondary distribution points throughout the world. Commercially pure argon is produced at many large air-separation plants and is transported to bulk users up to several hundred kilometers away by tmck, by railcar, and occasionally by dedicated gas pipeline (see Pipelines). Normally, only cmde grades of neon, krypton, and xenon are produced at air-separation plants. These are shipped to a central purification faciUty from which the pure materials, as well as smaller quantities and special grades of helium and argon, are then distributed. Radon is not distributed commercially. 

If possible comparisons are focused on energy systems, nuclear power safety is also estimated to be superior to all electricity generation methods except for natural gas (30). Figure 3 is a plot of that comparison in terms of estimated total deaths to workers and the pubHc and includes deaths associated with secondary processes in the entire fuel cycle. The poorer safety record of the alternatives to nuclear power can be attributed to fataUties in transportation, where comparatively enormous amounts of fossil fuel transport are involved. Continuous or daily refueling of fossil fuel plants is required as compared to refueling a nuclear plant from a few tmckloads only once over a period of one to two years. This disadvantage appHes to solar and wind as well because of the necessary assumption that their backup power in periods of no or Httie wind or sun is from fossil-fuel generation. Now death or serious injury has resulted from radiation exposure from commercial nuclear power plants in the United States (31). 

Economic Aspects. To be useful the raw materials must be recoverable at a cost not greater than the cost of similar terrestrial materials. These costs must include transportation to the point of sale. Comparative costs of recovery are strongly influenced by secondary environmental or imputed costs, such as legal costs or compensatory levies. 

Examination of equation 5 shows that if there are no chemical reactions, (R = 0), or if R is linear in and uncoupled, then a set of linear, uncoupled differential equations are formed for determining poUutant concentrations. This is the basis of transport models which may be transport only or transport with linear chemistry. Transport models are suitable for studying the effects of sources of CO and primary particulates on air quaUty, but not for studying reactive pollutants such as O, NO2, HNO, and secondary organic species. 

The influx of Ca(Il) across the presynaptic membrane is essential for nerve signal transmission involving excitation by acetylcholine (26). Calcium is important in transducing regulatory signals across many membranes and is an important secondary messenger hormone. The increase in intracellular Ca(Il) levels can result from either active transport of Ca(Il) across the membrane via an import channel or by release of Ca(Il) from reticulum stores within the cell. More than 30 different proteins have been linked to regulation by the calcium complex with calmoduhn (27,28). 

Much more carbon dioxide is generated daily than is recovered (44). The decision whether or not to recover by-product carbon dioxide often depends on the distance and cost of transportation between the carbon dioxide producer and consumer. For example, it has become profitable to recover more and more carbon dioxide from C02-rich natural gas weUs in Texas as the use of carbon dioxide in secondary oil recovery has increased. The production levels for enhanced oil recovery are generally not reported because of the captive nature of the appHcation. 

SASOLII a.ndIII. Two additional plants weie built and aie in operation in South Africa near Secunda. The combined annual coal consumption for SASOL II, commissioned in 1980, and SASOL III, in 1983, is 25 x 10 t, and these plants together produce approximately 1.3 x lO" m (80,000 barrels) per day of transportation fuels. A block flow diagram for these processes is shown in Figure 15. The product distribution for SASOL II and III is much narrower in comparison to SASOL I. The later plants use only fluid-bed reactor technology, and extensive use of secondary catalytic processing of intermediates (alkylation, polymerisation, etc) is practiced to maximise the production of transportation fuels. 

Pneumatic-Conveying Devices See Sec. 21 for descriptions, ratings, and design factors on these devices. Use is primarily for transport purposes, and heat transfer is a very secondary consideration. 

Location of Transfer Stations Whenever possible, transfer stations should be located (1) as near as possible to the weighted center of the individual solid-waste-produciion ares to be served, (2) within easy access of major arterial highways as well as near secondary or supplemental means of transportation, (3) where there will be a minimum of pubhc and environmental objection to the transfer operations, and (4) where construction and operation will be most economical. Additionally, if the transfer-station site is to be used for processing operations involving material recovery and/or energy production, the requirements for those operations must be considered. 

Modern central stations use the other burner-furnace configurations shown in Fig. 27-16, in which the coal and air are mixed rapidly in and close to the burner. The primary air, used to transport the pulverized coal to the burner, comprises 10 to 20 percent of the total combustion air. The secondary air comprises the remainder of the total air and mixes in or near the burner with the primary air and coal. The velocity of the mixture leaving the burner must be high enough to prevent flashback in the primaiy air-coal piping. In practice, this velocity is maintained at about 31 m/s (100 ft/s). 

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