Active transport definition

Although several allelochemicals (primarily phenolic acids and flavonoids) have been shown to inhibit mineral absorption, only the phenolic acids have been studied at the physiological and biochemical levels to attempt to determine if mineral transport across cellular membranes can be affected directly rather than indirectly. Similar and even more definitive experiments need to be conducted with other allelochemicals that are suspected of inhibiting mineral absorption. Membrane vesicles isolated from plant cells are now being used to elucidate the mechanism of mineral transport across the plasma membrane and tonoplast (67, 68). Such vesicle systems actively transport mineral ions and thus can serve as simplified systems to directly test the ability of allelochemicals to inhibit mineral absorption by plant cells. 

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). 

This simple experiment was important in that it clearly established the key notion that cellular extrusion of sodium ions by the sodium pump was coupled to metabolism. Because in this and subsequent experiments of the same sort the electrochemical gradient for sodium was known precisely, and since the fluxes of sodium (and later potassium) both into and out of the cell could be measured independently, this study also laid the groundwork for a theoretical definition of active transport, a theory worked out independently by Ussing in the flux ratio equation for transepithelial active transport of ions (see below). 

Active Transport. By definition, active transport occurs in the absence of any electrochemical potential originating in a concentration gradient (4,6). Active transport is driven by a coupled chemical reaction. Distinction is made between primary and secondary active transport. 

Biological membranes show anisotropy, as their molecules are preferentially ordered in a definite direction in the plane of the membrane, and the coupling between chemical reactions (scalar) and diffusion flow (vectorial) can take place. Almost all outer and inner membranes of the cell have the ability to undergo active transport. Sodium and potassium pumps operate in almost all cells, especially nerve cells, while the active transport of calcium takes place in muscle cells. The proton pumps operate in mitochondrial membranes, chloroplasts, and the retina. 

The active uptake of K+ and Cl-, together with an active extrusion of Na+, as for Nitella (Fig. 3-13), occurs for many plant cells. We might ask Why does a cell actively transport K+ and Cl- in and Na+ out Although no definitive answer can be given to such a question, we shall speculate on possible reasons, based on the principles that we have been considering. 

Facilitated diffusion has certain general characteristics. As already mentioned, the net flux is toward a lower chemical potential. (According to the usual definition, active transport is in the energetically uphill direction active transport may use the same carriers as those used for facilitated diffusion.) Facilitated diffusion causes fluxes to be larger than those expected for ordinary diffusion. Furthermore, the transporters can exhibit selectivity (Fig. 3-17) that is, they can be specific for certain molecules solute and not bind closely related ones, similar to the properties of enzymes. In addition, carriers in facilitated diffusion become saturated when the external concentration of the solute transported is raised sufficiently, a behavior consistent with Equation 3.28. Finally, because carriers can exhibit competition, the flux density of a solute entering a cell by facilitated diffusion can be reduced when structurally similar molecules are added to the external solution. Such molecules compete for the same sites on the carriers and thereby reduce the binding and the subsequent transfer of the original solute into the cell. 

Such a large osmotic pressure, caused by the high concentrations of sucrose and other solutes, suggests that active transport is necessary at some stage to move certain photosynthetic products from leaf mesophyll cells to the sieve elements of the phloem. From the definition of water potential, = P — II + pwgh (Eq. 2.13a), we conclude that the hydrostatic pressure in the phloem of a leaf that is 10 m above the ground is... 

While other terms and definitions may be used in speeifie transportation regulations and for other hazardous material activities, the definitions in Table 3.1 are consistent with those found in the Guidelines for Chemical Transportation Risk Analysis (CCPS, 1995) and Guidelines for Chemical Process Quantitative Risk Analysis, Second Edition (CCPS, 2000). These definitions are further defined and developed below. 

The question of the mechanism of active or metabolically coupled transport is, of course, concerned with the relationship between chemical reaction and transport. An interesting paradox has arisen in connection with the definition of active transport because coupling between the chemical reactions of metabolism and the processes of transport seems to require that scalar chemical processes should drive vector transport processes, thus contravening a principle attributed to Curie which is usually described in some such form as that it is impossible for a force of a given tensorial order to be associated with a flow of a higher tensorial order . This matter has been discussed for some years (see refs. 7-9), and in view of the attention that it has recently received, we shall attempt some clariflcation of the fundamental issues. 

Kedem ° attempted to circumvent the paradox implicit in the driving of vectorial transport processes with supposedly scalar chemical forces by introducing a vectorial cross coefficient in a non-equilibrium thermodynamic definition of active transport. Jardetzky , however, stated that the direct coupling between a metabolic reaction and a transport process, implied by Kedem s vectorial cross coefficient, was impossible because it would contravene the Curie principle (see also ref. 11). Katchalsky and Kedem, later supported by Moszynski et al, answered the criticism of Jardetzky by saying that Langeland had shown that the principle of Curie applies... 

Dmgs pass across barriers by a number of mechanisms. Passive mechanisms utilize the forces of concentration differences or pressure differences to move substances from one site to another. Active transport of a drug is typically via a specific transporter, requires energy, and moves solute against its electrochemical gradient. Figure 4 illustrates these definitions. 

Let us now consider in more detail the definitions of free energy related to enzyme reactions. Under steady state conditions, functioning enzyme complexes undergo cyclic transitions between a number of different states. These states can differ in the composition of a complex (enzyme molecule with ligands, substrates, products, low-molecular aflfectors, etc.), as well as in the conformations of an enzyme molecule. The complex s transitions are coupled with the chemical transformations of substrate molecules, the processes of association-dissociation of substrates and products, active transport of various substances, muscle contraction, etc. Most of these processes are of course associated with the energy transduction from one form to another. 

In the actual state of our knowledge we should carefully avoid the application of the definition of active transport, given below, to any translocation of a chemical group or radical from a donor to an acceptor. More appropriately, we should consider some of the relationships established between an organism and its surroundings For instance, how does a frog, sitting in tap-water, keep its interior of different ionic composition (Na and a ) from the outside medium ... 

Rosenberg, T., 1954, The concept and definition of active transport, Symp. Soc. Exp. Biol. 8 27. 

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