Membrane, transport across

Facilitated diffusion (channel), molecule moves down its electrochemical gradient. Active transport (pump), molecule moves up its electrochemical gradient (requires energy input). Pumps use energy (usually ATP hydrolysis). Na high outside/K high inside. 

Because membranes are impermeable to most molecules, you must have a transporter (a protein) in the membrane to help molecules or ions move through it (Fig. 3-5). They are also called channels because they behave like selective holes in fee membrane. Transporters are selective 

Pumps move ions and molecules up their electrochemical gradient. Pumps require energy, usually in the form of ATP hydrolysis. Sodium-potassium ATPase is an example of a pump. Cells maintain a higher concentration of potassium inside the cell than they do outside the cell. Sodium is maintained low inside, high outside. Sodium-potassium ATPase pumps three sodium ions from inside the cell to outside. This is the unfavorable direction—Na moves from low concentration to a higher one and against the membrane potential. At the same time, it also 

If the species has a charge on it, there s another AG associated with moving it tlu-ough a membrane. 

There are lots of questions that can be asked about your understanding of membrane potential. One common question is to give you a membrane potential, a few concentrations of ions, and ask which way will things move spontaneously. Which way will an ion move It will move so that the AG is 0 (favorable). Let s do an example. Assume that the membrane potential of the cell is —60 mV (inside). This is near the potential that the cell normally maintains and denotes that there are more negative ions inside the cell than outside. Let s also assume that the concentration of Na is 20 mM outside and 100 mM inside. Now, if we were to punch a hole in the membrane that would allow Na to move (a chaimel), which way would it move  

The considerable advance achieved in recent years in the discrete treatment of transport processes is primarily due to the development of the bilayer lipid membranes (BLM) modifiable with various ionophores. The most important results directly concerned with the functioning of excitable membranes are highlighted in Sections 4.2 and 4.3. 

Biolc ica] Inoi anic Chemistry, 2nd Edition. DOI 10.1016/B978-0-444-53782-9.00009-7. Copyright 2012 EIscvict B.V. All rights resCTved. 

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In subsequent studies attempting to find a correlation of physicochemical properties and antimicrobial activity, other parameters have been employed, such as Hammett O values, electronic distribution calculated by molecular orbital methods, spectral characteristics, and hydrophobicity constants. No new insight on the role of physiochemical properties of the sulfonamides has resulted. Acid dissociation appears to play a predominant role, since it affects aqueous solubiUty, partition coefficient and transport across membranes, protein binding, tubular secretion, and reabsorption in the kidneys. An exhaustive discussion of these studies has been provided (10). 

All of the transport systems examined thus far are relatively large proteins. Several small molecule toxins produced by microorganisms facilitate ion transport across membranes. Due to their relative simplicity, these molecules, the lonophore antibiotics, represent paradigms of the mobile carrier and pore or charmel models for membrane transport. Mobile carriers are molecules that form complexes with particular ions and diffuse freely across a lipid membrane (Figure 10.38). Pores or channels, on the other hand, adopt a fixed orientation in a membrane, creating a hole that permits the transmembrane movement of ions. These pores or channels may be formed from monomeric or (more often) multimeric structures in the membrane. 

Energy for maintenance is the energy required for survival, or non-growth related purposes. It includes activities such as active transport across membranes and turnover (replacement synthesis) of macromolecules. 

Unlike mass transport across membranes, which relates to chemical structure in predictable ways, the potencies of drugs as seen in pharmacological, pharmacodynamic, or other tests are highly structurally specific within a class of drugs and are without commonality across classes. A drug s activity involves a complex merging of these separate structural influences, with bioavailability always one of the concerns. Such concern is minimal when a truly superficial effect is involved, however. For example, the most potent antiseptic as measured in the test tube is likely to have... 

An exciting area in inclusion chemistry is the design and synthesis of molecules which could behave as ion channels. Future developments in this field offer the potential for developing new synthetic antibiotic molecules, model systems for investigating transport across membranes, and ion channels specific for particular ions. Such studies are so far only in their infancy. 

Ion transport across membranes can be evaluated by using mucosal and serosal electrodes to read transepithelial current (I) and potential difference OP). With these parameters, equivalent circuit analysis can be utilized to account for the relative contributions of transcellular and paracellular pathways. Ionic flux (J) is defined by the Nernst-Planck equation,... 

Movement of Ions and Molecules Across Membranes Transport Across Membranes The Nernst Equation... 

Lazdunski, C. J. (1995). Colicin import and pore formation a system for studying protein transport across membranes Mol. Microbiol., 16, 1059-1066. 

Phagocytosis is an important mechanism for the organism to rid itself of bacteria and pathogenic material, as well as cell debris and remnants of apoptosis. However, it can also provide a route for the uptake of pollutant particulate material. It is seen to be especially important in the incorporation of airborne particulate material, which often has serious health consequences (see Section 6.4). In terrestrial invertebrates, food is obtained either from particulate matter in the soil or from molecules dissolved in interstitial water. Most of these organisms have extracellular digestion, with nutrients and foreign material being absorbed by one or more of the routes available for transport across membranes, such as diffusion, channels or pinocytosis. There have been few studies to establish which route is taken. 

Hynn, G.L. and Yalkowsky, S.H. 1972, Correlation and prediction of mass transport across membranes I Influence of alkyl chain length on flux-determining properties of barrier and diffusant. J. Pharm. Sci. 61 838-852. 

Bath, B. D. White, H. S. Scott, E. R. Imaging molecular transport across membranes, John Wiley New York, 2001. 

The major antibody in external secretions such as milk, tears, sahva, mucus of the bronchial, intestinal and nrogenital tracts. It is secreted along with a transport protein to aid transport across membranes, which also protects the antibody against digestion or denaturation in extraceUnlar enviromnents... 

This agrees to internal VolSurf models derived for PAMPA membrane transport [163] to understand passive transcellular transport across membranes. One of our internal models based on 29 compounds characterized by immobilized artificial membrane chromatography by Salminen etal. ]164] shows an of 0.81 and = 0.70 for two PLS components derived using VolSurf descriptors. This is one of the rare examples where ionized starting molecules led to slightly better PLS statistics, while the general chemical interpretation is not affected. 

Molecular mechanism of ion transport across membrane Cells are enclosed by a membrane of about 70 A thickness and composed of double layers of protein separated by lipids. Cation cannot pass through the lipid layer without encapsulation and thus the enclosed cation presents an organic, lipid soluble surface to the membrane. 

Dick DAT, Naylor GJ, Dick EG Effect of lithium on sodium transport across membranes, in Dthium in Medical Practice. Edited by Johnson EN, Johnson S. Lancaster, England, MTP Press, 1978, pp 183-192 Dick P, Eerrero E A double-blind comparison study of the clinical efficacy of fluvoxamine and chlorimipramine. Br J Chn Pharmacol 15 [suppl 3) 419S-425S, 1983... 

Figure 19 Photochemical control of cation transport across membranes...

In conclusion, then, when true steric effects have been eliminated by proper choice of the members of the data set the bulk parameter constitutes a measure of ii (dispersion) and di interactions involved in either transport across membranes or in the formation of a substrate — receptor complex. 

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