Transport across membranes facilitated

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. 

Be familiar with the composition and structure of biologic membranes. Be able to place the various phospholipids in the membrane bilayer. Know the function and position of membrane proteins and their possible movements. Know how membrane fluidity is controlled. Know the nature of various mechanisms to transport substances across membranes, receptor-mediated endocytosis, active and facilitated transport, ionophores, and the various types of channels. Be able to solve simple mathematical problems by creating solute gradients across membranes. Know the names of substances that inhibit the various modes of transport across membranes. 

The placenta is both a transport and a metabolizing organ. Transport is accomplished by simple diffusion, facilitated diffusion, active transport across membranes, and by special processes such as pinocytosis, phagocytosis, specific transport molecules, and channels in the barrier . The placenta also contains a full complement of mixed function oxidases located in the microsomal and mitochondrial subcellular fractions capable of induction and metabolism of endogenous and exogenous chemicals. 

Okadaic acid (OA), being a polyether, presents ionophoretic properties (facilitation of ion transport across membranes) as does CTX. It has been found that OA causes contraction in smooth muscles even in the absence of Ca (Ozaki and Karaki 1987 Shibata 1985). Ozaki and Karaki (1987) studied the mechanism of action of OA compared to calyculin A (another polyether isolated from a marine sponge). The results of this work suggest that OA has two separate effects activation of calcium channels as well as activating contractile elements to induce smooth muscle contraction. Recently, OA, in addition to other compounds from marine sources, has been found to be a tumor promoter that is, an agent that promotes tumor formation on already initiated cells (Fujiki 1988). It has been found that the OA class of tumor promoters bind to their own receptors which are present in particulate as well as cytosol fractions. The mechanism of action of these compounds has been partially elucidated. 

Synthetic ionophore ligands, mostly based on cyclic and branched polyethers, have been widely used in the last decade to study carrier facilitated cation transport across membranes. It has recently been observed that the organometallic ligand (C H )Co[P0(0C2H )2]2 Iso has... 

Kolomeisky, A.B., 2007. Channel-facilitated molecular transport across membranes Attraction, repulsion, and asymmetry, Phys. Rev. Lett., 98, 048105. 

Example 1.3-6 Facilitated transport across membranes Some membranes contain a mobile carrier, a reactive species that reacts with diffusing solutes, facilitating their transport across the membrane. Such membranes can be used to concentrate copper ions from industrial waste and to remove carbon dioxide from coal gas. Diffusion across these membranes does not vary linearly with the concentration difference across them. The diffusion can be highly selective, but it is often easily poisoned. Should this diffusion be described with mass transfer coefficients or with diffusion coefficients ... 

Facilitated transport membranes can be used to separate gases membrane transport is then driven by a difference in the gas partial pressure across the membrane. Metal ions can also be selectively transported across a membrane driven by a flow of hydrogen or hydroxyl ions in the other direction. This process is sometimes called coupled transport. 

Adenosine and inosine can be transported across cell membranes in either direction, facilitated by a membrane-associated nucleoside transport protein. Concentrative transporters have also been identified. Messenger RNA for a pyrimidine-selective Na+-nucleoside cotransporter (rCNTl) and a purine-selective Na+-nucleoside cotransporter (rCNT2) are found throughout the rat brain. Most degradation of adenosine is intracellular, as evidenced by the fact that inhibitors of adenosine transport, such as dipyridamole, increase interstitial levels of adenosine. Dipyridamole is used clinically to elevate adenosine in coronary arteries and produce coronary vasodilation. In high doses, dipyridamole can accentuate adenosine-receptor-mediated actions in the CNS, resulting in sedation and sleep, anticonvulsant effects, decreased locomotor activity and decreased neuronal activity. 

Ambient concentrations of COj are very low and usually biolimiting. Hence phytoplankton generally rely on bicarbonate as their carbon source. Phytoplankton must convert this bicarbonate to COj to enable production of organic matter. This conversion is facilitated by the Zn-containing enzyme, carbonic anhydrase (Table 11.4). Some phytoplankton release carbonic anhydrase into seawater with the resulting COj then transported across their cell membrane. 

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