Surfaces membranes

The flow along the membranes also improves the mass transport there, and the separators between the membranes are constmcted to provide good flow distribution and mixing on the membrane surfaces. Membrane sizes are often about 0.5 x 1 m, spaced about 1 mm apart. Many types of polymers are used to manufacture these ion-exchange-selective membranes, which are often reiaforced by strong fabrics made of other polymers or glass fibers. 

Factor VII. This is a vitamin K-dependent serine protease that functions in the extrinsic coagulation pathway and catalyzes the activation of Factors IX and X. Factor VII is present constitutively in the surface membrane of pericytes and fibroblasts in the adventitia of blood vessels, vascular endothehum, and monocytes. It is a single-chain glycoprotein of approximately 50,000 daltons. 

A receptor is a surface membrane component, usually a protein, which regulates some biological event in response to reversible binding of a relatively small molecule40 . The precise three-dimensional structures of the binding sites of receptors still remain unknown today. Thus, this section mainly describes the correlation of shape similarity between the molecules which would bind to a given receptor with their biological activity. 

Arencibia, I., Pedari, L., Sundqvist, K.G. (1987). Induction of motility and alteration of surface membrane polypeptides in lymphocytes by contact with autologous and allogenic fibroblasts. Expt. Cell Res. 172, 124-133. 

Voluntary muscle contraction is initiated in the brain-eliciting action potentials which are transmitted via motor nerves to the neuromuscular junction where acetylcholine is released causing a depolarization of the muscle cell membrane. An action potential is formed which is spread over the surface membrane and into the transverse (T) tubular system. The action potential in the T-tubular system triggers Ca " release from the sarcoplasmic reticulum (SR) into the myoplasm where Ca " binds to troponin C and activates actin. This results in crossbridge formation between actin and myosin and muscle contraction. 

While the fluid mosaic model of membrane stmcture has stood up well to detailed scrutiny, additional features of membrane structure and function are constantly emerging. Two structures of particular current interest, located in surface membranes, are tipid rafts and caveolae. The former are dynamic areas of the exo-plasmic leaflet of the lipid bilayer enriched in cholesterol and sphingolipids they are involved in signal transduction and possibly other processes. Caveolae may derive from lipid rafts. Many if not all of them contain the protein caveolin-1, which may be involved in their formation from rafts. Caveolae are observable by electron microscopy as flask-shaped indentations of the cell membrane. Proteins detected in caveolae include various components of the signal-transduction system (eg, the insutin receptor and some G proteins), the folate receptor, and endothetial nitric oxide synthase (eNOS). Caveolae and lipid rafts are active areas of research, and ideas concerning them and their possible roles in various diseases are rapidly evolving. 

As an example, the low-density lipoprotein (LDL) molecule and its receptor (Chapter 25) are internalized by means of coated pits containing the LDL receptor. These endocytotic vesicles containing LDL and its receptor fuse to lysosomes in the cell. The receptor is released and recycled back to the cell surface membrane, but the apoprotein of LDL is degraded and the choles-teryl esters metabolized. Synthesis of the LDL receptor is regulated by secondary or tertiary consequences of pinocytosis, eg, by metabolic products—such as choles-... 

MORE IS KNOWN ABOUT THE MEMBRANE OF THE HUMAN RED BLOOD CELL THAN ABOUT THE SURFACE MEMBRANE OF ANY OTHER HUMAN CELL... 

Neutrophils play a major role in the body s defense mechanisms. Integrins on their surface membranes determine specific interactions with various cell and tissue components. 

Most living cells, including muscle, maintain the cytoplasmic Ca concentration at submicromolar levels, against steep gradients of [Ca ], both at the cell surface and across the endoplasmic reticulum membrane [17]. In the musele cell two membrane systems are primarily involved in this function the sarcoplasmic reticulum and the surface membrane. 

The Ca transport ATPase of the surface membrane is a Ca -calmodulin-dependent enzyme of approximately 138-kDa mass that is structurally distinct from the sarcoplasmic reticulum Ca -ATPase, but shares with it some similarities in the mechanism of Ca translocation [2,3,34]. In both enzymes the Ca -dependent phosphorylation of an aspartyl-carboxyl-group by ATP leads to the formation of an acyl phosphate intermediate that provides the coupling between ATP hydrolysis and Ca translocation. 

Our discussion here will concentrate on the various forms of the Ca " transport ATPases that occur in the sarcoplasmic reticulum of muscle cells of diverse fiber types and in the endoplasmic reticulum of nonmuscle cells (SERCA). The structure of these enzymes will be compared with the Ca transport ATPases of surface membranes (PMCA) [3,29-32,34] and with other ATP-dependent ion pumps that transport Na, K, andH [46,50-52]. 

Classification of sarco(endo)plasmic reticulum (SERCA) and surface membrane (PMCA) Ca -ATPases... 

Both the intracellular and the plasma membranes are actively involved in the cell s vital functions. In the surface membranes of axons, processes of information transfer in the form of electrical signals (nerve impulses) lake place. Bioenergy conversion processes occur at the intracellular membranes of the mitochondria and chloroplasts. 

Any living cell continuously receives information about its surroundings. Its surface membrane has numerous protein receptors, which interact with practically all vitally important molecules. 

Our knowledge of biological membrane ultrastructure has increased considerably over the years as a result of rapid advances in instrumentation. Although there is still controversy over the most correct biological membrane model, the concept of membrane structure presented by Davson and Danielli of a lipid bilayer is perhaps the one best accepted [12,13]. The most current version of that basic model, illustrated in Fig. 7, is referred to as the fluid mosaic model of membrane structure. This model is consistent with what we have learned about the existence of specific ion channels and receptors within and along surface membranes. 

CD62P or P-selectin, which is a component of the platelet a granule membrane of resting platelets. It is expressed on the platelet surface membrane only after a granule secretion. P-selectin mediates the adhesion of activated platelets to neutrophils and monocytes. 

Blank, M. (Ed.), Bioelectrochemistry Ions, Surfaces, Membranes, American Chemical Society, Washington, 1980. 

Zarling, D.A., Miskimen, J.A., Fan, D.P., Fujimoto, E.K., and Smith, P.K. (1982) Association of Sendai virion envelope and a mouse surface membrane polypeptide on newly infected cells Lack of association with H-2K/D or alteration of viral immunogenicity. /. Immunol. 128, 251-257. 

To what extent does the SR contribute to the rise of [Ca2+]j that activates contraction In other words, what are the relative contributions of the SR and the surface membrane In contrast to the situation in striated muscle where inhibition of SR function abolishes most of contraction, there are several examples in smooth muscle of large amounts of force remaining under these conditions. The SR is an intracellular store of finite capacity. Release of Ca2+ from such a store is well suited to producing transient contractions. However, maintained contraction can be produced by steady state changes in Ca2+ fluxes across the surface membrane. Does the SR make different contributions during different phases of contraction ... 

Recent work using confocal microscopy has found localized increases of [Ca2+]j named Ca2+ sparks which are due to the release of Ca2+ from one or a small number of RyRs (Jaggar et al 2000). These localized releases of Ca2+ activate Ca2+-dependent channels in the surface membrane (Perez et al 2001). Activation of the Ca2+-activated K+ current will hyperpolarize the membrane potential (Herrera et al 2001) and thereby decrease Ca2+ entry into the cell on voltage-dependent Ca2+ channels. This provides a mechanism whereby Ca2+ release from the SR can decrease contraction. It is therefore important, in different smooth muscles, to consider to what extent SR Ca2+ release activates rather than decreases contraction. It is, of course, possible that, in the same smooth muscle, SR release may sometimes directly activate contraction and, at other times, decrease it by activating K+ channels. 

FIG. 4. Mechanisms to lower [Ca] in rat uterine myocytes. The cells were stimulated with carbachol and then the rate of decay of the Ca2+ transient determined. The traces show the effects of inhibiting Na/Ca exchange (top traces), surface membrane Ca-ATPase (middle traces) and SR Ca-ATPase (bottom traces). (Taken from Shmigol et al 1999.)... 

In single uterine cells measuring Ca2+ ], simultaneously with ICa and eliciting Ca2+ transients by repetitive membrane depolarizations, blocking the surface membrane Ca-ATPase with carboxyeosin decreased the rate of the decay of the Ca2+ transient by around 30—50%. 

Thus in the uterus the SR contributes to the decay of the [Ca2+] transients but, acting alone, it cannot significantly reduce Ca2+. The conclusion reached is that the SR takes up Ca2+ and then releases it close to the plasmalemmal Ca2+ extrusion sites (i.e. the SR is acting in series with the surface membrane extrusion mechanisms). This translocation is thought to be mediated by the SR forming a superficial barrier in the narrow space between the SR and plasmalemma and the SR releasing the Ca2+ in a vectorial manner (van Breemen 1977). 

In summary, in the uterus where L-type Ca entry represents the most significant mechanism for elevating Ca2+], then it is necessary that surface membrane extrusion mechanisms exist to remove the Ca2+ gained by the cell. In the uterus this task is performed equally by the Na+/Ca2+ exchanger and the Ca-ATPase. However the SR contributes to this process by taking up Ca2+ and vectorially releasing it to the extrusion sites, to maximize their efficiency. 

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