General Membrane Function

The water-soluble vitamins generally function as cofactors for metabolism enzymes such as those involved in the production of energy from carbohydrates and fats. Their members consist of vitamin C and vitamin B complex which include thiamine, riboflavin (vitamin B2), nicotinic acid, pyridoxine, pantothenic acid, folic acid, cobalamin (vitamin B12), inositol, and biotin. A number of recent publications have demonstrated that vitamin carriers can transport various types of water-soluble vitamins, but the carrier-mediated systems seem negligible for the membrane transport of fat-soluble vitamins such as vitamin A, D, E, and K. 

The inorganic poly(phosphazene) backbone has received attention as a PEM candidate. This is an attractive system for study due to its ease of synthesis and subsequent modification by many functional groups. However, these membranes generally show low glass transition temperatures and somewhat poor mechanical properties, and they require cross-linking to enhance their performance in hydrated environments. 

Is it possible for Professor McConnell to give a description of his observed A-point transition on a molecular basis, or does he believe that the complexity is so great that he can only describe the system in general functional thermodynamic terminology It is sometimes possible to describe A-point transitions in pure solids (e.g., NH4C1) in a molecular fashion, but can we hope to do this for a biological membrane ... 

Generalized representations of the internal structures of animal and plant cells (eukaryotic cells). Cells are the fundamental units in all living systems, and they vary tremendously in size and shape. All cells are functionally separated from their environment by the plasma membrane that encloses the cytoplasm. Plant cells have two structures not found in animal cells a cellulose cell wall, exterior to the plasma membrane, and chloroplasts. The many different types of bacteria (prokaryotes) are all smaller than most plant and animal cells. Bacteria, like plant cells, have an exterior cell wall, but it differs greatly in chemical composition and structure from the cell wall in plants. Like all other cells, bacteria have a plasma membrane that functionally separates them from their environment. Some bacteria also have a second membrane, the outer membrane, which is exterior to the cell wall. 

The discussion of synthetic membranes can be structured in terms of the function or the structure of the membrane used in a particular application. For instance, one can consider whether a membrane is used to separate mixtures of gas molecules vs particles from liquids (function) vs whether the membrane structure is primarily microporous or dense (structure). In fact, function and stmcture are linked, but to facilitate the consideration of physical science issues related to membranes appropriate for this reference, emphasis on functional aspects are probably most appropriate. This approach reflects the fact that the use of a membrane generally involves one or more physical sci-... 

Be able to describe the structures and general functions of cell walls, plasma membranes, nucleus and nucleoid region, mitochondria, lysosomes, peroxisomes, endoplasmic reticulum, cytoskeleton, and extracellular matrix. 

SPOT synthesis Structure determination not needed convenient on-paper assay moderately expensive spot synthesis equipment and custom-spot peptide membrane are commercially available. Peptide library is relatively small limited to binding and simple functional assay if bound peptides are used for the assay releasable peptides possible, but each in small quantity and peptide spot membrane generally not recyclable for subsequent use. 

From studies of lipid-water mixtures and isolated membranes the general functional features of the bilayer are known barrier properties, lateral diffusion, acyl chain disorder and protein association. To vmderstand the mechanisms behind a wide spectrum of membrane functions, a detailed picture at the level of local curvature is needed. Examples are fusion processes, cooperativity in receptor/ligand binding or transport through the bilayer of the proteins that are constantly synthesised for export from the endoplasmic reticulum. Some preliminary discussions of the possibilities of curved, rather than flat, membremes follow. 

Cubic membranes can be involved in curvature-controlled activation of certain enzymes, as well as control of enzyme activity. It is tempting to suggest the latter as a general function of cubic membranes since proteins could conceivably be located at regular points in the lattice. This could enhance transport efficiency of both product and substrate. Such mechanisms are particularly well suited for mass cooperative synthesis, such as those... 

FIGURE 2.37 Thoracic duct and other vessels of the thorax. Lymphatic capillaries are most numerous just beneath body surfaces, such as the skin and the mucus membranes of the gastrointestinal and respiratory tracts. The mucus membrane of the gastrointestinal tract is called the gut mucosa. The general function of these capillaries is to absorb interstitial fluid that has leaked from the circulatory system and to return it to the bloodstream. The function of the l)miphatic capillaries that end in the lacteals of the small intestine is to transport absorbed dietary lipids. These capillaries coalesce and eventually deliver their contents to the thoracic duct. The lymph collected from other parts of the body, as indicated by the "collecting trunk," also is transferred to the thoracic duct. [Redrawn with permission, from "Grant s Atlas of Anatomy," Williams Wilkins Co., Baltimore, 1978.]... 

The structures of F class and V class ion pumps are sIm liar to one another but unrelated to and more complicated than P-class pumps. F- and V-class pumps contain several different transmembrane and cytosolic subunits. All known V and F pumps transport only protons. In a process that does not Involve a phosphoprotein Intermediate. V-class pumps generally function to maintain the low pH of plant vacuoles and of lysosomes and other acidic vesicles In animal cells by pumping protons from the cytosolic to the exoplasmic face of the membrane against a proton electrochemical gradient. F-class pumps are found In bacterial plasma membranes and In mitochondria and chloroplasts. In contrast to V pumps, they generally function to power the synthesis of ATP from ADP and Pj by movement of protons from the exoplasmic to the cytosolic face of the membrane down the proton electrochemical gradient. Because of their Importance In ATP synthesis in chloroplasts and mitochondria, F-class proton pumps, commonly called ATP synthases, are treated separately In Chapter 8. 

Before we consider the specifics of membrane-protein function, we will consider some general principles of membrane transport. Two factors determine whether a molecule will cross a membrane (1) the permeability of the molecule in a lipid bilayer and (2) the availability of an energy... 

The following sections describe various organelles and membrane systems found in most human cells and outline the relationship between their properties and function. Each organelle has different enzymes and carries out different general functions. For example, the nucleus contains the enzymes for DNA and RNA synthesis. 

In this chapter, the authors describe the composition, structural organization, and general functions of biological membranes. After outlining the common features of membranes, a new class of biomolecules, the lipids, are introduced in the context of their role as membrane components. The authors focus on the three main kinds of membrane lipids—the phospholipids, glycolipids, and cholesterol. The amphi-pathic nature of membrane lipids and their ability to organize into bilayers in water are then described. An important functional feature of membranes is their selective permeability to molecules, in particular the inability of ions and most polar molecules to cross membrane bilayers. This aspect of membrane function is discussed next and will be revisited when the mechanisms for transport of ions and polar molecules across membranes is discussed in Chapter 13. 

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