Selective Permeability

8 Key excerpts on "Selective Permeability"

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  Plant Disease: An Advanced Treatise

  How Plants Suffer from Disease

  • James G. Horsfall(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  In addition, a comprehensive review of the role of permeability and host membranes in disease specificity is forthcoming (Hanchey and Wheeler, 1978). This plethora makes it easy to comply with the editors' request that this chapter not be a review of the literature. At the same time, it makes their challenge—to produce fresh ideas and to synthesize new concepts—more demanding. II. CONCEPTS OF C E L L PERMEABILITY Any valid concept or theory of the nature of cell permeability must account for the phenomenon of Selective Permeability mentioned in the preceding section as one characteristic of living organisms. A well-known and much-studied example of the phenomenon is the ability of cells, both plant and animal, to accumulate potassium and to exclude sodium in de-fiance of the laws of ordinary diffusion. Algal cells growing in seawater provide a striking example of Selective Permeability. Inside such cells the concentration of potassium is 40 times higher, whereas that of sodium is 5 times lower, than the concentration of these elements in the surround-ing seawater (Osterhout, 1922). The ability of algae to accumulate potassium is even more striking when cells are grown in solutions which contain only trace amounts of this element. Under these conditions, accumulation ratios (ratio of the concentration of potassium in the cell sap to that in the external solution) in excess of 2,000 have been ob-served (Hoagland and Davis, 1923). Selective Permeability of living cells is also reflected by their bioelectric properties. The potential dif- 15. P E R M E A B I L I T Y AND M E M B R A N E S 329 ference (PD) between the interior of a plant cell held in a dilute salt solution and the surrounding medium usually falls in the range of —50 to —150 mV. Maintenance of one component of this PD requires meta-bolic energy (Higinbotham, 1973). Three general theories of cell permeability have been proposed.

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  Physicochemical and Biomimetic Properties in Drug Discovery

  Chromatographic Techniques for Lead Optimization

  • Klara Valko(Author)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  Chapter 7

  Molecular Physicochemical Properties that Influence Absorption and Distribution—Permeability

  Biological Membranes

  Biological membranes cover every cell. They protect the inner environment against the changes occurring in the outside world. Biological membranes are very flexible and represent a physical state between liquid and solid. The membrane is formed from phospholipid bilayer. In 1972, Singer and Nicolson described the unique structure of biological membranes as a “fluid mosaic structure.” The polar head groups of phospholipids cover the outer sides of the membrane, while the inner part consists of completely nonpolar hydrocarbons. The polar head group is often covered with carbohydrates and proteins and hydrated with structured water molecules. The inner part of the membrane contains saturated or unsaturated hydrocarbon chains. The hydrophobic forces between the fatty acid chains hold the membrane firmly, but at the same time they make it very flexible too. There are many “holes” in the membrane that are usually covered with proteins. While polar water molecules cannot go through the inner hydrocarbon part of the phospholipid bilayer, they can go through the pores of the membrane together with other small polar molecules. The surface characteristics of the membrane depend on the nature of the polar head groups. There are several books and reviews that discuss the structure and physicochemical properties of cell membranes [1–6].

  As the membranes form a selective barrier to exogenous molecules, including drugs, it is important to understand the fundamental physicochemical principles of the passive membrane transport processes. The measurement of a compound's ability to pass through biological membranes is essential for successful drug discovery. Although the importance of active membrane transport processes is well known, the active transport processes are difficult to explain only by the physicochemical properties of compounds; therefore, it is not discussed here in detail.

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  Drug Delivery Across Physiological Barriers

  • Silvia Muro(Author)
  • 2016(Publication Date)
  • Jenny Stanford Publishing

    (Publisher)

  The entry of a substance in a cell depends on the properties of the membrane and the substance (ion or molecule) itself [12, 13]. From here, a fundamental concept emerges: plasma membranes are selectively permeable. In other words, the lipoprotein composition of a cell membrane determines which ions/molecules will enter and exit a 44 Plasma Membrane as a Semipermeable Barrier Membrane Protein Required Figure 2.2 Chemical and physical forces rule the transport of ions and molecules across the plasma membrane. Simple diffusion and facilitated diffusion are considered equilibrating transport, while the primary and secondary active transports are considered nonequilibrating. The driving force ( C) of the secondary active transport comes from the concentration gradient generated by primary active transport. cell. If a membrane allows a substance to cross it, the membrane is said to be permeable to that substance. Conversely, if a membrane does not allow a substance to cross the membrane, it is impermeable to the said substance. More generally, the permeability of a membrane is variable and can be modified by changing lipid and/or protein composition. Water, gas (oxygen, carbon dioxide), and some lipids move without any difficulty. Conversely, ions and polar molecules will not move freely. Finally, the proteins may cross a plasma membrane with extreme difficulty, or not cross it at all. The movement of ions and molecules through a membrane is defined as transport, a process influenced by two parameters: the size of the substance and its solubility in lipids. Small or some fat-soluble substances can pass through the phospholipid bilayer. In contrast, ions and molecules of moderate size or not soluble in lipids need transport systems such as those provided by a class of transmembrane proteins that form ion channels. Finally, large molecules and polar ones are transported via vesicles (Fig. 2.2).

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  Comprehensive Neurosurgery Board Review

  • Jonathan Stuart Citow, R. Loch Macdonald, Daniel Refai(Authors)
  • 2011(Publication Date)
  • Thieme

    (Publisher)

  I. Cellular Molecular Transport A. Cell membrane — a semipermeable lipid bilayer containing channel and carrier proteins that regulate the fl ow of ions and other molecules across the membrane. It also functions as a capacitor to store charge. B. Simple di ff usion — characterized by kinetic movement of ions or molecules across the cell membrane without the necessity for binding to carrier proteins in the membrane. It may occur either directly through the lipid bilayer of the membrane or through protein channels that are highly selective for speci fi c ions/molecules based on their shape, size, and charge. Simple di ff usion is limited to small ions/molecules (e.g., H 2 O) and lipid-soluble molecules (e.g., O 2 , N 2 , CO 2 , and alcohols). Other molecules exhibit limited di ff usion owing to their larger size (e.g., glucose) or electrostatic charge. C. Selective Permeability — the highly selective property of protein channels to transport speci fi c ions/molecules across the membrane based on shape, size, and charge. For example, Na + channels are small in diameter and contain a negatively charged inner surface to attract positively charged Na + ions. Once a Na + ion is inside the channel, it may then di ff use out in either direction. In contrast, K + channels are not negatively charged and are even smaller than Na + channels. These properties confer Selective Permeability to the channels for the transport of their speci fi c ionic species. D. Gating — mechanism responsible for controlling the permeability of a protein channel to the passage of ionic or molecular species. Gates are structural components of the protein molecule that may open or close over the opening of a channel in response to conformational changes in the protein molecule. The opening or clos-ing of these gates is dictated by either voltage changes or the binding of ligands.

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  Ion Transport Across Membranes

  Incorporating Papers Presented at a Symposium Held at the College of Physicians & Surgeons, Columbia University, October, 1953

  • Hans T. Clarke(Author)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  Ion Transport Across Biological Membranes HANS H. USSING DIFFUSION T H R O U G H BIOLOGICAL M E M B R A N E S At the outset it might be worth while to define briefly what we ex-perimental biologists mean by a biological membrane. I think most of us can agree upon a formulation like this: Whenever we meet, in a liv-ing organism or part thereof, a boundary that presents a diffusion re-sistance to solutes higher than that of the phases separated by the boundary, it is called a membrane. The membrane is often, but not always, anatomically discernible. The objects we study under the name of biological membranes are extremely diverse. Thus.we have membranes on the multicellular level like the gastric mucosa or the frog skin epithelium. Then there are the cell membranes like, for instance, the membranes of the nerve fiber. Finally, the work of the last few years tends to show that even membranes on the subcellular level are highly important. Notably, the surface of the mitochondria shows membrane-like properties, such as the ability to maintain, and under certain circumstances to create, within the mitochondrium concentrations of a number of substances which differ from those of the surroundings. As an example we may take a table from a recent paper by Bartley and Davies (1952) (Table I). It is seen that the Na ion undergoes a conspicuous concentration in the mitochondria as compared to the surrounding medium. At first sight there seems very little in common between the nerve fiber membrane and the skin of a frog, or between the tip of a plant root and the gill of a crab. Nevertheless, these different structures show many similarities in the way they handle inorganic ions. Formerly it was generally assumed that the similarities stemmed from the fact that the element determining the behavior of ions was in all cases a cell membrane, or possibly a number of cell membranes placed in series.

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  Aquaporins in Health and Disease

  New Molecular Targets for Drug Discovery

  • Graca Soveral, Soren Nielsen, Angela Casini(Authors)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  The present chapter describes established in vitro assays to assess AQP function in cells and tissues as well as the experimental strategies required to reveal functional regulation. The basic principles involved in water and solute permeation through membranes are described along with the theoretical models to access parameters defining water and solute transport, such as osmotic water permeability ( P f ), solute permeability ( P S ) and Arrhenius activation energies ( E a ). 1.1 INTRODUCTION Water homeostasis is central to the physiology of all living cells. Exchanges of water and sol-utes between environment and intracellular compartments require their passage through a membrane barrier composed of a hydrophobic lipid bilayer with specific transmembrane proteins facilitating permeation of polar and charged species. Channels that facilitate water permeation through cell membranes were first described on red blood cells in the late 1950s (Paganelli and Solomon 1957) and later on renal epi-thelia (Whittembury 1960). The first recognized water channel protein was identified in red blood cells by Agre and coworkers (Preston et al. 1992) and was named aquaporin 1 (AQP1). Now it is generally accepted that water crosses cell membranes by two parallel pathways, with distinct mechanisms for permeation: partition/diffusion of water mole-cules across the hydrophobic bilayer (with high activation energy for transport) and water molecule diffusion through aquaporins (AQPs) (with low activation energy) (Verkman 2000) (Figure 1.1). AQPs belong to a highly conserved group of membrane proteins called the major intrin-sic proteins that form a large family comprising more than 1700 integral membrane proteins found in virtually all living organisms (Abascal et al. 2014).

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  Infrastructure and Activities of Cells

  Biotechnology by Open Learning

  • M.C.E. van Dam-Mieras, B C Currell, R C E Dam-Mieras(Authors)
  • 2016(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  60 Chapter 3 The structure and function of membranes 3.1 Introduction Cells in multicellular organisms are maintained as distinct entities by the plasma membrane. This acts both as a barrier, separating the cell's internal solutions from those around it, and as a transport system, allowing the passage of certain compounds into or out of the cell. The functioning of this membrane is of paramount importance to the organism as a whole because the specialised functioning of cells depends upon the proper regulation of what goes in and what goes out. If cells are treated with chemicals which alter the permeability properties of the plasma membrane they can no longer maintain the correct internal environment and cease to function properly. In this chapter we will explore the structure of the plasma membrane, and indirectly, that of other cellular membranes, to see if we can understand how it carries out its various functions. 3.2 Early studies of the membrane needed a readily available source of cells If a scientist is planning to study plasma membranes a suitable source of material is necessary. One particular source has been used much more than any other. n Can you suggest what source of plasma membranes have been used more than any other? Would it be of plant or animal origin? Ideally this source will be readily available and it must be reasonably easy to obtain plasma membrane material from it. If the plasma membrane can be obtained in relatively pure form so much the better. I I Do you know of a cell type which satisfies these criteria? The answer is the human red blood cell. This is a quite unusual cell type because when it is fully mature it no longer contains a nucleus and has virtually no other internal membranes. It is also available from blood banks in considerable quantity. Thus red blood cells are a potentially useful source of membranes. Having got the cells, we now break them open by lysis, using the process of osmosis.

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  Biocultural Creatures

  Toward a New Theory of the Human

  • Samantha Frost(Author)
  • 2016(Publication Date)
  • Duke University Press Books

    (Publisher)

  Having dispelled the possibility of an überbio-logical anything, I will explore how the porosity of cell membranes enables the environment to shape how organisms develop from fertilized eggs through birth and growth to reproductive maturity. I will then trace some of the mechanisms through which that environmental shaping extends through and is realized in the development and growth of subsequent generations of organisms. In both cases, I will show that in living organ-isms, prior responses to habitat inputs are a limiting as well as enabling condition for further responses. The activities that distinguish the inside from the outside of a cell, or the inside from the outside of a living organ-ism, have a history. It is this history, as a set of patterns of responsiveness and activity in and between cells, that sets a living organism apart from its habitat to an extent such that even as there is a constant traffic of chem-icals and stimuli across the permeable cell membranes, the organism is not reducible to that environmental interchange. Cell Membranes Are Porous: A Reprise Like many scholars of culture and politics, contemporary scientists are trying to reconceptualize and rearticulate the relationship between organ-isms and their habitats. Questions about the extent and significance of the body’s porosity are at the center of such efforts. As more scientists let go of the assumption that there is some facet of the body that is sequestered from environmental influence, they contribute to a growing acknowledg-ment and an emerging consensus that the development and transforma-tion of organisms across generations is made possible and shaped by their porosity. The fact that this is a growing acknowledgment rather than a long-held insight needs explanation.

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