Selective Permeability

3 Key excerpts on "Selective Permeability"

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  eBook - ePub

  Physicochemical and Biomimetic Properties in Drug Discovery, Enhanced Edition

  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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  eBook - ePub

  General Cytology

  A Textbook of Cellular Structure and Function for Students of Biology and Medicine

  • Edmund Vincent Cowdry, Edmund Vincent Cowdry(Authors)
  • 2014(Publication Date)
  • University of Chicago Press

    (Publisher)

  SECTION III PERMEABILITY OF THE CELL TO DIFFUSING SUBSTANCES By MERKEL H. JACOBS University of Pennsylvania Passage contains an image PERMEABILITY OF THE CELL TO DIFFUSING SUBSTANCES MERKEL H. JACOBS

  According to the kinetic theory, molecules, small molecular aggregates, and ions in an essentially liquid medium such as protoplasm, or the various fluids which surround most living cells, have the tendency to become uniformly distributed. This tendency, which is as universal and fundamental as that of heat to flow from a warm to a cool body or electricity from a region of higher to one of lower potential, is utilized by every living cell in securing and distributing materials necessary to its metabolism and in getting rid of waste products. But in no case in a living cell does diffusion proceed in a free and unrestricted manner. Such unrestricted diffusion would soon put an end to that high degree of chemical heterogeneity which is characteristic of all protoplasm and on which life depends. It is a universal property of cells to limit and to modify in a manner which is frequently very complex the diffusion of dissolved substances. The extent of this limitation varies with different cells, with different substances, and with different external and internal conditions, but it is always present, and it is an important task for the physiologist and the cytologist to discover what factors are concerned in determining its characteristic features.

  The fundamental nature of the problem of cell permeability—using this term in general to cover the penetration of the cell in either direction by diffusing substances—is at once apparent when the activities of any organism are systematically reviewed. Because of their general interest, a number of examples from human and mammalian physiology may be mentioned in this connection, though such examples might equally well be chosen from a variety of other fields.

  Beginning with the alimentary system, the problem of cell permeability arises in many forms. Why, for example, does practically no absorption, even of water, occur in the stomach, while taking place with the greatest ease in the small intestine? Why, in the latter, are some substances absorbed much more rapidly than others; for example, NaCl more rapidly than Na2 SO4

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  eBook - ePub

  Membrane Technology in Separation Science

  • Mihir K. Purkait, Randeep Singh(Authors)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  1 Introduction to Membranes 1.1 An Overview of Separation Processes Life on Earth depends on various membrane processes and membranes play an important role in the lives of all of the living beings present on this planet. Examples are the process of respiration through the lungs, which continuously allows the diffusion of O 2 and CO 2 to and from our bodies to the outside; the purification of groundwater because of the different layers of earth that act as membranes of different porosities; and the skin of living organisms also works as a semipermeable membrane. The skin does not allow microorganisms to enter into our bodies, but allows diffusion of water in the form of sweat and other toxic substances from the body to the outside. Similarly, there are an infinite number of other examples that confirm the presence of different kinds of membranes around all of us. The word membrane is derived from the Latin word membrana, which means “skin.” Membrane is a sort of barrier that separates things and allows materials to be passed selectively [ 1 ]. There are many definitions available for a membrane. A general definition could be: A membrane is a thin barrier, placed between two phases, or mediums, which allow one or more constituents to selectively pass from one medium to the other in the presence of an appropriate driving force while retaining the rest [ 2 ]. This definition is based on a macroscopic level, but it should be taken care that the separation is at the microscopic level. Accordingly, it can be said that a membrane process is a combination of both mass and momentum transfer. A membrane can be homogeneous or heterogeneous, symmetric or asymmetric, solid or liquid; it can carry a positive or negative charge or be neutral or bipolar. Transport through a membrane can be affected by convection or by diffusion of individual molecules, induced by an electric field or concentration, pressure, or temperature gradient

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