Chapter 4 Outline.docx - Chapter 4 TRANSPORT OF SUBSTANCES...

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Chapter 4 TRANSPORT OF SUBSTANCES THROUGH CELL MEMBRANE The Cell Membrane consists of a lipid bilayer with cell membrane transport proteins. This membrane consists almost entirely of a lipid bilayer. Lipid bilayer is not miscible with either the extracellular fluid or the intracellular fluid, therefore it constitutes a barrier. Barrier against movements Protein molecules in membrane have different properties for transporting substances. o Their structure interrupt the continuity of the bilayer leads to alternative pathway through the cell membrane. Transport proteins Figure 4.2 Channel proteins (selective)= have watery spaces all the way through the molecule and allow free movement of H2O and selected ions/molecules Carrier proteins (selective)= binds with molecules or ions that are to be transported, conformational changes in the protein move the molecules inside Lipid soluble substances can penetrate this lipid bilayer Active Transport : mvmt of ions/substances across the membrane in combinations with carrier protein in such a way that the carrier protein causes the substances to move against an energy gradient o Area of low concentration higher concentration state o Requires more energy than just kinetic energy Diffusion The continual mvmt of molecules among one another in liquids or in gases motion of the particles causes “heat” o Greater the motion greater the temp o Motion never ceases except at absolute zero temp Random molecular mvmt of substances, molecule by molecule, either through intermolecular spaces or in combination with a carrier protein. Diffusion through the cell membrane Diffusion is divided into two subtypes: o Simple diffusion: kinetic mvmt of molecules or ions through a membrane opening or through intermolecular spaces WITHOUT any interaction with carrier proteins The rate of diffusion is determined by:
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The amt of substance available The velocity of kinetic motion Number and size of openings in the membrane Can occur through two pathways: 1. Interstices of the lipid bilayer (if substance is lipid soluble) 2. Through watery channels that penetrate all the way through some of the large transport proteins (refer to Figure 4.2) o Facilitated diffusion : REQUIRES interaction of a carrier protein Binds chemically with them and shuttles them through the membrane in this form Important factor that determines how rapidly a substance diffuses through the lipid bilayer is the lipid solubility. o rate of diffusion of each of these substances through the membrane is directly proportional to its lipid solubility Many of the body’s cell membranes contain protein “Pores” called aquaporins that selectively permit rapid passage of H2O through the membrane.
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