Javad Jamshidi Fasa University of Medical Sciences, October 2015 The Biomembrane Structure & Function

2 The plasma membrane defines the cell and separates the inside from the outside. These biomembranes all have the same basic architecture-a phospholipid bilayer in which proteins are embedded They can bend and flex in three dimensions while still maintaining their integrity 2

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4 4 Image From: Lodish, Molecular Cell Biology 7e. 2013

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7 Differ in their chemical structures, abundance, and functions in the membrane Phosphoglycerides Sphingolipids Sterols 7

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9 glycerol 3-phosphate 9

10 All are derived from sphingosine, an amino alcohol In sphingomyelin, the most abundant sphingolipid, phosphocholine is attached to the terminal hydroxyl group of sphingosine 10

11 The major sterols in animals (cholesterol), fungi (ergosterol), and plants (stigmasterol) Like other membrane lipids, sterols are amphipathic. 11

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13 Image From: Lodish, Molecular Cell Biology 7e

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16 A characteristic of all biomembranes is an asymmetry in lipid composition across the bilayer Unlike particular phospholipids, cholesterol is relatively evenly distributed in both leaflets of cellular membranes How the asymmetric distribution of phospholipids in membrane leaflets arises is still unclear. When cells die, lipid asymmetry is no longer maintained 16

17 Yellow= PhosphatidylEthanolamine Green= PhosphatidylSerine Red= PhosphatidylCholine Brown= Sphingomyelin 17

18 Because cholesterol and sphingomyelin are found in more ordered, less fluid bilayers, they can form microdomains, termed lipid rafts 18

19 Proteins associated with a particular membrane are responsible for its distinctive activities. The kinds and amounts of proteins associated with biomembranes vary depending on cell type and subcellular location. Membrane proteins are defined by their location within or at the surface of a phospholipid bilayer 19

20 Membrane proteins can be classified into three categories on the basis of their position with respect to the membrane Integral Lipid-anchored Peripheral 20

21 Transmembrane proteins, span a phospholipid bilayer and comprise three segments. The cytosolic and exoplasmic domains have hydrophilic exterior surfaces The membrane-spanning segments usually contain many hydrophobic amino acids The membrane-spanning domains consist of one or more α helices or of multiple β strands. 21

22 (1)a single α helix (2) multiple α helices, (3) as a rolled-up β sheets(β barrel). 22

23 They are bound covalently to one or more lipid molecules. The hydrophobic segment of the attached lipid is anchors the protein to the membrane. The polypeptide chain itself dose not enter the phospholipid bilayer. Examples include bacterial lipoproteins, G proteins and certain kinases 23

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25 Do not directly contact the hydrophobic core of the phospholipid bilayer They are bound to the membrane either indirectly by interactions with integral or lipid-anchored membrane proteins or directly by interactions with lipid head groups Peripheral proteins can be bound to either the cytosolic or the exoplasmic face of the plasma membrane 25

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27 Every type of transmembrane protein has a specific orientation, known as its topology, with respect to the membrane faces Transmembrane glycoproteins are always oriented so that all carbohydrate chains are in the exoplasmic 27

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30 Movement of virtually all small molecules and ions across cell membranes is mediated by membrane transport proteins Each protein transports a particular class of molecule (such as ions, sugars, or amino acids) and often only certain molecular species of the class. Membrane transport proteins usually found to be multipass transmembrane proteins 30

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35 Table From: Lodish, Molecular Cell Biology 7e

36 Structure of Biomembrane Lipid Components of Biomembrane Different types of membrane Proteins Various types of transport in Biomembranes 36