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Chapter 6 Notes - Chapter 6 Notes Lipids Membranes and the...

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Chapter 6 Notes Lipids, Membranes, and the First Cells Most biologists believe life began with an RNA molecule that could copy itself that was stable and an efficient catalysis When this replicator became enclosed within a membrane, it was a big deal Plasma membrane or cell membrane separates life from nonlife o Layer of molecules that surrounds the cell interior and separates it from the external environment Plasma membrane: selective barrier keeps damaging compounds out of the cell and allows entry of compounds needed by the cell Because the plasma membrane sequesters the appropriate chemicals in an enclosed area, reactants collide more frequently – the chemical reactions necessary for life occur more efficiently The first cell functioned as an efficient and dynamic reaction vessel; responsible for the presence of a barrier that defines the cell and regulates the passage of materials Research suggests that the evolution of the first self-replicating molecule and the first membrane could have occurred simultaneously 6.1 Lipids Lipid: carbon-containing compounds that are found in organisms and are largely nonpolar and hydrophobic (do not dissolve readily in water) o Do dissolve in liquids consisting of nonpolar organic compounds Hydrocarbons: molecules that contain only carbon and hydrogen o Nonpolar electrons are shared equally in C-H bonds; equal electronegativity of carbon and hydrogen The reason lipids do not dissolve in water is that they have a significant hydrocarbon component Fatty acid: a hydrocarbon chain bonded to a carboxyl functional group A Look at Three Types of Lipids Found in Cells Unlike amino acids, nucleotides, and monosaccharides, lipids are characterized by a physical property – their solubility – instead of a shared chemical structure. The structure of lips varies widely Fats o Fats: composed of three fatty acids that are linked to a three-carbon molecule called glycerol Also called triacylglycerols or triglycerides o Fats form when a dehydration reaction occurs between a hydroxyl group of a glycerol and the carboxyl group of a fatty acid o Ester linkage: joins the glycerol and fatty-acid molecules Steroids o Steroids: a family of lipids distinguished by the bulky, four-rings structure o Differ because of the functional groups or side groups attached to the rings Phospholipids
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o Phospholipids: consist of a glycerol that is linked to a phosphate group and to either two chains of isoprene or two fatty acids o The phosphate group is bonded to a small, organic molecule that is charged or polar o Phospholipids with isoprene tails are found in the domain Archaea o Phospholipids composed of fatty acids are found in the domains Bacteria and Eukarya o Phospholipids are important components of the plasma membrane The Structures of Membrane Lipids The most important function of a lipid is their role in the plasma membrane
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