Biochemistry by Mary Campbell

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Biochemistry
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Lipids and Membranes
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Lipids

• Lipids a heterogeneous class of naturally
  occurring organic compounds classified together
  on the basis of common solubility properties
• insoluble in water, but soluble in aprotic
  organic solvents including diethyl ether,
  chloroform, methylene chloride, and acetone
• Lipids include
• fatty acids, triacylglycerols, sphingolipids,
  phosphoacylglycerols, glycolipids,
• lipid-soluble vitamins
• prostaglandins, leukotrienes, and thromboxanes
• cholesterol, steroid hormones, and bile acids

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Fatty Acids

• Fatty acid an unbranched-chain carboxylic acid,
  most commonly of 12 - 20 carbons, derived from
  hydrolysis of animal fats, vegetable oils, or
  phosphodiacylglycerols of biological membranes
• In the shorthand notation for fatty acids
• the number of carbons and the number of double
  bonds in the chain are shown by two numbers,
  separated by a colon

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Fatty Acids
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Fatty Acids

• Among the fatty acids most abundant in plants and
  animals
• nearly all have an even number of carbon atoms,
  most between 12 and 20, in an unbranched chain
• the three most abundant are palmitic (160),
  stearic (180), and oleic (181) acids
• in most unsaturated fatty acids, the cis isomer
  predominates the trans isomer is rare
• unsaturated fatty acids have lower melting points
  than their saturated counterparts the greater
  the degree of unsaturation, the lower the melting
  point

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Triacylglycerols

• Triacylglycerol (triglyceride) an ester of
  glycerol with three fatty acids
• natural soaps are prepared by boiling
  triglycerides (animal fats or vegetable oils)
  with NaOH, in a reaction called saponification
  (Latin, sapo, soap)

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Soaps

• Soaps form water-insoluble salts when used in
  water containing Ca(II), Mg(II), and Fe(III) ions
  (hard water)

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Phosphoacylglycerols

• Phosphoacylglycerols (phosphoglycerides) are the
  second most abundant group of naturally occurring
  lipids
• found almost exclusively in plant and animal
  membranes, which typically consist of 40 -50
  phosphoacylglycerols and 50 - 60 proteins
• the most abundant phosphoacylglycerols are
  derived from phosphatidic acid, a molecule in
  which glycerol is esterified with two molecules
  of fatty acid and one of phosphoric acid
• the three most abundant fatty acids in
  phosphatidic acids are palmitic (160), stearic
  (180), and oleic (181)

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Phosphoacylglycerols

• A phosphatidic acid
• further esterification with a low-molecular-weight
  alcohol gives a phosphoacylglycerol
• the most common of these low-molecular-weight
  alcohols are

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Phosphoacylglycerols
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Phosphoacylglycerols
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Phosphoacylglycerols

• A lecithin

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Waxes

• An ester of a long-chain fatty acid and alcohol
• from the Old English word weax honeycomb

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Sphingolipids

• contain sphingosine, a long-chain aminoalcohol
  from which this class is named

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Glycolipids

• Glycolipid a compound in which a carbohydrate is
  bound to an -OH of the lipid
• many glycolipids are derived from ceramides

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Steroids

• Steroids a group of plant and animal lipids that
  have this tetracyclic ring structure

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Steroids

• The features common to the ring system of most
  naturally occurring steroids are illustrated here

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Androgens

• Androgens male sex hormones
• synthesized in the testes
• responsible for the development of male secondary
  sex characteristics

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Estrogens

• Estrogens female sex hormones
• synthesized in the ovaries
• responsible for the development of female
  secondary sex characteristics and control of the
  menstrual cycle

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Cholesterol

• The steroid of most interest in our discussion of
  biological membranes is cholesterol

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Biological Membranes

• In aqueous solution, phosphoglycerides
  spontaneously form into a lipid bilayer, with a
  back-to-back arrangement of lipid monolayers (see
  Figure 7.10)
• polar head groups are in contact with the aqueous
  environment
• nonpolar tails are buried within the bilayer
• the major force driving the formation of lipid
  bilayers is hydrophobic interaction
• the arrangement of hydrocarbon tails in the
  interior can be rigid (if rich in saturated fatty
  acids) or fluid (if rich in unsaturated fatty
  acids)

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Biological Membranes

• Figure 7.10 A lipid bilayer

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Biological Membranes

• the presence of cholesterol increases rigidity
• with heat, membranes become more disordered the
  transition temperature is higher for more rigid
  membranes it is lower for less rigid membranes
• plant membranes have a higher percentage of
  unsaturated fatty acids than animal membranes
• the presence of cholesterol is characteristic of
  animal rather than plant membranes
• animal membranes are less fluid (more rigid) than
  plant membranes
• the membranes of prokaryotes, which contain no
  appreciable amounts of steroids, are the most
  fluid

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Membrane Proteins

• Functions transport substances across membranes
  act as receptor sites, and sites of enzyme
  catalysis
• Peripheral proteins
• bound by electrostatic interactions
• can be removed by raising the ionic strength
• Integral proteins
• bound tightly to the interior of the membrane
• can be removed by treatment with detergents or
  ultrasonification
• removal generally denatures them

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Fluid Mosaic Model

• Fluid there is lateral motion of components in
  the membrane
• proteins, for example, float in the membrane
  and can move along its plane
• Mosaic components in the membrane exist
  side-by-side as separate entities
• the structure is that of a lipid bilayer with
  proteins, glycolipids, and steroids such as
  cholesterol embedded in it
• no complexes, as for example, lipid-protein
  complexes, are formed

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Membrane Transport

• Passive transport
• driven by a concentration gradient
• simple diffusion a molecule or ion moves through
  an opening created by a channel protein
• facilitated diffusion a molecule or ion is
  carried across a membrane by a carrier protein
• Active transport
• a substance is moved against a concentration
  gradient
• primary active transport transport is linked to
  the hydrolysis of ATP or other high-energy
  molecule for example, the Na/K ion pump
  (Figures 7.24 and 7.25)
• secondary active transport driven by H gradient