Chapter 18: Lipids

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Chapter 18Lipids
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• IMPORTANT FUNCTIONS OF LIPIDS
• Protective wax coatings found on some plants
• Energy-rich compounds with low densities
• Storage form of energy for plants and animals
• Structural components, especially in cellular
  membrane formation

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• LIPID
• A biological compound that is soluble only in
  nonpolar solvents.

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• FATTY ACIDS
• Building blocks of many lipids
• Long chain carboxylic acids

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• FATTY ACIDS, cont.
• In water, fatty acids will form micelles.

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• FATTY ACIDS COMMON CHARACTERISTICS
• Usually straight chains (no branching)
• Sizes usually range from C10 to C20
• Usually have an even number of carbons
• Can be saturated (no CC bonds) or unsaturated
  (has CC bonds, usually in the cis configuration)
  
• Examples of saturated, monounsaturated, and
  polyunsaturated fatty acids containing 18 carbon
  atoms

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• FATTY ACID MOLECULAR SHAPES
• CC causes kinking of the carbon chain

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• FATTY ACIDS MOLECULAR SHAPES, cont.
• Fatty acids with CC bonds cannot pack closely
  together because of shape. This leads to
  decreased intermolecular attractions and lower
  melting points.
• Fatty acid melting points decrease as the number
  of CC bonds increases.
• Most unsaturated fatty acids are liquids at room
  temperature.

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• ESSENTIAL FATTY ACIDS
• Those needed by the body, but not synthesized
  within the body in adequate amounts.
• For humans, linoleic and linolenic acid are
  essential, but easily obtainable from plant and
  fish oils.

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• STRUCTURE OF FATS AND OILS
• Fats Usually from animal sources Solids at room
  temperature Contain a high degree of saturated
  fatty acids
• Oils Usually from plant and fish
  sources Liquids at room temperature Contain
  more unsaturated fatty acids

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• STRUCTURE, cont.
• Fats and oils are triglycerides (triglycerols)
  which are triesters of glycerol.

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• STRUCTURE, cont.
• An example of the esterification process

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• REACTIONS OF FATS AND OILS
• Hydrolysis important for fat and oil digestion

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• REACTIONS, cont.
• Saponification the commercial production of the
  salts of fatty acids (soaps)

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• REACTIONS, cont.
• Hydrogenation decreases the degree of
  unsaturation, used to make margarines from oils

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• WAXES
• Esters of fatty acids and long chain
  alcohols Water insoluble and not easily
  hydrolyzed Often found in protective coatings

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• PHOSPHOGLYCERIDES
• A phospholipid that contains an alcohol (usually
  an amino alcohol)

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• IMPORTANT PHOSPHOGLYCERIDES
• Lecithins Contain the amino alcohol
  choline Important cell membrane
  component Emulsifying agent
• Cephalins Contain ethanolamine or serine as the
  alcohol Found in most cell membranes, especially
  brain tissue

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• SPHINGOLIPIDS
• Sphingolipids contain sphingosine rather than
  glycerol.
• Two types of sphingolipids exist
• Sphingomyelin contains a phosphate and choline
  group. It is found in the myelin sheath
  surrounding nerve cells.
• Glycolipids contain carbohydrates unit. They
  are often called cerebrosides because of their
  abundance in brain tissue.

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• SPHINGOLIPID STRUCTURE

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• CELL MEMBRANES
• Prokaryotic Cells simple unicellular organisms
  with no internal membrane structures
• Eukaryotic Cells cells containing
  membrane-enclosed organelles, particularly a
  nucleus

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• MEMBRANE STRUCTURE
• Most are 60 lipid, 40 protein
• Lipids in membranes phosphoglycerides,
  sphingomyelin, and cholesterol
• Lipids are organized in a bilayer with
  hydrophobic (long carbon chain) portions inside
  and hydrophilic (polar groups) exposed to the
  water environment.

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• FLUID MOSAIC MODEL OF MEMBRANE STRUCTURE

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• STEROIDS
• A broad class of compounds that have the same
  structural feature
• Like all lipids, steroids are soluble in nonpolar
  solvents.

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• STEROIDS, cont.
• Cholesterol the most abundant steroid in the
  human body Essential component of cell
  membranes Precursor of other important
  steroids Synthesized by liver and present in
  foods Strong correlation between cholesterol
  blood levels and atherosclerosis

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• STEROIDS, cont.
• Bile salts bile is produced by the liver,
  stored in the gallbladder, and emptied into the
  intestine Bile salts emulsify lipids in the
  intestine, breaking
• apart lipid globules Increased surface area
  allows lipids to be more easily hydrolyzed

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• STEROID HORMONES
• Hormone a chemical messenger secreted by
  specific glands and carried through the blood to
  a target tissue, where it triggers a particular
  response
• There are two major categories of steroid
  hormones adrenocorticoid hormones and sex
  hormones.

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• ADRENOCORTICOID HORMONES
• Produced by the adrenal glands
• Two classes
• Mineralocorticoids regulate the concentration
  of ions in bodily fluids Aldosterone increases
  absorption of Na and Cl-
• Glucocortiocids enhance carbohydrate
  metabolismCortisol increases glucose and
  glycogen concentrations in the body

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• SEX HORMONES
• Androgens Male sex hormones produced by testes,
  including testosterone
• Estrogen and Progesterone female sex hormones
  produced by ovaries, including estradiol and
  estrone (the most important estrogens)

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• PROSTAGLANDINS
• Cyclical compounds synthesized from arachidonic
  acid
• Similar to hormones in that they are intimately
  involved in a host of body processes
• Involved in almost every phase of reproduction
• Participate in blood clotting

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• THERAPEUTIC POTENTIAL OF PROSTAGLANDINS
• PGE2 and PGF2 induce labor and used for
  therapeutic abortion in early pregnancy
• PGE2 in aerosol form used to treat asthma - opens
  up bronchial tubes by relaxing the surrounding
  muscles
• Others inhibit gastric secretions and used to
  treat peptic ulcers
• Many researchers believe that when they are fully
  understood, prostaglandins will be found useful
  for treating a much wider variety of ailments