biochemistry Chapter 2 lipid

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  • Introduction to lipids

    • Lipids are organic compounds that are insoluble in water and soluble in organic solvents

    • Fats and oils are examples of lipids found in plant and animal tissues

  • Functions of lipids

    • Source of energy for the body

    • Storage and transport of fatty acids

    • Structural components of membranes

    • Precursor of steroid hormones

    • Facilitate absorption of fat-soluble vitamins

    • Act as emulsifying agents for lipid digestion

    • Provide waterproof coating in plants, feathers, and insect secretions

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  • Classification of lipids

    • Simple lipids

      • Esters of fatty acids with different alcohols

      • Sub-classified into neutral lipids and waxes

    • Compound lipids

    • Derived lipids

  • Simple lipids

    • Neutral lipids

      • Esters of fatty acids with glycerol (acylglycerols)

      • Monoacylglycerol, diacylglycerol, and triacylglycerol

    • Waxes

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  • Triglycerides

    • Major storage and transport forms of fatty acids

    • Simple triglycerols contain the same type of fatty acids in all three positions

    • Mixed triglycerols contain two or three different fatty acids

  • Fats and oils

    • Fats are solid at room temperature and contain a higher percentage of saturated fatty acids

    • Oils are liquid at room temperature and rich in unsaturated fatty acids

    • Oils can be converted to solid fats by saturating the fatty acids

  • Chemical properties of triglycerides

    • Hydrolysis of triacylglycerol to obtain fatty acids and glycerol

  • Saponification of triglycerides

    • Reaction with strong bases to form carboxylate salts (soaps)

    • Soaps have both hydrophobic and hydrophilic properties

    • Micelles are formed in aqueous solutions, important for lipid transport and soap actions

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  • Fatty acids

    • Long hydrocarbon chains with a terminal carboxylic acid group

    • Obtained from hydrolysis of lipids

    • Saturated, unsaturated, branched, and cyclic acids

    • Serve as a major fuel for cells and precursors of other lipid classes

  • Saturated fatty acids

    • All carbon atoms in the chain are saturated with hydrogen atoms

  • Monounsaturated fatty acids

    • Have one double bond in their structure

  • Polyunsaturated fatty acids

    • Have two or more double bonds separated by at least one methylene group

  • Importance of double bonds in fatty acids

    • Causes a bend or kink in the fatty acid chain

    • Affects the structure of biological membranes

  • Example: Olive oil contains a high percentage of oleic acid

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  • Most abundant fatty acids in animal fats, vegetable oils, and biological membranes

  • Saturated fatty acids and naturally occurring unsaturated fatty acids

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  • Nomenclature of fatty acids

    • Common (trivial) names and systematic names accepted in official nomenclature

  • Saturated fatty acids end with the suffix (-anoic)

  • Unsaturated fatty acids end with the suffix (-enoic)

  • Structure of fatty acids represented by two numbers separated by a colon

  • Representation of double bonds in fatty acids

    • C-System and ω-System

    • C-System indicates the position of double bonds using the symbol ∆ and subscript

    • Example: Oleic acid (C18) with one double bond at C-9 is represented as C:18:1:∆9

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  • ω-System: Refers to the carbon of the terminal methyl group in a fatty acid

  • Linolenic acid (C18) with three double bonds (C:18:3:ω-3)

  • Linoleic acid (C18) with two double bonds (C:18:2:ω-6)

  • Omega-6 and Omega-3 Fatty Acids

  • The first double bond in vegetable oils is at carbon 6 (omega-6)

  • The first double bond in fish oils is at carbon 3 (omega-3)

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  • The properties of fatty acids depend on chain length and number of double bonds

  • Shorter chain length fatty acids have lower melting temperatures

  • Unsaturated fatty acids have lower melting temperatures than saturated fatty acids of the same chain length

  • Essential fatty acids cannot be synthesized in the body and must be obtained from the diet

  • Essential fatty acids include linoleic, linolenic, and arachidonic acid

  • Arachidonic acid is the precursor of eicosanoids

  • Eicosanoids include prostaglandins, leukotrienes, thromboxanes, and lipoxins

  • Prostaglandins, leukotrienes, and thromboxanes regulate various biological processes

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  • Prostaglandins and related compounds regulate blood clotting, the inflammatory response, reproductive system, gastrointestinal tract, kidneys, and respiratory tract

  • Waxes are esters of long-chain fatty acids with long-chain alcohols

  • Waxes have higher melting points than triacylglycerols

  • Waxes are found as protective coatings on skin, fur, feathers, leaves, fruits, and exoskeletons

  • Sebum contains waxes that keep skin soft and prevent dehydration

  • Waxes are used in cosmetics, candles, ointments, and protective polishes

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  • Complex or Compound lipids are esters of fatty acids with alcohol containing additional groups

  • Sub-classifications of complex lipids include phospholipids, glycolipids, and lipoproteins

  • Phospholipids are lipids that contain fatty acids, alcohol, and phosphoric acid residue

  • Phospholipids can be classified as glycerophospholipids or sphingophospholipids

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  • Glycerophospholipids are the most abundant phospholipids found in cell membranes

  • Glycerophospholipids are derived from phosphatidic acid

  • Phosphatidic acid is composed of glycerol-3-phosphate esterified with two fatty acids

  • Glycerophospholipids are classified based on the alcohol esterified to the phosphate group

  • Examples of glycerophospholipids include phosphotidylcholin (Lecithin), phosphatidyl ethanol amine (Cephalin), phosphatidyinositol, and phosphatidylglycerol (Cardiolipin)

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  • Lecithin is the most abundant phospholipid in cell membranes and has both polar and non-polar ends

  • Lecithin can be hydrolyzed to yield choline, phosphoric acid, glycerol, and fatty acids

  • Cephalin and phosphatidyinositol are other types of phospholipids with different nitrogen bases

  • Phosphatidylglycerol (Cardiolipin) is found in mitochondrial membranes and myocardium

  • Phosphatidyl glyceracetals (Plasmalogens) are phospholipids with an aliphatic long chain unsaturated in C-1

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  • Phospholipids serve various biological functions including being major components of biological membranes, regulating cell membrane permeability, maintaining protoplasmic structure, transporting other lipids in the bloodstream, and acting as donors of arachidonic acid for the synthesis of prostaglandins and thromboxanes

  • Sphingophospholipids are derived from alcohol sphingosine and are structural components of cell membranes

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  • Sphingomyelins are phospholipids found in most animal cell membranes

    • They contain sphingosine, fatty acid, phosphoric acid, and choline

    • Abundant in the myelin sheath of the central nervous system

  • Glycolipids are molecules that contain carbohydrate and lipid

    • Glycosphingolipids are glycolipids that contain sphingosine

    • Ceramides are derived from sphingosine and have sugar residues

    • Cerebrosides are the simplest glycosphingolipids, containing a monosaccharide (galactose or glucose)

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  • Plant membranes contain sulfolipids, which have a sulfonated glucose residue

  • Gangliosides are glycolipids with oligosaccharide groups, including N-acetylneuraminic acid

  • Chloroplast thylakoid membranes in plant cells have a distinctive lipid composition

    • 40% galactolipids, 4% sulfolipids, and 10% phospholipids

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  • Lipoproteins are responsible for transporting lipid molecules in the bloodstream

  • Lipoproteins contain a core of hydrophobic lipids surrounded by proteins, phospholipids, and cholesterol

  • Lipoproteins are classified based on density:

    • Chylomicrons: transport dietary triglycerides from the intestine to tissues

    • Very low density lipoproteins (VLDL): transport endogenous triglycerides to tissues

    • Low density lipoproteins (LDL): carry cholesterol from the liver to tissues

    • High density lipoproteins (HDL): transport cholesterol from tissues to the liver

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  • Derived lipids are derived from other lipids, including steroids, terpenoids, fatty acids, and glycerol

  • Sterols are a class of steroids found in the membranes of most cells

  • Cholesterol is the major sterol in animal tissues and a precursor for steroid hormones, vitamin D, and bile salts

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  • Plants contain little cholesterol but have other sterols like stigmasterol and β-sitosterol

  • Terpenes are lipids formed from combinations of isoprene units

  • Monoterpenes, sesquiterpenes, and diterpenes are classes of terpenes

  • Triterpenes contain 30 carbon atoms and include squalene and lanosterol

  • Tetraterpenes contain 40 carbon atoms and include carotenes, precursors of vitamin A

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  • Rancidity is an unpleasant odor and taste in fats and oils when stored for a long time

  • There are two types of rancidity: hydrolytic and oxidative

  • Hydrolytic rancidity is caused by the growth of microorganisms that secrete lipases

  • Oxidative rancidity occurs due to the autooxidation of unsaturated fatty acids

  • Hydrogenation is a reaction used to combine hydrogen with unsaturated fats and oils to produce hydrogenated shortening

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  • Hydrogenation is used to enhance the keeping quality of vegetable oils

  • Oxidative rancidity can be prevented by adding compounds like vitamin E

  • Oxygen adds to the double bonds of unsaturated fatty acids in oxidative rancidity

  • The reaction of hydrogen with fats and oils is used to produce hydrogenated shortening and oleomargarine commercially.