Lipids

1
Lipids

• General Characteristics
• Classification
• Biological function
• Lipids and detergents (HW)

2
Characteristics (use in conjunction with
Interactive Concepts in Biology)

• Composed of a hydrocarbon chain (R or alkyl
  group), hydrogen groups and carboxyl group at the
  end of the chain
• As the size of the alkyl group increases, the
  relative contribution of the carboxyl group to
  the physical properties of the molecule decreases
• Insoluble in water, soluble in organic solvents,
  fat-soluble

3
What are lipids?

• Are fat molecules or formed by dehydration or
  condensation reactions of fatty acids (an even
  of carbons, typically 10-24 carbons in the chain)
  and glycerol
• Are involved in
• energy storage (triglycerols in fat cells) and
  utilization (some fatty acids are used as
  building blocks, most are used for energy
  production
• structural elements of membranes,
• hormones (steroids, sex hormones),
• some aid in digestion and nutrient transport, may
  be part of proteins and carbohydrates1
• 1 Campbell, N. et al 2002 Biology Fifth
  Edition. Benjamin Cummings Publishing Company.
  United States

4
Classification of Lipids

• Lipids are classified according to solubility
• Fats and oils (triglycerides), waxes,
  phospholipids, sterols
• Formation of triglycerides
• Glycerol 3 Fatty Acids TRIGLYCERIDE
• Ester Linkage
• Fatty acid chain could be saturated or
  unsaturated

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• Saturated fatty acids
• Saturated with H
• Contain single carbon- carbon covalent bonds
• Examples
• 160 Palmitic acid CH3(CH2)14COOH
• 180 Stearic acid CH3(CH2)16COOH
• Unsaturated fatty acids
• Contain 1 or more C-C double bonds (mono-poly)
• Example
• 181 Oleic acid CH3(CH2)7 CHCH (CH2)
  7COOH

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or according to behavior in basic
environments

• Lipids are often classified as saponifiable and
  nosaponifiable.
• Saponifiable are hydrolyzed by bases (strong
  bases NaOH, KOH) to give salts and alcohol.
  Examples include tryglycerols, glycolipids,
  phospholipids, and some waxes
• Nonsaponifiable not hydrolyzed by base.
  Examples steroids, prostaglandins, some waxes.
• Saponification base catalyzed hydrolysis

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Lipids and Detergents

• Soaps are salts of fatty acids (Na or K salts)
• The hydrocarbon portion (the tail) of the fatty
  acid salt is water-insoluble but the ionic is
  water soluble, even more than alcohol!!
• Grease (and dirt) have the hydrocarbon character
  and so it dissolves in the hydrocarbon chains of
  the soap molecules

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• Water Hydrocarbons
• Hydrophilic and Hydrophobic and lipophilic
  (tail)
• Lipophobic portion of soap portion of the soap
• Soluble in hydrocarbon part of
• 
  fatty acid

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Then how does it help dissolve lipids?

• As a consequence oil (and dirt) bits are
  collected in the center of a micelle.
• Micelles stay in basic solutions as a colloid and
  do not come together to precipitate because of
  the ion repulsion

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Biological Function of Lipids

• Triglycerides and energy storage
• High in energy and water-insoluble
• In general, the less oxygen the molecule
  contains, the more energy it contains, then fats
  and oils have more energy than sugars and
  proteins (contain numerous hydroxyl atoms)2
• Fats and oils have about 9 kcal (Cal) of energy
  per gram, twice as effective as compared to dry
  carbohydrates and proteins
• Fats and oils are stored dry in fat cells, but
  glycogen is hydrated by numerous water
  molecules3
• 2 Bradford, M. et al. 1993 Organic and
  Biological Chemistry. Saunders College
  Publishing, United States. p. 301
• 3 Ibid

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Biological functions

• Fatty tissue serves a protective role
• Help to cushion and protect organs from injury
• Large amounts of fatty acids are required in
  animals
• These are used in insulation against cold
  temperatures and energy source in animals that
  hibernate
• Subcutaneous fat in humans also has an insulating
  role