Lipids

• Lipids are a diverse group of hydrophobic molecules
• Lipids are one class of large biological molecule that don't include polymers and are not large enough to be considered macromolecules
• The compounds called lipids are grouped with each other because they are all hydrophobic; they mix poorly, if at all with water
• Lipids are hydrophobic because they consist mostly of hydrocarbons, which form nonpolar covalent bonds
• The most biologically important lipids are fats, phospholipids, and steroids
• Lipids

  Any group of large biological molecules including fats, phospholipids, and steroids that mix poorly, if at all with water
• Although fats are not polymers, they are large molecules assembled from smaller molecules by dehydration synthesis
• Fat

  A lipid consisting of three fatty acids linked to one glycerol molecule
• Glycerol is a three-carbon alcohol with a hydroxyl group (OH) attached to each carbon
• Fatty Acids

  • A fatty acid consists of a carboxyl group (COOH) attached to a long carbon skeleton
  • The many nonpolar C—H bonds in the long hydrocarbon skeleton make fats hydrophobic
• Fats separate from water because the water molecules hydrogen bond to one another and exclude the fats
• In making a fat, each fatty acid molecule is joined to glycerol by a dehydration reaction

  
  

  This results in an ester linkage, which is a bond between a hydroxyl group (OH) and carbonyl group (C=O)
• Whether a fat is saturated or unsaturated depends on the structure of hydrocarbon chains of the fatty acids
• Saturated fatty acid

  A fatty acid in which all carbons in the hydrocarbon tail are all connected by single bonds, thus maximizing the number of hydrogen atoms that are attached to the carbon skeleton
• Unsaturated fatty acid

  A fatty acid that has one or more double bonds between carbons in the hydrocarbon tail. Such bonding reduces the number of hydrogen atoms attached to the carbon skeleton
• Fats made from saturated fatty acids are saturated fats.

  • Most animal fats are saturated.
  • Saturated fats are solid at room temperature.
• Fats made from unsaturated fatty acids are unsaturated fats.

  • Plant and fish fats are liquid at room temperature and are known as oils.
• Hydrogenation

  The process of converting unsaturated fats to saturated fats by adding hydrogen

  
  

  Hydrogenating vegetable oils also creates unsaturated fats with trans double bonds
• The major function of fats is energy storage
• Many unsaturated fats are liquid at room temperature because their molecules are unable to closely pack together due to the kinks in some of their fatty acid hydrocarbon chains
• Phospholipids are major components of cell membranes
• •In a phospholipid, two fatty acids and a phosphate group are attached to glycerol

  •The two fatty acid tails are hydrophobic, but the phosphate group and its attachments form a hydrophilic head
• When phospholipids are added to water, they self-assemble into assemblages with the hydrophobic tails pointing toward the interior.

  This type of structure is called a micelle.
• Steroids

  A type of lipid characterized by a carbon skeleton consisting of four fused rings with various chemical groups attached
• Cholesterol

  • Cholesterol, an important steroid, is a component in animal cell membranes.
  • Cholesterol is also the precursor from which all other steroids are synthesized.
  • Many of these other steroids are hormones, including the vertebrate sex hormones.
  • While cholesterol is an essential molecule in animals, high levels of cholesterol in the blood may contribute to cardiovascular disease.