carbohydrates

• Carbohydrates


  • A major source of energy from our diet
  • Made from the elements carbon, hydrogen, and oxygen
  • Also called saccharides, which means "sugars"
• Carbohydrate synthesis


  1. Produced by photosynthesis in plants

  2. Synthesized in plants from CO2, H2O, and energy from the sun

  3. Oxidized in living cells to produce CO2, H2O, and energy

  
• Types of carbohydrates

  • Monosaccharides, the simplest carbohydrates
  • Disaccharides, which consist of two monosaccharides
  • Polysaccharides, which contain many monosaccharides

  
• Monosaccharides


  • Contain several hydroxyl groups attached to a chain of three to eight carbon atoms
  • That contain an aldehyde group are classified as aldoses
  • That contain a ketone group are classified as ketoses
  • Have hydroxyl groups on all carbons except the carbonyl carbon

  
• Types of monosaccharides


  • Triose (three C atoms)
  • Tetrose (four C atoms)
  • Pentose (five C atoms)
  • Hexose (six C atoms)

  
• Aldopentose


  A five-carbon saccharide with an aldehyde group
• Ketohexose


  A six-carbon saccharide with a ketone group
• Achiral


  An object whose mirror image is identical to the original and can be superimposed on it
• Chiral


  Objects such as hands that have nonsuperimposable mirror images
• Structural isomers


  Molecules with the same molecular formula but different bonding arrangements
• Stereoisomers


  Molecules with the atoms bonded in the same sequence but differ in the way they are arranged in space
• Chiral carbon atoms


  Carbon atoms with four different atoms or groups
• Enantiomers


  Stereoisomers that cannot be superimposed
• Achiral carbon atoms


  Molecules with two or more identical atoms bonded to the same atom that can be superimposed
• Fischer projection


  A two-dimensional representation of a molecule that places the most oxidized group at the top, uses vertical lines for bonds that go back, and uses horizontal lines for bonds that go forward
• Drawing Fischer projections


  1. Places the carbonyl group, the most highly oxidized group, at the top

  2. Represents the chiral carbon atom at the center, with horizontal lines for bonds that project forward and vertical lines for bonds that point away

  
• Molecules in nature have mirror images, and often one stereoisomer has a different biological effect than the other one
• 1. Glucose, D-galactose, and D-fructose


  The most important monosaccharides, all hexoses with the molecular formula C6H12O6 and isomers of each other

  
• 1. Glucose


  The most common hexose, found in fruits, vegetables, corn syrup, and honey, also known as dextrose and blood sugar, a building block of disaccharides and polysaccharides
• 1. Galactose


  An aldohexose obtained from the disaccharide lactose, important in the cellular membranes of the brain and nervous system
• 1. Fructose


  A ketohexose, the sweetest of the carbohydrates, twice as sweet as sucrose, obtained as one of the hydrolysis products of sucrose, used to produce high-fructose corn syrup
• Hyperglycemia and hypoglycemia


  • Hyperglycemia occurs when the pancreas is unable to produce sufficient quantities of insulin, allowing glucose levels to rise as high as 350 mg/dL
  • Hypoglycemia occurs when the blood glucose level rises and then decreases rapidly to levels as low as 40 mg/dL, causing symptoms like dizziness, weakness, and muscle tremors
• Haworth structures


  The most stable forms of pentose and hexose sugars are five- or six-atom rings, produced from the reaction of a carbonyl group and a hydroxyl group in the same molecule
• Drawing Haworth structures


  1. Turn the Fischer projection clockwise by 90°, with the -H and -OH groups on the right below the horizontal chain and those on the left above

  2. Fold the horizontal chain into a hexagon and bond the O on carbon 5 to carbon 1

  3. Draw the -OH group on carbon 1 below the ring for the α isomer or above the ring for the β isomer
• Mutarotation


  When placed in solution, cyclic structures open and close, with α-D-glucose converting to β-D-glucose and vice versa, with only a small amount of open chain form present at any time
• Haworth structure of galactose

  An aldohexose with the -OH group on carbon 4 drawn above the ring
• Haworth structure of fructose

  A ketohexose forming a five-atom ring structure with carbon 2 at the right corner, with the -OH group on carbon 5 reacting with the carbonyl group on carbon 2
• Sugar alcohols


  Such as D-sorbitol, D-xylitol, and D-mannitol, used as sweeteners in many sugar-free products
• Oxidation of monosaccharides


  Monosaccharides have small amounts of the open-chain form present, with the aldehyde group and adjacent hydroxyl group that can be oxidized to carboxylic acid by an oxidizing agent

  
• Draw the new — OH group on carbon 1

  1. Below the ring to give the α isomer

  2. Above the ring to give the β isomer
• Identify the following structure of a monosaccharide as D- or L-ribulose

  
• Sugar alcohols


  • D-sorbitol, D-xylitol from D-xylose, and D-mannitol from D-mannose
  • Used as sweeteners in many sugar-free products such as diet drinks and sugarless gum as well as products for people with diabetes
• Oxidation of Monosaccharides


  1. Monosaccharides, found mostly as cyclic forms in solution, have small amounts of the open-chain form present

  2. Have an aldehyde group with an adjacent hydroxyl group that can be oxidized to carboxylic acid by an oxidizing agent such as Benedict's solution

  
• Sugar acids


  • Produced from the oxidation of the aldehyde as Cu2+ is reduced to Cu+
  • Named by replacing the ose ending of the monosaccharide with onic acid
• Reducing sugar


  A carbohydrate (such as the open-chain form of D-glucose) that reduces another substance
• Oxidation: Fructose to Glucose

  1. Fructose, a ketohexose, contains a ketone group, which usually cannot be oxidized

  2. Can be oxidized in a basic Benedict's solution when a rearrangement occurs between the ketone group on carbon 2 and the hydroxyl group on carbon 1

  3. Is then converted to glucose, which has an aldehyde group with an adjacent hydroxyl that can be oxidized
• Fructose, a ketohexose, rearranges to form glucose and is then oxidized in Benedict's solution
• Reduction of Monosaccharides


  1. Converts an aldehyde group to alcohol producing sugar alcohols, which are also called alditols

  2. Converts D-glucose to the sugar alcohol D-sorbitol
• Sugar alcohols


  • Named by replacing the ose ending of the monosaccharide with itol
  • Include D-sorbitol, D-xylitol from D-xylose, and D-mannitol from D-mannose
  • Used as sweeteners in many sugar-free products such as diet drinks and sugarless gum
• Lactose

  • A disaccharide found in milk and milk products
  • Contains the monosaccharides galactose and glucose