Carbohydrates

• Carbohydrates


  A class of food material, the most abundant organic compounds in nature
• Carbohydrate synthesis


  Plants synthesize carbohydrates via the process of photosynthesis
• Carbohydrates are divided into three classes: monosaccharides, disaccharides and polysaccharides

  
• Monosaccharides


  Simple sugars that cannot be hydrolysed to simpler compounds

  
• Monosaccharides


  • Can be further divided into polyhydroxyaldehydes (aldoses) and polyhydroxyketoses (ketoses)
  • Glucose is an aldose sugar with a terminal aldehydic group
  • Fructose is a ketose sugar with an internal carbonyl group

  
• Disaccharides


  Sugars formed from two monosaccharides or that can be hydrolysed to two monosaccharides

  
• Polysaccharides


  Carbohydrates that can be hydrolysed to many monosaccharide units, naturally occurring polymers of carbohydrates

  
• Polysaccharides


  • Starch
  • Cellulose
• Starch


  A polysaccharide whose carbohydrate units are easily added to store energy or removed to provide energy to cells
• Cellulose


  A polysaccharide that is a major structural component of plants
• Hydrolysis of either starch or cellulose gives many molecules of glucose

  
• Criteria for classifying monosaccharides


  • Whether the sugar contains a ketone or an aldehyde group
  • The number of carbon atoms in the carbon chain
  • The stereochemical configuration of the chiral carbon atom farthest from the carbonyl group

  
• Aldose


  Monosaccharides with a terminal aldehydic group
• Ketose


  Monosaccharides with an internal carbonyl group
• Triose, tetrose, pentose, hexose, heptose


  Monosaccharides with 3, 4, 5, 6, or 7 carbon atoms respectively

  
• Glucose


  An aldohexose
• Fructose


  A ketohexose
• Monosaccharides


  • Can exist in straight chain and cyclic forms
  • Cyclic forms include Haworth projection and chair conformation

  
• Anomeric carbon


  The carbon atom that forms the cyclic structure
• Alpha (α) and beta (β) anomers


  Diastereomers that differ in the orientation of the hydroxyl group on the anomeric carbon
• Anomers have different physical properties like melting point and specific rotation
• Mutarotation


  The change in specific rotation observed when one anomer dissolves in water, as the two anomers interconvert in solution
• Reduction of monosaccharides


  1. Aldoses and ketoses can be reduced to the corresponding polyalcohols (sugar alcohols or alditols)

  2. Glucose is reduced to sorbitol using NaBH4

  
• Mutarotation


  When a pure anomer of a monosaccharide dissolves in water, its rotation gradually changes to an intermediate rotation that results from equilibrium concentrations of the anomers
• Mutarotation of Monosaccharides


  1. Equilibrium mixture of α and β anomers forms

  2. Crystallisation below 98°C gives pure α-anomer

  3. Crystallisation above 98°C gives pure β-anomer

  
• Reduction of Monosaccharides


  1. Aldoses and Ketoses can be reduced to corresponding polyalcohols (sugar alcohols or alditols)

  2. Glucose is reduced to sorbitol using NaBH4

  
• Oxidation of Monosaccharides


  1. Tollens reagent can oxidise the aldehydic group of open-chain monosaccharides

  2. Non-reducing sugars don't react with Tollens reagent
• Acetal


  General formula: O-CH-OR'-OR''
• Glycoside


  Sugars in the form of acetals, names end in -oside suffix
• Glucoside


  Glycoside of glucose
• Glucopyranoside


  Glucoside with six-membered ring
• Riboside


  Glycoside of ribose
• Ribofuranoside


  Riboside with five-membered ring
• Acetal/Glycoside Formation


  1. Aldehydes and ketones are converted to acetals via treatment with alcohol and acid catalyst

  2. This also converts aldoses and ketoses to glycosides
• Aglycone


  Group bonded to the anomeric carbon atom of a glycoside
• Ether Formation


  Treating monosaccharide with methyl iodide and silver oxide converts hydroxyl groups to methyl ethers
• Ester Formation


  Monosaccharides can be converted to esters using acetic anhydride and pyridine
• Disaccharide


  Formed by condensation of two monosaccharide units
• Polysaccharide


  Formed by condensation of many monosaccharide units
• Starch


  • Made up of two polymers: Amylose (linear) and Amylopectin (branched)