Carbohydrates

• Carbohydrates oxidation provides energy
• Carbohydrate storage, in the form of glycogen, provides a short-term energy reserve
• Carbohydrates supply carbon atoms for the synthesis of other biochemical substances (proteins, lipids, and nucleic acids)
• Carbohydrates form part of the structural framework of DNA and RNA molecules
• Carbohydrates link to lipids are structural components of cell membranes
• carbohydrates linked to proteins function in a variety of cell-cell and cell-molecule recognition processes
• A carbohydrate is a polyhydroxy aldehyde, a polyhydroxy ketone, or a compound that yields polyhydroxy aldehydes or polyhydroxy ketones upon hydrolysis
• Classification of Carbohydrates

  • Monosaccharides
  • Disaccharides
  • Oligosaccharides
  • Polysaccharides
• Monosaccharides are classified as aldose or ketose on the basis of the type of carbonyl present
• Disaccharides are glycosides formed from the linkage of two monosaccharides
• Oligosaccharides are carbohydrates that contain three to ten monosaccharide units
• Polysaccharides are polymers in which monosaccharides are the monomers
• Most monosaccharides exist in two forms: a "left handed" and "right handed" form
• superimposable mirror images are images that coincide at all points when the images are laid upon each other
• nonsuperimposable mirror images are images where not all points coincide when the images are laid upon each other
• The carbohydrate glucose is a polyhydroxy aldehyde, and the carbohydrate fructose is a polyhydroxy ketone.
• Monosaccharides:

  • Carbohydrates containing a single polyhydroxy aldehyde or polyhydroxy ketone unit
  • Cannot be broken down into simpler units by hydrolysis reactions
  • Glucose and fructose are examples of monosaccharides
  • Naturally occurring monosaccharides have 3 to 7 carbon atoms, with 5- and 6-carbon species being common
  • Pure monosaccharides are water-soluble, white, crystalline solids
• Disaccharides:

  • Carbohydrates containing two monosaccharide units covalently bonded
  • Examples include sucrose (table sugar) and lactose (milk sugar)
  • Hydrolysis of a disaccharide produces two monosaccharide units
  • Like monosaccharides, disaccharides are crystalline, water-soluble substances
• Oligosaccharides:

  • Carbohydrates containing three to ten monosaccharide units covalently bonded
  • "Free" oligosaccharides are rare in biochemical systems
  • Usually found associated with proteins and lipids in complex molecules with structural and regulatory functions
  • Complete hydrolysis of an oligosaccharide produces several monosaccharide molecules
• Polysaccharides:

  • Polymeric carbohydrates containing many monosaccharide units covalently bonded
  • Number of monosaccharide units in a polysaccharide can range from a few hundred to over a million
  • Polysaccharides, like disaccharides and oligosaccharides, undergo hydrolysis to produce monosaccharides
  • Examples of naturally occurring polysaccharides are cellulose and starch
• Cellulose:

  • A prevalent polysaccharide in plants
  • Found in paper, cotton, and wood
  • Main component of the paper, clothing fabrics, and wood used in home construction
• Starch:

  • Another naturally occurring polysaccharide
  • Found in various foods like bread, pasta, potatoes, rice, corn, beans, and peas
• Types of carbohydrates are related to each other through hydrolysis.
• Molecules possess handedness if they contain a carbon atom with four different groups bonded to it in a tetrahedral orientation
• A chiral center is a carbon atom in a molecule with four different groups bonded to it in a tetrahedral orientation
• A molecule with a chiral center is chiral and its mirror images are not superimposable
• Chiral molecules have handedness
• An achiral molecule is a molecule whose mirror images are superimposable
• Achiral molecules do not possess handedness
• • Carbon atoms in a ring system, if not involved in multiple bonding, can be chiral centers. These carbon atoms have four bonds—two to neighboring atoms in the ring and two to substituents on the ring. Chirality occurs when both the two substituents are different and the two “halves” of the ring emanating from the chiral center are different
• Guidelines for Identifying Chiral Centers:

  • A carbon atom involved in a multiple bond (double or triple bond) cannot be a chiral center since it has fewer than four groups bonded to it. All bonds about the chiral center must be single bonds to have four groups present
  • A carbon atom with two like groups bonded to it cannot be a chiral center since it does not have four different groups. CH3 and CH2 in a structural formula do not involve chiral centers due to the presence of two or more like hydrogen atoms
• In human body chemistry, right-handed and left-handed forms of a molecule often elicit different responses within the body
• Sometimes both forms of a molecule are biologically active, each form giving a different response
• Sometimes both forms of a molecule elicit the same response, but one form's response is many times greater than that of the other
• Sometimes only one of the two forms of a molecule is biochemically active
• Monosaccharides, the simplest type of carbohydrate and the building block for more complex types of carbohydrates, are almost always "right-handed"
• Plants, the main dietary source of carbohydrates, produce only right-handed monosaccharides
• The building blocks for proteins, amino acids, are always left-handed molecules
• Stereoisomers have the same molecular and structural formulas but differ in the orientation of atoms in space
• Constitutional isomers have atoms connected to each other in different ways