Carbohydrates


Carbohydrates are the most abundant of the four main classes of organic molecules found in nature

They contain carbon, hydrogen and oxygen in an approximate ratio of (CH2O)n with the number of carbons usually ranging from 3 - 7 per monomer

Monomers usually adopt a cyclic ring structure in aqueous solutions and their names often end with the suffix 'ose'


Structure of Glucose


Types of Carbohydrates

  • Monosaccharides:  Simple monomeric sugars, forms the basic subunits of more complex sugars
    • Examples include glucose, fructose and galactose
  • Disaccharides:  Two sugar subunits joined together (often used for transport as contains twice the energy yield while still remaining soluble)
    • Examples include maltose (glucose + glucose), sucrose (glucose + fructose) and lactose (glucose + galactose)
  • Polysaccharides:  Many sugar subunits joined together in a long chain or polymer (are typically insoluble in water due to their size)
    • Examples include glycogen (glucose storage in animals), starch (glucose storage in plants) and cellulose (component of plant cell wall)
    • Complex polysaccharides are those that consist of different monosaccharide subunits (e.g. murein in bacterial cell walls)


Different Polymers of Glucose


Functions of Carbohydrates

Carbohydrates can serve many functions within the cell, including:

  • Principle source of chemical energy for living organisms (e.g. glucose)
  • Storage of energy reserves in plant and animal tissue (e.g. starch and glycogen respectively)
  • Structural components of cells (e.g. cellulose in the cell wall of plants)
  • Function as cell membrane receptors for cell recognition and communication (e.g. glycoproteins and glycolipids)


Biosynthesis of Carbohydrates

  • In carbohydrates, monosaccharides can be joined together in a condensation reaction to form a disaccharide and water
  • This results in the formation of a covalent bond known as a glycosidic link
  • Under appropriate conditions, monosaccharides may link up via glycosidic bonds to form polysaccharides
  • The synthesis of polysaccharides can occur at various locations within a cell, such as the smooth ER, the golgi apparatus or plastids (in plants)
  • Polysaccharides and disaccharides can be broken down into monosaccharides via a hydrolysis reaction, which requires water to reverse the process


A Condensation Reaction between Two Monosaccharides