Why are carbohydrates generally optically active?

• When unpolarized light (vibrating in all directions) passes through a polarizer, the emerging polarized light vibrates in only one direction. Certain substances can rotate this plane of polarized light either to the right or to the left. Molecules capable of rotating plane-polarized light are called optically active molecules.
• A chiral carbon (chiral centre) is a carbon atom bonded to four different atoms or groups.
• Carbohydrates, which often contain one or more chiral centres, are generally optically active.
  • If they rotate the plane of polarized light to the left, they are called laevorotatory (–) substances. Example: (–)-glucose
  • If they rotate the plane to the right, they are called dextrorotatory (+) substances. Example: (+)-glucose
• Most biologically important molecules possess chiral centres and therefore show optical activity. In living systems, usually only one optical form ((+) or (–)) is present.
• Molecules with more than one chiral centre can exist in different forms:
  • Dextrorotatory (+)
  • Laevorotatory (–)
  • Meso form (optically inactive due to internal compensation)
  • Example: Tartaric acid, which exists in L(+), D(–), and meso forms.
• Whether a substance is laevorotatory or dextrorotatory is determined experimentally by measuring its effect on plane-polarized light.

Thus, carbohydrates are generally optically active because they contain multiple asymmetric carbon atoms, making their molecules chiral.