Carbohydrates and Fischer Projections

Carbohydrates

• Carbohydrates have four main functions: providing energy through oxidation, supplying carbon for cell components, serving as a stored form of energy, and forming structural elements in some cells and tissues.
• Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield these compounds upon hydrolysis.
• Glyceraldehyde, the simplest carbohydrate, exists in two isomeric forms (mirror images) called enantiomers.
• A chiral carbon is a carbon atom bonded to four different groups.
• The carbon atom in glyceraldehyde is chiral, leading to two enantiomers (D and L).
• Enantiomers are nonsuperimposable mirror images.
• Organic molecules, like carbohydrates, may have more than one chiral carbon.

Fischer Projections

• Fischer projections represent molecules in two dimensions to show mirror images.
• L-Glyceraldehyde and D-Glyceraldehyde are enantiomers.
• The D-isomer is when the -OH group is on the right of the chiral carbon.
• The L-isomer is when the -OH group is on the left of the chiral carbon.
• This system helps to determine the D and L configuration of sugars.
• Isomers have the same chemical formula but different spatial arrangement of their atoms.

Physical Properties and Optical Activity

• D and L isomers generally have the same physical properties except for rotation of polarized light.
• Enantiomers that rotate polarized light to the left are levorotatory (-).
• Enantiomers that rotate polarized light to the right are dextrorotatory (+).
• Optical activity is the property of rotating the plane of polarized light.
• Optical activity is useful for differentiating between stereoisomers.
• Stereoisomers are important in living organisms.

Stereoisomers in Living Organisms

• Living organisms often use only one stereoisomer (like L-amino acids).
• D-glucose is a common carbohydrate.
• Not all molecules with chiral carbons only contain one isomer.

Classification of Carbohydrates

• Classification is based on the number of molecules:
  • Monosaccharides: one molecule
  • Disaccharides: two molecules
  • Oligosaccharides: 3-10 molecules
  • Polysaccharides: many molecules (hundreds or thousands)
• Monosaccharides are classified by the number of carbons: Trioses, Tetroses, Pentoses, Hexoses and etc.
• Glucose is an aldohexose, a six-carbon sugar with an aldehyde group.
• Ribulose is a ketopentose, a five-carbon sugar with a ketone group