Carbohydrate Chemistry Quiz

Biomolecules

• Biomolecules are the essential organic molecules that make up living organisms.
• They range from small molecules (like metabolites, vitamins, hormones) to large macromolecules (like proteins, nucleic acids, carbohydrates, lipids).

Carbohydrates

• Carbohydrates (also called saccharides) are the most abundant organic molecules in nature.
• They have various functions, including providing dietary calories for organisms.
• Also act as energy storage in the body.
• Serve as components of cell membranes for intercellular communication.
• Act as structural components in organisms (e.g., cell walls of bacteria, cellulose of plants).

Carbohydrate Classification

• Carbohydrates are categorized according to the number of sugar units they contain.
  • Monosaccharides: one sugar unit
  • Disaccharides: two sugar units
  • Oligosaccharides: 3-9 sugar units
  • Polysaccharides: 10 or more sugar units

Monosaccharides

• Monosaccharides are simple sugars that are classified by the number of carbon atoms.
• Examples in humans include trioses (3 carbons), tetroses (4 carbons), pentoses (5 carbons), hexoses (6 carbons), heptoses (7 carbons), and nonoses (9 carbons). Specific examples are provided in the text.

Isomers

• Isomers are compounds with the same chemical formula but different structures.
• Glucose, fructose, mannose, and galactose are examples of isomers with the same chemical formula (C₆H₁₂O₆).

Epimers

• Epimers are isomers that differ in the configuration around only one specific carbon atom (not the carbonyl carbon).
• Glucose and galactose are C-4 epimers, differing only in the position of the -OH group at carbon 4.

Enantiomers

• Enantiomers are isomers that are mirror images of each other.
• D-sugars and L-sugars are examples of enantiomers.
• The vast majority of sugars in humans are D-sugars.

Cyclization of Monosaccharides

• Most monosaccharides with five or more carbons exist as cyclic (ring) structures in solution, rather than open-chain structures.
• The ring closure is formed by reaction of the carbonyl group with an alcohol group on the sugar.
• The carbon involved in the reaction becomes the anomeric carbon.

Anomers

• Anomers are a specific type of isomer, created when a cyclic sugar forms, different cyclic forms are possible as the anomeric carbon (which was the carbonyl carbon) can now be either up (“α”) or down (“β”).
• These anomers are diastereomers (not mirror images).
• The a and β forms of a sugar spontaneously change back and forth in solution, a process called mutarotation.

Reducing Sugars

• Reducing sugars can act as reducing agents.
• This capability stems from the anomeric carbon (which can be in an open chain form) enabling oxidation of the aldehyde.
• Most monosaccharides are reducing sugars.

Disaccharides

• Disaccharides are formed by the joining of two monosaccharides via a glycosidic linkage, via a dehydration reaction; that is, water is released.
• Common examples are sucrose, lactose, and maltose.

Sucrose

• Sucrose (table sugar) is a disaccharide made of glucose and fructose.
• Sucrose is a non-reducing sugar; the reducing groups are involved in the glycosidic bond.

Maltose

• Maltose is a disaccharide made up of two glucose units linked through a specific carbon-carbon bond configuration.
• It is a reducing sugar.

Lactose

• Lactose (milk sugar) is a disaccharide made up of galactose and glucose.
• It also is a reducing sugar.

Other Disaccharides

• Trehalose: Two glucose units linked differently. Found in various organisms.
• Lactulose: Formed from fructose and galactose. Helpful in treating certain digestive issues.
• Cellobiose: Two glucose units linked differently. Found in cellulose.
• Chitobiose: Made of two glucosamine units. Found in exoskeletons.