Carbohydrates Overview

Carbohydrates

• Major source of energy in our diet
• Composed of the elements carbon (C), hydrogen (H), and oxygen (O)
• Also called saccharides, meaning "sugars"
• Produced by photosynthesis in plants, such as glucose, using CO2, H2O, and energy from the sun
• Oxidized (respiration) in living cells to produce CO2, H2O, and energy
• "Hydrate of carbon"
• Derived from the formula Cn(H2O)m
• Glucose: C6H12O6 or C6(H2O)6
• Polyhydroxy aldehydes, polyhydroxy ketones, or compounds that yield them after hydrolysis

Function

• Provides energy
• Glycogen provides short-term energy reserves
• Supplies carbon for the synthesis of other biochemical substances
• Part of the structure of DNA and RNA
• Linked to lipids in cell membranes
• Linked to proteins in biological recognition processes
• Regulates blood sugar
• Spares protein use for energy
• Breaks down fatty acids and prevents ketosis
• Provides flavor and sweeteners
• Source of dietary fiber

Isomerism

• Isomers are compounds with identical molecular formulas but different structures
• Types of isomerism:
  • Structural isomerism (different arrangement of atoms)
  • Stereoisomerism (same arrangement of atoms but different spatial arrangement)
    • Types of stereoisomerism associated with glucose are D and L isomerism, Optical isomerism, Epimerism, a and β anomerism
    • Enantiomers: Stereoisomers whose molecules are nonsuperimposable mirror images of each other
    • Diastereomers: Stereoisomers whose molecules are not mirror images of each other

D and L Isomerism

• D-glucose is right-handed
• L-glucose is left-handed

Optical Isomerism

• Optical activity is the capacity of a substance to rotate the plane-polarized light passing through it
• Clockwise direction: Dextrorotatory (d) or (+)
• Counterclockwise direction: Levorotatory (l) or (-)
• Chiral compounds rotate polarized light clockwise or counterclockwise through a certain angle.

Epimerism

• If two monosaccharides differ in their configuration around a single specific carbon (other than the anomeric carbon) atom.

Anomerism

• Isomers formed by the change of position of a hydroxyl group attached to the anomeric carbon (e.g., α and β glucose are anomers)
• Also α and β fructose are anomers

Mutarotation

• Change in the specific optical rotation by interconversion of α and β forms of D-glucose to an equilibrium mixture.

Classification (by size)

• Monosaccharides: The simplest carbohydrates
• Disaccharides: Consist of 2 monosaccharides
• Oligosaccharides: Consist of 3-10 monosaccharides
• Polysaccharides: Contain many monosaccharides

Monosaccharide

• Consist of 3 to 6 carbon atoms, typically
• A carbonyl group (aldehyde or ketone)
• Several hydroxyl groups
• Colorless, crystalline solids

Aldose

• Monosaccharides with an aldehyde group
• Types include triose (3 carbon atoms), tetrose (4 carbon atoms), pentose (5 carbon atoms), hexose (6 carbon atoms)
• Examples: Erythrose (aldo-tetrose)

Ketose

• Monosaccharides with a ketone group
• Types include triose, tetrose, pentose, hexose
• Example: Fructose (ketohexose)

Important Monosaccharides

• D-Glucose: Found in fruits, corn syrup, and honey; known as blood sugar.
• D-Fructose: Sweetest carbohydrate, found in fruits, juices and honey.
• D-Galactose: Found in lactose, a disaccharide.

Important Disaccharides

• Maltose: Malt sugar; composed of two glucose molecules; from starch hydrolysis.
• Lactose: Found in milk and milk products; comprised of glucose and galactose.
• Sucrose: Table sugar; obtained from sugar cane and sugar beets; composed of glucose and fructose.

Relative Sweetness

• Fructose is sweeter than sucrose.
• Lactose has little sweetness.

Classification (by reaction)

• Reducing sugars: React positively with Benedict's and Tollen's test
  • Examples: Lactose and Maltose
• Non-reducing sugars: React negatively with Benedict's and Tollen's test
  • Examples: Sucrose

Reducing Sugars

• Carbohydrates with carbonyl group that oxidizes to a carboxylic acid
• Undergo reaction with Benedict's reagent (Cu2+) to give the corresponding carboxylic acid.
• Examples include Glucose, Galactose, Fructose

Oxidation of D-Glucose

• Glucose is oxidized to a carboxylic acid.

Reduction of D-Glucose

• Involves the carbonyl group and produces sugar alcohols (alditols)
• Example: D-glucose produces D-glucitol (sorbitol)

Polysaccharides

• Formed by the condensation of multiple monosaccharides with the removal of water molecules.
• Polysaccharides are non-reducing
• Types include:
  • Homopolysaccharides (one type of monosaccharide monomer unit)
  • Heteropolysaccharides (more than one, usually two, types of monosaccharide monomer units)
  • Branched polysaccharides
  • Unbranched polysaccharides
  • Examples:
    • Amylose and amylopectin (starches)
    • Glycogen (animal starch)
    • Cellulose

Amylose

• Polymer of α-D-glucose
• Linked by α-1,4 glycosidic bonds
• Unbranched chain

Amylopectin

• Polymer of α-D glucose
• Linked by a-1,4 glycosidic bonds
• Highly branched chain

Glycogen

• Polysaccharide that stores α-D-glucose in muscle.
• Similar to amylopectin but more highly branched.

Cellulose

• Polysaccharide of glucose units in unbranched chains
• Has β-1,4-glycosidic bonds
• Cannot be digested by humans

Fischer Projection

• Developed by Hermann Emil Fischer (German)
• Used to represent carbohydrates
• Places the most oxidized group at the top
• Shows chiral carbons as the intersection of vertical and horizontal lines

Explore the essential role of carbohydrates in our diet through this quiz. Learn about their composition, functions, and the concept of isomerism. Test your knowledge on how carbohydrates provide energy and their importance in biological processes.