Carbohydrate&itsPhysico-chemical Properties

1. Introduction to Carbohydrates

  • Definition: Organic molecules made of carbon, hydrogen, and oxygen in a 1:2:1 ratio (CH₂O)ₙ. where n>_3

  • Functions:

    • Provide dietary calories.

    • Serve as energy storage (e.g., glycogen).

    • Function as cell membrane components.

  • Nomenclature: "Saccharide" is derived from the Greek word for sugar.

2. Classification of Carbohydrates

  • By complexity:

    1. Monosaccharides: Simplest form, cannot be hydrolyzed further. Examples: Glucose, fructose, galactose.

    2. Disaccharides: Two monosaccharides linked by glycosidic bonds. Examples: Sucrose, lactose, maltose.

    3. Oligosaccharides: 3–10 monosaccharide units.

    4. Polysaccharides: >10 monosaccharide units. Examples: Starch, glycogen, cellulose.

  • By functional group:

    • Aldoses: Contain an aldehyde group.

    • Ketoses: Contain a ketone group.

  • By number of carbons:

    • Trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C).

3. Structural Variations

  1. Stereoisomers: Same chemical formula but differ in spatial arrangement. Examples: Glucose, fructose, galactose.

  2. Epimers: Differ in configuration around a single carbon atom. Example: Glucose and galactose (C-4 epimers).

  3. Anomers: Differ in the configuration around the first carbon in cyclic forms. Examples: α-D-glucose and β-D-glucose.

  4. D- and L-Isomers: Determined by the orientation of the hydroxyl group on the reference carbon. Only D-sugars are metabolized in humans.

4. Physico-Chemical Properties

  1. Formation of Glycosides: Glycosidic bonds link monosaccharides to other compounds or each other.

    • α or β glycosidic bonds determine digestibility. Example: Starch (α-1,4) vs. Cellulose (β-1,4).

  2. Ring Structures:

    • Monosaccharides like glucose predominantly exist in cyclic forms (pyranose or furanose).

    • Haworth and chair projections depict these forms.

5. Disaccharides

  1. Sucrose: Glucose + Fructose; broken down by sucrase.

  2. Lactose: Glucose + Galactose; hydrolyzed by lactase.

  3. Maltose: Glucose + Glucose (α-1,4 linkage); a product of starch digestion.

6. Polysaccharides

  1. Starch:

    • Reserve carbohydrate in plants.

    • Composed of amylose (linear) and amylopectin (branched).

    • Digested by salivary and pancreatic α-amylases.

  2. Glycogen:

    • Reserve carbohydrate in animals, stored in liver and muscle.

    • Composed of glucose with α-1,4 and α-1,6 linkages.

  3. Heteroglycans (e.g., GAGs):

    • Contain different sugar units (e.g., hyaluronic acid in connective tissue, heparin as an anticoagulant).