Carbohydrates: Definition and Importance

Carbohydrate Definition

• Carbohydrates are biomolecules primarily composed of carbon (C), hydrogen (H), and oxygen (O).
• The common ratio of hydrogen to oxygen is 2:1, similar to water (H2O), leading to the historical term "hydrates of carbon."
• However, many carbohydrates deviate from this strict ratio, as illustrated by deoxyribose (C5H10O4).

Chemical Definition

• Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield these compounds upon hydrolysis.
• Polyhydroxy signifies the presence of multiple hydroxyl groups (-OH).
• The defining functional group in carbohydrates is the carbonyl group, which can either be an aldehyde group (H-C=O) or a ketone group (C=O).

Importance of Carbohydrates

• Carbohydrates are the most abundant organic molecules found in nature.
• They serve as a primary energy source for cells.
• Plants store excess energy in the complex carbohydrate starch.
• Humans and other vertebrates store excess energy as glycogen, another complex carbohydrate.
• Carbohydrates contribute to structural components in living organisms:
  • Cellulose forms the cell walls of plants.
  • Chitin forms the exoskeletons of insects.
  • Carbohydrates also contribute to the cell walls of microorganisms.
• They are a part of RNA, DNA, and various coenzymes like NAD and FAD.

Classification of Carbohydrates

• Carbohydrates are categorized based on the number of saccharide (sugar) units:
  • Monosaccharides: Contain one saccharide unit.
  • Disaccharides: Contain two monosaccharide units.
  • Oligosaccharides: Contain 3 to 10 monosaccharide units.
  • Polysaccharides: Contain more than 10 monosaccharide units.

Monosaccharides

• Monosaccharides are classified based on:
  • The number of carbon atoms:
    • Trioses (3 carbons) - Glyceraldehyde, Dihydroxyacetone
    • Tetroses (4 carbons) - Erythrose, Erythrulose
    • Pentoses (5 carbons) - Ribose, Ribulose
    • Hexoses (6 carbons) - Glucose, Fructose, Galactose, Mannose
  • The presence of aldehyde or ketone groups:
    • Aldoses: Contain an aldehyde group.
    • Ketoses: Contain a ketone group.

Derivatives of Monosaccharides

• These are derived from monosaccharides through modifications:
  • Sugar alcohols (Reduction Products):
    • Formed by reducing the carbonyl carbon of monosaccharides to alcohols.
    • Glucose is reduced to sorbitol (sweetener, laxative).
    • Mannose is reduced to mannitol (sweetener).
    • Galactose is reduced to dulcitol (used in culture media).
  • Sugar Acids (Oxidation Products):
    • Produced by oxidizing either the aldehyde group, primary alcohol group, or both.
    • Aldonic acids: Oxidation of the aldehyde group (H-C=O) to a carboxyl group (COOH). Glucose forms gluconic acid.
    • Uronic acids: Oxidation of the primary alcohol group (CH2OH) to a carboxyl group (COOH). Glucose forms glucuronic acid.
    • Aldaric acids: Oxidation of both the aldehyde and primary alcohol groups to carboxyl groups. Glucose forms glucaric acid (a dicarboxylic acid).
  • Deoxy-sugars:
    • Sugars where a hydroxyl group is replaced by a hydrogen atom, resulting in the loss of an oxygen atom.
    • Ribose is converted to deoxyribose by removing the hydroxyl group on carbon 2 and replacing it with hydrogen. It is present in DNA.
  • Amino-sugars:
    • The hydroxyl group at carbon 2 is replaced by an amino group (NH2).
    • Glucosamine is found in heparin and hyaluronic acids.
    • Galactosamine is found in chondroitin sulfate.

Optical Activity of Monosaccharides

• The asymmetric carbon atom is a carbon atom bonded to four different atoms or groups, as exemplified by the middle carbon in glyceraldehyde.
• Plane-polarized light (PPL) vibrates in a single plane, achieved by passing ordinary light through a calcite prism (polarizing filter).
• Optical activity refers to the ability of an asymmetric carbon to rotate plane-polarized light.
• When PPL passes through sugars, they rotate the light either to the right or left:
  • Dextrorotatory (+) substances rotate PPL to the right.
  • Levorotatory (-) substances rotate PPL to the left.
• This rotation is measured with a polarimeter.

Important Monosaccharides

• Pentoses:
  • Ribose: Found in nucleic acids (RNA, DNA) and coenzymes like NAD and FAD.
• Hexoses:
  • Glucose: Primary blood sugar and the main energy source for mammals.
  • Galactose: Converted into glucose in the liver. Synthesized in mammary glands to form lactose (milk sugar).
  • Fructose: Converted into glucose in the liver. Present in seminal fluid.

Explore the definition and significance of carbohydrates in this quiz. Learn about their chemical structure, common ratios, and their crucial role as an energy source for living organisms. This quiz covers the fundamental concepts associated with carbohydrates systematically.