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MONOSACCHARIDES

Monosaccharides are the simplest carbohydrates as they cannot be hydrolyzed into smaller carbohydrates. Defined chemically as aldehydes or ketones with two or more hydroxyl groups, monosaccharides serve as crucial fuel molecules and the fundamental building blocks for nucleic acids. The general formula for monosaccharides is (CH<em>2O)</em>n(CH<em>2O)</em>n, where nn is any integer greater than or equal to 3.

CLASSIFICATION OF MONOSACCHARIDES

Monosaccharides can be classified based on two primary features:

  1. Location of Carbonyl Groups:

    • Aldoses: If the carbonyl group is at the end of the carbon chain (aldehyde group), the monosaccharide is classified as an aldose.

    • Ketoses: If the carbonyl group is located at the second carbon (ketone group), the monosaccharide is classified as a ketose.

  2. Number of Carbon Atoms:

    • Trioses: Monosaccharides with three carbon atoms.

    • Tetroses: Monosaccharides with four carbon atoms.

    • Pentoses: Monosaccharides with five carbon atoms.

    • Hexoses: Monosaccharides with six carbon atoms.

HAWORTH FORMULA

The Haworth formula is a depiction for cyclic structures of monosaccharides. For example, D-glucopyranose and D-fructose are two common structures shown in Haworth form.

ISOMERISM IN CARBOHYDRATES

Types of Isomerism

  1. Aldose-Ketose Isomerism: This encompasses functional group isomerism where aldoses (e.g., glucose) and ketoses (e.g., fructose) share the same molecular formula but differ in the placement of the carbonyl group.

  2. Stereoisomerism: Compounds with the same structural formula but different spatial configurations are known as stereoisomers. The spatial arrangement of OHOH and HH groups is important due to the presence of asymmetric carbon atoms.

    • Enantiomers: These are non-superimposable mirror images of each other. An example includes D-glucose and L-glucose.

    • Diastereoisomers: Stereoisomers that are not mirror images, like D-glucose and D-mannose, differing at one or more stereocenters.

    • Epimers: Sugars differing at a single carbon's configuration (e.g., glucose and galactose are epimers at C-4).

    • Anomers: Cyclic monosaccharides differing at the anomeric carbon (C-1 for aldoses and C-2 for ketoses).

DISACCHARIDES

Disaccharides are carbohydrates composed of two monosaccharide units linked by glycosidic bonds. Key disaccharides include:

  1. Maltose: Composed of two glucose units (found in sprouting grains; it has a sweet taste). The linkage is referred to as an  1,4 glycosidic bond.

  2. Sucrose: Composed of glucose and fructose (often obtained from cane sugar); the bond is a  1,2 glycosidic bond. Sucrose is a non-reducing sugar.

  3. Lactose: Made of glucose and galactose (commonly found in milk); it features a  1,4 glycosidic bond and is a reducing sugar.

  4. Trehalose: Composed of two glucose units, linked through an  1,1 bond; found in mushrooms and insects; it is a non-reducing sugar.

OLIGOSACCHARIDES

Oligosaccharides consist of 3 to 10 monosaccharide units. They are often attached to proteins and lipids to form glycoproteins and glycolipids, which are essential for cell signaling and recognition.

POLYSACCHARIDES

Polysaccharides are long chains of monosaccharides, composed of more than ten sugar units. Examples include:

  • Storage Polysaccharides: Starch in plants and glycogen in animals, which serve as energy reserves.

  • Structural Polysaccharides: Cellulose in plant cell walls and chitin in arthropods' exoskeletons, which provide structural support.

STRUCTURAL REPRESENTATIONS OF CARBOHYDRATES

Fischer Projection and Haworth Projections:
These representations illustrate the spatial arrangement of atoms in monosaccharides. The Fischer projection is a two-dimensional representation focusing on the linear structure, while the Haworth projection shows the cyclic form, which is predominant in solutions.

FUNCTIONS OF CARBOHYDRATES

Carbohydrates play various roles in biological systems:

  1. Energy Source: They are the body’s primary energy source, providing energy for brain cells and red blood cells, which primarily depend on carbohydrates.

  2. Energy Storage: Excess carbohydrates are stored as glycogen in animals and starch in plants, which can be mobilized for energy when needed.

  3. Structural Role: Certain carbohydrates, like cellulose in plants and chitin in arthropods, provide structural support.

  4. Precursor for Biomolecules: Carbohydrates are involved in forming glycoproteins and glycolipids, essential for cell signaling and recognition.

  5. Regulation of Physiological Processes: They influence osmotic pressure and hydration status, important for proper physiological function.

  6. Detoxification: Monosaccharides like glucuronic acid conjugate with toxins to enhance water solubility for excretion.

REACTIONS OF MONOSACCHARIDES

Monosaccharides undergo several types of chemical reactions:

  1. Oxidation Reaction: Leads to the formation of sugar acids, converting aldehyde groups into carboxyl groups (aldonic acids), or terminal carbons into uronic acids when shielded.

  2. Reduction Reaction: The conversion of sugar into alditols.

  3. Esterification: Formation of esters by reacting with alcohols.

  4. Glycoside Formation: The reaction of a monosaccharide to form a glycoside.

  5. Enediol Formation: A reaction involving the transformation of monosaccharides into enediols through tautomerization.

Overall, carbohydrates are vital biomolecules that serve functional and structural roles in living organisms, highlighting their significance in biochemistry and cell biology. Each classification, structure, and function illustrates their complexity and essential nature in biological systems.