Carbohydrates

Carbohydrates are a major class of biological molecules that are fundamental to life, containing only the elements carbon (C), hydrogen (H), and oxygen (O). The hydrogen and oxygen atoms are typically present in a 2:1 ratio, similar to water (H₂O), which gives them their characteristic name. Their general formula can be represented as Cₓ(H₂O)y. Carbohydrates are a key energy source for cells, and their structure is closely related to their function.

1. Monosaccharides (Simple Sugars):

   • Definition: These are the simplest sugars and serve as the building blocks (monomers) for larger carbohydrates.

   • Properties: Monosaccharides are typically sweet-tasting and soluble in water. All monosaccharides are reducing sugars, meaning they can reduce Benedict's solution upon heating.

   • Examples: Common monosaccharides include glucose, fructose, and galactose. Glucose is particularly important as a hexose sugar (containing six carbon atoms) and exists in two isomeric forms: alpha (α-glucose) and beta (β-glucose), which differ in the orientation of a hydroxyl (-OH) group on carbon atom 1.

   • Functions: Monosaccharides are primarily used as a direct source of energy in respiration (e.g., glucose) and as building blocks for disaccharides and polysaccharides.

2. Disaccharides:

   • Definition: These are formed when two monosaccharides are joined together.

   • Formation: The bonding occurs through a condensation reaction, where a glycosidic bond (a strong covalent C-O-C link) is formed between the two monosaccharides, and a molecule of water is eliminated.

   • Examples: Common disaccharides include maltose (two glucose molecules), sucrose (glucose and fructose), and lactose (glucose and galactose).

   • Properties: Disaccharides are also generally soluble and sweet-tasting. However, not all disaccharides are reducing sugars; sucrose is a common non-reducing sugar.

3. Polysaccharides:

   • Definition: These are large, complex molecules (macromolecules) formed when many monosaccharides are joined together by numerous glycosidic bonds through condensation reactions.

   • Properties: Polysaccharides are generally insoluble in water, do not affect water potential, and are typically not sweet-tasting. Their insolubility makes them good for storage.

   • Functions: They serve as energy storage compounds or as structural components in organisms.

   • Examples:

     • Starch: The main energy storage material in plants. It's a mixture of two α-glucose polysaccharides: amylose (unbranched, helical, 1,4 glycosidic bonds) and amylopectin (branched, 1,4 and 1,6 glycosidic bonds).

     • Glycogen: The main energy storage material in animals and fungi. It is a polymer of α-glucose, highly branched (more so than amylopectin), and stored as granules in liver and muscle cells. Its high branching allows for quick glucose release.

     • Cellulose: The major component of plant cell walls. It is formed from β-glucose monomers, which are linked in straight, unbranched chains. These chains are held together by hydrogen bonds to form strong microfibrils, providing structural support and rigidity to plant cell walls.

Breakdown of Carbohydrates: Polymers (disaccharides and polysaccharides) are broken down into their monomer units by hydrolysis reactions, which involve the addition of a water molecule to break the glycosidic bonds. This process is vital for digestion of large food molecules so they can be absorbed. For example, amylase catalyses the hydrolysis of starch into maltose, and disaccharidases like sucrase hydrolyse disaccharides into monosaccharides.

Tests for Carbohydrates:

• Benedict's test: Used for reducing sugars (all monosaccharides and some disaccharides like maltose). A positive result is a coloured precipitate (green, orange, yellow, or brick red, depending on concentration) upon heating with Benedict's reagent (which is blue). A negative result remains blue.

• Non-reducing sugars test: For sugars like sucrose, which don't react directly with Benedict's. The sample is first hydrolysed using dilute hydrochloric acid (to break it into reducing monosaccharides), then neutralised, and finally the Benedict's test is performed.

• Iodine test: Used for starch. Adding orange/brown iodine in potassium iodide solution to a sample will result in a dark blue-black colour if starch is present; a negative result remains browny-orange.

Unlike lipids, which are not polymers, carbohydrates are largely formed by repeating monomer units.