The simplest and most essential unit of all carbohydrates is the monosaccharide, often referred to as a simple sugar. These molecules follow the general formula CnH₂nOn, consisting of carbon (C), hydrogen (H), and oxygen (O).

Examples of monosaccharides:

1. Galactose – a key component of milk sugar (lactose)
2. Glucose – the universal cellular fuel
3. Fructose – the natural sugar found in fruits and honey

Monosaccharides are the sweet sparks of life—tiny but mighty! They link up to form dynamic duos (disaccharides like sucrose and lactose) or join forces into carb superstructures (polysaccharides like energy-packed starch and ultra-strong cellulose). Nature’s ultimate building blocks for everything from quick energy to plant armor!

The major difference lies in the position of the carbonyl group (C=O) within the sugar molecule:

1. Aldoses have their carbonyl group at the end of the carbon chain, forming an aldehyde group (-CHO). Examples: Glucose, Galactose
2. Ketoses have the carbonyl group on an internal carbon, forming a ketone group (=CO). Example: Fructose

This structural variation plays a vital role in classifying sugars and determining whether they are reducing or non-reducing.

A glycosidic bond connects two monosaccharides through a dehydration reaction, which removes a water molecule.
For example:

• Glucose + Fructose → Sucrose (α-1→2 bond)
• Glucose + Glucose → Maltose (α-1→4 bond)

The type and position of these bonds influence the structure and digestibility of the resulting sugar.

Though both consist of glucose units, the glycosidic bonds differ:

• Cellulose: Built with β(1→4) bonds that form rigid, straight chains. Humans lack cellulase, the enzyme needed to digest it—so cellulose passes as dietary fiber.
• Starch: Contains α(1→4) and α(1→6) bonds (e.g., amylose and amylopectin), which human enzymes like amylase can break down.