Biomolecules and Carbohydrates: Definition, Structure, Classification, Types & Functions

What Are Biomolecules?

• Biomolecules are complex organic compounds that govern and facilitate the fundamental biological activities within living organisms. They act as the functional units that drive the physiological processes necessary to sustain life.
• The living systems are made up of various complex biomolecules like carbohydrates, proteins, nucleic acids, lipids, etc.
• In addition, some simple molecules like vitamins and mineral salts also play an important role in the function of organisms.

Understanding Carbohydrates: Nature and Origin

Carbohydrates, commonly known as sugars, are a vast class of naturally occurring organic compounds primarily synthesized by plants.
 Common examples include:

• Cane sugar
• Glucose
• Starch

General Formula and Its Limitations

• Carbohydrates were historically classified using the general formula Cx(H₂O)y, earning them the label "hydrates of carbon." However, this definition has notable limitations. Not every compound fitting this formula is a carbohydrate, and not every carbohydrate fits this formula."
• Exception example: Rhamnose has the formula C₆H₁₂O₅, yet it is still classified as a carbohydrate despite not satisfying the Cx(H₂O)y ratio.

NOTE: Rhamnose, C6H12O5 is a carbohydrate but does not fit in this definition.

Chemical Definition of Carbohydrates

From a strict chemical standpoint, carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that produce these units on hydrolysis.

Commonly encountered sugars include:

• Sucrose – The most common household sugar
• Lactose – The specific sugar found in milk

Classification of Carbohydrates

Carbohydrates are classified under three categories 

Classification: Monosaccharides

Monosaccharides, or simple sugars, are the foundational category of carbohydrates.

Core Characteristics

• Irreducibility: They are the simplest carbohydrates because they cannot be hydrolyzed further into smaller sugar units.
• Structural Role: They serve as the fundamental basic units all other carbohydrate types.
• Chemical Formula: Monosaccharides follow the general formula CₙH₂ₙOₙ, where n typically ranges from 3 to 7.

Classification by Carbon Atom 

Carbon Count	Type	Example
3	Triose	Glyceraldehyde
4	Tetrose	Erythrose
5	Pentose	Ribose & Xylose
6	Hexose	Glucose, Fructose           & Galactose
7	Heptose	Sedoheptulose

Classification by Functional Group

Monosaccharides are also grouped by their primary functional group:

• Aldoses: Contain an aldehyde group (-CHO), for example; glucose.
• Ketoses: Contain a ketone group (C=O) , for example; fructose.

NOTE : Glucose and fructose are isomers (C₆H₁₂O₆) but differ in functional group

Classification: Oligosaccharides

Oligosaccharides are defined by their intermediate structural complexity.

Type	Units	Examples
Disaccharide	2	Sucrose, Maltose & Lactose
Trisaccharide	3	Raffinose
Tetrasaccharide	4	Stachyose

Key Characteristics

• Formation: Synthesized through condensation of 2 to 10 monosaccharide units.
• Bonding: Units are linked by glycosidic bonds.
• Hydrolysis: Break down into 2 to 10 monosaccharide units upon hydrolysis.

Classification: Polysaccharides

• Molecular Composition: Composed of hundreds to thousands of  monosaccharide units.
• Physical Properties: Large, insoluble molecules serving storage or structural roles.
• Hydrolysis: Yield many monosaccharide units upon breakdown.

 Classification Based on Reducing Properties

Type	Definition	Example
Reducing Sugar	Reducing sugars are carbohydrates that contain a free aldehyde group or a free ketone group capable of reducing Cu²⁺ to Cu⁺ in Benedict's test or Fehling's test.	Glucose Fructose Maltose Lactose
Non-Reducing Sugar	Non-reducing sugars are carbohydrates that do not have a free aldehyde or ketone group available for reaction, and therefore cannot reduce Cu²⁺ to Cu⁺.	Sucrose

Cyclic Structure of Glucose: The Pyranose Form

Glucose does not always exist in its open-chain form in solution, it predominantly adopts a six-membered cyclic structure known as the pyranose structure, named by analogy with the compound pyran.

This pyranose ring can exist in two interconvertible forms:

• α-form (alpha)
• β-form (beta)

 Understanding Anomers

The two cyclic hemiacetal forms of glucose differ only in the configuration of the hydroxyl group (-OH) at the C1 position.

• This C1 carbon is called the anomeric carbon  because it was the aldehyde carbon before cyclisation occurred.
• The resulting α-form and β-form  are known as anomers.

References     

• Lehninger Principles of Biochemistry
• Harper's Illustrated Biochemistry
• Biochemistry

Suggested Readings 

• What Are Carbohydrates? Definition, Functions, Types & Classification

• Monosaccharides: Definition, Classification, Structure, Types, Examples & Properties