define the terms monomer, polymer, macromolecule, monosaccharide, disaccharide and polysaccharide

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology 9700 – Carbohydrates and Lipids

Carbohydrates and Lipids – Key Terminology

Learning Objective

Define the following terms and understand their relationships:

  • Monomer

  • Polymer

  • Macromolecule

  • Monosaccharide

  • Disaccharide

  • Polysaccharide

Definitions

TermDefinitionExample (Biological Context)
MonomerA single, simple molecule that can bind chemically to other identical or different monomers to form a larger structure.Glucose (a monosaccharide) is a monomer for many polysaccharides.
PolymerA large molecule composed of repeated monomer units linked by covalent bonds (typically condensation or dehydration synthesis).Starch, glycogen and cellulose are polymers of glucose.
MacromoleculeA very large molecule, usually a polymer, whose size and complexity give it distinct biological functions. The term is often used for the four major classes of biomolecules: carbohydrates, lipids, proteins and nucleic acids.Cellulose is a macromolecule that provides structural support in plant cell walls.
MonosaccharideThe simplest form of carbohydrate; a single sugar unit that cannot be hydrolysed into smaller carbohydrate molecules.Glucose (\$\mathrm{C6H{12}O_6}\$), fructose and galactose.
DisaccharideA carbohydrate formed by the condensation (dehydration) of two monosaccharide units, linked by a glycosidic bond.Sucrose (glucose + fructose), maltose (glucose + glucose), lactose (glucose + galactose).
PolysaccharideA carbohydrate polymer consisting of many (often hundreds or thousands) of monosaccharide units.Starch (energy storage in plants), glycogen (energy storage in animals), cellulose (structural component in plants).

Relationships Between the Terms

The hierarchy of carbohydrate terminology can be visualised as follows:

  1. Monosaccharides are the monomers of carbohydrate polymers.

  2. When two monosaccharides join, they form a disaccharide.

  3. When many monosaccharides join, they form a polysaccharide, which is a type of polymer.

  4. Polysaccharides, together with proteins, nucleic acids and some lipids, are classified as macromolecules because of their large size and functional importance.

Structural Overview of a Typical Monosaccharide

A monosaccharide such as glucose can be represented in its open‑chain form as:

\$\mathrm{HOCH2-(CHOH)4-CHO}\$

or in its cyclic hemiacetal form as a six‑membered ring (pyranose). The ability of the carbonyl carbon to react with a hydroxyl group on the same molecule creates the ring structure that is common to most biologically relevant sugars.

Suggested diagram: Open‑chain and cyclic forms of D‑glucose, showing the α‑ and β‑anomers.

Key Points to Remember

  • Monomers join by dehydration synthesis (loss of \$\mathrm{H2O}\$) and can be broken down by hydrolysis (addition of \$\mathrm{H2O}\$).

  • Polysaccharides differ in the type of glycosidic bond (α or β) and in branching, which determines their biological role (e.g., storage vs. structural).

  • Macromolecules are essential for cell structure, energy storage, and information transfer.