state the role of covalent bonds in joining smaller molecules together to form polymers

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology 9700 – Carbohydrates and Lipids

Carbohydrates and Lipids

Learning Objective

State the role of covalent bonds in joining smaller molecules together to form polymers.

Key Concepts

  • Polymers are large molecules formed by the repeated linking of monomer units.

  • The linking is achieved through specific covalent bonds that are formed by condensation (dehydration) reactions.

  • In carbohydrates the bond is a glycosidic bond; in lipids the bond is an ester bond.

Carbohydrates

Monomers: simple sugars (monosaccharides) such as glucose (\$C6H{12}O_6\$).

Polymerisation: two monosaccharides join by a dehydration reaction, releasing a molecule of water and forming a covalent glycosidic bond (\$\ce{C-O-C}\$).

Example: two glucose molecules → maltose.

Lipids

Monomers: fatty acids (carboxylic acids) and glycerol (a tri‑hydroxy alcohol).

Polymerisation: each fatty‑acid carboxyl group reacts with a hydroxyl group of glycerol, releasing water and forming an ester linkage (\$\ce{R-CO-O-CH_2}\$).

Example: glycerol + three fatty acids → triglyceride.

Comparison of Polymerisation

FeatureCarbohydratesLipids
Monomer unitMonosaccharide (e.g., glucose)Fatty acid + glycerol
Type of covalent bond formedGlycosidic bond (\$\ce{C-O-C}\$)Ester bond (\$\ce{R-CO-O-CH_2}\$)
Reaction typeCondensation (dehydration) reactionCondensation (dehydration) reaction
By‑productWater (\$\ce{H2O}\$)Water (\$\ce{H2O}\$)
Resulting polymerPolysaccharide (e.g., starch, cellulose)Triglyceride or phospholipid

Step‑by‑Step: Formation of a Glycosidic Bond

  1. Two monosaccharides approach each other.

  2. The hydroxyl group on carbon‑1 of one sugar attacks the hydroxyl on carbon‑4 (or carbon‑6) of the second sugar.

  3. A water molecule is eliminated (condensation).

  4. A covalent \$\ce{C-O-C}\$ glycosidic bond is created, linking the sugars.

Step‑by‑Step: Formation of an Ester Bond

  1. A fatty‑acid carboxyl group (\$\ce{-COOH}\$) aligns with a hydroxyl group of glycerol.

  2. The hydroxyl hydrogen and the carboxyl hydroxyl hydrogen combine to form water.

  3. The remaining oxygen of the carboxyl group forms a covalent bond with the carbon of the glycerol hydroxyl, creating an ester linkage (\$\ce{-CO-O-CH_2-}\$).

  4. Repeating this process with three fatty acids yields a triglyceride.

Suggested diagram: schematic of (a) glycosidic bond formation between two glucose molecules and (b) ester bond formation in a triglyceride.

Why Covalent Bonds Matter

Covalent bonds provide the strong, stable connections required for the structural integrity and functional properties of polymers. In carbohydrates, the orientation of glycosidic bonds determines whether a polysaccharide is digestible (e.g., starch) or indigestible (e.g., cellulose). In lipids, the length and saturation of fatty‑acid chains attached via ester bonds influence fluidity, energy storage, and membrane behaviour.

Summary

  • Covalent bonds join monomers to form polymers through condensation reactions.

  • Glycosidic bonds link sugars; ester bonds link fatty acids to glycerol.

  • The nature of the covalent bond dictates the physical and biological properties of the resulting polymer.