Carbohydrates and Lipids: Covalent Bonds in Polymer Formation

Objective

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

Introduction

Biological molecules, such as carbohydrates and lipids, are often large molecules called polymers. Polymers are formed by the repeated joining of smaller, identical or similar subunits called monomers. This joining process is achieved through the formation of covalent bonds.

Covalent Bonds: The Key to Polymerization

A covalent bond is a strong chemical bond formed by the sharing of one or more pairs of electrons between atoms. In the context of polymer formation, covalent bonds are the forces that hold the monomers together in the polymer chain.

The formation of a covalent bond requires energy to initiate the reaction, but the bond itself is relatively stable and requires significant energy to break.

Polymer Formation: A Step-by-Step Process

1. Dehydration Synthesis: When monomers join to form a polymer, a molecule of water (H₂O) is removed. This process is called dehydration synthesis.
2. Bond Formation: The atoms within the monomers form covalent bonds to link them together. For example, in carbohydrates, the hydroxyl (-OH) group of one monomer reacts with the hydrogen (-H) atom of another monomer, releasing water and forming a glycosidic bond.
3. Chain Growth: This process of monomer addition continues, leading to the formation of long polymer chains.
4. Cross-linking (in some polymers): In some polymers, additional covalent bonds can form between different polymer chains, creating a network structure. This can increase the strength and rigidity of the polymer.

Examples in Carbohydrates and Lipids

Carbohydrates (Polysaccharides)

Carbohydrates like starch and cellulose are polysaccharides. They are formed from many glucose monomers linked together by glycosidic bonds, which are covalent bonds. The formation of these glycosidic bonds involves the removal of a water molecule for each bond formed.

Lipids (Polymers - e.g., Triglycerides)

While lipids are not typically considered polymers in the same way as carbohydrates, triglycerides are formed by the covalent bonding of glycerol and three fatty acid monomers. Ester bonds, which are a type of covalent bond, form between the glycerol and each fatty acid.

Table Summarizing Covalent Bond Role

Bond Type	Role in Polymer Formation
Covalent Bonds	Form the strong bonds that link monomers together to create polymer chains. Dehydration synthesis reactions involve the formation of covalent bonds and the removal of water molecules.
Specific Examples	Glycosidic bonds (in carbohydrates), Ester bonds (in triglycerides).

Conclusion

Covalent bonds are essential for the formation of polymers from monomers. The sharing of electrons between atoms creates strong and stable bonds that hold the polymer chains together, giving polymers their characteristic properties.