Chapter 5: Biomolecules – Structure and Function of Macromolecules in Biology

Biomolecules

Introduction to Biomolecules

Living organisms are composed of four major classes of large biological molecules, known as macromolecules. These macromolecules are essential for structure, function, and regulation of the body's tissues and organs.

• Lipids

• Carbohydrates

• Proteins

• Nucleic acids

Macromolecules and Polymers

Building Macromolecules

Macromolecules are formed by linking smaller units into larger structures. Many macromolecules are polymers, which are long molecules made up of repeating subunits called monomers.

• Polymer: A long molecule consisting of many similar or identical building blocks (monomers) linked by covalent bonds.

• Monomer: The smaller molecules that serve as the repeating units in a polymer.

• Only three classes of macromolecules are true polymers: carbohydrates, proteins, and nucleic acids.

• Lipids are not true polymers.

Lipids

Structure and Properties of Lipids

Lipids are a diverse group of hydrophobic molecules that do not form true polymers. Their unifying feature is that they are hydrophobic, consisting mostly of hydrocarbon regions. The most biologically important lipids are fats, phospholipids, and steroids.

• Hydrophobic: Repels water; does not dissolve in water.

• Hydrocarbon regions: Long chains of carbon and hydrogen atoms.

Fats: Structure and Synthesis

Fats are constructed from two types of smaller molecules: glycerol and fatty acids.

• Glycerol: A three-carbon alcohol with a hydroxyl group (-OH) attached to each carbon.

• Fatty acid: Consists of a carboxyl group (-COOH) attached to a long hydrocarbon skeleton.

• In a fat molecule, three fatty acids are joined to glycerol by an ester linkage, forming a triacylglycerol (triglyceride).

• Fatty acids in a fat can be identical or of two or three different kinds.

Diagram: The synthesis of a fat involves three dehydration reactions, each joining a fatty acid to glycerol via an ester linkage.

Saturated vs. Unsaturated Fatty Acids

Fatty acids vary in length (number of carbons) and in the number and location of double bonds.

• Saturated fatty acids: Have the maximum number of hydrogen atoms possible and no double bonds.

• Unsaturated fatty acids: Have one or more double bonds in the hydrocarbon chain.

Example: Animal fats (such as butter) are typically saturated and solid at room temperature, while plant and fish fats are usually unsaturated and liquid at room temperature (oils).

*Additional info: Saturated fats are associated with cardiovascular disease due to plaque formation in arteries. Hydrogenation can convert unsaturated fats to saturated fats and may create trans fats, which are also linked to health risks.*