Living organisms are built from four major classes of biomolecules: carbohydrates, lipids, proteins, and nucleic acids. These molecules supply energy, form cell structures, carry information, and control chemical reactions. Understanding them helps explain nutrition, genetics, cell membranes, enzymes, and many processes in the human body.
Each class has characteristic building blocks, bonds, and biological roles.
Most large biomolecules are polymers made by linking smaller units called monomers. Cells often build these molecules through dehydration synthesis, which forms a covalent bond while removing H2O. The reverse process, hydrolysis, uses water to break large molecules into smaller units.
Comparing the four biomolecule groups makes it easier to predict their structure and function from their chemical features.
Key Facts
- Carbohydrate monomers are monosaccharides such as glucose, with a common formula near C6H12O6.
- Carbohydrate polymers include starch, glycogen, and cellulose, which store energy or provide structural support.
- Lipids are not true polymers, but many are built from glycerol and fatty acids and are mostly hydrophobic.
- Proteins are polymers of amino acids joined by peptide bonds, and their function depends on 3D shape.
- Nucleic acids are polymers of nucleotides, and DNA stores genetic information using the base-pair rules A = T and C = G.
- Dehydration synthesis builds polymers by removing water, while hydrolysis breaks polymers by adding water.
Vocabulary
- Monomer
- A monomer is a small molecular building block that can bond with others to form a larger molecule.
- Polymer
- A polymer is a large molecule made of many repeating or linked monomers.
- Dehydration synthesis
- Dehydration synthesis is a reaction that joins molecules by forming a bond and releasing water.
- Hydrolysis
- Hydrolysis is a reaction that breaks a bond in a large molecule by adding water.
- Macromolecule
- A macromolecule is a very large biological molecule such as a carbohydrate, lipid, protein, or nucleic acid.
Common Mistakes to Avoid
- Calling all biomolecules polymers is wrong because lipids are large biomolecules but are not usually made of repeating monomer chains.
- Mixing up starch, glycogen, and cellulose is wrong because starch stores energy in plants, glycogen stores energy in animals, and cellulose provides plant cell wall structure.
- Thinking proteins are mainly for energy is wrong because proteins are best known for enzymes, structure, transport, signaling, and movement, even though they can be used for energy in some conditions.
- Forgetting water in biomolecule reactions is wrong because dehydration synthesis releases H2O when bonds form, while hydrolysis consumes H2O when bonds break.
Practice Questions
- 1 A cell joins 12 glucose molecules to make one polysaccharide chain. How many water molecules are released during dehydration synthesis?
- 2 A short protein contains 85 amino acids in one chain. How many peptide bonds does it contain?
- 3 A student says fats, cellulose, and DNA all store genetic information because they are large biomolecules. Explain which molecule stores genetic information and why the others do not.