Living things are built from four major classes of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids. These molecules store energy, build cell structures, speed up reactions, and carry genetic information. Understanding their parts and functions helps explain how cells grow, communicate, and maintain life.
Each macromolecule has a pattern of smaller building blocks and chemical bonds that gives it special properties.
Key Facts
- Carbohydrates are often built from monosaccharides and commonly follow the ratio (CH2O)n.
- Proteins are polymers of amino acids joined by peptide bonds.
- Nucleic acids are polymers of nucleotides joined by phosphodiester bonds.
- Lipids are mostly nonpolar molecules such as fats, phospholipids, and steroids, and they are not true polymers in the same way proteins and DNA are.
- Dehydration synthesis builds larger molecules by forming bonds and releasing water: monomer + monomer -> polymer + H2O.
- Hydrolysis breaks polymers by adding water: polymer + H2O -> monomers.
Vocabulary
- Macromolecule
- A large biological molecule that is important for cell structure, function, energy storage, or information storage.
- Monomer
- A small molecular building block that can be linked with others to form a larger molecule.
- Polymer
- A large molecule made of many repeating or similar monomer units bonded together.
- Dehydration synthesis
- A chemical reaction that joins smaller molecules by forming a bond and releasing a water molecule.
- Hydrolysis
- A chemical reaction that breaks a bond in a larger molecule by adding water.
Common Mistakes to Avoid
- Calling all lipids polymers, which is wrong because many lipids are assembled from components like glycerol and fatty acids but do not form long repeating chains like proteins or nucleic acids.
- Mixing up dehydration synthesis and hydrolysis, which is wrong because dehydration synthesis removes water to build bonds while hydrolysis adds water to break bonds.
- Thinking proteins are made of nucleotides, which is wrong because proteins are made of amino acids and nucleic acids are made of nucleotides.
- Assuming carbohydrates only provide quick energy, which is incomplete because carbohydrates also provide structural support, such as cellulose in plant cell walls and chitin in fungi and arthropods.
Practice Questions
- 1 A cell links 12 amino acids into one polypeptide chain. How many peptide bonds form, and how many water molecules are released?
- 2 A polysaccharide is broken into 25 monosaccharides by hydrolysis. How many water molecules are used to break all the glycosidic bonds in this unbranched polymer?
- 3 A student says that DNA and proteins are similar because both are polymers, but they must have the same monomers. Explain why this reasoning is incorrect and name the monomers of each.