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Biological macromolecules are the molecular toolkit that makes life possible—and understanding them is foundational to nearly everything else you'll encounter in AP Biology. From cellular respiration to gene expression to membrane transport, these four classes of molecules show up again and again. The College Board expects you to connect structure to function: why does a phospholipid form a bilayer? How does the sequence of amino acids determine what an enzyme can do? Why does DNA's structure make it ideal for storing hereditary information?
Here's the key insight: you're being tested on relationships, not just definitions. The exam will ask you to explain how a molecule's chemical properties enable its biological role, or to compare how different macromolecules solve similar problems (like energy storage). Don't just memorize that "carbohydrates provide energy"—know why glucose is quick energy while fats store more energy per gram, and when cells use each. Master the structure-function connections, and you'll be ready for any FRQ they throw at you.
Most macromolecules are built from smaller subunits through dehydration synthesis and broken down through hydrolysis. This modular design allows cells to construct diverse molecules from a limited set of building blocks.
Compare: Carbohydrates vs. Nucleic Acids—both are polymers built from monomers via dehydration synthesis, but carbohydrates primarily store energy and provide structure, while nucleic acids store and transmit genetic information. If an FRQ asks about information storage, nucleic acids are your answer; if it asks about energy storage, think polysaccharides.
Proteins demonstrate the structure-function relationship more dramatically than any other macromolecule. The precise three-dimensional shape of a protein determines exactly what it can do.
Compare: Proteins vs. Nucleic Acids—both rely on precise sequences (amino acids vs. nucleotides) that determine function, and both can be disrupted by environmental changes. However, proteins perform diverse cellular work (enzymes, transport, structure), while nucleic acids specialize in information storage and transfer. FRQs often ask how mutations in DNA lead to altered protein function—trace the path from nucleotide sequence to amino acid sequence to protein shape.
Unlike the other macromolecules, lipids are defined by their physical property—hydrophobicity—rather than a common monomer structure. Their water-fearing nature drives membrane formation and enables long-term energy storage.
Compare: Lipids vs. Carbohydrates for energy storage—glycogen and starch provide quick, accessible energy (easily hydrolyzed), while fats store more energy per gram but require more steps to mobilize. Animals store both: glycogen in liver and muscles for immediate needs, fat in adipose tissue for long-term reserves. This is a classic FRQ comparison topic.
The Central Dogma—DNA → RNA → Protein—represents the flow of genetic information in cells. Nucleic acids store and transmit information; proteins execute the instructions.
Compare: DNA vs. RNA—both are nucleic acids with sugar-phosphate backbones and nitrogenous bases, but DNA is double-stranded with deoxyribose and thymine (stable, long-term storage), while RNA is typically single-stranded with ribose and uracil (temporary, functional). When asked about heredity, emphasize DNA; when asked about gene expression, focus on RNA's roles.
| Concept | Best Examples |
|---|---|
| Energy storage (quick access) | Glucose, glycogen, starch |
| Energy storage (long-term) | Triglycerides, fats, oils |
| Structural support | Cellulose, chitin, collagen |
| Membrane formation | Phospholipids, cholesterol |
| Information storage | DNA double helix |
| Information transfer | mRNA, tRNA, rRNA |
| Catalysis and regulation | Enzymes (proteins), ribozymes (RNA) |
| Structure-function relationship | Protein folding levels, enzyme active sites |
Which two macromolecules are both built from monomers through dehydration synthesis but serve completely different primary functions? Explain what those functions are.
Compare and contrast how carbohydrates and lipids each contribute to energy storage. Under what circumstances would an organism preferentially use each?
A mutation changes one nucleotide in a gene. Trace how this change could affect the final protein's function, identifying each macromolecule involved in the process.
Both phospholipids and proteins are essential components of cell membranes. How does the structure of each contribute to membrane function and selective permeability?
If an FRQ asks you to explain why DNA is better suited for long-term genetic storage than RNA, what three structural differences would you cite, and how does each contribute to stability?