Properties of Biological Macromolecules — AP Biology
1. Polymerization: Dehydration Synthesis and Hydrolysis ★★☆☆☆ ⏱ 4 min
All biological macromolecules are assembled and broken down via two core reaction types that rely on the reactivity of functional groups. Dehydration synthesis (condensation) builds larger polymers from smaller monomer subunits, while hydrolysis breaks large polymers into smaller monomers.
n\ \text{Monomer} \rightarrow (\text{Monomer})_n + (n-1)\ H_2O
Exam tip: Always count bonds first, not monomers. AP questions frequently trick students into using $n$ instead of $n-1$, so pause to ask 'how many linkages between monomers do I have?' before writing your answer.
2. Directionality of Macromolecule Backbones ★★★☆☆ ⏱ 5 min
Every linear biological polymer has inherent directionality: the two ends of the backbone are chemically distinct, and the order of monomers is always read from one end to the other. Directionality arises from the way monomers are linked: each monomer contributes asymmetric functional groups to the backbone bond, so the chain cannot be reversed without changing the chemical identity of the molecule.
- **Proteins**: Amino acids are linked by peptide bonds between the amino group of one amino acid and the carboxyl group of the next. One end has a free amino group ($-NH_3^+$) called the N-terminus, and the other end has a free carboxyl group ($-COO^-$) called the C-terminus. Sequence is always read N → C.
- **Nucleic acids**: Nucleotides are linked by phosphodiester bonds between the 3' hydroxyl of one nucleotide sugar and the 5' phosphate of the next. One end has a free 5' phosphate, the other has a free 3' hydroxyl. Sequence is always read 5' → 3'.
- **Carbohydrates**: Polysaccharides have directionality, with distinct chemical groups at each end, and enzymes only add/remove monomers from one end.
Directionality means reversing the monomer sequence creates a different molecule with different biological function.
Exam tip: Always label ends and note direction when writing sequences on FRQs. AP graders will dock points for unlabeled reversed sequences, even if the monomer order is correct.
3. Structure-Function Relationships and Sequence Variation ★★★☆☆ ⏱ 5 min
The core enduring understanding for this topic is that macromolecule function is directly determined by monomer sequence and resulting three-dimensional structure. Any change to monomer sequence, branching, or bonding can alter folding and function. For example, starch and cellulose are both polymers of glucose, but the orientation of their glycosidic bonds differs: starch uses α-1,4 linkages that form helical, easily hydrolyzed structures for energy storage, while cellulose uses β-1,4 linkages that form straight, hydrogen-bonded fibers for structural support. The single difference in bond orientation creates completely different functions, even with identical monomers. For proteins, a single amino acid change can alter folding: in sickle cell anemia, a nonpolar valine replaces a charged glutamic acid in hemoglobin, causing the protein to aggregate and distort red blood cells. Changes that preserve R-group chemistry (e.g., replacing one nonpolar amino acid with another) often have little to no effect on function.
Exam tip: When explaining function differences between macromolecules made of the same monomers, always link structural differences to interactions with other molecules (like enzymes). AP exam answers that only state a structural difference without connecting it to function will not earn full points.
4. AP-Style Concept Check ★★★★☆ ⏱ 3 min
Common Pitfalls
Why: Students memorize 'dehydration makes water' but forget each bond connects two monomers, so the number of bonds is one less than the number of monomers.
Why: Students group all four macromolecules together as polymers, but polymers are defined as chains of repeating monomer subunits, which lipids do not have.
Why: Students confuse the direction of the complementary strand with the standard convention for writing and reading sequence.
Why: Students generalize that any change alters function, but some substitutions do not change amino acid identity or replace an amino acid with a chemically similar one.
Why: Students mix up naming conventions for proteins vs nucleic acids.