How do changes to the subunits of a polymer change the structure and function of a macromolecule?
Topic 1.5 Structure and Function of Biological Macromolecules: explain how a change in the subunit composition or sequence of a polymer may affect its structure and function.
A focused answer to AP Biology Topic 1.5, covering how the sequence and composition of monomers determine the structure and function of macromolecules, illustrated with proteins, sickle-cell haemoglobin, and the directionality of polymers.
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What this topic is asking
The College Board (Topic 1.5) wants you to explain how a change in the composition or sequence of a polymer's subunits changes its structure and therefore its function. This is the central structure-function idea of the course, and it links to information storage and transmission: the sequence carries information.
Sequence determines structure determines function
For proteins, the gene specifies the order of amino acids (primary structure). Interactions between the R groups fold the chain into a specific tertiary shape. Because each R group has particular properties (charged, polar, nonpolar, able to form disulfide bridges), changing even one can change how the chain folds and how it interacts with other molecules.
Why a single change can matter
A change in one subunit can be:
- Silent, if the substituted monomer has similar properties and sits in a non-critical region, leaving structure and function unchanged.
- Significant, if the change is at a functional site (such as an enzyme's active site) or alters folding, changing or abolishing function.
The same idea in other macromolecules
- Nucleic acids: the sequence of nitrogenous bases is information; changing a base (a mutation) can change the amino acid specified, and thus the protein. This links Unit 1 to gene expression.
- Carbohydrates: changing the type of glycosidic bond turns digestible starch into indigestible cellulose.
- Lipids: adding or removing double bonds (saturated versus unsaturated) changes whether a fat is solid or liquid and how it behaves in membranes.
Directionality and reading
Polymers are directional (proteins N to C, nucleic acids 5' to 3'). The cell reads and builds them in a defined direction, so the position of a change within the sequence, not just its identity, affects the outcome.
Try this
Q1. Explain why a substitution that swaps one hydrophobic amino acid for another similar hydrophobic amino acid might have no effect on a protein. [2 points]
- Cue. The new R group has similar properties, so folding and the functional sites are unchanged; structure and therefore function are preserved.
Q2. Predict how a change in the base sequence of a gene could change the protein it encodes, and justify. [2 points]
- Cue. A base change can alter the codon and thus the amino acid inserted; because sequence determines folding and function, the protein's shape and activity may change.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 20214 marksSection II (short FRQ). In sickle-cell anaemia, a single nucleotide change replaces glutamic acid (hydrophilic) with valine (hydrophobic) in the beta-globin protein. Explain how this single subunit change alters the structure and function of haemoglobin.Show worked answer →
A 4-point concept-explanation FRQ on structure-function and information.
Point 1 (subunit change): the substituted valine has a hydrophobic R group, unlike the original hydrophilic glutamic acid.
Point 2 (structure): the hydrophobic patch on the surface causes haemoglobin molecules to stick together (aggregate) when oxygen is low, changing the protein's behavior.
Point 3 (cell shape): the aggregated haemoglobin distorts red blood cells into a sickle shape.
Point 4 (function): sickled cells carry oxygen poorly and block capillaries, so a single amino-acid change reduces oxygen transport.
Markers reward connecting the changed R-group property to aggregation, then to cell shape, then to impaired function.
AP 20244 marksSection I-style data question rewritten as a short FRQ. Two enzymes differ at only one position: enzyme X has a charged amino acid in its active site, enzyme Y has a nonpolar one. At pH 7 the reaction rate is 40 micromoles/min for X and 6 micromoles/min for Y. (a) Calculate the percentage decrease in rate from X to Y. (b) Explain, in terms of subunit change, why the rates differ.Show worked answer →
A 4-point quantitative and concept FRQ.
(a) Calculate (1 point): decrease ; (1 point) percentage decrease.
(b) Explain (1 point): the single change from a charged to a nonpolar R group alters the chemistry and shape of the active site; (1 point) the substrate (or transition state) is no longer bound or stabilized as effectively, so the catalytic rate falls.
Markers reward correct working for the percentage and a mechanism that links the subunit change to altered active-site interactions, not just "the shape changed."
Related dot points
- Topic 1.4 Properties of Biological Macromolecules: describe the properties of carbohydrates, lipids and proteins, including the directionality of their structures and how their subunits and bonding give rise to their functions.
A focused answer to AP Biology Topic 1.4, covering carbohydrates, lipids and proteins, the four levels of protein structure, saturated versus unsaturated fats, and how subunits and bonding determine properties and function.
- Topic 1.3 Introduction to Biological Macromolecules: describe the chemical reactions that build and break biological macromolecules and the structure and function of the four classes.
A focused answer to AP Biology Topic 1.3, covering dehydration synthesis and hydrolysis, monomers and polymers, and the four classes of macromolecule (carbohydrates, lipids, proteins, nucleic acids).
- Topic 1.6 Nucleic Acids: describe the structural similarities and differences between DNA and RNA and explain how the directionality and base pairing of nucleic acids support their function.
A focused answer to AP Biology Topic 1.6, covering nucleotide structure, the antiparallel double helix, base pairing, the 5' to 3' directionality, and the structural differences between DNA and RNA.
- Topic 1.2 Elements of Life: describe the composition of macromolecules required by living organisms and the role of carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur in forming them.
A focused answer to AP Biology Topic 1.2, covering the major elements of life (C, H, O, N, P, S), why carbon is the backbone of organic molecules, and which elements each class of macromolecule contains.
- Topic 1.1 Structure of Water and Hydrogen Bonding: explain how the properties of water that result from its polarity and hydrogen bonding affect its biological function.
A focused answer to AP Biology Topic 1.1, covering the polarity of water, hydrogen bonding, and the emergent properties (cohesion, adhesion, high specific heat, evaporative cooling and the solvent role) that make water essential to life.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)