What are the properties of carbohydrates, lipids and proteins, and how do their subunits and bonds determine them?
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.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
What this topic is asking
The College Board (Topic 1.4) wants you to describe the properties of carbohydrates, lipids and proteins, including the directionality of their structures, and to explain how the subunits and the bonds between them determine each macromolecule's function. The four levels of protein structure are a heavily examined idea.
Carbohydrates
- Storage: starch (plants) and glycogen (animals) coil so they are compact and easily hydrolyzed for energy.
- Structure: cellulose (plant cell walls) has a different glycosidic linkage that makes straight, hydrogen-bonded chains, giving great tensile strength that most animals cannot digest.
The same monomer (glucose) gives storage or structural polymers depending only on how the monomers are bonded, a clear case of bonding determining function.
Lipids
Lipids are largely nonpolar (hydrophobic) because they are mostly carbon and hydrogen with little oxygen.
Proteins
Proteins are polymers of amino acids joined by peptide bonds. An amino acid has a central carbon, an amino group, a carboxyl group, a hydrogen, and a variable R group (side chain). The R groups give each amino acid its chemical character.
The four levels of protein structure:
- Primary: the sequence of amino acids, determined by the gene. It is directional, running from the amino (N) terminus to the carboxyl (C) terminus.
- Secondary: local folding into alpha helices and beta-pleated sheets, held by hydrogen bonds along the backbone.
- Tertiary: the overall three-dimensional shape, set by interactions between R groups (hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges).
- Quaternary: the assembly of two or more polypeptide subunits (for example haemoglobin's four chains).
The sequence determines the folding, and the folded shape determines the function, so a single amino-acid change can change shape and therefore function (as in sickle-cell haemoglobin).
Directionality
Many biological polymers are directional: proteins run N-terminus to C-terminus, and nucleic acids run 5' to 3'. Directionality matters because synthesis, reading and function all proceed in a defined direction. AP questions sometimes test that you know the ends and why they matter.
Try this
Q1. Identify the bond joining two amino acids and the level of protein structure it creates. [2 points]
- Cue. A peptide bond; it creates the primary structure (the amino acid sequence).
Q2. Explain why unsaturated fats are usually liquid at room temperature. [2 points]
- Cue. Cis double bonds put kinks in the fatty-acid chains, so they cannot pack tightly, leaving the fat liquid.
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 20174 marksSection II (long FRQ excerpt). A mutation changes one amino acid in the active site of an enzyme. Explain how this single change could alter the protein's function, referring to the levels of protein structure.Show worked answer →
A 4-point concept-explanation FRQ on structure-function and information flow.
Point 1 (primary): the amino acid sequence is the primary structure, set by the gene; one substitution changes a single R group at the active site.
Point 2 (folding): R-group interactions (hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bridges) determine tertiary structure, so a different R group can alter local folding.
Point 3 (shape of active site): a changed active-site shape can no longer bind the substrate by complementary fit (or binds it differently).
Point 4 (function): the enzyme's catalytic function is reduced or lost because the substrate cannot bind, lowering reaction rate.
Markers reward tracing the chain from primary sequence to three-dimensional shape to loss of function, not just naming the levels.
AP 20204 marksSection I-style data question rewritten as a short FRQ. A food label lists 9 g of saturated fat and 3 g of unsaturated fat per serving. (a) Calculate the percentage of the fat that is saturated. (b) Predict which fat is more likely to be solid at room temperature and justify using molecular structure.Show worked answer →
A 4-point quantitative and concept FRQ.
(a) Calculate (1 point): total fat g; (1 point) saturated percentage .
(b) Predict (1 point): the saturated fat is more likely solid at room temperature. Justify (1 point): saturated fatty acids have no carbon-carbon double bonds, so the straight chains pack tightly together, whereas unsaturated chains have kinks (cis double bonds) that prevent tight packing and keep the fat liquid.
Markers reward the correct percentage with working and a packing-based structural justification.
Related dot points
- 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 1.5 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.
- 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 1.6 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 2.4 Plasma Membranes: describe the roles of each of the components of the cell membrane in maintaining the internal environment of the cell.
A focused answer to AP Biology Topic 2.4, covering the fluid-mosaic model, the phospholipid bilayer, membrane proteins, cholesterol and carbohydrates, and how each component maintains the cell's internal environment.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)