The Four Macromolecules
General Biology · Section 1.3 · 18 study cards
Dehydration synthesis and hydrolysis, then carbohydrates, lipids, proteins and nucleic acids, their monomers, bonds and functions.
Practice this set → Spaced repetition, card by card. No account needed.
Method
Use one grid for all four classes
Answer every macromolecule question by filling the same five columns: monomer, bond that joins the monomers, key structural variation, major functions, and how it is broken down. Once the grid is memorised you can reconstruct any comparison question rather than recalling it.
- Carbohydrates: monosaccharide, glycosidic linkage, alpha versus beta and branching, energy and structure.
- Lipids: no true monomer, ester linkage in fats, saturated versus unsaturated tails, storage, membranes, signalling.
- Proteins: amino acid, peptide bond, R group chemistry, almost every cellular job.
- Nucleic acids: nucleotide, phosphodiester bond, sugar and base identity, information storage and transfer.
Track the water
- Building a polymer removes water, one molecule per bond formed, and requires energy.
- Breaking a polymer adds water, one molecule per bond broken, and releases energy.
- To join n monomers into one chain you form n minus 1 bonds and release n minus 1 water molecules.
That last count is a favourite exam calculation, so practise it until it is automatic.
Let structure predict function
Do not learn functions as a separate list. A branched polymer such as glycogen has many free ends, so it can be mobilised fast. A straight beta-linked chain such as cellulose forms hydrogen-bonded sheets, so it is strong. An amphipathic phospholipid has two chemistries in one molecule, so it forms a boundary. In each case the function is a consequence you can state in one sentence from the structure.
For proteins, separate backbone from side chain
Secondary structure is backbone hydrogen bonding and nothing else. Tertiary and quaternary structure are side chain interactions. If a question describes a disulfide bridge or a hydrophobic core, it is asking about tertiary structure, no matter how the helices are described.
Definitions and theorems
Worked example
A linear polypeptide is assembled from 100 amino acids. (i) How many peptide bonds does it contain and how many water molecules were released? (ii) The chain is then heated to 90 °C and loses all catalytic activity, but analysis shows the amino acid sequence is unchanged. Identify which levels of structure were lost, name three specific interactions that were disrupted, and predict whether cooling will restore activity.
Joining n monomers in a single unbranched chain forms n minus 1 bonds, so 100 amino acids give 99 peptide bonds.
Each dehydration synthesis releases one water molecule, so 99 water molecules were released.
Heating supplies kinetic energy that disrupts weak interactions but is far too little to break covalent peptide bonds, which is confirmed by the sequence being unchanged.
Primary structure is therefore intact. Secondary, tertiary and any quaternary structure are lost: the protein is denatured.
The disrupted interactions include backbone hydrogen bonds holding alpha helices and beta sheets, hydrogen bonds and ionic bonds between R groups, and hydrophobic clustering of nonpolar side chains in the core. Disulfide bridges are covalent and would survive heating alone.
Activity is lost because the active site is a precise three-dimensional arrangement of residues that may be far apart in the sequence; unfolding destroys that geometry.
Prediction: because the sequence is intact, refolding is possible in principle, and some small proteins do renature on cooling. In practice large proteins usually aggregate instead through exposed hydrophobic surfaces, so activity is often not recovered.
Common mistakes
- Calling lipids polymers. A triglyceride is not a chain of repeating monomers, and lipids as a class are defined by hydrophobicity rather than by a building plan. Questions asking which macromolecule is not a polymer appear on almost every exam; the answer is lipids.
- Mixing up the levels of protein structure. Students assign hydrogen bonds to secondary structure and then forget that tertiary structure also uses hydrogen bonds. The clean rule is that secondary structure uses backbone hydrogen bonds only, while tertiary structure uses R group interactions of every kind.
- Thinking denaturation breaks peptide bonds. It does not. Primary structure survives heat and pH extremes; what is lost is the folded shape. Writing that a denatured protein has been broken into amino acids is a hydrolysis answer given to a denaturation question.
- Forgetting that starch and cellulose are both glucose. Asked why humans cannot digest cellulose, students answer that it is a different sugar. It is the same sugar in a beta linkage, and human enzymes are specific for the alpha linkage. Say linkage, not sugar.
- Losing directionality in nucleic acids. Writing a DNA sequence without labelling the 5-prime and 3-prime ends, or forgetting that the two strands are antiparallel, makes replication and transcription questions unanswerable later in the course. Label the ends every time.
Practice it
Reading the method is not the same as being able to recall it under pressure. This set drills 18 cards one at a time and schedules each card separately, so the ones you keep missing come back sooner.