Core: Molecules
IB Biology HLΒ· Theme D: Continuity and ChangeΒ· 5 min read
1. Monomers, Polymers, and Core Reactionsβ β ββββ± 15 min
Macromolecule
Large biological molecules assembled from smaller subunits. Four core classes are carbohydrates, proteins, nucleic acids, and lipids; lipids are not true polymers.
Example:
A DNA strand is a macromolecule made of repeating nucleotide monomers.
Condensation & Hydrolysis
Condensation bonds two monomers together, releasing one water molecule per bond. Hydrolysis is the reverse reaction that breaks polymers into monomers by adding water to break bonds.
What reaction breaks a 12-glucose starch chain into individual glucose monomers, and how many water molecules are required?
- 1
First, identify the reaction type: Breaking polymers into monomers requires hydrolysis, which uses one water molecule per bond broken.
- 2
A chain of monomers has covalent bonds between monomers. For 12 glucose monomers, that is bonds.
- 3
Each bond needs one water molecule to break, so 11 water molecules are required.
2. Carbohydrates and Lipidsβ β ββββ± 20 min
Carbohydrates have the general formula and are divided into monosaccharides (single sugars), disaccharides (two sugars), and polysaccharides (long chains). They function in energy storage and structural support.
Glycosidic Bond
A covalent bond formed between two monosaccharides via a condensation reaction.
How do structural differences between starch, glycogen, and cellulose lead to different functions?
- 1
Starch (plants) is made of alpha-glucose monomers, stored for energy. It has linear amylose and branched amylopectin forms.
- 2
Glycogen (animals) is also made of alpha-glucose, but is much more highly branched. This allows for rapid breakdown to release glucose for energy.
- 3
Cellulose (plants) is made of beta-glucose monomers, with alternating orientations to form straight unbranched chains. Hydrogen bonding between chains creates strong fibers that form plant cell walls for structural support.
Lipids are non-polar, hydrophobic molecules. Triglycerides (energy storage) are made of one glycerol and three fatty acids linked by ester bonds. Saturated fatty acids have no double bonds between carbons, while unsaturated fatty acids have one or more double bonds.
3. Proteins: Levels of Structureβ β β βββ± 25 min
Proteins are polymers of amino acids, with an enormous range of functions including enzymes, structural support, cell signaling, and transport. All amino acids have a shared core structure: a central carbon bonded to an amino group, carboxyl group, hydrogen, and a variable R-group that defines each amino acid's properties.
Peptide Bond
A covalent bond formed between the carboxyl group of one amino acid and the amino group of another via condensation.
Describe the four levels of protein structure and the bonds that stabilize each level.
- 1
- Primary structure: The linear sequence of amino acids in the polypeptide chain, stabilized exclusively by peptide bonds.
- 2
- Secondary structure: Repeating folding patterns (alpha-helices, beta-pleated sheets) formed by hydrogen bonding between the polypeptide backbone (not R-groups).
- 3
- Tertiary structure: The overall 3D shape of a folded polypeptide, stabilized by interactions between R-groups: hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions.
- 4
- Quaternary structure: The structure of proteins made of multiple separate polypeptide chains, stabilized by the same R-group interactions as tertiary structure. Not all proteins have this level.
4. Nucleic Acidsβ β β βββ± 20 min
Nucleic acids are polymers of nucleotides that function in storing and transmitting genetic information. There are two types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
Nucleotide
The monomer of nucleic acids, made of three components: a pentose sugar, a phosphate group, and a nitrogenous base. Adjacent nucleotides are linked by phosphodiester bonds formed via condensation.
What are three key structural differences between DNA and RNA?
- 1
- Sugar: DNA contains deoxyribose sugar, which lacks a hydroxyl group on the 2' carbon. RNA contains ribose, which has a 2' hydroxyl group.
- 2
- Nitrogenous base: DNA uses the base thymine, while RNA uses uracil instead of thymine.
- 3
- Overall structure: DNA is almost always a double-stranded helix, while RNA is typically single-stranded (though it can fold into complex 3D shapes).
5. Common Pitfalls
Wrong move:
Claiming lipids are polymers made of repeating fatty acid monomers.
Why:
IB explicitly marks this incorrect. Lipids are macromolecules but not true polymers with a repeating monomer chain.
Correct move:
State that lipids are a group of hydrophobic macromolecules that are not true polymers.
Wrong move:
Saying condensation reactions use water as a reactant.
Why:
This reverses the two core reactions. Condensation releases water to build polymers, hydrolysis uses water to break them.
Correct move:
Remember the mnemonic: Condensation creates water, hydrolysis uses water.
Wrong move:
Stating peptide bonds stabilize secondary protein structure.
Why:
Peptide bonds only form the primary linear sequence. Secondary structure is held together by hydrogen bonds between the polypeptide backbone.
Correct move:
Link primary structure to peptide bonds, secondary structure to hydrogen bonds.
Wrong move:
Claiming all proteins have quaternary structure.
Why:
Quaternary structure only exists in proteins made of multiple separate polypeptide chains.
Correct move:
Only describe quaternary structure when referring to multi-chain proteins.
Wrong move:
Confusing glycosidic, ester, peptide, and phosphodiester bonds.
Why:
Exam questions regularly test this distinction, and mixing them up is a common cause of lost marks.
Correct move:
Memorize: Glycosidic = carbohydrates, ester = lipids, peptide = proteins, phosphodiester = nucleic acids.
6. Quick Reference Cheatsheet
Molecule Class | Monomer/Subunit | Bond Type | Core Function |
|---|---|---|---|
Carbohydrates | Monosaccharide | Glycosidic | Energy storage, structure |
Lipids | Glycerol + fatty acids | Ester | Energy, membranes, hormones |
Proteins | Amino acid | Peptide | Enzymes, structure, signaling |
Nucleic acids | Nucleotide | Phosphodiester | Genetic information storage |
7. Frequently Asked
Do I need to memorize molecular structures for exams?
Yes, HL requires you to memorize the general structure of glucose, amino acids, fatty acids, and nucleotides.
Are lipids considered polymers?
No, IB marks the statement 'lipids are polymers' as incorrect. They are macromolecules but not true polymers, as they have no repeating monomer chain.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 Β· Paper 1
Multiple choice on carbohydrate structure
- 2024 Β· Paper 2
Protein structure extended response
- 2023 Β· Paper 1
Hydrolysis reaction calculation
Going deeper
What's Next
Core biological molecules are the foundation of every other topic in IB Biology HL. All cellular processes, from enzyme catalysis to DNA replication, cell division, and immune response, depend on the structure and properties of the molecules covered in this subtopic. Mastering the distinctions between molecule classes, bond types, and structure-function relationships will make all subsequent topics much easier to understand. You will next build on this core knowledge to explore more specific processes related to molecular biology, cell function, and genetics.