# Core: Molecules

> IB Biology HL · IB Biology HL 2025 Syllabus
> Source: https://www.owlsprep.com/study/ib-biology-hl-u4-core-molecules/

This core subtopic explores the key biological molecules that underpin all cellular processes, including carbohydrates, lipids, proteins, and nucleic acids. It covers how monomers assemble into polymers and the link between molecular structure and function.

**Prerequisites:** [Basic cell structure and organization](https://www.owlsprep.com/study/ib-biology-hl-intro-to-cells/)

## Learning objectives

- Identify the structure and function of the four core classes of biological molecules
- Distinguish between monomers and polymers, and between condensation and hydrolysis reactions
- Explain how molecular structure determines biological function
- Name and correctly assign covalent bonds to different classes of molecules

## Monomers, Polymers, and Core Reactions

**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.

**Worked example:** 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 $n$ monomers has $(n-1)$ covalent bonds between monomers. For 12 glucose monomers, that is $12-1 = 11$ bonds.
3. Each bond needs one water molecule to break, so 11 water molecules are required.

> **mnemonic**
>
> Condensation = Create (releases water), Hydrolysis = H2O to lyse (break, uses water)

## Carbohydrates and Lipids

Carbohydrates have the general formula $C_n(H_2O)_m$ 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.

**Worked example:** 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.

## Proteins: Levels of Structure

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.

**Worked example:** Describe the four levels of protein structure and the bonds that stabilize each level.

1. 1. Primary structure: The linear sequence of amino acids in the polypeptide chain, stabilized exclusively by peptide bonds.
2. 2. Secondary structure: Repeating folding patterns (alpha-helices, beta-pleated sheets) formed by hydrogen bonding between the polypeptide backbone (not R-groups).
3. 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. 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.

> **info**
>
> Changes in pH or temperature break the interactions that hold protein shape together, causing denaturation, which is almost always irreversible.

## Nucleic Acids

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.

**Worked example:** What are three key structural differences between DNA and RNA?

1. 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. 2. Nitrogenous base: DNA uses the base thymine, while RNA uses uracil instead of thymine.
3. 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).

## Common pitfalls

- **Wrong:** Claiming lipids are polymers made of repeating fatty acid monomers.
  - Why it fails: IB explicitly marks this incorrect. Lipids are macromolecules but not true polymers with a repeating monomer chain.
  - Correct: State that lipids are a group of hydrophobic macromolecules that are not true polymers.
- **Wrong:** Saying condensation reactions use water as a reactant.
  - Why it fails: This reverses the two core reactions. Condensation releases water to build polymers, hydrolysis uses water to break them.
  - Correct: Remember the mnemonic: Condensation creates water, hydrolysis uses water.
- **Wrong:** Stating peptide bonds stabilize secondary protein structure.
  - Why it fails: Peptide bonds only form the primary linear sequence. Secondary structure is held together by hydrogen bonds between the polypeptide backbone.
  - Correct: Link primary structure to peptide bonds, secondary structure to hydrogen bonds.
- **Wrong:** Claiming all proteins have quaternary structure.
  - Why it fails: Quaternary structure only exists in proteins made of multiple separate polypeptide chains.
  - Correct: Only describe quaternary structure when referring to multi-chain proteins.
- **Wrong:** Confusing glycosidic, ester, peptide, and phosphodiester bonds.
  - Why it fails: Exam questions regularly test this distinction, and mixing them up is a common cause of lost marks.
  - Correct: Memorize: Glycosidic = carbohydrates, ester = lipids, peptide = proteins, phosphodiester = nucleic acids.

## 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 |

## 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.

- [Membrane Structure and Transport](https://www.owlsprep.com/study/ib-biology-hl-u4-core-cells/)
- [Core: Classification and Cladistics of Organisms](https://www.owlsprep.com/study/ib-biology-hl-u4-core-organisms/)
- [Core: Ecosystems](https://www.owlsprep.com/study/ib-biology-hl-u4-core-ecosystems/)

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