# Core: Molecules

> IB Biology HL · Theme A: Unity and Diversity
> Source: https://www.owlsprep.com/study/ib-biology-hl-u1-core-molecules/

This foundational sub-topic introduces the core biological molecules that underpin all life, covering macromolecule classes, key synthetic reactions, and the unifying role of molecules in supporting the unity of life across all organisms.

**Prerequisites:** [Basic chemical principles (covalent bonding, organic compounds)](https://www.owlsprep.com/study/ib-biology-hl-preliminary-chemical-basics/)

## Learning objectives

- Identify the four main classes of biological macromolecules
- Distinguish between monomers and polymers
- Explain the role of condensation and hydrolysis in molecule synthesis/breakdown
- Relate core molecular structure to key biological functions

## Key Definitions: Monomers, Polymers and Macromolecules

**Biological Macromolecule** — Large organic molecules synthesized by living organisms, grouped into four core classes common to all life. While lipids are not true polymers, they are classified as macromolecules due to their size and biological role.

*Example:* Cellulose, insulin, DNA, cholesterol

All macromolecules are built from smaller subunits: monomers are small repeating units that link together to form long chains called polymers. Only three of the four core macromolecule classes are true polymers.

**Worked example:** Classify each molecule as monomer, polymer, or neither: glucose, hemoglobin, carbon dioxide, triglyceride, nucleotide

1. 1. Recall definitions: monomers are small repeating subunits that form polymers. Glucose (a sugar subunit for carbohydrates) and nucleotide (a subunit for nucleic acids) fit this definition.
2. 2. Hemoglobin is a protein made of 4 linked chains of amino acid monomers, so it is a polymer.
3. 3. Carbon dioxide is a small inorganic molecule, not a monomer or polymer, so it is neither.
4. 4. Triglyceride is a large lipid made of one glycerol and three fatty acids, with no chain of repeating monomers, so it is neither.

> **Exam tip:** Examiners regularly test whether you know lipids are not true polymers — always remember this distinction.

## Condensation and Hydrolysis Reactions

**Condensation Reaction** — An anabolic (building) reaction where two molecules are covalently joined, with the release of one water molecule per bond formed.

**Hydrolysis Reaction** — A catabolic (breaking) reaction where a covalent bond between two molecules is broken via the addition of one water molecule per bond broken.

All polymerization and breakdown of biological molecules follows these two reaction types, both catalyzed by specific enzymes in cells. For a polymer made of $n$ monomers, $n-1$ condensation reactions (and thus $n-1$ water molecules released) are required to build the full polymer.

**Worked example:** How many water molecules are released when a polypeptide 150 amino acids long is synthesized?

1. 1. A polypeptide is a polymer of amino acid monomers, each peptide bond between two monomers forms via one condensation reaction that releases one water molecule.
2. 2. The number of bonds in a polymer of $n$ monomers equals $n-1$.
3. $$n - 1 = 150 - 1 = 149$$
4. 3. 149 water molecules are released during synthesis.

**Check your understanding**

Check your understanding:

1. Which reaction breaks down starch into individual glucose molecules?

   - A: Condensation
   - B: Hydrolysis
   - C: Oxidation
   - D: Reduction

   *Why:* Hydrolysis uses water to break the glycosidic bonds between glucose monomers in starch, a polysaccharide polymer.

*Calculator:* forbidden

## Overview of Core Macromolecule Classes

| Macromolecule Class | Monomer Subunit | Key Biological Functions | Example |
| --- | --- | --- | --- |
| Carbohydrates | Monosaccharides | Energy storage, structural support | Cellulose, starch |
| Proteins | Amino acids | Catalysis, transport, structure, signaling | Enzymes, hemoglobin |
| Nucleic Acids | Nucleotides | Genetic information storage/transfer | DNA, RNA |
| Lipids | No true repeating monomer | Energy storage, membrane structure, insulation | Phospholipids, triglycerides |

A core unifying principle of biology is that all living organisms use the same four classes of macromolecules for the same core functions, which directly supports the theory of common ancestry and the unity of life, a central theme of IB Biology.

**Worked example:** A new molecule isolated from algae is a long chain of identical 6-carbon sugar subunits linked by covalent bonds. What class of macromolecule is this?

1. 1. Analyze the structure: long chain of identical repeating sugar subunits.
2. 2. Match to the core classes: carbohydrates are polymers of monosaccharide (sugar) monomers.
3. 3. Eliminate other options: proteins are made of amino acids, nucleic acids of nucleotides, lipids have no repeating monomer chain.
4. Conclusion: The molecule is a carbohydrate (specifically a polysaccharide).

> **Exam tip:** Always link structure to function when asked for explanations — this is the most common marking requirement for macromolecule questions.

## Exam Phrasing for Molecule Questions

**Exam command terms**

IB Biology uses specific command terms for this topic, with different expectations for full marks:

- **Distinguish** — Provide clear differences between concepts, not just individual definitions *(Distinguish between condensation and hydrolysis requires you to compare both reaction types directly)*

- **Identify** — Only require the correct name/classification, no further explanation needed unless asked *(Identify the class of molecule = just state the correct class)*

- **Explain** — Link cause and effect, connect structure to function *(Explain why lipids are not polymers = state they lack repeating monomers)*

## Common pitfalls

- **Wrong:** Claiming that all macromolecules are polymers
  - Why it fails: Students assume all large biological molecules are polymers, but lipids do not fit the definition
  - Correct: State that lipids are macromolecules but not true polymers, as they have no chain of repeating monomer subunits
- **Wrong:** Calculating $n$ water molecules released for a polymer of $n$ monomers
  - Why it fails: Students forget that each bond forms between two monomers, so one fewer bond than the number of monomers
  - Correct: Use the formula $n-1$ for the number of water molecules released during synthesis of a polymer of $n$ monomers
- **Wrong:** Mixing up condensation and hydrolysis, claiming condensation breaks bonds
  - Why it fails: Similar names and reaction types are often confused by students
  - Correct: Remember: condensation *builds* and releases water; hydrolysis *breaks* and uses water
- **Wrong:** Claiming all biological molecules are organic
  - Why it fails: Students focus only on macromolecules and forget essential inorganic biological molecules
  - Correct: Acknowledge that both organic macromolecules and inorganic molecules (water, minerals) are critical for life

## Cheatsheet

| Category | Details |
| --- | --- |
| Condensation Reaction | Joins monomers, releases water, anabolic |
| Hydrolysis Reaction | Breaks polymers, uses water, catabolic |
| Monosaccharide → | Polysaccharide (Carbohydrate) |
| Amino acid → | Polypeptide (Protein) |
| Nucleotide → | Nucleic Acid |
| Lipids | Macromolecule, not a polymer |

## What's next

This sub-topic provides the foundational biochemistry you need for all subsequent units in IB Biology HL. Each core macromolecule class is explored in detail in upcoming sub-topics, with their specific structure-function relationships frequently tested in both Paper 1 and Paper 2 exams. Mastering the core definitions and reaction types here will make understanding more complex topics like enzyme activity, cell membrane structure, and DNA replication much easier.

- [Core: Cells](https://www.owlsprep.com/study/ib-biology-hl-u1-core-cells/)
- [Core: Organisms](https://www.owlsprep.com/study/ib-biology-hl-u1-core-organisms/)
- [Core: Ecosystems](https://www.owlsprep.com/study/ib-biology-hl-u1-core-ecosystems/)

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