# Core: Molecules

> IB Biology Higher Level · IB Biology HL 2025+ First Assessment
> Source: https://www.owlsprep.com/study/ib-biology-hl-u3-core-molecules/

This module covers the fundamental biological molecules that make up living organisms, including monomer-polymer structure, chemical reactions that build and break molecules, and the core link between structure and function.

**Prerequisites:** [Basic atomic structure and covalent bonding](https://www.owlsprep.com/study/ib-biology-hl-u1-chemical-basis-of-life/)

## Learning objectives

- Identify the four main classes of biological macromolecules and their key properties
- Distinguish between condensation and hydrolysis reactions in polymer assembly and breakdown
- Recognize common functional groups and their chemical properties
- Explain the core relationship between molecular structure and biological function

## Key Classes of Biological Macromolecules

**Macromolecule** — Large organic molecules produced by living organisms, grouped into four core classes. Most are polymers built from repeating monomer subunits, lipids are the exception.

*Example:* The protein hemoglobin is a macromolecule built from amino acid monomers.

All biological macromolecules are organic, meaning they contain carbon, with additional elements depending on the class. Each class has distinct structures and core biological functions:

- **Carbohydrates**: Composed of C, H, O in ~1:2:1 ratio, function in energy storage and structural support
- **Lipids**: Non-polar hydrophobic molecules, function in energy storage, membrane structure, and signaling
- **Proteins**: Polymers of amino acids, with functions including catalysis, transport, structure, and signaling
- **Nucleic acids**: Polymers of nucleotides, function in storing and transmitting hereditary information

**Worked example:** Classify each molecule as one of the four core biological molecule classes: glucose, hemoglobin, cholesterol, DNA

1. Step 1: Recall the defining features of each class.
2. Step 2: Glucose is a single sugar (monosaccharide), so it is classified as a carbohydrate.
3. Step 3: Hemoglobin is a oxygen-binding molecule made of amino acid monomers, so it is a protein.
4. Step 4: Cholesterol is a hydrophobic steroid that regulates membrane fluidity, so it is a lipid.
5. Step 5: DNA is a polymer of nucleotides that stores genetic information, so it is a nucleic acid.

> **Exam tip:** IB questions often test the fact that lipids are not true polymers — always remember this distinction.

## Condensation and Hydrolysis Reactions

**Condensation Reaction** — An anabolic reaction that covalently links two smaller subunits to form a larger molecule, with the release of one water molecule per bond formed.

*Example:* Two glucose molecules condense to form maltose, releasing one $H_2O$ molecule.

Hydrolysis is the reverse catabolic reaction that breaks the covalent bond between two subunits, by adding a water molecule to the bond. Hydrolysis occurs during digestion, when large macromolecules are broken down into absorbable monomers.

**Worked example:** Write the reaction that forms a dipeptide from two amino acids, name the reaction type and the new bond formed.

1. Step 1: Linking two amino acids is a condensation reaction.
2. Step 2: The carboxyl group ($-COOH$) of the first amino acid reacts with the amine group ($-NH_2$) of the second.
3. Step 3: $-OH$ from the carboxyl and $-H$ from the amine combine to release $H_2O$, leaving a covalent bond between the carbon and nitrogen.
4. $$H_2N-CHR-COOH + H_2N-CHR'-COOH \rightarrow H_2N-CHR-CO-NH-CHR'-COOH + H_2O$$
5. Step 4: The new covalent bond is called a peptide bond.

> **mnemonic**
>
> Condensation *Creates* polymers, Hydrolysis *Hydrolyzes* (breaks) them. C for Create, H for Hydrolyze.

## Common Functional Groups

Functional groups are specific atom groups that give biological molecules distinct chemical properties. They determine polarity, acidity/basicity, and reactivity of the molecules they are part of.

| Functional Group | Formula | Common Location | Key Property |
| --- | --- | --- | --- |
| Hydroxyl | $-OH$ | Carbohydrates, lipids | Polar |
| Carboxyl | $-COOH$ | Amino acids, fatty acids | Acidic |
| Amine | $-NH_2$ | Amino acids, nucleic acids | Basic |
| Phosphate | $-PO_4^{3-}$ | Nucleic acids, ATP, phospholipids | Polar, negative charge |
| Methyl | $-CH_3$ | Lipids, modified bases | Non-polar |

**Worked example:** Which functional group gives amino acids their acidic property?

1. Step 1: Acidic molecules donate $H^+$ ions to solution.
2. Step 2: Carboxyl groups ($-COOH$) readily dissociate to release $H^+$, forming $-COO^-$, so they are acidic.
3. Step 3: Amine groups are basic, they accept $H^+$ to become positively charged.
4. Conclusion: The carboxyl group is responsible for the acidic property of amino acids.

## Structure Determines Function

This is a core unifying theme in biology that appears repeatedly across IB Biology assessments. Any change to the linear sequence of monomers (primary structure) changes the overall 3D shape of a macromolecule, which usually alters or destroys its function.

> **info**
>
> Approximately 20% of IB Biology exam marks require you to connect structure to function, so always practice this link for every molecule you learn.

**Worked example:** Explain how phospholipid structure makes it ideal for forming cell membranes

1. Step 1: Recall phospholipid structure: one hydrophilic (water-loving) phosphate head, and two hydrophobic (water-fearing) fatty acid tails.
2. Step 2: When placed in aqueous (water-based) environments, phospholipids spontaneously arrange into a bilayer.
3. Step 3: Hydrophilic heads face outward to interact with water on both sides of the membrane, while hydrophobic tails face inward, away from water.
4. Step 4: This bilayer forms a semi-permeable barrier that controls movement of substances into and out of the cell, the core function of cell membranes.

## Common pitfalls

- **Wrong:** Claiming lipids are polymers made of repeating fatty acid monomers
  - Why it fails: Lipids are not formed from identical repeating monomers, so they are not true polymers unlike carbohydrates, proteins, and nucleic acids
  - Correct: State that lipids are a group of hydrophobic macromolecules, not true polymers
- **Wrong:** Mixing up condensation and hydrolysis, saying condensation breaks polymers
  - Why it fails: Students often forget that condensation releases water to build new bonds, while hydrolysis uses water to break bonds
  - Correct: Remember: condensation builds polymers (releases water), hydrolysis breaks polymers (uses water)
- **Wrong:** Claiming only carbohydrates contain carbon, hydrogen, and oxygen
  - Why it fails: All biological macromolecules are organic, so they all contain C, H, O; proteins add nitrogen, nucleic acids add nitrogen and phosphorus
  - Correct: Recall all macromolecules have C, H, O, with additional elements for proteins and nucleic acids
- **Wrong:** Assuming all carbohydrates function only in energy storage
  - Why it fails: Many carbohydrates have critical structural roles that are often tested in exams
  - Correct: Remember both energy storage (starch, glycogen) and structural roles (cellulose, chitin) for carbohydrates

## Cheatsheet

| Molecule Class | Subunit | Key Bond | Core Function |
| --- | --- | --- | --- |
| Carbohydrates | Monosaccharides | Glycosidic | Energy storage, structure |
| Lipids | Fatty acids + glycerol | Ester | Energy storage, membranes |
| Proteins | Amino acids | Peptide | Diverse (catalysis, structure) |
| Nucleic acids | Nucleotides | Phosphodiester | Store genetic information |

## What's next

Understanding core biological molecules is the foundation for all subsequent topics in IB Biology HL. This sub-topic sets up the detailed exploration of each biomolecule class that follows, and mastery of these basics will make complex topics like enzyme kinetics, cell membrane transport, DNA replication, and protein synthesis much easier to grasp. The core theme of structure determining function introduced here runs through every topic in Theme C, so it is important to master this concept before moving on to more specific content.

- [Membrane Structure (Core: Cells)](https://www.owlsprep.com/study/ib-biology-hl-u3-core-cells/)
- [Core: Organisms](https://www.owlsprep.com/study/ib-biology-hl-u3-core-organisms/)
- [Core: Ecosystems](https://www.owlsprep.com/study/ib-biology-hl-u3-core-ecosystems/)

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