Core: Molecules

IB Biology Higher Level· Theme B: Form and Function, Unit 2· 15 min read

1. Classification of Biological Molecules★★☆☆☆⏱ 5 min

📘 Definition

Biological Macromolecule

Large organic molecules synthesized by living organisms; most are polymers built from smaller repeating monomer subunits

Example:

DNA is a nucleic acid macromolecule built from repeating nucleotide monomers

There are four core classes of biological macromolecules universal to all life. Three of the four form polymers, while lipids are not true polymers because they lack a repeating monomer structure.

Carbohydrates: Built from monosaccharide monomers, used for energy storage and structural support
Proteins: Built from amino acid monomers, carry out most cellular catalytic and structural roles
Nucleic acids: Built from nucleotide monomers, store and transmit genetic information
Lipids: Non-polar hydrophobic molecules, used for energy storage, membrane structure and signalling

📐 Worked Example

A student identifies a triglyceride (three fatty acids bonded to glycerol) as a protein polymer. Is the student correct? Explain your answer.

1
First, recall the definition of a protein and how triglycerides are classified.
2
Proteins are polymers made of repeating amino acid monomers. A triglyceride is a type of lipid.
3
Lipids are not true polymers because they do not have a chain of identical repeating monomers.
4
Conclusion: The student is incorrect; this molecule is a lipid, not a protein.

Exam tip:

Examiners frequently test whether you know lipids are not polymers — don't forget this distinction!

2. Condensation and Hydrolysis Reactions★★★☆☆⏱ 5 min

🚫 No Calculator

📘 Definition

Condensation Reaction

An anabolic reaction that forms a covalent bond between two molecules, releasing one water molecule as a byproduct

Example:

Joining two glucose molecules to form maltose releases one water molecule

📘 Definition

Hydrolysis Reaction

A catabolic reaction that breaks a covalent bond between two molecules, using one water molecule to split the bond

Example:

Breaking down sucrose into glucose and fructose uses one water molecule

All polymer synthesis occurs via condensation, and all polymer breakdown occurs via hydrolysis. These reactions are catalyzed by enzymes in cells, with one water molecule produced or consumed per bond.

📐 Worked Example

How many water molecules are released when a polypeptide of 12 amino acids is synthesized via condensation?

1
A polypeptide is a linear chain of amino acids linked by peptide bonds.
2
For a linear chain of $n$ monomers, the number of covalent bonds between monomers is always $n - 1$ .
3
$\text{Number of bonds} = 12 - 1 = 11$
4
One water molecule is released per bond formed during condensation.
5
Conclusion: 11 water molecules are released.

3. The Form-Function Relationship★★★☆☆⏱ 5 min

The core theme of IB Biology is that a molecule's biological function is directly determined by its three-dimensional structure (form). Even small changes to molecular structure can completely alter or eliminate function.

📐 Worked Example

Explain why phospholipid structure makes them ideal for forming cell membranes.

1
Phospholipids have a unique amphipathic structure: a polar hydrophilic phosphate head, and two non-polar hydrophobic fatty acid tails.
2
Cell environments are aqueous (water-based) on both the inside and outside of the cell.
3
Hydrophobic interactions cause the fatty acid tails to turn inward away from water, while hydrophilic heads face outward toward water on both sides.
4
This spontaneous arrangement forms a phospholipid bilayer, a selectively permeable barrier that matches the required function of a cell membrane.

4. Inorganic Ions in Biology★★☆☆☆⏱ 4 min

In addition to organic macromolecules, inorganic ions (minerals) are critical for biological function, with each ion carrying out specific roles based on its chemical properties.

Magnesium (Mg²⁺): Core component of chlorophyll
Iron (Fe²⁺): Oxygen binding cofactor in hemoglobin
Sodium (Na⁺): Nerve impulse transmission and osmotic balance
Calcium (Ca²⁺): Muscle contraction and bone structure

📐 Worked Example

Explain why leaves of a magnesium-deficient plant turn yellow.

1
Magnesium is an essential inorganic ion required to synthesize chlorophyll.
2
Chlorophyll is the photosynthetic pigment that gives leaves their green color.
3
Without magnesium, the plant cannot produce enough chlorophyll.
4
Lack of green chlorophyll reveals yellow carotenoid pigments already present in leaves, causing yellowing.

5. Common Pitfalls

Wrong move:

Claiming lipids are polymers made of fatty acid monomers

Why:

Lipids do not have a repeating chain of identical monomers, so they are not true polymers

Correct move:

State that lipids are biological macromolecules, but not true polymers

Wrong move:

Calculating $n$ water molecules for an $n$ -monomer polymer

Why:

$n$ monomers only form $n - 1$ bonds, so $n - 1$ water molecules are released

Correct move:

Use the formula (number of monomers $- 1$ ) for condensation water count

Wrong move:

Confusing condensation and hydrolysis: saying condensation breaks bonds

Why:

Hydro- means water, lysis means breaking: hydrolysis uses water to break bonds

Correct move:

Remember: condensation joins molecules and releases water; hydrolysis breaks molecules using water

Wrong move:

Claiming all biological molecules are organic

Why:

Inorganic molecules like water and ions are critical core biological molecules

Correct move:

Distinguish between organic and inorganic biological molecules when answering questions

Wrong move:

Answering function questions without referencing molecular structure

Why:

IB exams expect explicit links between form and function for full marks

Correct move:

Always connect a molecule's structural features to its biological role in your answer

6. Quick Reference Cheatsheet

Molecule Class	Monomer	Key Function	Synthesis Reaction
Carbohydrates	Monosaccharides	Energy storage / structure	Condensation
Proteins	Amino acids	Catalysis / signalling / structure	Condensation
Nucleic Acids	Nucleotides	Genetic information storage	Condensation
Lipids	None (not a polymer)	Membranes / energy storage	Condensation
Water	N/A	Solvent / temperature regulation	N/A

7. Frequently Asked

Are lipids considered biological macromolecules? Are they polymers?

Lipids are classified as biological macromolecules due to their large size, but they are not true polymers because they lack a repeating chain of identical monomer subunits.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · 1
Identify polymer from molecule list
2024 · 2
Relate molecular structure to function
2023 · 1
Compare condensation and hydrolysis

Going deeper

syllabusIB Biology 2025 Official SyllabusFull theme B reference

What's Next

Understanding core biological molecules is the foundation for almost all subsequent units in IB Biology HL. The form-function relationship you learned here will reappear when you study enzyme action, membrane transport, DNA replication, and protein synthesis, as all these processes depend entirely on the structure of the molecules involved. Next, you will dive deeper into the structure and function of each class of biomolecule, starting with carbohydrates and lipids, then proteins and nucleic acids. Mastering the basics of condensation and hydrolysis now will make it much easier to remember the details of each macromolecule's structure when you encounter them in more depth.