Unit Overview

Theme A: Unity and Diversity

IB Biology SL· 5 min read 📊 23-27% of overall IB Biology SL exam

1. Unit at a Glance

This unit moves from molecular to cellular scale, following a logical learning arc that connects small-scale building blocks to large-scale patterns of unity and diversity across life. We start with the most abundant and important molecule for life: water, before moving on to nucleic acids, the universal genetic material shared by all living things.

Next, we explore the cell, the fundamental unit of life, looking at conserved core structures and key differences between cell types. We end the unit by investigating how cells first arose on Earth, and the unique diversity of unicellular life and viruses, tying all concepts back to the overarching theme of unity and diversity.

Sub-topics in this unit, ordered for learning:

Water

Covers the unique chemical and physical properties of water and their biological importance.

★⏱ 10 min

Nucleic acids

Introduces the structure and function of DNA and RNA, the universal genetic material of life.

★★⏱ 12 min

Cell structure

Compares prokaryotic and eukaryotic cell structures and introduces endosymbiotic theory.

★★⏱ 15 min

Cell membranes and transport

Explains membrane structure and mechanisms of passive and active transport across cell membranes.

★★★⏱ 14 min

Origin of cells and unicellular diversity

Covers evidence for abiogenesis and explores functional diversity across unicellular organisms.

★★★⏱ 13 min

Viruses

Explores viral structure, replication, and the unique diversity of viruses as biological entities.

★★⏱ 10 min

2. Common Pitfalls

Wrong move:

Claiming viruses are living organisms, or that they do not share any traits with life

Why:

IB examiners regularly test this distinction, and both extremes lead to lost marks

Correct move:

Remember viruses lack cellular structure and cannot replicate independently, so they are not considered alive, but they do share genetic material (DNA/RNA) with cellular life

Wrong move:

Mixing the order of processes required for abiogenesis

Why:

This is a common error in short answer questions about the origin of cells

Correct move:

Recall the correct order: 1. Synthesis of simple organic molecules 2. Assembly into polymers 3. Formation of membranes 4. Development of self-replicating molecules

3. Quick Reference Cheatsheet

Key Concept	Core Summary
Water polarity	Polar molecule with partial charges, forms hydrogen bonds that give water its unique biological properties
Nucleotide structure	All nucleotides are made of three parts: phosphate group, pentose sugar, nitrogenous base
Cell theory postulates	All organisms are made of cells; cells are the basic unit of life; all cells come from pre-existing cells
Membrane transport	Passive transport (diffusion, osmosis) moves down the concentration gradient and requires no energy; active transport moves against the gradient and requires ATP
Endosymbiotic theory	Mitochondria and chloroplasts evolved from free-living prokaryotes engulfed by larger host cells
Basic virus structure	All viruses contain genetic material (DNA or RNA) surrounded by a protein capsid, some also have an outer envelope

What's Next

Start your study of Theme A with the first sub-topic on water, the fundamental molecule of all life. Each sub-topic builds on the previous one, so working through them in order will help you connect concepts effectively. Once you complete all six sub-topics in this unit, you will be ready to progress to the first sub-topic of the next unit covering Theme B: Molecular Biology.