IB Biology · Theme A

Unity
and Diversity

How the staggering variety of life ultimately rests on a small number of shared ideas — water, cells, descent, biodiversity.

9Topics
95Sub-topics
143Key terms
4Levels of organisation
A
Theme A · Four levels of organisation

From molecules to ecosystems.

IB Biology organises each theme across four scales — molecules, cells, organisms, ecosystems. Browse below.

A1Molecules

2 topics
A1.1

Water

8 sub-topics · 16 key terms

A1.1Theme A
A1.2

Nucleic Acids

15 sub-topics · 19 key terms

A1.2Theme A

A2Cells

3 topics
HL only

Origins of Cells

9 sub-topics · 0 key terms

A2.1Theme A
A2.2

Cell Structure

14 sub-topics · 48 key terms

A2.2Theme A
HL only

Viruses

6 sub-topics · 0 key terms

A2.3Theme A

A3Organisms

2 topics
A3.1

Diversity of Organisms

15 sub-topics · 19 key terms

A3.1Theme A
HL only

Classification and Cladistics

9 sub-topics · 0 key terms

A3.2Theme A

A4Ecosystems

2 topics
A4.1

Evolution and Speciation

11 sub-topics · 14 key terms

A4.1Theme A
A4.2

Conservation of Biodiversity

8 sub-topics · 27 key terms

A4.2Theme A