Theme A: Unity and diversity

What all living things share, and how they differ: the molecules of life such as water and nucleic acids, the structure and origin of cells, viruses, how organisms are classified, and how evolution produces and threatens biodiversity.

Molecules

A1.1 Water
SL and HL, with AHL content · 8 understandings · visuals: high priority
Which features of the water molecule explain why life depends on it?
A1.2 Nucleic acids
SL and HL, with AHL content · 15 understandings · visuals: high priority
How can a chain of just four kinds of building block hold the instructions for a whole organism?

Cells

A2.1 Origins of cells
AHL only · 9 understandings · visuals: medium priority
How could non-living chemistry on the young Earth have given rise to the first cells?
A2.2 Cell structure
SL and HL, with AHL content · 14 understandings · visuals: high priority
What do all cells have in common, and where do different kinds of cell part ways?
A2.3 Viruses
AHL only · 6 understandings · visuals: high priority
How do viruses manage to reproduce while carrying so little genetic information?

Organisms

A3.1 Diversity of organisms
SL and HL, with AHL content · 15 understandings · visuals: low priority
Where should we draw the line between one species and another?
A3.2 Classification and cladistics
AHL only · 9 understandings · visuals: medium priority
How can we sort millions of species into groups that reflect their real evolutionary history?

Ecosystems

A4.1 Evolution and speciation
SL and HL, with AHL content · 11 understandings · visuals: medium priority
What lines of evidence convince biologists that species change over time?
A4.2 Conservation of biodiversity
SL and HL · 8 understandings · visuals: low priority
Why are species disappearing so quickly today, and how do we know?