IB Biology – Theme Summaries (SL & HL)

A free overview of IB Diploma Biology organised by the four themes of the current course. HL students study every theme in more depth; the key ideas below apply to both SL and HL.

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1. Unity and diversity

All life shares water-based chemistry, nucleic acids and cells. Water’s properties support life; DNA and RNA store and carry information. Cells range from prokaryotic to eukaryotic, and viruses sit at the edge of life. Diversity is organised by classification, from species to domains, and is shaped by evolution. Conservation of biodiversity matters because species loss reduces ecosystem stability.

Key terms: water properties, nucleotides, cell theory, species, classification, cladistics, biodiversity

2. Form and function

Structure fits function at every level. Carbohydrates, lipids and proteins have shapes that match their jobs. Membranes and organelles separate processes inside cells. Organisms exchange gases, transport materials and support themselves with specialised systems, such as lungs, blood vessels, xylem and phloem, whose surface area and structure make exchange efficient.

Key terms: protein structure, membrane structure, organelles, gas exchange, transport, adaptation

3. Interaction and interdependence

Molecules, cells and organisms interact. Enzymes and metabolism control reactions; cell signalling and hormones coordinate the body. Nerves and muscles allow fast responses, and plants respond to light and gravity. At the ecosystem level, populations interact through competition, predation and mutualism, energy flows through trophic levels and carbon cycles through the living and non-living world.

Key terms: enzymes, metabolism, signalling, neurons, homeostasis, trophic levels, carbon cycle

4. Continuity and change

Life continues through DNA replication, protein synthesis, cell division and reproduction. Inheritance follows patterns explained by meiosis and genetics. Mutations and gene expression changes create variation, which natural selection acts on to drive evolution and speciation. Homeostasis keeps internal conditions stable while ecosystems change over time through succession and human impact.

Key terms: replication, transcription, meiosis, inheritance, mutation, natural selection, homeostasis

These summaries are written by Dabour Hub from the published course outline. Full notes, diagrams, quizzes and question banks are available to registered students.

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