IB Biology HL/SL
Comprehensive coverage of the 2025 IB Biology syllabus across all four themes — Unity & Diversity, Form & Function, Interaction & Interdependence, and Continuity & Change — for both Higher Level and Standard Level students. Includes molecules, cells, gas exchange and transport, ecology, genetics, evolution, and HL-only extensions.
Topics Covered
What you get
A1 — Water & Nucleic Acids
THE BIG PICTURE. Theme A opens with the molecular foundations of life: WATER (A1.1), the medium of all metabolism, and NUCLEIC ACIDS (A1.2), the molecules that store and transmit genetic information. Together they answer two questions that recur throughout IB Biology — what physical conditions are required for life? and how is biological information passed from generation to generation?
Sample Flashcards
Explain why water is a polar molecule and how this leads to hydrogen bonding.
Oxygen is more electronegative than hydrogen, causing unequal sharing of electrons in each O–H bond. This makes the oxygen end δ− and the hydrogen ends δ+. Hydrogen bonds form between the δ+ H of one molecule and a lone pair on the δ− O of an adjacent molecule.
State four biologically significant properties of water and give a biological example for each.
1. COHESION: water molecules stick together via H-bonds → continuous columns transported in xylem under tension. 2. HIGH SPECIFIC HEAT CAPACITY: resists temperature change → buffers aquatic habitats and the bodies of organisms (which are mostly water). 3.
Sample Key Terms
Hydrogen Bond
A weak electrostatic attraction between a δ+ hydrogen atom bonded to an electronegative atom (N, O, F) and a lone pair on another electronegative atom. In water: between adjacent molecules.
Cohesion
Attraction between water molecules due to hydrogen bonding. Creates surface tension and allows water to be pulled under tension in xylem vessels.
Adhesion
Attraction between water molecules and polar/charged surfaces. Enables capillary action in soil and plant cell walls.
What's Covered
- A1 — Water & Nucleic Acids
- A2 — Cell Origins & Structure
- A3–A4 — Diversity, Evolution & Conservation
- B1 — Carbohydrates, Lipids & Proteins
- B2 — Membranes, Organelles & Cell Specialisation
- B3 — Gas Exchange, Transport & Movement
- B4 — Adaptation & Ecological Niches
- C1 — Enzymes, Cell Respiration & Photosynthesis
- C2–C3 — Signalling & Body Systems
- C4 — Populations, Communities & Energy Transfer
- D1 — DNA Replication, Protein Synthesis & Mutations
- D2–D3 — Cell Division, Inheritance & Homeostasis
- D4 — Natural Selection, Stability & Climate Change
- Exams, IA & Skills
14 topics · 171+ flashcards · quizzes & matching games included
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