IB DP Biology Theme B: Form and Function -- Revision Notes

These notes go deeper into Theme B – Form and function (26 hours SL / 39 hours HL) than the full syllabus guide, which lists every theme at a glance. Use this alongside the Theme A revision notes already on the site. Where Theme A asks what living things have in common, Theme B connects biological structure at every scale to the job that structure performs – its organising question throughout is “how does this structure’s form suit its function?”

Carbohydrates and lipids

Revise both macromolecule groups through the same structure-function lens: carbohydrates (built from monosaccharide units) vary in structure between simple sugars used for quick energy and polymers such as starch, glycogen or cellulose used for energy storage or structural support, with each polymer’s specific bonding arrangement explaining its role (cellulose’s structure gives it strength for plant cell walls; starch’s structure makes it compact and readily broken down for stored energy). Lipids (including triglycerides and phospholipids) are revised similarly – their hydrophobic properties explain both their efficiency as long-term energy stores and, for phospholipids specifically, their role in forming membranes.

Proteins

The link between amino acid sequence, three-dimensional folded shape, and function is this sub-topic’s central idea – revise being able to explain that a protein’s specific shape (determined by its sequence of amino acids and how that sequence folds) is what allows it to perform a specific job, whether structural (e.g. collagen), transport (e.g. haemoglobin), or catalytic (enzymes). A common exam trap is describing a protein’s function without connecting it back to its shape, when the syllabus’s own framing expects shape and function to be explained together.

Membranes and membrane transport

Covers how substances cross cell membranes by passive means (diffusion, facilitated diffusion, osmosis, none requiring energy input) and active means (active transport, requiring ATP to move substances against a concentration gradient). Revise the phospholipid bilayer’s structure specifically: its hydrophilic heads and hydrophobic tails explain why the membrane is selectively permeable, allowing small non-polar molecules through readily while requiring specific transport proteins for larger or charged molecules.

Organelles and compartmentalization

Why dividing a eukaryotic cell into membrane-bound compartments (organelles) is functionally useful – revise this as an efficiency argument: compartmentalization lets a cell run chemically incompatible processes simultaneously in separate spaces, and lets each organelle maintain the specific internal conditions (pH, enzyme concentration) its function requires, rather than every reaction occurring in a single shared cytoplasm.

Cell specialization

How cells differentiate to perform specific roles despite sharing an identical genome. Revise the underlying logic: differentiation occurs through differential gene expression, not through cells gaining or losing genetic material, so a nerve cell and a muscle cell in the same organism contain the same genes but express different subsets of them, producing very different structures suited to very different functions.

Gas exchange and transport

Covers how organisms exchange gases with their environment and move substances internally, at scales from single cells to large multicellular bodies. Revise the structure-function connections specifically: gas exchange surfaces (such as alveoli or gill lamellae) share common structural features – large surface area, thin barrier, good blood supply, and ventilation – each of which directly increases the rate or efficiency of diffusion, and being able to explain WHY each feature helps (not just naming it) is what a strong answer does. Transport systems (such as circulatory systems) are revised similarly: their structural features suit the specific demand for moving substances over the distances a given organism’s body size requires.

Muscle and motility (HL only)

The structural and biochemical mechanics of movement, including muscle contraction. This is HL-only and builds directly on Proteins (contractile proteins’ structure enabling the sliding-filament mechanism) and Membranes (ion movement across membranes triggering contraction) – revising Muscle and motility in isolation from those two earlier sub-topics leaves genuine gaps in understanding the underlying mechanism.

Adaptation to environment and ecological niches

How an organism’s form suits it to where and how it lives – the sub-topic that most directly closes the loop on Theme B’s whole organising question. Revise by being able to take a described organism’s environment and explain, structurally, why a specific feature suits that environment (for example, why a desert plant’s reduced leaf surface area limits water loss), rather than describing adaptations in general terms disconnected from a specific environmental pressure.

How Theme B connects to the rest of the syllabus

Theme B’s structure-function logic is the foundation Theme C (interaction and interdependence) and Theme D (continuity and change) both assume: respiration and photosynthesis (Theme C) depend on organelle structure (Theme B); protein synthesis (Theme D) depends on understanding protein structure already established here. Treat Theme B as building the structural vocabulary the rest of the course reuses, not as a self-contained block to revise once and set aside.

Exam traps

• Describing a protein’s function without connecting it back to its specific folded shape.
• Explaining a gas-exchange surface’s features without stating why each feature increases diffusion rate specifically.
• Treating cell specialization as if cells with different roles contain different genes, rather than the same genes expressed differently.
• Revising Muscle and motility (HL) in isolation from Proteins and Membranes, missing the mechanism that connects all three.

Self-test

1. Why does cellulose’s structure suit it to a structural role while starch’s structure suits it to energy storage?
2. What determines a protein’s specific function, according to this theme’s central idea?
3. Name one passive and one active method of membrane transport, and state the key difference between them.
4. Why is compartmentalization functionally useful for a eukaryotic cell?
5. Explain, in terms of gene expression, how a nerve cell and a muscle cell can perform very different functions despite sharing the same genome.
6. Name two structural features shared by efficient gas exchange surfaces, and explain why each increases diffusion rate.

Answers: 1. Cellulose’s specific bonding arrangement gives long, strong, non-branching chains suited to providing structural strength in cell walls; starch’s structure is more compact and readily broken down, suiting it to storing and releasing energy as needed. 2. Its three-dimensional folded shape, which is itself determined by its sequence of amino acids. 3. Passive: diffusion, facilitated diffusion, or osmosis (no energy required, movement down a concentration gradient); active: active transport (requires ATP, can move substances against a concentration gradient) – the key difference is whether energy input is required. 4. It lets a cell run chemically incompatible processes simultaneously in separate spaces and lets each organelle maintain the specific internal conditions its function needs, rather than requiring every process to occur under one shared set of conditions. 5. Both cells contain the same genes, but differential gene expression means each cell type expresses only the subset of genes relevant to its own specialised function, producing structurally and functionally different cells from identical genetic material. 6. Any two of: large surface area (more area for diffusion to occur across at once), thin barrier (shorter diffusion distance), good blood supply (maintains a steep concentration gradient by removing/supplying gas), ventilation (maintains a steep concentration gradient by refreshing the gas on one side of the exchange surface).

Official syllabus

International Baccalaureate Organization, Diploma Programme Subject Brief – Sciences: Biology, first assessment 2025, published January 2022 – the same source already cited by the full syllabus guide, which first reproduced this theme’s sub-topic names and HL-only markers from it.