IB DP Biology Theme B: Form and Function — Study Guide

This guide covers Theme B – Form and function (26 hours SL / 39 hours HL) of IB Diploma Programme Biology, first assessment 2025. It complements the full syllabus guide and the Theme B revision notes, which condense this content for late-stage recall. This guide explains each sub-topic in full, for first study or wherever a fuller explanation is needed than a condensed recap can provide.

Where this theme fits

Where Theme A asked what living things have in common and where their diversity comes from, Theme B connects biological structure at every scale – molecular, cellular, and whole-organism – to the specific job that structure performs. Its organising question runs through every sub-topic below: how does this structure’s form suit its function? This structure-function logic, once established here, becomes the foundation both Theme C (interaction and interdependence) and Theme D (continuity and change) build on directly.

Carbohydrates and lipids

Both macromolecule groups are best understood through the same structure-function lens. Carbohydrates, built from monosaccharide units, range from simple sugars used for quick, accessible energy to polymers such as starch, glycogen, and cellulose. Each polymer’s specific bonding arrangement explains its distinct role: cellulose’s long, strong, non-branching chains suit it to a structural role in plant cell walls, while starch’s more compact structure suits it to storing energy that can be readily broken down when needed. Lipids, including triglycerides and phospholipids, share a broadly hydrophobic character that explains both their efficiency as long-term energy stores (more energy-dense per gram than carbohydrates) and, specifically for phospholipids, their role in forming biological membranes.

Proteins

A protein’s specific three-dimensional folded shape, determined by its underlying sequence of amino acids, is what allows it to perform a specific job – structural (such as collagen), transport (such as haemoglobin), or catalytic (enzymes). This link between sequence, shape, and function is this sub-topic’s central idea, and a common exam trap is describing a protein’s function without connecting it explicitly back to its shape, when the syllabus’s own framing expects shape and function to be explained together, not as two separate facts about the same molecule.

Membranes and membrane transport

Substances cross cell membranes by passive means (diffusion, facilitated diffusion, and osmosis, none of which require energy input, since each moves substances down a concentration gradient) or active means (active transport, which requires ATP to move substances against a concentration gradient). The phospholipid bilayer’s own structure explains why membranes are selectively permeable: hydrophilic phosphate heads face outward on both surfaces while hydrophobic fatty-acid tails face inward, allowing small non-polar molecules to pass through relatively freely while larger or charged molecules require specific transport proteins embedded in the membrane.

Organelles and compartmentalization

Dividing a eukaryotic cell into membrane-bound compartments is functionally useful for two connected reasons: it allows chemically incompatible processes to occur simultaneously in separate parts of the same cell, and it allows each organelle to maintain the specific internal conditions – pH, enzyme concentration, ion balance – that its particular function requires, rather than every reaction in the cell having to tolerate a single shared set of conditions.

Cell specialization

Cells within the same multicellular organism can look and behave very differently despite containing an identical genome. This is explained by differential gene expression: differentiation occurs because different cells express different subsets of their genes, not because they gain or lose genetic material. A nerve cell and a muscle cell in the same organism therefore contain exactly the same genes, but express different genes, producing structurally and functionally distinct cell types from identical genetic starting material.

Gas exchange and transport

Organisms exchange gases with their environment, and move substances internally, at scales ranging from a single cell to a large multicellular body. The IB Biology guide (first assessment 2025) lists four properties of gas-exchange surfaces (B3.1.2): permeability, a thin tissue layer, moisture and a large surface area. A strong exam answer explains why each property helps rather than simply listing them: a permeable surface lets oxygen and carbon dioxide pass through; a thin tissue layer shortens the diffusion distance; moisture lets gases dissolve before they diffuse across; and a large surface area provides more area for diffusion to occur across simultaneously. Separately, animals maintain the concentration gradients across these surfaces (B3.1.3) through dense networks of blood vessels, continuous blood flow, and ventilation with air for lungs and with water for gills: blood flow carries absorbed oxygen away and brings carbon dioxide to the surface, while ventilation replaces the air or water on the outer side, so the gradient stays steep. Keep the two lists apart – blood supply and ventilation are not properties of the surface itself. Internal transport systems, such as circulatory systems, are revised with the same logic: their structural features suit the specific demand of moving substances over the distances a given organism’s body size requires.

Muscle and motility (HL only)

This sub-topic covers the structural and biochemical mechanics of movement, including the mechanism of muscle contraction. It is HL-only and builds directly on two earlier sub-topics: Proteins (the structure of contractile proteins enables the sliding-filament mechanism of contraction) and Membranes (the movement of ions across membranes triggers contraction). Studying Muscle and motility in isolation from those two sub-topics leaves genuine gaps in understanding the underlying mechanism, since the content here assumes both are already understood.

Adaptation to environment and ecological niches

This sub-topic most directly closes the loop on Theme B’s organising question, examining how an organism’s form suits it to where and how it lives. The key applied skill is being able to take a described organism and its environment and explain, structurally, why a specific feature suits that particular environmental pressure – for example, explaining why a desert plant’s reduced leaf surface area limits water loss through transpiration – rather than describing adaptations in general terms disconnected from a specific environment.

How Theme B is examined

As with every DP Biology theme, Theme B content is drawn on throughout Paper 1 and Paper 2 rather than tested in a single isolated section, and it also frequently underpins the internally assessed scientific investigation, since structure-function relationships (in membranes, enzymes, or gas exchange surfaces, for example) make for well-defined, testable investigation questions. Theme B’s sub-topics also connect forward explicitly: respiration and photosynthesis in Theme C depend on the organelle structure introduced here, and protein synthesis in Theme D depends on the protein structure already established in this theme – treat Theme B as building a structural vocabulary the rest of the course reuses, not as a self-contained block to revise once and set aside.

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 and the Theme B revision notes, which first reproduced this theme’s sub-topic names and HL-only markers from it.