Form and Function
This domain examines how biological structures at molecular, cellular and organ-system levels enable particular functions. The exam tests structure–function relationships, membrane transport, exchange surfaces, transport systems and organismal adaptations to physical constraints.
Core rules and concepts
- As an object or organism increases in size, its surface area-to-volume ratio decreases. Diffusion is favoured by a large surface area, a short diffusion distance and a steep concentration gradient; Fick’s law combines these factors.
- Membranes follow the fluid mosaic model. Small nonpolar molecules may diffuse directly; ions and polar molecules use channels or carriers. Osmosis is the diffusion of water across a selectively permeable membrane. Active transport and cotransport use energy or electrochemical gradients.
- Compartmentalisation allows incompatible reactions to occur simultaneously. Cristae increase the inner membrane surface area for oxidative phosphorylation; villi and microvilli increase absorption; alveoli provide a large, thin gas-exchange surface.
- Transport systems reflect metabolic demand: closed circulatory systems maintain pressure and flow, xylem transports water and minerals, and phloem translocates assimilates. Countercurrent exchange in fish gills maintains a gradient along the exchange surface.
Worked example
A cube with 1 cm sides has surface area 6 cm² and volume 1 cm³, so its surface area-to-volume ratio is 6:1. A cube with 2 cm sides has surface area 24 cm² and volume 8 cm³, a ratio of 3:1. Doubling the side length halves the ratio. A larger organism therefore has relatively less surface for exchange per unit volume and needs specialised exchange surfaces, ventilation or circulation.
Traps that cost marks
Do not say that surface area-to-volume ratio increases with body size. Do not treat all membrane transport as active. Distinguish osmosis from diffusion of solutes, and link every named structure explicitly to the function it performs.