Form and Function
This section covers the IB Biology SL domain Form and Function. The exam tests how biological structures at molecular, cellular, tissue, organ and organism level are adapted to perform specific functions.
Core rules
- Protein function depends on three-dimensional conformation stabilised by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions; the primary sequence determines folding, and denaturation destroys function.
- Enzymes are globular catalysts with an active site; they are specific, lower activation energy and show induced fit. Temperature, pH and substrate concentration affect rate.
- Membranes follow the fluid mosaic model: a phospholipid bilayer with proteins, cholesterol and glycoproteins. Diffusion, osmosis and facilitated diffusion are passive; active transport requires ATP and a carrier protein.
- As an object increases in size, its surface area to volume ratio decreases. Effective exchange surfaces therefore have a large area, a thin barrier, moisture and a maintained concentration gradient.
- Compartmentalisation increases efficiency: mitochondrial cristae, chloroplast thylakoids, villi and microvilli, and alveoli all increase surface area.
- Transport systems match body size and demand: double circulation in mammals, transpiration in xylem and translocation in phloem.
Worked example
A 1 cm cube has surface area 6 cm2 and volume 1 cm3, so its surface area to volume ratio is 6:1. A 3 cm cube has surface area 54 cm2 and volume 27 cm3, so the ratio is 2:1. The smaller cube can rely on diffusion because every point is close to the surface; the larger cube cannot, which is why larger organisms need specialised exchange surfaces and transport systems.
Traps
- Osmosis is the movement of water across a partially permeable membrane, not the movement of solute.
- Facilitated diffusion uses proteins but no ATP; active transport uses both.
- Enzymes lower activation energy; they do not change the overall energy change of a reaction.
- Arteries carry blood away from the heart, not always oxygenated blood; the pulmonary artery carries deoxygenated blood.