Form and Function

Every structure in this theme — from a single organelle to a whole gas exchange surface — exists because it solves a defined physical problem. Cells need to keep incompatible chemistry apart, organisms need to move substances faster than diffusion alone allows, and molecules need shapes that match their bonding pattern. Once you can state the problem a structure solves, most exam questions become straightforward description rather than guesswork.

A eukaryotic cell runs thousands of reactions at once, many of which would destroy each other if mixed — lysosomal enzymes would digest cytosolic proteins, and the alkaline chloroplast stroma would disrupt cytosolic pH. Membrane-bound organelles solve this by giving each process its own sealed environment with its own pH, enzymes and ion concentrations. Key players: the nucleus (DNA storage, pores control mRNA/protein traffic), rough ER (protein synthesis and folding), smooth ER (lipid synthesis, detox), Golgi apparatus (modifies and packages proteins, cis to trans face), and mitochondria (folded cristae house the electron transport chain for ATP synthesis).

IB loves testing the fixed route a secreted protein takes: ribosome → RER lumen → transport vesicle → Golgi (cis to trans face) → secretory vesicle → exocytosis at the plasma membrane. Getting into the RER in the first place depends on a signal recognition particle (SRP), which binds a signal sequence on the growing polypeptide and pauses translation until the ribosome docks onto the RER membrane — this is co-translational translocation.

Every cell shares the same genome, but differentiation switches genes on or off so cells build different organelle inventories matched to their job. A secretory cell (pancreatic cell, plasma cell) is packed with RER, Golgi and secretory vesicles; an adipocyte is dominated by lipid droplets with little RER; a macrophage is rich in lysosomes rather than secretory machinery. Stem cell potency ranges from totipotent (any cell type, including extra-embryonic tissue) through pluripotent (any body cell) to multipotent (a limited lineage, e.g. blood stem cells).

Diffusion alone is too slow to supply a large organism with oxygen, so gas exchange surfaces are structurally optimised: large surface area (millions of alveoli, roughly a tennis court's worth of surface), short diffusion distance (alveolar and capillary walls each one cell thick, pressed together), a steep concentration gradient maintained by continuous ventilation and blood flow, and a moist surface for gases to dissolve into before crossing.