Figure 1. Schematic representation of a typical somatic animal cell, highlighting the organization of the microtubule cytoskeleton (blue lines), and the relative positions of four subcellular compartments. Typically, the nucleus is found near the centre of the cell (1). The Golgi apparatus appears as a stack of pancakes close to the nucleus (2), whereas the tubular network of the endoplasmic reticulum extends throughout the cell (3). The mitochondria appear randomly dispersed through the cell (4).

Figure 2. Specific and nonspecific effects on the free-energy landscape of biochemical reactions in cells. A reference is provided by an uncatalysed reaction in a dilute solution (a). Enzymes lower the free-energy level of intermediate states to catalyse the conversion between specific reactants and products, without altering the free-energy levels of the reactants and products themselves (b). Macromolecular crowding effects change the free-energy levels of all reactant, products and intermediate states in nonspecific way (c).

Figure 3. Bringing macromolecules together. (a) Depletion forces lead to an enhanced attraction between large molecules in the presence of small molecules. The total volume excluded to the small molecules (dashed lines) is reduced when the excluded volumes of large molecules overlap (purple shaded area). (b) Molecules in solution perform a random walk owing to random collisions with the solvent molecules. This leads to diffusive motion of the molecules. (c) Molecular motors mediate the directed transport of molecules or small vesicles filled with molecules along cytoskeletal filaments. (d) The confinement of molecules to domains in membranes (or other structures) enhances the efficiency of diffusion as a mechanism for bringing molecules together.