Role of cytoarchitecture in cytoplasmic transport

Cytoplasm is a thick viscoelastic gel consisting of a highly concentrated protein solution interspersed through a matrix of cytoskeletal filaments and organellar structures. Diffusion of soluble low-molecular weight molecules through cytoplasm is slowed by a factor of 5-10 relative to a dilute aqueous solution, reflecting increased solvent viscosity, molecular crowding, tortuosity imposed by cytoplasmic membranes and organelles, and transient binding to immobile structures. Larger molecules are further slowed by cytoskeletal sieving. Messenger RNA and ribosomes may be prevented from entering certain parts of cytoplasm due to their size and may follow preferential channels as they exit the nucleus. Mechanisms also exist for energy-dependent transport of larger molecules and organelles.