Ribosomes, cytoskeleton, cilia, flagella and centrioles

George Palade first observed ribosomes under the electron microscope in 1953 as dense particles. They are made of RNA and proteins and have no surrounding membrane.

Eukaryotic ribosomes are 80S (60S + 40S subunits); prokaryotic ones are 70S (50S + 30S). The 'S' stands for Svedberg unit, the sedimentation coefficient, which indirectly reflects density and size, so subunit values do not simply add up.

The cytoskeleton is an elaborate network of protein filaments (microtubules, microfilaments and intermediate filaments) giving mechanical support, motility and maintenance of cell shape.

Cilia are small, oar-like structures that move either the cell or the fluid around it; flagella are comparatively longer and move the cell. Prokaryotic flagella are structurally different from eukaryotic ones.

The eukaryotic cilium or flagellum is covered by plasma membrane. Its core, the axoneme, has nine doublets of microtubules around the edge and a pair of single microtubules in the centre: the 9+2 array.

The central pair is linked by bridges and enclosed by a central sheath, which connects to one tubule of each peripheral doublet through a radial spoke, giving nine radial spokes. Linkers join neighbouring peripheral doublets. Cilia and flagella arise from centriole-like basal bodies.

The centrosome usually has two cylindrical centrioles surrounded by amorphous pericentriolar material, lying perpendicular to each other.

Each centriole is built like a cartwheel: nine evenly spaced peripheral fibrils of tubulin, each a triplet, with neighbouring triplets linked. In the proximal region the centre is a proteinaceous hub joined to the triplets by protein radial spokes; there is no central pair of microtubules.

Centrioles form the basal bodies of cilia and flagella and give rise to the spindle fibres of the spindle apparatus during division in animal cells.