Cell: The Unit of Life

Biology · Class 11

Simulation · Biology · Class 11

Cilium axoneme or centriole?

From the lesson Ribosomes, cytoskeleton, cilia, flagella and centrioles in Cell: The Unit of Life. Change the values and watch what happens.

Cilium axoneme or centriole?Biology · Class 11

The idea behind it

NCERT §8.5.6–§8.5.9

  • 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.