Biology · Ch 5 — Cell Structure and Organization
Cytoskeleton
Cytoskeleton
With advances in light and electron microscopy, scientists discovered a network of fine fibrils running throughout the cytoplasm of a cell. This network is called the cytoskeleton, and it is made up of three kinds of filament: microtubules, built from the protein tubulin; microfilaments, built from the protein actin; and intermediate filaments, built from various fibrous proteins. Between them, these filaments help the cell maintain its shape, allow the cell to contract, give the cell and its organelles the ability to move, permit changes in cell shape, and play a part in cell division.
Cilia and flagella
Cilia and flagella are fine, hair-like, contractile, membrane-bound outgrowths of the cell's cytoplasm found on the free surface of certain cells; they generate a current in the surrounding fluid medium that helps move material or move the cell itself. Cilia are small and occur in large numbers, beating together like oars to produce coordinated movement of the cell. Flagella are longer and occur in smaller numbers; the flagella of prokaryotic bacteria are structurally different from those of eukaryotic cells. A cilium or flagellum is made up of a basal body, a basal plate, and a shaft. The basal body sits in the outer part of the cytoplasm, is derived from a centriole, and has nine peripheral triplets of fibrils. The shaft is the visible, exposed part of the cilium or flagellum, made of a covering sheath and an inner axoneme.
The axoneme core contains 11 fibrils running parallel to the long axis of the structure -- nine peripheral doublets of microtubules arranged around two single central fibrils, a pattern usually described as the '9+2' arrangement. The two central tubules are enclosed by a central sheath, which is connected by a radial spoke to one tubule of each peripheral doublet; the two central tubules are themselves linked to each other by bridges, while neighbouring peripheral doublets are connected to one another through linkers, sometimes called interdoublet bridges.
Centrioles and centrosomes …
What this figure shows. A cross-section through the axoneme of a cilium showing the '9+2' arrangement -- nine peripheral doublet microtubules, each with an A-tubule and B-tubule and dynein arms, arranged around two central single microtubules, linked by radial spokes and nexin connections, the whole structure enclosed …
What this figure shows. A cross-sectional diagram of a centriole showing its cartwheel-like arrangement -- nine peripheral microtubule triplets connected to a central hub by radial spokes/fibres, with connecting fibres linking adjacent triplets to …