Microbodies, cytoskeleton and movement

Microbodies are small organelles of eukaryotic cells with a single membrane. There are two kinds.

• Peroxisomes form from the ER and have no DNA of their own. They contain oxidase enzymes and catalase. In liver cells they break down alcohol and other harmful substances; in plants they take part in photorespiration.
• Glyoxysomes are special peroxisomes in fatty seeds (like castor). As the seed sprouts, they turn its stored fat into sugar, through fatty acid breakdown, the glyoxylate cycle and gluconeogenesis, feeding the young plant.

The cytoskeleton is the cell's inner framework of protein fibres. It gives shape and support and allows movement. Three kinds:

• Microtubules (25 nm wide), made of tubulin: they form the spindle and move cilia and flagella.
• Actin filaments (6 nm), made of actin: they help in muscle contraction and in dividing the cytoplasm (cytokinesis).
• Intermediate filaments (10 nm): they give mechanical strength.

Cilia are short and many; they beat in rhythm, like the oars of a boat, as on Paramecium and on the lining of our airways. Flagella are long and few (one or two) and move like a whip, as in sperm and Chlamydomonas. Both have a core, the axoneme, of microtubules in a 9 + 2 pattern (nine pairs around two in the centre), and both grow from a basal body.

The centrosome, near the nucleus, holds two centrioles at right angles to each other. Each centriole is made of nine triplets of microtubules. The centrosome doubles during the S phase of the cell cycle, and organises the spindle that separates chromosomes in cell division.