Download Figure 1-21: Microtubules in a dividing cell.

CYTOSKELETON IS RESPONSIBLE FOR CELL MOVEMENTS
Figure 1-21: Microtubules in a dividing cell.
When a cell divides, its nuclear envelope breaks down and its DNA condenses into
pairs of visible chromosomes, which are pulled apart into separate cells by
microtubules. The microtubules radiate from foci at opposite ends of the dividing cell.
(Photograph courtesy of Conly L. Rieder.)
CYTOSKELETON IS RESPONSIBLE FOR CELL MOVEMENTS:
Figure 1-20: The cytoskeleton. Cytosol isnot just a structureless soup of chemicals.
Filaments made of protein provide all eucaryotic cells with an internal framework that
helps organize the internal activities of the cell and underlies its movements and
changes of shape. Shown here are (A) actin filaments, (B) microtubules, and (C)
intermediate filaments, in cultured cells. The different types of filaments are
revealed with different fluorescent stains. (A, courtesy of Simon Barry and Chris
D'Lacey; B, courtesy of Nancy Kedersha; C, courtesy of Clive Lloyd.)
CELLS VARY ENORMOUSLY in APPEARANCE AND FUNCTION
Figure 1-22: Some examples of the variety of cell shapes and sizes.
(A) A nerve cell from the cerebellum (a part of the brain that controls movement). This cell has
a huge branching tree of processes, through which it receives signals from as many as 100,000
other nerve cells. (B) Paramecium. This protozoan swims by means of the beating cilia that cover
its surface. (C) A section of a young plant stem in which cellulose is stained orange and lignin
red. The outermost layer of cells is at the top of the photo. (D) A tiny bacterium, Bdellovibrio
bacteriovorus, with a single terminal flagellum. This bacterium attacks, kills, and feeds on other
larger bacteria. (E) A human white blood cell (a neutrophil) approaching and engulfing a red
blood cell.
Cells are also enormously diverse in their chemical requirements and
activities.
Some require oxygen to live, for others ist is a deadly poison.
Some consume little more than air, sunlight and water as their raw materials;
others need a complex mixture of molecules produced by other cells.
Some appear to be specialized factories for the production of a particular
substances such as hormones, starch, fat, etc. Some are like engines, like muscle,
burning fuel to the mechanical work, or electricity generators like the modified
muscle in the electric eel.