We often think of the plasma membrane as a physically separable part of the cell. It is chemically distinct: a thin oily film of lipids and hydrophobic proteins that is immiscible with the water-based cytoplasm it encloses. In questions of cell physiology and metabolism the outer membrane is conveniently regarded as an isolated permeability barrier controlling the entry and egress of ions and small molecules. However, when we come to consider the mechanical properties of cells and their movements, any attempt to segregate the plasma membrane from the rest of the cell is misleading. It is true that the forces necessary for motile phenomena such as locomotion and mitosis are generated mainly in the cytoplasm, by a system of protein filaments known collectively as the cytoskeleton. But the cytoskeleton does not move within the flexible enclosure of the plasma membrane like a cat in a bag. There are extensive and...

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