cells, or that of diffusion within them. Movement of macromolecules lial cells 20 µm tall and 10 µm wide (all measurements are approximate).
can be much accelerated and also directed by processes of active trans- Some cells are much larger than this: e.g. megakaryocytes of the bone
port across the plasma membrane and by transport mechanisms within marrow and osteoclasts of the remodelling bone are more than 200 µm
the cell. According to the location of absorptive or transport functions, in diameter. Neurones and skeletal muscle cells have relatively extended
apical microvilli (Fig. 1.1) or basolateral infoldings create a large shapes, some of the former being over 1 m in length.
surface area for transport or diffusion.
Motility is a characteristic of most cells, in the form of movements
of cytoplasm or specific organelles from one part of the cell to another. CELLULAR ORGANIZATION
It also includes: the extension of parts of the cell surface such as pseu-
dopodia, lamellipodia, filopodia and microvilli; locomotion of entire Each cell is contained within its limiting plasma membrane, which