The Cell as a Growing System

Abstract

Growth of a cell may be described in physical terms as an irreversible increase in volume with time. This process is completely different in plants and animals. The animal cell grows primarily by an increase in protoplasm, above all, through the biosynthesis of cell proteins. In contrast, the increase of protoplasm plays a minor role in the growth of plant cells. The increase of volume in plant cells results primarily from the uptake of water into the vacuoles, which are formed from the endoplasmic membrane system of the meristematic cells (see Fig. 3.1). Vacuoles, as they enlarge in the process of cell growth, coalesce to form a central vacuole which occupies over 90% of the cell volume (Fig. 8.1). The characteristic structure of the plant cell allows this type of growth in volume: The protoplast together with the extensible cell wall forms an osmo-mechanical system which develops a hydrostatic pressure (turgor) on the basis of the difference in osmotic potential between the vacuolar solution and the solution in the cell wall (see Fig. 4.7).