18.16: Enlargement of the Plasma Membrane
Cell division and enlargement are processes that require precise control. The control ensures that cell division cannot proceed unless the cell has grown to a specific size. A spherical, dividing cell requires an approximately 1.6X increase in its surface area to double its volume. The secretory pathway also has a significant role in cell membrane enlargement. Secretory vesicles that bud off from the Golgi apparatus and later fuse with the plasma membrane during exocytosis are a major source of the additional membrane. In animal cells, plasma membrane expansion occurs throughout the cell cycle. Between metaphase and cytokinesis, the surface area of the plasma membrane increases by approximately 30%. As a cell approaches cytokinesis, membrane growth mainly occurs to separate the dividing cells.
Plasma membrane enlargement also is used to repair wounds in the cell membrane. This process is stimulated by an influx of calcium which turns on the regulated exocytosis of lysosomes. Proteins, including lysosome-associated membrane protein, or Lamp1, dock the lysosomes on the plasma membrane. A calcium-sensor, synaptotagmin VII, regulates the fusion of lysosomes with the plasma membrane and exocytosis of the molecules inside the lysosomes.
A 25-fold increase in the plasma membrane area occurs during cellularization–the formation of a layer of epithelial cells in a Drosophila embryo. In this process, the syncytium, a mass of cytoplasm containing 6000 nuclei surrounded by a single plasma membrane, is converted into a similar number of separate cells, each with a single nucleus. Cytoplasmic vesicles provide the additional membrane required for this vast expansion.
