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The plasma membrane expands when organelle membranes or cytoplasmic vesicles fuse with the cell membrane, adding their molecules.

Expansion of the plasma membrane is essential for maintaining the required membrane composition and surface area for events such as cell division, phagocytosis, or mechanical stress.

Membrane expansion during cell division is precisely controlled so that cells divide only after reaching a specific size.

In eukaryotic organisms such as yeast, the plasma membrane expands uniformly in the early G1 phase. From late G1 until the end of mitosis, membrane growth is polarized to the region where cell separation occurs.

Similarly, in phagocytosis, the plasma membrane expands to extend the pseudopodia. The cell undergoes a change in shape to surround a particle to be engulfed.

If there is mechanical stress to a cell, such as a wound, extracellular calcium enters the cell and triggers regulated exocytosis of organelles, including lysosomes.

During exocytosis, the lysosomal membrane fuses with the cell membrane. This fusion expands the ruptured membrane and helps reseal it.