Describe two situations where apoptosis plays a crucial role

Apoptosis is important for programmed cell destruction when a cell begins to grow abnormally, or when cells become inefficient at completing the cell cycle and providing the specific set of functions to the organism.
(cancer/abnormal cells, and old cells)

The size of the molecule determines that. There are bigger molecules that can't just cross the membrane that is why there are protiens specialized for allowing large molecules that the cell needs. Small molecules like gas molecules can just go in through the phosooholipids and inside because they are very small