1) It acts as a barrier, protecting and surrounding the protoplasm. This barrier regulates which molecules enter and leave the cell.
The structure of the membrane is essentially a phospholipid bilayer which means that only hydrophobic molecules and small polar molecules (Oxygen, Water, etc) can pass through the cell. This is why it is considered semi-permeable, it prevents hydrophillic and large polar molecules from entering the cell without help from carrier proteins (proteins that help transpot these molecules accross the membrane)
2) The cell membrane has many different receptors that function in cell signalling, ligand binding initiates and is part of many signal transduction pathways, almost all of the important functions and changes of the body are regulated and done by these pathways.
3) Exocytosis is the process by which molecules synthesized inside the cell are packaged into vesicles and secreted from the cell, this secretion is done via the cell membrane in which the membrane of the vesicle fuses with the cell membrane in order to release the contents into the extracellular environment.
