What protects the cell membrane from being catabolized?

Question

What exactly prevents the catabolization of our own body cells?

Answer 1

Nothing. The cell membrane in living cells is constantly being turned over. Phospholipids and bilayer components are brought back into the cell by vesicle formation at the membrane. Membrane components can be recycled back to the membrane, or sorted to other vesicles for catabolic actions. Also, brand new membrane components are made in the ER, modified in the Golgi apparatus and then trafficked to the membrane for fusion. Voila, new membrane.

Since we know that we can metabolize membrane components, maybe what you're getting at is how come this doesn't happen all in one direction. Basically, there is an equilibrium between the rate of catabolism of the membrane and the rate of replenishment or membrane molecule anabolism. But the reason catabolism doesnt become a runaway train that destroys the cell is due to compartmentalization and regulation of enzymes. Enzymes that break down phospholipids (phospholipases) are carefully controlled and segregated from their substrates to minimize the chance of unintentionally initiating a self-destruct sequence. Interestingly, the reason why many snakes' venom is poisonous to humans is because it contains phospholipases that rapidly destroy cellular membranes. The result is cell lysis and death.