HOW DOES THE PLASMA MEMBRANE WORK? How does it act as a "gatekeeper"?

Answer 1

Because of its unique double layer,
the membrane keeps out most compounds. It has proteins imbedded that allow important molecules through - like water.

This is called facilitated diffusion.

Ions and small molecules that are dissolved in water can go through these channels with the water.

Larger molecules need another way in...

Things that are very important to the cell will have their own proteins to bring them into the cell (Usually active transport).

Some others will be brought in via pincytosis and phagocytosis. In this case, the cell engulfs the material to be brought in, and then the bubble of membrane that forms will fuse with a lysosome or a vacuole.

Things work pretty much the same way on the way out....

Answer 2

A cell membrane or plasma membrane is a physical barrier and consists of a lipid bilayer. It has pores, receptors, intramembrane proteins, ion pumps, and binding sites for a number of compunds. Some of these binding site work through active transport to bring certain substances (like glucose) into the cell. So in essence, the membrane, by virtue of these functions, decides what is allowed in and out of the cell.

Answer 3

the plasma membrane is semi-permeable so it only allows certain substances into the cell, as a gate keeper only allows certain people to enter.