Plasma Membrane of a Cell: Definition, Function & Structure

Cells are made of many crucial parts that work together and perform specific functions. In this lesson, you will learn about the plasma membrane of a cell to gain an understanding of its structure and function.
What is a Plasma Membrane?
Imagine a water balloon. Prick it with a needle and the water would burst out everywhere; no more water balloon. Now, imagine a new water balloon in a bathtub full of water. What if the balloon itself was porous and could choose when water could flow in and out of the balloon? That’s sort of like what a plasma membrane is to a cell.
The plasma membrane is the boundary between the cell and its environment. It regulates what enters and exits the cell. Cells must maintain an appropriate amount of molecules to function inside them. They must also have a way to keep things out or to allow things to enter. This is the job of the plasma membrane. The plasma membrane is like the guard at a gated community. The guard must inspect those who enter and those who leave to make sure that only the people and things needed in the community are there.
Plasma Membrane Structure
The plasma membrane is composed of a phospholipid bilayer, which is two layers of phospholipids back-to-back. Phospholipids are lipids with a phosphate group attached to them. The phospholipids have one head and two tails. The head is polar and hydrophilic, or water-loving. The tails are nonpolar and hydrophobic, or water-fearing.
Since water is the key component of living organisms, both inside and outside of the cell, this causes the phospholipids to line up in two layers with the heads pointing outward and the tails hidden in the middle. It basically looks like a phospholipid sandwich with the heads forming the bread and the tails forming the meat in the middle.
Located all throughout the surface of the plasma membrane are cholesterol molecules. These molecules help stabilize the phospholipids and keep them in position. The plasma membrane is also dotted with proteins. Some of the proteins are peripheral proteins that only go halfway through the membrane, while others are integral proteins that go entirely through the membrane. These proteins serve as channels to allow the molecules to enter and leave the cell.
Function of the Plasma Membrane of a Cell
You are comfortable in your house largely because the thermostat maintains the temperature within a certain range regardless of what is happening outside. Similarly, all living cells must maintain a balance regardless of external or internal conditions. This includes regulating what can enter and exit the cell. The plasma membrane serves as a gateway to allow or block the entry or exit of materials.
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