2.20: Cell Membrane

Who guards your cells?

Not everything can make it into your cells. Your cells have a plasma membrane that helps to guard your cells from unwanted intruders.

The Plasma Membrane and Cytosol

If the outside environment of a cell is water-based, and the inside of the cell is also mostly water, something has to make sure the cell stays intact in this environment. What would happen if a cell dissolved in water, like sugar does? Obviously, the cell could not survive in such an environment. So something must protect the cell and allow it to survive in its water-based environment. All cells have a barrier around them that separates them from the environment and from other cells. This barrier is called the plasma membrane, or cell membrane.

The Plasma Membrane

The plasma membrane (Figure below) is made of a double layer of special lipids, known as phospholipids. The phospholipid is a lipid molecule with a hydrophilic ("water-loving") head and two hydrophobic ("water-hating") tails. Because of the hydrophilic and hydrophobic nature of the phospholipid, the molecule must be arranged in a specific pattern as only certain parts of the molecule can physically be in contact with water. Remember that there is water outside the cell, and the cytoplasm inside the cell is mostly water as well. So the phospholipids are arranged in a double layer (a bilayer) to keep the cell separate from its environment. Lipids do not mix with water (recall that oil is a lipid), so the phospholipid bilayer of the cell membrane acts as a barrier, keeping water out of the cell, and keeping the cytoplasm inside the cell. The cell membrane allows the cell to stay structurally intact in its water-based environment.

The function of the plasma membrane is to control what goes in and out of the cell. Some molecules can go through the cell membrane to enter and leave the cell, but some cannot. The cell is therefore not completely permeable. "Permeable" means that anything can cross a barrier. An open door is completely permeable to anything that wants to enter or exit through the door. The plasma membrane is semipermeable, meaning that some things can enter the cell, and some things cannot.

Figure \(\PageIndex{1}\): Plasma membranes are primarily made up of phospholipids (red/yellow). The hydrophilic ("water-loving") head and two hydrophobic ("water-hating") tails are shown. The phospholipids form a bilayer (two layers). The middle of the bilayer is an area without water. There can be water on either side of the bilayer. There are many proteins (blue) scattered throughout the membrane.

Cytosol

The inside of all cells also contain a jelly-like substance called cytosol. Cytosol is composed of water and other molecules, including enzymes, which are proteins that speed up the cell’s chemical reactions. Everything in the cell sits in the cytosol, like fruit in a jello mold. The term cytoplasm refers to the cytosol and all of the organelles, the specialized compartments of the cell. The cytoplasm does not include the nucleus. As a prokaryotic cell does not have a nucleus, the DNA is in the cytoplasm.

Summary

• The plasma membrane is formed by a phospholipid bilayer.
• The plasma membrane controls what moves inside and outside the cell.
• The cytosol is the jelly-like material in which the contents of the cell are suspended.

Explore More

Use the resource below to answer the following questions.

• The Plasma Membrane at http://www.youtube.com/watch?v=moPJkCbKjBs (5:16)

1. What makes up the "head" region of a phospholipid? Is it hydrophobic or hydrophilic?
2. What makes up the "tail" region of a phospholipid? Is it hydrophobic or hydrophilic?
3. What happens when you drop a phospholipid in water?
4. How are phospholipids arranged in a plasma membrane?
5. What is a glycoprotein? What is one of the uses of glycoproteins?
6. What is "Brownian movement"? How is this movement related to the cell membrane?

Review

1. What is the plasma membrane?
2. Describe a phospholipid.
3. What are the components of the cytosol?
4. What is meant by the description of the plasma membrane as “semipermeable”?
5. What is the difference between the cytosol and the cytoplasm?