You are made up of trillions and trillions of cells, and each one is separated from its surroundings by a cell membrane. Learn about the structure and functions of the cell membrane in this lesson.
The Cell Membrane is a Fluid Mosaic
A cell is the basic unit of life, and all organisms are made up of one or many cells. One of the things that all cells have in common is a cell membrane. It is a barrier that separates a cell from its surrounding environment. This outer boundary of the cell is also called the plasma membrane. It is composed of four different types of molecules:
The fluid mosaic model describes the structure of a cell membrane. It indicates that the cell membrane is not solid. It is flexible and has a similar consistency to vegetable oil, so all the individual molecules are just floating in a fluid medium, and they are all capable of moving sideways within the cell membrane. Mosaic refers to something that contains many different parts. The plasma membrane is a mosaic of phospholipids, cholesterol molecules, proteins and carbohydrates.
Phospholipids make up the basic structure of a cell membrane. A single phospholipid molecule has two different ends: a head and a tail. The head end contains a phosphate group and is hydrophilic. This means that it likes or is attracted to water molecules.
The tail end is made up of two strings of hydrogen and carbon atoms called fatty acid chains. These chains are hydrophobic, or do not like to mingle with water molecules. This is just like what happens when you pour vegetable oil in water. The vegetable oil will not mix with the water.
This arrangement of phospholipid molecules makes up the lipid bilayer.
The phospholipids of a cell membrane are arranged in a double layer called the lipid bilayer. The hydrophilic phosphate heads are always arranged so that they are near water. Watery fluids are found both inside a cell (intracellular fluid) and outside a cell (extracellular fluid). The hydrophobic tails of membrane phospholipids are organized in a manner that keeps them away from water.
Cholesterol, Proteins and Carbohydrates
When you hear the word cholesterol, the first thing you probably think of is that it is bad. However, cholesterol is actually a very important component of cell membranes. Cholesterol molecules are made up of four rings of hydrogen and carbon atoms. They are hydrophobic and are found among the hydrophobic tails in the lipid bilayer.
Cholesterol molecules are important for maintaining the consistency of the cell membrane. They strengthen the membrane by preventing some small molecules from crossing it. Cholesterol molecules also keep the phospholipid tails from coming into contact and solidifying. This ensures that the cell membrane stays fluid and flexible.
Some plasma membrane proteins are located in the lipid bilayer and are called integral proteins. Other proteins, called peripheral proteins, are outside of the lipid bilayer. Peripheral proteins can be found on either side of the lipid bilayer: inside the cell or outside the cell. Membrane proteins can function as enzymes to speed up chemical reactions, act as receptors for specific molecules, or transport materials across the cell membrane.
Carbohydrates, or sugars, are sometimes found attached to proteins or lipids on the outside of a cell membrane. That is, they are only found on the extracellular side of a cell membrane. Together, these carbohydrates form the glycocalyx.
The glycocalyx of a cell has many functions. It provides cushioning and protection for the plasma membrane, and it is also important in cell recognition. Based on the structure and types of carbohydrates in the glycocalyx, your body can recognize cells and determine if they should be there or not. The glycocalyx can also act as a glue to attach cells together.
This is the fluid mosaic model of a cell membrane.
Fluid Mosaic Model
Cell Membrane Functions
The plasma membrane of a cell has two main roles:
It is a physical barrier.
It regulates exchange of materials with its surroundings.
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