Cell Membrane: Functions, Role & Structure

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:

  1. Phospholipids
  2. Cholesterol
  3. Proteins
  4. Carbohydrates

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.

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Phospholipids

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.
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:

  1. It is a physical barrier.
  2. It regulates exchange of materials with its surroundings.

The cell membrane is important because it separates and protects a cell from its surroundings. This allows the intracellular conditions of a cell to be very different from the extracellular conditions. For example, nerve cells in your body will maintain a high concentration of potassium inside. Outside, in the extracellular fluid, there is very little potassium and lots of sodium. These concentration differences are absolutely necessary for the function of nerve cells, which is to send signals or nerve impulses.

A cell membrane’s structure and properties, like having hydrophilic outer areas and hydrophobic inner regions, prevents many substances from entering or leaving a cell. This is good because it means that unwanted materials don’t accidentally get inside a cell. However, many materials, like the nutrient glucose, do need to cross the cell membrane. Also, waste substances need to get out of a cell. If they did not, the wastes would build up and become toxic to the cell.

The cell membrane is able to regulate what comes in and what goes out of a cell. This is called selective permeability. Only very small molecules, such as water, oxygen or carbon dioxide, can easily pass through the lipid bilayer of a cell membrane. Any other substances that must cross the cell membrane must pass through the transport proteins. These proteins are very specific about what they transport. For example, your cell membranes have transporters that will only allow for the movement of glucose molecules. There are others with different structures that only transport sodium.

The selective permeability of a cell membrane: Some molecules can pass through the phospholipid layer. Those that cannot, must have their own transport proteins.
Selective Permeability

Lesson Summary

The cell membrane, or plasma membrane, surrounds and protects the internal environment of a cell; however, this is not its only function. A cell membrane also determines what materials enter or leave the cell. This ensures that the cells will be able to get rid of wastes and take up important nutrients and gases.

The plasma membrane is a fluid mosaic. This means that it is flexible and made up of many different types of molecules. Phospholipids form the basic structure of a cell membrane, called the lipid bilayer. Scattered in the lipid bilayer are cholesterol molecules, which help to keep the membrane fluid consistent. Membrane proteins are important for transporting substances across the cell membrane. They can also function as enzymes or receptors. On the extracellular fluid side of a cell membrane, you find carbohydrates. They help a cell to be recognized as a certain type of cell and are important for holding cells together.

Learning Outcomes

Once you’ve finished with this lesson, you will have the ability to:

  • Identify the cell membrane’s functions
  • Describe the four types of molecules that make up the cell membrane
  • Explain the fluid mosaic model of the cell membrane
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