Book : Cell Biology
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Posted by: CHELSEA
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Writer CHELSEA

Cell Membrane

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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:

  • Phospholipids 
  • Cholesterol
  • Proteins
  • 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. The cell membrane is important because it separates and protects a cell from its surroundings.

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.


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