Cell Biology: Bacterial Cell Membrane

Bacterial cell membrane

  

A cell membrane is a thin structure that is also known as the plasma membrane

 The main functions of the cell membrane are:

• to maintain the physical integrity of the cell - that is to mechanically enclose the contents of the cell, and also
• to control the movement of particles e.g. ions or molecules, into and out of the cell.

The plasma membrane

Both prokaryotic and eukaryotic cells have a plasma membrane, a double layer of lipids that separates the cell interior from the outside environment. This double layer consists largely of specialized lipids called phospholipids.

A phospholipid is made up of a hydrophilic, water-loving, phosphate head, along with two hydrophobic, water-fearing, fatty acid tails. Phospholipids spontaneously arrange themselves in a double-layered structure with their hydrophobic tails pointing inward and their hydrophilic heads facing outward. This energetically favorable two-layer structure, called a phospholipid bilayer, is found in many biological membranes. 

As shown below, proteins are also an important component of the plasma membrane. Some of them pass all the way through the membrane, serving as channels or signal receptors, while others are just attached at the edge. Different types of lipids, such as cholesterol, may also be found in the cell membrane and affect its fluidity. An image of plasma membrane shows the phospholipid bilayer, embedded proteins, and cholesterol molecules. The membrane separates the extracellular space, outside of the cell, from the cytosol inside the cell.

                                        
                                                         (Fluid Mosaic Model)

The plasma membrane   is the border between the interior and exterior of a cell. As such, it controls passage of various molecules—including sugars, amino acids, ions, and water—into and out of the cell. How easily these molecules can cross the membrane depends on their size and polarity. Some small, nonpolar molecules, such as oxygen, can pass directly through the phospholipid portion of the membrane. Larger and more polar, hydrophilic, molecules, such as amino acids, must instead cross the membrane by way of protein channels, a process that is often regulated by the cell.

The Functions of Components in Plasma Membrane

· Ion channels responsible for the selective permeability of the cell membrane.

·  Membrane pumps that enable active transport of specific molecules across the cell membrane

·  Carrier proteins and receptor proteins that transport specific substance(s) across the cell membrane and receive (chemical) signals from outside the cell that tell the cell to take a specific action, respectively. 

·  Cholesterol molecules and carbohydrate chains that support the structure of the membrane and its attachment to other tissues, respectively.