Plasma Membranes

Overview of Plasma Membrane

The plasma membrane, also known as the cell membrane, is a thin layer that surrounds a cell.
It acts as a barrier separating the internal content of the cell from the external environment.
The plasma membrane is composed of a phospholipid bilayer, which contains proteins, carbohydrates and cholesterol.

The phospholipid bilayer is made up of two layers of phospholipid molecules. Each molecule has a hydrophilic (water-loving) head and two hydrophobic (water-hating) tails.
Within the phospholipid bilayer, some proteins are embedded, which are known as integral proteins. These proteins can either span the entire membrane (transmembrane proteins) or only partially penetrate one layer.
On the surface of the plasma membrane, there are peripheral proteins. They are not embedded within the bilayer, but rather, attached to integral proteins.
The carbohydrates on the membrane surface are often attached to proteins, forming glycoproteins, or to lipids, forming glycolipids.
Cholesterol molecules are embedded within the bilayer, which help to regulate membrane fluidity.

Acts as a boundary to keep the cell’s contents inside and keep unwanted substances out.
Regulates the movement of substances in and out of the cell through processes such as diffusion, facilitated diffusion, osmosis, and active transport.
The proteins on the membrane deliver signals between the cell and its environment. These proteins can act as receptors to respond to molecular signals from outside the cell.
Identifies the cell to other cells. This is primarily the function of the carbohydrates on the plasma membrane’s outer surface.

The Fluid Mosaic Model describes the plasma membrane as a fluid, dynamic structure with proteins embedded in or associated with the phospholipid bilayer.
This model suggests that the membrane is flexible and the lipids, proteins, and carbohydrates within it are constantly moving.
Cholesterol within the membrane contributes to fluidity by preventing fatty acid chains of phospholipids from sticking together.

Plasma membrane fluidity is vital for various cellular processes, such as cell division, endocytosis, exocytosis and membrane repair.
It also allows membrane proteins to move within the bilayer to facilitate their function as transport channels or signal transducers.