Plasma Membrane: Concept, Structure and Functions

We explain what the plasma membrane is and how it is its structure.In addition, the main functions of this layer of lipids.

The plasma membrane is not visible under an optical microscope.

What is the plasma membrane?

It is called a plasma membrane, cell membrane, plasmalema or cytoplasmic membrane to a double layer of lipids that covers and delimits the cells , serving as a border between the inside and outside of it, and also allowing a physicochemical balance between the environment and cell cytoplasm.

The plasma membrane is not visible under an optical microscope (if under an electronic one), since has an average thickness of 7 , 3 cubic nanometers .In plant and fungal cells, this membrane is located below the cell wall.

Selective permeability is the main characteristic of the plasma membrane , that is, its ability to allow or reject the entry of certain molecules into the cell, thus regulating the passage of water, nutrients or ionic salts, and maintaining the cytoplasm always in optimal conditions in terms of electrochemical potential (negatively charged), pH or concentration refers.

The latter occurs through two elementary processes of absorption (or endocytosis ) or expulsion (or exocytosis ) of cellular substances, and can also release to medium environment metabolic waste materials , the result of cellular respiration.For this purpose, small vesicles are formed in the plasma membrane.They serve as a means of cellular transport.

An important dynamic in the case of cells or unicellular organisms that use their membrane to wrap (or phagocyte r ) nutrients or prey, or to expel from a multicellular organism those harmful agents (as do lymphocytes or white blood cells).

See also: Meiosis.

Structure of the plasma membrane

The plasma membrane has about 20% protein.

The plasma membrane is composed of two layers of lipids, which orient their hydrophilic polar heads (that is, they have an affinity for water) inwards the cell, keeping its hydrophobic parts (which reject water) in contact, in the manner of a sandwich.These lipids are primarily cholesterol, phosphoglycerides and sphingolipids.

At the same time it has about 20% of proteins , which fulfill functions of connection, transport and catalysis: diverse forms of biochemical communication and cellular transport of nutrients and wastes.Likewise, the membrane has several glucides (sugars), in its outermost part, serving as support material and intercellular identification.These sugars represent only 8% dry of the total membrane weight.

Function of the plasma membrane

The plasma membrane serves as a filter and transport.

The plasma membrane has a different set of functions, such as:

• Delimit the cell.Define and protect the cell from its surroundings, separating the outside from the inside and one cell from another (in the case of cell tissues).It is the first defense barrier in case of invading agents, such as viruses.


• Nutrient administration.The selectivity of the membrane gives way to desired substances and denies them to unwanted ones, serving as a filter and transport between outside and inside, since it also allows the disposal of toxins and metabolic wastes (such as CO2).


• Preservation of life.By exchanging fluids and substances between the cytoplasm and the environment, the plasma membrane seeks to keep the concentration of water and other substances in the cytoplasm stable.This also implies maintaining its pH level and electrochemical charge.


• Cellular communication.Given certain stimuli from outside the cell, the plasma membrane is capable of reacting, transmitting information inside the cell and launching specific biochemical processes: cell division, cell movement or substance segregation.biochemistry.


• Cellular displacement.In some cases the cell membrane lengthens and allows the appearance of flagella (tails) or cilia (hairs) that allow the cell to physically move.