Plasma Membrane – Concept, structure and functions

We explain what the plasma membrane is and what its structure is like. In addition, the main functions of this lipid layer.

Plasma Membrane
The plasma membrane is not visible under a light microscope.

What is the plasma membrane?

It is called the plasma membrane, cell membrane, plasmalemma or cytoplasmic membrane a a double layer of lipids that covers and delimits 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 (yes 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 is concerned.

The latter occurs through two elementary processes of absorption (or endocytosis) or expulsion (or exocytosis) of cellular substances, being able to also release metabolic waste materials into the environment, fruit of cellular respiration. For this, small vesicles are formed in the plasma membrane and 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 phagocyter) nutrients or prey, or to expel harmful agents from a multicellular organism (such as lymphocytes or white blood cells).

Structure of the plasma membrane

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) towards the inside of the cell, keeping their 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 has around 20% protein, that fulfill functions of connection, transport and catalysis: diverse forms of biochemical communication and cellular transport of nutrients and wastes. Likewise, the membrane has various carbohydrates (sugars), in its outermost part, serving as support material and intercellular identification. These sugars represent only 8% dry of the weight of the total membrane.

Function of the plasma membrane

Plasma Membrane
The plasma membrane serves as a filter and for transport.

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

  • Delineate the cell. Define and protect the cell from its environment, separating the outside from the inside and one cell from another (in the case of cellular 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 it to unwanted ones, serving as a filter and transport between the outside and the inside, since it also allows to dispose of toxins and metabolic waste (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 conserving its pH level and its electrochemical charge.
  • Cellular communication. Faced with certain stimuli from outside the cell, the plasma membrane is capable of reacting, transmitting information inside the cell and setting in motion certain biochemical processes: cell division, cell movement or the segregation of biochemical substances.
  • Cell displacement. In some cases, the cell membrane lengthens and allows the appearance of flagella (tails) or cilia (hairs) that allow the cell to move physically.