Structure and functions of plasma membrane

What is plasma membrane or cell membrane?

The plasma membrane of the cell is a network of lipids and proteins that form the boundary between cell contents and the extracellular matrix. It is simply called the cell membrane. It is a subcellular structure, about 10 nm thick, and forms protective borders around the cell as well as cell organelles. Here you will read in detail the structure and functions of the plasma membrane.

The main function of the plasma membrane is to protect the cell from the surrounding environment. It is semi-permeable and regulates the substances entering and exiting the cell.

The plasma membrane consists of a phospholipid bilayer with embedded proteins, selectively performs ions and organic molecules and regulates the movement of substances within and outside cells. Plasma membranes must be extremely flexible to allow certain cells, such as red blood cells and white blood cells, to change their shape as they pass through narrow capillaries.

The plasma membrane also plays a role in stabilizing the cytoskeleton to provide the shape of the cell, and in the attachment to the extracellular matrix and other cells to help group the cells together to form tissues. The membrane also maintains the cell’s potential.

In short, if the cell is represented by a castle, the plasma membrane can be considered as a wall. Just as a hole in the wall of the castle can be a disaster for the castle, the rupture of the plasma membrane causes the cell to die and die.

Chemical composition of plasma membrane

Chemically, the plasma membrane consists of a wide range of proteins, lipids in the form of cholesterol, phospholipids, sphingolipids, and carbohydrates in the form of glycoproteins. Among different cell types, there may be differences not only in the type and amount of lipids, carbohydrates and proteins, but the amount of these chemicals may also vary.
Based on the chemical analysis of the cellular membrane of human red blood cells, it was believed that the cell membrane consists of lipid arranged in two layers. These lipids molecules are arranged inside the membrane with a polar head towards the outer sides and non-polar tails towards the inner side. This arrangement ensures that the non-polar tail of saturated hydrocarbons is protected from the aquatic environment.

Table showing the percentage of biomolecules in cell membranes of different cells

Plasma membrane of	Protein %	Lipid %	Carbohydrate %
Mouse Liver	44%	52%	4%
Human RBC	52%	40%	8%
Amoeba	54%	42%	4%

A) Proteins: Proteins are the main components of all biological membranes that may be about 50% of the components of the plasma membrane. The amount of proteins in the membrane varies greatly depending on the location and function of the cell. The nerve cell membrane contains less than 25 percent protein, while the inner membranes of cells involved in energy transfer such as mitochondria and chloroplasts contain about 75% protein. Proteins may act as enzymes, antigens, receptor molecules, regulatory molecules, etc.

Depending on the placement of the protein and the ease of extraction, membrane proteins can be classified into two types: integral proteins or peripheral proteins.

Peripheral proteins

These proteins are also called exogenous proteins.
It lies on the surface of the membrane.
Peripheral proteins have a weak association with the membrane. They are linked to membrane lipids by electrostatic interaction.
Structure of plasma membrane showing proteins, lipids and carbohydrates

Integrated proteins

These proteins are also called intrinsic proteins.
They are partially or completely submerged in the membrane.
Intrinsic proteins have a strong relationship with the membrane.
Peripheral and integral proteins may be ectproteins (proteins located on the outer side of the cell surface of the plasma membrane) or endoproteins (proteins on the inner side of the surface of the plasma cell membrane).

B) Carbohydrates: Membrane carbohydrates are found in the chemically linked form such as glycolipids and glycoproteins. However, some membrane carbohydrates are part of the proteoglycans that insert their amino acid chain among fatty acids. Although some carbohydrates are found associated with intracellular membranes most of these are found in the outer monolayer of the plasma membrane, which faces extracellular space.

Three types of glycolipids are found in the membranes: glycosphingolipids, which are the most abundant in animal cells, glycoglycerolipids, and glycophosphatidylinositol. Glycoglycerides are more frequent in the plasma membrane than plant cells. However, most membrane carbohydrates are found linked to proteins, known as glycoproteins. Almost all membrane proteins contain carbohydrates, but only 5% of lipids are glycolipids.

Carbohydrates in the plasma membrane as a whole are referred to as glycocalyx. In some cell types, glycocalyx has been developed so that it can be observed by electron microscopy. For example, in erythrocytes, glycocalyx can be extended by more than 1 µm in length. In this way, the cell is covered with a layer of carbohydrates that can reach 2 to 10% of the membrane weight. The development of glycocalyx depends on cell type.

C) Lipids: Plasma membranes and internal membranes are comped of glycerophospholipids, molecules consisting of glycerol, a phosphate group, and two chains of fatty acids. Glycerol is a tri-carbon molecule that acts as the backbone of this membrane lipid.

Within individual glycerophospholipid, fatty acids are attached to the first and second carbons, and the phosphate group is bound to the third carbon of the glycerol backbone. Different head groups are attached to phosphates. The space-filling models of these molecules reveal their cylindrical shape, a geometrical pattern that allows glycerophospholipids to align side by side to form broad sheets.

Glycerophospholipids are by far the most abundant lipid molecules in cell membranes. Like all lipids, they are insoluble in water, but their unique geometry makes them aggregate in bilayers without any energy input. This is because they are two-sided molecules, with hydrophobic phosphate heads and a hydrocarbon tail (afraid of water) of fatty acids. In water, these molecules are automatically aligned with their heads outward and their tails lined up inside the bilayer. Thus, the hydrophilic heads of the glycerophospholipids in the plasma membrane of the cell face both the water-based cytoplasm and the outer part of the cell.

In general, lipids represent about half the mass of cell membranes. Cholesterol molecules, although less abundant than glycerophospholipids, account for about 20 percent of the lipids present in animal cell plasma membranes. However, cholesterol is not present in bacterial membranes or mitochondrial membranes.

Cholesterol helps regulate the hardness of membranes, while other less prominent lipids play roles in cell signaling and cell recognition.

Functions of the plasma membrane

Function of proteins
Depending on the function of proteins in the membrane, they may be of three main types: structural proteins, transport proteins, and enzymes.

Structural proteins

They are very lipophilic by nature. They form the backbone of the plasma membrane.

Transport proteins

These are proteins that help transport specific substances across the plasma membrane as well as other cell membranes.

Enzymes

Enzymes are protein molecules present in the cell and act as catalysts. Plasma membrane enzymes can be either ectoenzymes (enzymes located on the outer side of the plasma membrane cell surface) or endoenzymes (enzymes located on the inner side of the plasma membrane cell surface). Examples of enzymes present on the plasma membrane are phospholipase A, maltase, lactase, alkali phosphatase, acetyl phosphatase, etc.

Function of carbohydrates

Membrane carbohydrates perform two main functions:

Participation in cell identification and adhesion, either cell signaling or cell pathogen interactions
Structural role as a physical barrier

Blood groups are determined by the cell surface carbohydrates of erythrocytes and also have the ability to stimulate immune responses.

Cell identification and adhesion: After an infection, endothelial cells close to the affected tissue expose a type of protein, known as selectins, in their plasma membranes. They recognize the carbohydrates in the plasma membrane of the lymphocytes that pass through the blood and bind them. In this way, lymphocytes are connected to the walls of blood vessels, and they can cross the endothelium and move to the infection point.
Carbohydrates as recognition molecules are also important during embryonic development.
Cell pathogen interactions: Carbohydrates in the plasma membrane are major binding and recognition sites for the pathogens during infection. The virus, such as the influenza virus, the pathogenic E. coli bacteria, and some types of protozoa need to bind to the cell surface before they enter the cell, otherwise, they will be swept by cleaning mechanisms of the body. These pathogens contain proteins, known as lectins, that bind to specific carbohydrates in certain cells. Thus, the type of infected cell depends on the carbohydrates that appear in the plasma membrane. Vertebrates, invertebrates, and protozoa carry a different set of carbohydrates in their cells.

The glycan parts of glycoconjugates on cell surfaces regulate many types of interactions between cells and their immediate environments. Changes in glycosylated glycoproteins associated with cancer are responsible for changes in their molecular interactions and biological functions.

Function of lipid

The lipid bilayer structure forms an impermeable barrier for essential water-soluble substances in the cell and forms the basis of the compartmentalization function of biological membranes.
In the case of lipid-soluble substances, lipid bilayer serves as the best way to move across the membrane.
Cholesterol has been shown to regulate ion pumps, which in some cases show absolute dependence on cholesterol for its activity.
Cholesterol in the mammalian cells allows essential membrane enzymes to provide the function necessary for cell survival.

Models of plasma membrane

The following points highlight the four main historical plasma membrane models. The models are as follows:
1. 1. Lipid and Lipid Bilayer Model 2. Unit membrane model (protein-lipid bilayer -lipid) 3. Fluid mosaic model 4. Danielle model.

1. Lipid and Lipid Bilayer Model

This model explains the structure of the plasma membrane has been given by Overton, Gorion, and Grendel. In 1926, Gorter and Grendel observed that the amount extracted from erythrocyte membranes was twice that expected if a single layer was present on the entire surface of these cells. On this basis, they stated that the plasma membrane is composed of a double layer of lipid molecules. These Gorter and Grendel models could not explain the proper structure of the plasma membrane, but they laid the foundation for future membrane structure models.

2. Unit membrane model (Protein-Lipid Bilayer-Protein):

This is also called the unit membrane model. This model was proposed by Davson Daniell and Robertson. On this basis, Davson and Danielli proposed that the plasma membrane contains a lipid bilayer with proteins on both surfaces. They observed that the plasma membrane of most cells appeared to be three-layered. They developed the model in which the protein appears to be spread on the hydrophilic ends of the lipid bilayer. This model has been popular for a long time.

3. Fluid mosaic model

: It was proposed by Singer and Nicholson (1912). This model postulates that lipid and integrated proteins are eliminated in a kind of mosaic pattern and that all biological membranes have a quasi-fluid structure where lipid and protein components are able to perform a transitional movement within the lipid bilayer.