The cell membrane: structure, composition and functions.

The cells have multiple organelles and structures that are important for the functioning of the cell, but if one of them is fundamental and vital is the cell membrane, also called the plasma membrane.

The rupture of the cell membrane would lead to the death of the cell in a few seconds since it acts as a physical barrier that isolates the inside of the cell from the outside. Due to this important function, the plasma membrane has self-healing mechanisms although they do not solve all the problems that the cell may face.

There are other plasma membranes in addition to the membrane that externally surrounds the cell: they are those that make up organelles and the one that surrounds the nucleus of the cell.

However, this is not the only one of its functions but we will develop it at a later point. We are going to start with the definition, composition and structure of the cell membrane.

What is the cell membrane?

The cell membrane is a layer that surrounds the cell to separate it from the external environment . It is a thin layer of around  75 amstrongs, or 7.5 nanometers,  flexible as we will see later, a quality that is very important to be able to carry out part of its functions.

It is not visible under the light microscope but it is visible under the electron microscope as a double thin line, a double structure that is only visible under the electron microscope.

As we said, the plasma membrane is not only limiting the cell from the outside, but is also part of the organelles that have a cell membrane such as mitochondria or the nucleus. 

At the chemical level, it is a double layer of lipids, as we will see later. If we had to define the cell membrane, …

What is the definition of membrane?

The definition of plasma membrane is that the plasma membrane is a lipid double bilayer with embedded proteins floating in the membrane that limits the cell separating the interior of the cell, the cytoplasm, from the external environment.

Composition of the plasma membrane

Next, we are going to see the components of the plasma membrane and then describe how they are structured forming it.

There are three types of components in the cell membrane: lipids, proteins, and carbohydrates. We are going to see each of them in more detail.

Lipids

The major components of the plasma membrane are lipids, which make up about 50% of the membrane’s mass , although this percentage varies depending on whether it is the outer cell membrane or the membrane of some of the organelles inside the cell. For example, the mitochondrion has a higher proportion of proteins due to the amount of chemical reactions that take place in its membrane.

Phospholipids

The lipids of the plasma membrane are phospholipids that have a very important characteristic for the formation of the membrane: they are amphiphilic (or also called amphipathic). 

What do you mean that phospholipids are amphipathic or amphiphilic?

This means that they have two parts: a head that shows a preference for water and is therefore called hydrophilic and a tail that repels water and is therefore hydrophobic. In this way they tend to spontaneously form membranes in the presence of water.

The hydrophilic head is made up of an amino acid (depending on the phospholipid) and a phosphate group bound to a glycerol which in turn is bound to the hydrophobic tail which is made up of two fatty acid chains.

There are different types of phospholipids that can be part of the membranes depending on the organism to which they refer. For example, the membrane of bacteria such as Escherichia coli has a predominant phospholipid which is phosphatidylethanolamine. 

In contrast, in mammalian animal cells we can find up to four different phospholipids : phosphatidylcholine, phosphatidylserine, phosphatidylamine and sphingomyelin. The percentage in which each of the phospholipids are present in mammalian cells depends on the cell type and the particular organelle, but the most frequent is phosphatidylcholine.

Other lipids in the cell membrane

Among other lipids that can be found in the membrane are sterols and glycolipids.

Sterols

Among the sterols, the best known and most relevant is cholesterol . Cholesterol plays an important role in the fluidity of the membrane since, on the one hand, it provides rigidity at the points where it is located, but on the other hand, when there are low temperatures, it maintains fluidity.

How does cholesterol do this function in the membrane? 

Well, cholesterol is formed by a rigid hydrocarbon ring, with a part that is formed by hydroxyl groups (OH-, which are apolar and show affinity for water) that are located in the same region as the phosphate groups of phospholipids. . The hydrocarbon ring of cholesterol interacts with the fatty acids of phospholipids, being rigid, it provides rigidity to the membrane . This same interaction is what makes the membrane fluid at low temperatures.

Glycolipids

Glycolipids are another of the lipids found in cell membranes. They are made up of a short-chain carbohydrate and a ceramide, which in turn is made up of sphingosine plus a fatty acid. For this reason, they are also called sphingoglycolipids.

They behave in the same way as membrane phospholipids with the carbohydrate and fatty acid as the hydrophilic part and the hydrocarbon tails as the hydrophobic part. The carbohydrate has important functions such as cell recognition or also as an antigenic receptor . Glycolipids are found on the outside of the membrane, not in the cytosol.

Some types of glycolipids are:

• Cerebrosides : they can be of two types, those that have galactose and are called galactocerebrosides and are found in animal nervous tissues as part of the myelin sheath, and glucocerebrosides that have glucose instead of galactose and are found in non-nervous tissues .
• Globosides : they are a type of sphingolipids that only appear in the progenitor cells of erythrocytes, in myocytes of the heart, endothelial cells and in some other organs such as the liver, kidney and spleen. Its function is involved in the recognition of the membrane.
• Gangliopsids : they are the most complex and appear in large numbers in ganglion cells of the central nervous system, especially in nerve endings. They account for up to 6% of the membrane lipids of the gray matter of the human brain. Their main function is to be membrane receptors. 

Membrane proteins

Proteins are another of the main constituents of the membrane and are of two types: integral membrane and peripheral. One of the main differences between them is that the integral membrane proteins can only be eliminated by the breakdown of the lipid bilayer by the action of a detergent, while the peripheral proteins can be separated with other more complex and delicate types of extraction but which remain intact. the bilayer. 

Integral membrane proteins

The integral membrane proteins are embedded within the membrane and many of them are able to cross it leaving parts exposed on each side of the membrane, others are anchored in one way or another to the membrane.

These integral membrane proteins are also called transmembrane proteins . As with phospholipids, these proteins are amphipathic, being exposed to the extracellular and intracellular hydrophilic part. Many of the transmembrane proteins have carbohydrates added that are exposed on the cell surface to perform specific functions such as cell-cell interactions.

There are other proteins that are only found in the internal part of the cell , and are associated with the internal part of the lipid bilayer, forming a part of them of the monolayer, since being amphipathic, a part is embedded in the lipid part of the membrane and the other part is exposed to the cytosol as it is hydrophilic.

Other proteins are found entirely outside the lipid bilayer (either inside or outside the cell), and are anchored by one or two lipid groups.

Peripheral membrane proteins

Peripheral membrane proteins are not inserted into the membrane but are indirectly associated through interaction with membrane proteins.

Carbohydrates or membrane carbohydrates

The carbohydrates that are part of the cell membrane are either attached to the lipids of the membrane or attached to proteins , when proteins are attached to short chains of sugars (oligosaccharides) they are called glycoproteins while if they are attached to chains Longer polysaccharides are called proteoglycans.

In any case, carbohydrates are always on the extracellular side, never on the cytosolic side . All this general coating of carbohydrates is called the carbohydrate layer and is responsible for some of the functions of the cell membrane that we will see later.

This layer of carbohydrates plays a very important role since it is responsible for characterizing cells specialized in a certain function, and is responsible for cell recognition in their interactions.

On the other hand, the carbohydrate layer absorbs water thanks to the carbohydrates themselves, which is why it serves to confer a viscous surface to the cell that allows mobile cells such as red and white blood cells … to easily pass through narrow channels and not adhere to the walls of blood vessels.

What is the structure of the plasma membrane?

Previously, in the definition, we explained that the plasma membrane has a lipid double layer structure, also called lipid bilayer, with proteins that float in it. 

We are going to see in particular the lipid bilayer and the fluid mosaic pattern.

Lipid bilayer

It is said to be a bilayer because there are two layers of amphipathic phospholipids, where the hydrophilic heads are towards the outside and the hydrophobic tails are towards the inside, joined to form a double layer in the form of a palisade. This structure works because the cell with its lipid bilayer is in an aqueous medium that allows the phospholipids to spontaneously adopt this structure.

The inner and outer layers of the membrane differ in chemical composition, both in lipids and proteins. Depending on the type of cell, the outer layer may be richer in glycolipids and glycoproteins.

Fluid Mosaic Membrane Model

The fluid mosaic model was proposed by Jonathan Singer and Garth Nicolson in 1972. According to this model, proteins are embedded in the lipid bilayer and phospholipids confer the basic structural organization of the membrane.

Lipids and proteins seem to be anchored in a fixed and immovable position, but the structure is actually fluid. There are proteins that can move laterally through the bilayer, coupling and uncoupling according to the molecular interactions typical of the life of the cell. In this way, the structure of phospholipids and proteins changes over time, with a mosaic structure. 

Main functions of the plasma membrane

The plasma membrane has several functions that we break down below:

Limit the cell , separating the cytoplasm and organelles from the external environment.

Cell division : the membrane is involved in the control and development of cell division or cytokinesis.

Endocytosis and exocytosis : the membrane is related to the uptake of large particles (endocytosis) and the secretion of substances to the outside (exocytosis).

Regulate the exchange of substances between the cytoplasm and the outside of the cell to keep the inside of the cell stable. This is done through the proteins that are inserted in the membrane, which carry out transport functions for molecules that cannot cross the lipid bilayer. For this reason, the plasma membrane is said to have selective permeability .

It develops communication and adhesion functions with other cells through other proteins, receiving and sending chemical signals to other cells.

Participate in chemical reactions through proteins that act as enzymes. For example, the case of the respiratory chain in mitochondria or photosynthesis in chloroplasts.

Allows cell recognition and identification . Both to develop normal cell functions and for cellular immunity . Through oligosaccharides on the surface of the membrane, as is the case with blood groups, or through glycolipids that may have antigenic properties.

Serve as a fixation for surface receptors, being able to fix certain substances or enzymes.

It participates in neuronal neurotransmission , such as the transmission of the nerve impulse.

Serve as a structural support for the cell . The cytoskeleton is anchored to the cell membrane to maintain the structure of the cell.

Bibliography

Alberts, B.,  et al.  Introduction to cell biology . 3rd edition. 2011. Editorial Médica Panamericana.