PLASMA MEMBRANE: CHARACTERISTICS, FUNCTIONS AND STRUCTURE - BIOLOGY - 2024

The plasma membrane, cell membrane, plasmalemma or cytoplasmic membrane, is a lipidic structure that surrounds and delimits cells, being an indispensable component of their architecture. Biomembranes have the property of enclosing a certain structure with its exterior. Its main function is to serve as a barrier.

In addition, it controls the transit of particles that can enter and exit. Membrane proteins act as "molecular gates" with quite demanding gatekeepers. The composition of the membrane also has a role in cell recognition.

Structurally, they are bilayers made up of naturally arranged phospholipids, proteins, and carbohydrates. Similarly, a phospholipid represents a phosphor with a head and a tail. The tail is made up of carbon chains insoluble in water, these are grouped inwards.

Instead, the heads are polar and give to the aqueous cellular environment. Membranes are extremely stable structures. The forces that maintain them are those of van der Waals, among the phospholipids that compose them; this allows them to firmly surround the edge of the cells.

However, they are also quite dynamic and fluid. The properties of the membranes vary according to the cell type analyzed. For example, red blood cells must be elastic to move through blood vessels.

In contrast, in neurons the membrane (myelin sheath) has the necessary structure to efficiently allow the conduction of the nerve impulse.

General characteristics

By Jpablo cad, from Wikimedia Commons

Membranes are quite dynamic structures that vary widely depending on the cell type and the composition of its lipids. The membranes are modified according to these characteristics as follows:

Membrane fluidity

The membrane is not a static entity, it behaves like a fluid. The degree of fluidity of the structure depends on several factors, among these the lipid composition and the temperature to which the membranes are exposed.

When all the bonds that exist in the carbon chains are saturated, the membrane tends to behave like a gel and the van der Waals interactions are stable. On the contrary, when there are double bonds, the interactions are smaller and the fluidity increases.

In addition, there is an effect of the length of the carbon chain. The longer it is, the more interactions occur with its neighbors, thus increasing fluency. As the temperature increases, the fluidity of the membrane also increases.

Cholesterol plays an indispensable role in the regulation of fluidity and depends on cholesterol concentrations. When the queues are long, cholesterol acts as an immobilizer of the same, decreasing the fluidity. This phenomenon occurs at normal cholesterol levels.

The effect changes when cholesterol levels are lower. By interacting with the lipid tails, the effect it causes is their separation, reducing fluidity.

Curvature

Like fluidity, the curvature of the membrane is determined by the lipids that make up each particular membrane.

The curvature depends on the size of the lipid head and tail. Those with long tails and large heads are flat; those with relatively smaller heads tend to curve much more than the previous group.

This property is important in membrane evagination phenomena, vesicle formation, microvilli, among others.

Lipid distribution

The two "sheets" that make up each membrane - remember that it is a bilayer - do not have the same composition of lipids inside; for this reason it is said that the distribution is asymmetric. This fact has important functional consequences.

A specific example is the composition of the plasma membrane of erythrocytes. In these blood cells, sphingomyelin and phosphatidylcholine (which form membranes with greater relative fluidity) are found facing the outside of the cell.

Lipids that tend to form more fluid structures face the cytosol. This pattern is not followed by cholesterol, which is distributed more or less homogeneously in both layers.

Features

The function of the membrane of each cell type is closely related to its structure. However, they fulfill basic functions.

Biomembranes are responsible for delimiting the cellular environment. Similarly, there are membranous compartments within the cell.

For example, mitochondria and chloroplasts are surrounded by membranes and these structures are involved in the biochemical reactions that occur in these organelles.

The membranes regulate the passage of materials into the cell. Thanks to this barrier, the necessary materials can enter, either passively or actively (with the need for ATP). Also, unwanted or toxic materials do not enter.

The membranes maintain the ionic composition of the cell at adequate levels, through the processes of osmosis and diffusion. Water can flow freely depending on its concentration gradient. Salts and metabolites possess specific transporters and also regulate cellular pH.

Thanks to the presence of proteins and channels on the surface of the membrane, neighboring cells can interact and exchange materials. In this way, cells unite and tissues are formed.

Lastly, membranes house a significant number of signaling proteins and allow interaction with hormones, neurotransmitters, among others.

Structure and composition

The basic component of membranes are phospholipids. These molecules are amphipathic, they have a polar and an apolar zone. The polar allows them to interact with water, while the tail is a hydrophobic carbon chain.

The association of these molecules occurs spontaneously in the bilayer, with the hydrophobic tails interacting with each other and the heads pointing outwards.

In a small animal cell we find an incredibly large number of lipids, on the order of 10 ⁹ molecules. The membranes are approximately 7 nm thick. The hydrophobic inner core, in almost all membranes, occupies a thickness of 3 to 4 nm.

Fluid mosaic pattern

The current model of biomembranes is known as "fluid mosaic", formulated in the 1970s by researchers Singer and Nicolson. The model proposes that membranes are made not only of lipids, but also of carbohydrates and proteins. The term mosaic refers to this mixture.

The face of the membrane that faces the outside of the cell is called the exoplasmic face. In contrast, the inner face is the cytosolic.

This same nomenclature applies to the biomembranes that make up the organelles, with the exception that the exoplasmic face in this case points to the inside of the cell and not to the outside.

The lipids that make up the membranes are not static. These have the ability to move, with a certain degree of freedom in specific regions, through the structure.

Membranes are made up of three fundamental types of lipids: phosphoglycerides, sphingolipids, and steroids; all are amphipathic molecules. We will describe each group in detail below:

Types of lipids

The first group, made up of phosphoglycerides, comes from glycerol-3-phosphate. The tail, hydrophobic in nature, is composed of two fatty acid chains. The length of the chains is variable: they can have from 16 to 18 carbons. They can have single or double bonds between the carbons.

The subclassification of this group is given by the type of head they present. Phosphatidylcholines are the most abundant and the head contains choline. In other types, different molecules such as ethanolamine or serine interact with the phosphate group.

Another group of phosphoglycerides are plasmalogens. The lipid chain is linked to glycerol by an ester bond; in turn, there is a carbon chain linked to glycerol via an ether bond. They are quite abundant in the heart and in the brain.

Sphingolipids come from sphingosine. Sphingomyelin is an abundant sphingolipid. Glycolipids are made up of heads made of sugars.

The third and final class of lipids that make up membranes are steroids. They are rings made of carbons, joined in groups of four. Cholesterol is a steroid present in the membranes and particularly abundant in those of mammals and bacteria.

Lipid rafts

There are specific areas of the membranes of eukaryotic organisms where cholesterol and sphingolipids are concentrated. These domains are also known as lipid rafts.

Within these regions they also house different proteins, whose functions are cell signaling. Lipid components are believed to modulate protein components in rafts.

Membrane proteins

A series of proteins are anchored within the plasma membrane. These can be integral, anchored to lipids or located on the periphery.

The integrals go through the membrane. Therefore, they must have hydrophilic and hydrophobic protein domains in order to interact with all the components.

In proteins that are anchored to lipids, the carbon chain is anchored in one of the layers of the membrane. The protein does not actually enter the membrane.

Lastly, the peripheral do not interact directly with the hydrophobic zone of the membrane. Rather, they can be attached by means of an integral protein or by the polar heads. They can be located on both sides of the membrane.

The percentage of proteins in each membrane varies widely: from 20% in neurons to 70% in the mitochondrial membrane, since it needs a large amount of protein elements to carry out the metabolic reactions that occur there.

References

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4. Lombard, J. (2014). Once upon a time the cell membranes: 175 years of cell boundary research. Biology direct, 9 (1), 32.
5. Thibodeau, GA, Patton, KT, & Howard, K. (1998). Structure and Function. Elsevier Spain.