WHAT IS THE FLUID MOSAIC MODEL? - BIOLOGY - 2025

The fluid mosaic model states that cell membranes or biomembranes are dynamic structures that present fluidity of their different molecular components, which can move laterally. In other words, these components are in motion and not static, as previously believed.

This model was raised by S. Jonathan Singer and Garth. L. Nicolson in 1972 and today is widely accepted by the scientific community. All cells are contained by a cell membrane with particularities in its constitution and function.

Figure 1. Diagram of the fluid mosaic model. Source: By LadyofHats Mariana Ruiz, translation Pilar Saenz, via Wikimedia Commons

This membrane defines the limits of the cell, allowing the existence of differences between the cytosol (or cell interior) and the external environment. In addition, it regulates the exchange of substances between the cell and the outside.

In eukaryotic cells, inner membranes also define compartments and organelles with different functions, such as mitochondria, chloroplasts, the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, among others.

Structure of the cell membrane

Generalities

The cell membrane consists of a structure that is impermeable to water-soluble molecules and ions between 7 and 9 nanometers thick. It is observed in electron photomicrographs as a continuous and thin double line that surrounds the cell cytoplasm.

The membrane is composed of a phospholipid bilayer, with proteins embedded throughout its structure and arranged on the surface.

Furthermore, it contains carbohydrate molecules on both surfaces (internal and external) and in the case of eukaryotic animal cells, it also has cholesterol molecules interspersed inside the bilayer.

Phospholipid bilayer

Phospholipids are amphipathic molecules that have a hydrophilic end - end to water - and another hydrophobic - which repels water.

The phospholipid bilayer that makes up the cell membrane has hydrophobic (apolar) chains arranged towards the interior of the membrane and hydrophilic (polar) ends located towards the external environment.

Thus, the heads of the phosphate groups of phospholipids are exposed on the outer surface of the membrane.

Remember that both the external environment and the internal or cytosol, are aqueous. This influences the arrangement of the phospholipid double layer with its polar parts interacting with water and its hydrophobic parts forming the internal matrix of the membrane.

Cholesterol

In the membrane of eukaryotic animal cells, cholesterol molecules are found embedded in the hydrophobic tails of phospholipids.

These molecules are not found in the membranes of prokaryotic cells, some protists, plants, and fungi.

Integral membrane or transmembrane proteins

Interspersed within the phospholipid bilayer are integral membrane proteins.

These interact non-covalently through their hydrophobic parts, with the lipid bilayer, locating their hydrophilic ends towards the external aqueous medium.

Configuration of membrane proteins

They can have a simple rod-shaped configuration, with a folded hydrophobic alpha helix embedded in the membrane interior, and with hydrophilic portions extending to the sides.

They can also present a larger configuration, globular type and with a complex tertiary or quaternary structure.

The latter usually cross the cell membrane several times with their segments of repeated alpha helices arranged in a zigzag through the lipid bilayer.

Pores in membranes

Some of these globular proteins present hydrophilic interior portions, forming channels or pores through which the exchange of polar substances occurs from the outside of the cell to the cytosol and vice versa.

Peripheral proteins

On the surface of the cytoplasmic face of the cell membrane, there are peripheral membrane proteins, linked to the protruding parts of some integral proteins.

These proteins do not penetrate the hydrophobic core of the lipid bilayer.

Carbohydrate cover

There are carbohydrate molecules on both surfaces of the membrane.

In particular, the outer surface of the membrane presents an abundance of glycolipids. Short chains of carbohydrates are also seen exposed and covalently attached to protruding protein parts, called glycoproteins.

Cell membrane fluidity

Saturated versus unsaturated fatty acid ratio

The fluidity of the membrane depends mainly on the ratio of saturated and unsaturated fatty acid phospholipids present. This membrane fluidity decreases as the proportion of saturated fatty acid chain phospholipids increases with respect to the unsaturated ones.

This is due to the fact that the cohesion between the long and simple chains of saturated fatty acids is greater, compared to the cohesion between the short and unsaturated chains of unsaturated fatty acids.

The greater the cohesion between its molecular components, the less fluid the membrane will present.

Cholesterol

The cholesterol molecules interact through their rigid rings with the hydrocarbon chains of the lipids, increasing the rigidity of the membrane and decreasing its permeability.

In the membranes of most eukaryotic cells, where there is a relatively high concentration of cholesterol, it prevents the carbon chains from binding at low temperatures. This provides for the freezing of the membrane at low temperatures.

Special features

The different types of cell membranes present particularities in their amount and type of proteins and carbohydrates, as well as in the variety of existing lipids.

These particularities are associated with specific cellular functions.

There are not only constitutive differences between the membranes of eukaryotic and prokaryotic cells, and between those of organelles, but also between regions of the same membrane.

Cell membrane function

Generalities

The cell membrane delimits the cell and allows it to maintain a stable condition in the cytosol, different from that of the external environment. This, through the active and passive regulation of the passage of substances (water, ions and metabolites) through each other, maintaining the electrochemical potential necessary for cell function.

It also allows the cell to respond to signals from the external environment through chemical receptors in the membrane and provides anchoring sites for the cytoskeletal filaments.

In the case of eukaryotic cells, it also participates in the establishment of internal compartments and organelles with specific metabolic functions.

Function of proteins in the membrane

There are different membrane proteins with specific functions, among which we can mention:

• Enzymes that catalyze (speed up) chemical reactions,
• Membrane receptors involved in the recognition and binding of signaling molecules (such as hormones),
• Substance transport proteins through the membrane (towards the cytosol and from it to the outside of the cell). These maintain an electrochemical gradient thanks to the transport of ions.

Function of the outer carbohydrate shell

Carbohydrates or glycolipids participate in the adhesion of cells to each other and in the process of recognition and interaction of the cell membrane with molecules such as antibodies, hormones, and viruses.

References

1. Bolsaver, SR, Hyams, JS, Shephard, EA, White HA and Wiedemann, CG (2003). Cell Biology, a short course. Second edition. Wiley-Liss pp 535.
2. Engelman, D. (2005). Membranes are more mosaic than fluid. Nature 438 (7068), 578-580. doi: 10.1038 / nature04394
3. Nicolson, GL (2014). The Fluid-Mosaic Model of Membrane structure. Still relevant to understanding the structure, function and dynamics of biological membranes after more than 40 years. Biochimica et Biophysica Acta (BBA) - Biomembranes, 1838 (6), 1451-1466. doi: 10.1016 / j.bbamem.2013.10.019
4. Raven, J. (2002). Biology. Sixth Edition. MGH. pp 1239.
5. Singer, SJ and Nicolson, GL (1972). The Fluid Mosaic Model of the Structure of Cell Membranes. Science, 175 (4023), 720-731. doi: 10.1126 / science.175.4023.720