The Fluid Mosaic Model: Phospholipid Bilayer

Phospholipid Bilayer

Phospholipid Bilayer: All cells are surrounded by the cell membranes, and this characteristic best portrayed by the Fluid Mosaic Model. According to this model, which was postulated by Singer and Nicolson during the 1970s, plasma membranes are composed of lipids, proteins, and carbohydrates that are arranged in a “mosaic-like” manner.

The fundamental structure of the plasma membrane is the phospholipid bilayer which act as a barrier and carry out other specific roles in the cell.

In particular, the main lipid component of the membrane are the phospholipids.

Find more about these molecules and the bilayer itself by scrolling down below.

What is a Phospholipid?

Phospholipid Molecule Structure
Phospholipid Molecule Structure (Source: cnx.org)
Phospholipids are amphipathic (double-charged) molecules composed of one glycerol molecule, one phosphate group, and two fatty acid units.

  • As portrayed in the diagrammatic illustration above, the glycerol molecule and the phosphate group makes up the “hydrophilic” head, or the water-loving part of the phospholipid. This head is such because of the negatively charged phosphate group that tends to attract the water molecules.
  • On the other hand, uncharged saturated and (some) unsaturated fatty acid chains make up the nonpolar tail, which is hydrophobic in nature. As such, these tails tend to repel water molecules.
  • As an amphiphilic molecule, the phospholipid can easily adapt to its environment. However, the structure of phospholipids can be affected by the salinity and pH of the cell’s surrounding environment.
  • When placed in water, phospholipids clump together to form micelles. Micelles are lipid molecules that form spherical aggregates in liquid solutions.

The Structure Of The Phospholipid Bilayer

Fluid Mosaic Model
Fluid Mosaic Model (Source: Link)
As its name implies, the phospholipid bilayer is made up of many phospholipids that lined up altogether. As shown in the diagram above, the bilayer is composed of hydrophilic (water-loving) heads that interact with the water on the outside environment, and a hydrophobic (water-fearing) tails that face each other in the inner structure of the membrane.

  • Interestingly, the phospholipid bilayer can form a closed sphere in order to completely remove any water molecule attached to its hydrophobic tail.
  • Basically, the “fluid” term in the Fluid Mosaic model pertains to the ability of the proteins and lipids to move in the membrane. In addition to that, the membrane is also “mosaic” due to the arrangement of the components themselves.  Because of this kind of structure of the phospholipid bilayer, the plasma membrane[1] can choose which molecules can enter or be rejected to pass through it.
  • For instance, proteins, which are amphipathic in nature (can be both water-loving and water-fearing) can easily establish connections with the similar molecules found in the bilayer.
  • Carbohydrates on the other hand can interact with the proteins and lipids which can result to the production of glycoproteins and glycolipids respectively.

For additional information, you may want to read S. G. Singer and G. L. Nichol’s journal article entitled The fluid mosaic model of the structure of cell membranes[2].

Functions Of The Phospholipid Bilayer

Contrary to the common notion that the plasma membrane is merely a boundary between the inside of the cell and its outside environment, this membrane is also responsible for other vital cell functions like the ones listed below.

1. Maintain The Shape Of The Cell

The inside part of the cell[3] is mainly composed of water. In the same manner, the outside environment where the cell resides is also surrounded by an almost similar fluid.

  • Without the double layer, the plasma membrane cannot remain stable and may either burst or shrink due to the difference in tonicity.

2. Act As A Semipermeable Membrane

Due to the unique physical and chemical properties of the phospholipids, the bilayer becomes a so-called semipermeable membrane which allows the entry of only certain molecules into the cell.

  • In particular, it only allows nonpolar molecules like oxygen, water, and others to pass through it. In addition, this property offers additional protection from foreign materials trying to invade the cell.

3. Important In Cell Recognition And Communication

In relation to its semipermeability, the phospholipid bilayer acting as a barrier between the interior and exterior cellular environment can in turn mediates[4] the recognition process, communication, and signaling process among neighboring cells.

4. Maintain Its Internal Environment

This part is very crucial because due to the difference in components, ion and protein concentrations, the environment inside the cell is very far from its extracellular environment. However, by being able to control the expression of some transmembrane proteins found in the phospholipid bilayer, the cell can somehow take control of its intracellular environment.

What else are found in the Plasma Membrane?

We’ve been talking about phospholipids since the start of this article. Is the bilayer also made up of other components as well? Of course, yes! The following are molecules that can also be found in the phospholipid bilayer.

1. Cholesterol

In animal cells, cholesterol is the component that helps strengthen the bilayer and help control some membrane proteins.

  • In addition to that, due to its nonpolar nature, cholesterol also decreases the permeability of the bilayer.

2. Glycoproteins

The glycoprotein, a protein with a sugar attached to it, is a component of the phospholipid bilayer responsible for the stabilization of its membrane structure.

  • This is mainly because of its ability to form strong hydrogen bonds with the water molecules found around the cell.
  • Furthermore, glycoproteins also act as receptors for molecules like hormones and signaling molecules.

3. Antigens

Aside from their roles with regards to immunity, antigens, like glycoproteins also act as cell recognition markers for chemicals and other molecules.

The component and structure of the phospholipid bilayer and the plasma membrane in general had proven to be a fruitful endeavor to explore. Although some known facts are subject to further scrutiny or falsification in the future, the idea remains that these components play major roles for the survival of the cell.

Wouldn’t there be much chaos if the phospholipid bilayer is not present at all?

Cite this article as: "The Fluid Mosaic Model: Phospholipid Bilayer," in Bio Explorer, February 12, 2017, http://www.bioexplorer.net/phospholipid-bilayer.html/.

References

  • [1]“Once upon a time the cell membranes: 175 years of cell boundary research”. Accessed February 12, 2017. Link.
  • [2]“The fluid mosaic model of the structure of cell membranes. – PubMed – NCBI”. Accessed February 12, 2017. Link.
  • [3]“Lipid Bilayer: Definition, Structure & Function – Video & Lesson Transcript | Study.com”. Accessed February 12, 2017. Link.
  • [4]“Lipid Bilayer – Fastbleep”. Accessed February 12, 2017. Link.
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