For many centuries, the biological composition of cell membrane structure remained one of the biggest scientific paradoxes that attracted considerable attention from biologists. In definition, a membrane refers to the outer cellular boundary of a living cell that appeared over 2.7 billion years, first as a prokaryotic cell, and then as a eukaryotic cell resulting from the evolution process (Lents and Hesterman). Subsequently, the new cell form is thus more complex, characterized by multicellular features. It has highly compartmentalized membrane-bound organelles that primarily regulate the intra-molecular movement between them and protect them from cytoplasm reactions. As aforementioned, scientists have devoted considerable attention to the study of cell membrane structure with the view of understanding their composition and functionality. To this end, Singer and Nicholson in 1972 formulated a model that proposed a descriptive analysis of the cell structure that included its semi-permeability and the two-dimensional liquid composition. In retrospect, the scientists suggested that these characteristics allowed free movement of the lipids and protein components, a position that remains plausible to date. The subsequent discussion of this paper will focus on the cell membrane structure proposed in the Singer and Nicholson model and similar scientific research hypotheses on the cellular biology of the cell membranes towards building an understanding of their composition and functionality spectrums.
Singer and Nicholson model started their experiment from the pre-existing evidence that supported their predisposition to cell membranes’ fluidity characteristics. One of the core research work that helped construct the framework of their model was Frye and Eddin’s research findings that proved the membranes’ fluidity theory using a cell-infusion of a human and mouse cells (Lents and Hesterman). In the experiment, the biologist had labeled the cell membranes of both the mouse and human cell with rhodamine and fluorescent dyes, respectively, and observed the migration process between the two cells proving the fluidity. The figure below illustrates the experiment’s finding that greatly helped to inform the fluid-mosaic model’s theoretical perspectives proposed by Singer and Nicholson.
Figure 1 Hybrid Cell Experiment
Singer and Nicholson (1972) articulated that the membrane structure was formed as a bi-layer composed of two lipid molecules layers with protein molecules embedded in the middle. In their explanation, the structural fo9rmation of the multilayered cell membrane was similar to a pool of mosaic tiles of both lipids and proteins that freely moved within the cell through the diffusion process. The scientists asserted, “The lipid molecules are like the ocean water, and the proteins are bobbing around like “icebergs…floating in a sea of lipid” (Lents and Hesterman). The scientists used a pictorial representation of the movement to illustrate the cellular membrane structure, as shown in figure 2 below.
Figure 2 Cell membrane proteins float in a sea of phospholipids
To show how the two cells related, they described lipid cells and the proteins using an example of an ocean of lipids molecules where the proteins cells float like icebergs. Using the example, Singer & Nicolson (1972) helped show how the cell membrane’s structural composition of lipid molecules and protein molecules allows the free movement of certain elements that help to plasma reactions in the cells. Thus, following the two biologists’ propositions, the scientific understanding of the composition of cells became clearly defined. By far, since the predispositions made in 1972, the cell membrane structure’s biological composition became understood and helped to form the foundations of future research on the topic (Lents and Hesterman). In more precise terms, the cell membranes’ amphipathic nature composed of a two-lipid layer and a single protein layer became well-defined. More importantly, discovering the special compositional characteristic of the lipids that features phosphate charge allows water solubility of the phospholipid molecules. In retrospect, this special characteristic allows them to allow the diffusion of water-soluble elements such as proteins and lipids, which explains the amphipathic structure of the cell membranes. Singer and Nicholson (1972) illustrated the phospholipid molecules’ concept, as the figure below shows.
Figure 3 Amphipathic Structure of Phospholipids
Thus, as the discussions mentioned above indicate, Singer and Nicholson build on the biological composition of the cell membranes in principle on the foundations of the cells’ multi-layer structure. In retrospect, they build the Fluidity-mosaic model by expounding on the structural composition of the cells’ plasma membranes (Lents and Hesterman). The scientists also identified the unique composition of the different types of lipid cells to explain the various ways that the cells overcome barriers of permeability of different elements that are both water-soluble and water-insoluble. In brief, the hydrophilic heads and hydrophobic tails described by Singer and Nicholson (1972) helped to validate the theoretical perspectives therein of how the cell membranes function and allow the free movement of proteins and lipids across the cells.
To conclude, the cell membranes structure, as Singer and Nicholson described helped explain the intracellular biological movements. As Lents and Hesterman avows, “We now understand that the plasma membrane is a very dynamic part of the cell and that is much more than just a barrier”. They are not just passive barriers but rather are very dynamic structures that regulate entry and exit of particular elements based on the findings of multiple experiments that led to the development of the fluid-mosaic model of the cellular model that described the compositional components of the membrane structure and their functionality paradigm.
Lents, N., and D. Hesterman. “Membranes I | Biology | Visionlearning.” Visionlearning, 19 May 2014, www.visionlearning.com/en/library/Biology/2/Membranes-I/198.