Biological membranes underpin all forms of life, playing vital roles as permeability barriers, transport mediators and structures enabling cellular complexity. Due to their prevalence and importance, membranes have been subject to extensive scientific research; this has lead to the development of a range of models over time. Proposed in 1972 by SJ Singer and GL Nicolson, the Fluid Mosaic Model (FMM) has become the most widely used, nanometer-scale description of its structure. The FMM model depicts biological membranes as lipid bilayers containing a homogenous and asymmetrical array of amphipathic protein, lipid and glycoprotein components - these are generally laterally mobile in the structure's fluidity. The model has been supported by a variety of experimental evidence, although - comparably to biological membranes themselves - the FMM remains dynamic and subject to adaptation as understanding of membrane organisation advances.
Biological Membranes are Lipid Bilayers
The central basis of the fluid mosaic model of membrane structure is the lipid bilayer. This acts as a matrix throughout which its lipid and protein components are arranged. Both in nature and in vitro, bilayers and micelles are observed to self-assemble due to the amphipathic nature of their constituent phospholipids. The spontaneity of this organisation indicates that a favourable minimisation of free energy (G) drives lipid bilayer formation, where ∆G depends on an entropic increase (+∆S) through the relationship ∆G = ∆H - T∆S. The structure of the bilayer shields hydrophobic fatty acid tails from the surrounding polar solvent, causing water molecules to adopt a more disordered and entropically favourable arrangement. This driving force is known as the hydrophobic effect, and is also observed in the folding of globular proteins in an aqueous environment. Despite the ability of other classes of lipid to self-assemble into structures, phospholipids are used preferentially in biological systems. Phospholipids favour the formation of bilayers over micelles to minimise hindrance between the pair of fatty acid moieties, whereas other lipids only contain one hydrophobic tail (Berg et al., 2015). As extensive, sheetlike-structures, lipid bilayers support the formation of larger biological compartments in comparison with size-restricted micelles.
Spontaneous lipid bilayer formation was proven by the black-lipid membrane painting experiment - even before the development of the FMM model - in 1961 by Müeller et al., and evidence from this procedure was later found to be consistent with Singer and Nicolson's propositions. A small aperture was prepared in an inorganic Teflon support and was pre-painted with brain lipid solution to create a hydrophobic edge. They dissolved the same brain lipid sample in an organic methanol solvent painted the solution over the aperture. Formation of a black spot over the aperture was observed over time as the lipid film thinned down to a double layer and destructive interference masked light. Analysis of the resultant, self-arranged bilayer returned a thickness of 40-80 Å. The BLM-painting experiment therefore provides evidence aligning with the FMM model.
Lipid bilayers have unique properties - as described by the FMM model - which make them well-suited to their roles in organisms. They are cooperative structures, held together by many strengthening noncovalent interactions. This gives them a tendency to be extensive and eventually close in on themselves to form cells and organelles without free energy input. Bilayers are also self-sealing due to the energetic unfavourability of exposing gaps to the surrounding polar medium. Solvent molecules are excluded from the core of the membrane, creating a hydrophobic central permeability barrier that restricts the movement of large and polar molecules. An ion passing through the bilayer's core must initially shed its hydration shell in a highly unfavourable process, significantly slowing the passage of charged/polar species between compartments separated by membranes (Berg et al., 2015).
Amphipathic Components Contribute to the Mosaic Structure of Biological Membranes
Another pertinent aspect of the FMM model related to the basic bilayer structure is its 'mosaic'-like arrangement. This refers to the random distribution of proteins, lipids and glycoproteins throughout the membrane bilayer. These molecules share a common amphipathic nature, despite having different potential means of interaction with the membrane, and are directed to the membrane by a specific sorting signal co-translationally. Singer and Nicolson outlined the distinction between two principal classes of membrane protein: integral and peripheral. Globular integral proteins undergo extensive stabilising interactions with the hydrophobic core and tend to span the membrane through one or multiple transmembrane helices. Further to the initial fluid mosaic model, it has been suggested that the lipid bilayer's varying degrees of thickness and curvature are able to allosterically modify the function of integral proteins (Stillwell, 2017). Peripheral proteins are attached to the membrane surface via weaker electrostatic interactions. In rarer cases, peripheral proteins are anchored to the membrane by a covalently-attached hydrophobic chain, such as a cysteine-linked palmitoyl group. Integral proteins are generally attached more strongly, requiring detergent to remove; peripheral proteins can be released without such disruption to the structure of the membrane.
Proteins have a range of functions as components of biological membranes. Integral proteins may establish channels lined with hydrophilic amino acid residues to mediate controlled flow of water and polar solutes. These membrane-spanning proteins are also involved signal transduction. Peripheral proteins act as sites of communication between the cell and its environment through an array of functions: signal particle reception, enzymatic reactions, cell-cell adhesion, stability, and structural support. Glycoproteins are essential for cell identity and antigen recognition. The importance of these proteins as initially outlined by the FMM model has been demonstrated by experimental analysis of the composition of various biological membranes to relate their structure to function. For example, myelin membranes surrounding nerve cell axons are only 18% protein as high lipid content more effectively insulates action potential transmission. Energy-transducing inner mitochondrial and chloroplast membranes are 75% protein, in contrast.
More modern analysis of membrane protein composition has required modification to the initial FMM model. The original approach classified the vast majority of membrane proteins as amphipathic in nature. Contrarily, a new class of protein - membrane-associated proteins (Nicolson, 2023) - surfaced in 1976 research, leading to a revision of the original model. These proteins are only functionally and not structurally integral to the membrane and are therefore not required to be amphipathic. Membrane-associated proteins play a role in controlling the stability and mobility of different regions without participating significantly in hydrophobic or electrostatic interactions with the membrane. Instead, these proteins regulate the membrane by indirect interactions via integral/peripheral protein links. Examples of membrane-associated proteins are broad and varied, and this category encompasses even the cell's cytoskeleton and the extracellular matrix. The importance of these proteins to the structure of the membrane means that the first proposed fluid mosaic model no longer completely and accurately applies to current understanding of membrane structure.
Biological Membranes are Fluid and Allow the Lateral Diffusion of their Components
A particularly interesting interpretation of the FMM model is that biological membranes behave as two-dimensional solutions. This refers to the ability of the protein and lipid components to laterally diffuse throughout the fluid bilayer structure, while being simulatneously restricted in their rotation as a result of their amphipathic quality. A third dimension of lipid movement is permissible, known as transverse diffusion (or 'flip-flopping'), in which lipids switch positions from one membrane leaftlet to the other (Stillwell, 2017). This transition is catalysed by a range of lipid translocase enzymes, and requires transient unfavourable interactions between both the hydrophilic head and the hydrophobic core, and the hydrophobic tail and polar medium. For this reason, the transverse diffusion of proteins has never been experimentally observed. In essence, membrane component movement is restricted to the two lateral dimensions. Importantly, this allows for the preservation of membrane asymmetry: the key property of membranes described by the FMM model that each opposing leaflet has a different composition. Therefore, cell polarity and different microenvironments supporting diverse functions are established either side of a biological membrane. Antiporters are notable examples of why this asymmetry is important, as they must face in a defined direction - random rotation perpendicular to the plane of the bilayer would prevent meaningful concentration gradients from being established. As new membranes are synthesised as part of pre-existing structures, membrane asymmetry is preserved.
Lateral mobility is quantified using the formula S = √(4Dt), where S is the rate of lateral diffusion and D is a diffusion constant. D for lipids is consistent at ~ 1 µm2 s-1, whereas the diffusion constant for proteins is more variable. The G-protein-coupled receptor rhodopsin has a D value around 3 orders of magnitude larger than that of fibronectin, a structural membrane-associated protein part of the extracellular matrix. This aligns with the different roles of the two proteins, and thus their differing mobilities in the membrane, although the relative rigidity of fibronectin to some extent contradicts the initial FMM model stating that proteins are arranged and free to move at random.
Membrane fluidity is also quantified in the FMM model through the specific melting temperature, Tm, referring to the temperature above which the membrane undergoes the sharp transition from a rigid to a fluid arrangement. Tm depends on the properties of the constituent phospholipid fatty acid tails. Fatty acid cis-unsaturation introduces kinks into the hydrophobic tail which push phospholipids further apart. Shorter fatty acid tails experience weaker van der Waals interactions and are held less tightly together. Both of these properties increase membrane fluidity and lower Tm. Cholesterol, an amphipathic steroid lipid, regulates fluidity within a narrow range. At T < Tm, it pushes fatty acid tails apart to maintain fluidity; at T > Tm, cholesterol interacts with the tails to prevent them from drifting apart.
The lateral mobility of membrane components has been elucidated through the technique Fluorescence Recovery After Photobleaching (FRAP) in 1976. Surface membrane components are fluorescently labelled, before a laser beam photobleaches a small membrane window. This prevents fluorescence in this region. The fluorescence of the photobleached section is then recorded as a function of time. It was observed through the FRAP procedure that membrane components drift rapidly and eventually recover the fluorescence in the bleached area. The lateral diffusion constant of the labelled membrane component is calculated using the formula D = W2/4t1/2, where W is the radius of the bleached region and t1/2 is the time taken for fluorescence to recover to 50% of the starting value. This experimental technique provides clear evidence for lateral diffusion in biological membranes, acting as a two-dimensional solution.
When Singer and Nicolson initially proposed the FMM model, it was believed that lipids and proteins were organised entirely homogenously and randomly as a consequence of this lateral mobility. The later discovery of lipid rafts has contradicted this statement (Nicolson, 2023). Lipid rafts are complexes formed from sphingosine-based lipids and are concentrated to dynamic, local regions. It is thought that they contribute to the control of fluidity by resisting phase transitions and are involved in cell signalling. Lipid rafts are more orderly and heterogenous than predicted by the fluid mosaic model, although the model has not been completely overturned by their discovery. Instead, biological membranes can be divided into discrete membrane domains with varying degrees of structure and sorting of lipids and proteins. Lipid rafts are liquid-ordered (L0) domains, while the bulk bilayer is liquid-disordered (LD) and resembles the original FMM model more closely.
Conclusion
The fluid mosaic model remains arguably the most accurate and detailed description of membrane structure, in comparison with earlier, more static theories. The FMM model has been supported by experimental evidence focusing on a range of properties, from the self-assembly of lipids into bilayers to the lateral diffusion of components of the membrane. In the decades following the original proposition of the FMM model by Singer and Nicolson, additional discoveries about membrane structure have revealed new information - such as the existence of lipid rafts - that have altered understanding of key ideas and assumptions made by the model. Even Singer and Nicolson themselves have published revisions to their previous literature. Despite this, the model still stands to describe and explain the structure, properties and functions of biological membranes in most cases, with numerical approaches allowing comparison between different membrane types. Overall, it is not a case of the fluid mosaic model being 'inaccurate'; it is simply a dynamic model that adapts to match the precision of contemporary research techniques and discoveries.
Bibliography
- Berg, J. M., Tymoczko, J. L., Gatto, Jr. G. J., Stryer, L. (2015). Biochemistry (8th ed.). W. H. Freeman & Company
- Nicolson, GL. (2023). The Fluid–Mosaic model of cell membranes: A brief introduction, historical features, some general principles, and its adaptation to current information. Biochimica et Biophysica Acta (BBA) - Biomembranes 1865(4), doi: https://doi.org/10.1016/j.bbamem.2023.184135
- Stillwell, W. (2017). Introduction To Biological Membranes. Elsevier Science.
- Müeller, P., Rudin, D. O., H. Ti Tien, & Wescott, W. C. (1962). Reconstitution of Excitable Cell Membrane Structure in Vitro. Circulation, 26(5), 1167–1171. https://doi.org/10.1161/01.cir.26.5.1167
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