Sandwich (Davson–Danielli) model of cell membrane

The cell membrane is a thin layer that separates the interior of a cell from its surroundings. It regulates the movement of substances in and out of the cell and maintains its shape and integrity. The cell membrane is also involved in various cellular processes such as signaling, adhesion, and recognition.

But what is the structure of the cell membrane? How is it composed of different molecules and how are they arranged? These questions have intrigued scientists for a long time and have led to the development of various models to explain the cell membrane structure.

One of the earliest and most influential models was proposed by Hugh Davson and James Danielli in 1935. They based their model on the observation that the cell membrane was electrically polarized, meaning that it had different charges on its two sides. They also considered the chemical composition of the cell membrane, which was known to contain lipids and proteins.

The Davson–Danielli model suggested that the cell membrane consisted of a lipid bilayer sandwiched between two layers of proteins. The lipid bilayer was composed of phospholipids, which are molecules that have a hydrophilic (water-loving) head and a hydrophobic (water-hating) tail. The hydrophobic tails faced each other in the middle of the bilayer, while the hydrophilic heads faced the aqueous environment on both sides. The protein layers were made of globular proteins that coated the lipid bilayer and provided stability and functionality to the membrane.

The Davson–Danielli model explained some of the properties and functions of the cell membrane, such as its selective permeability, electrical polarization, and thickness. It also received support from electron microscopy images that showed the cell membrane as two dark lines with a lighter region in between. However, it also faced some problems and limitations that eventually led to its rejection and replacement by a more accurate model.

In this article, we will explore the key features, support, problems, and falsification evidence of the Davson–Danielli model. We will also introduce the fluid-mosaic model that replaced it as the current accepted model of cell membrane structure.

The Davson–Danielli model, also known as the sandwich model, was a proposed structure for the cell membrane in which a lipid bilayer was coated on both sides with protein layers. The lipid bilayer consisted of phospholipids, which have hydrophilic heads and hydrophobic tails. The hydrophilic heads faced the aqueous environment inside and outside the cell, while the hydrophobic tails were hidden in the interior of the membrane. The protein layers were made of globular proteins that were hydrated and associated with the polar ends of the phospholipids by electrostatic interactions. The proteins did not penetrate the lipid bilayer, but formed a continuous and uniform layer on each surface of the membrane.

The Davson–Danielli model had several implications for the function and properties of the cell membrane. First, it suggested that the membrane was selectively permeable, meaning that it could regulate the passage of different molecules and ions based on their size and solubility. Second, it predicted that the membrane had a fixed thickness of about 8 nm, which was consistent with some electron microscopy observations at the time. Third, it implied that the membrane was symmetrical and had identical internal and external surfaces. Fourth, it did not account for any variation or diversity in the composition and distribution of membrane proteins.

The Davson–Danielli model was widely accepted for several decades until new evidence emerged that challenged its validity and led to the development of a more accurate and dynamic model of the cell membrane: the fluid-mosaic model.

One of the main sources of evidence for the Davson–Danielli model came from electron microscopy, a technique that uses beams of electrons to create images of very small structures. Electron microscopy revealed that membranes appeared as two dark parallel lines with a lighter colored region in between. This was consistent with the idea that membranes were composed of a lipid bilayer sandwiched between two protein layers. The proteins appeared dark in electron micrographs because they scattered more electrons than the phospholipids, which appeared light. The total thickness of the membranes measured by electron microscopy was also in agreement with the predicted thickness of about 8 nm.

Electron microscopy also showed that different types of membranes had different amounts and patterns of proteins on their surfaces. This suggested that the protein layers were not uniform and could vary according to the function and location of the membrane. For example, some membranes had more proteins on their inner surface than on their outer surface, indicating that the membrane was bifacial (having two different faces). Some membranes also had proteins that formed pores or channels across the lipid bilayer, allowing certain substances to pass through.

The Davson–Danielli model was able to explain these observations by proposing that the protein layers were not continuous and could have gaps or holes where the lipid bilayer was exposed. The model also suggested that some proteins could span the lipid bilayer and act as carriers or receptors for specific molecules. These proteins were called intrinsic or integral proteins, while those that only coated one surface of the membrane were called extrinsic or peripheral proteins.

The Davson–Danielli model was widely accepted for several decades as a plausible explanation for the structure and function of biological membranes. However, as new techniques and evidence emerged, some problems and limitations of the model became apparent.

Although the Davson–Danielli model was widely accepted for several decades, it faced some problems and limitations that could not be explained by its assumptions. Some of these problems were:

• The model assumed that all membranes had a uniform thickness and a constant lipid-protein ratio. However, different types of cells and organelles have different membrane compositions and structures, reflecting their specific functions and environments. For example, the inner mitochondrial membrane has a high protein content and many folds, while the plasma membrane of red blood cells has a low protein content and a smooth surface. The Davson–Danielli model could not account for this diversity and variability of membranes.
• The model assumed that all membranes had symmetrical internal and external surfaces. However, biochemical and biophysical studies showed that membranes are bifacial, meaning that they have distinct inner and outer leaflets with different properties and compositions. For example, the outer leaflet of the plasma membrane has more glycolipids and glycoproteins than the inner leaflet, which are involved in cell recognition and signaling. The Davson–Danielli model could not explain how such asymmetry was maintained and regulated.
• The model did not account for the permeability of certain substances. The Davson–Danielli model suggested that the lipid bilayer was impermeable to most molecules, except for those that were small and lipid-soluble. However, experiments showed that many substances that were large, polar, or charged could also cross the membrane, such as glucose, amino acids, ions, and water. The Davson–Danielli model did not recognize the need for hydrophilic pores or channels in the membrane that could facilitate the transport of these substances.
• The temperatures at which membranes solidified did not correlate with those expected under the proposed model. The Davson–Danielli model predicted that membranes would solidify at temperatures below the melting point of their lipids, which would depend on the degree of saturation of their fatty acid chains. However, studies showed that membranes remained fluid at much lower temperatures than expected, indicating that other factors besides lipid composition influenced their fluidity. The Davson–Danielli model could not account for these factors or their effects on membrane function.

These problems and limitations suggested that the Davson–Danielli model was oversimplified and inaccurate, and needed to be revised or replaced by a more realistic and comprehensive model of membrane structure.

The Davson–Danielli model was eventually falsified by several lines of evidence that contradicted its assumptions and predictions. Some of the most important evidence came from the following experiments:

• Membrane protein solubility and diversity: The Davson–Danielli model assumed that the proteins coating the lipid bilayer were globular and soluble in water, and that they formed a uniform and continuous layer. However, biochemical studies showed that many membrane proteins were insoluble in water and had hydrophobic regions that interacted with the lipid tails. Moreover, membrane proteins varied in size, shape and function, and could not form a regular layer on the membrane surface.
• Membrane protein mobility: The Davson–Danielli model implied that the proteins were fixed in place on the membrane surface. However, fluorescent antibody tagging of membrane proteins showed that they were mobile and could move within the membrane. For example, when two cells with different membrane proteins were tagged with red and green fluorescent markers respectively, and then fused together, the markers became mixed throughout the membrane of the fused cell. This demonstrated that the membrane proteins could diffuse laterally and did not form a static layer.
• Freeze-fracture microscopy: The Davson–Danielli model predicted that the membrane would have a smooth and symmetrical structure, with identical protein layers on both sides of the lipid bilayer. However, freeze-fracture microscopy revealed that the membrane had an irregular and asymmetrical structure, with different types of proteins embedded within the lipid bilayer. Freeze-fracture microscopy involved freezing the membrane and then splitting it open along the middle of the lipid bilayer. This exposed the inner surfaces of both halves of the membrane, which showed rough patches corresponding to transmembrane proteins. These proteins spanned across the lipid bilayer and disrupted its continuity.

These experiments provided strong evidence that the Davson–Danielli model was incorrect and needed to be replaced by a more accurate and realistic model of membrane structure.

The fluid-mosaic model is the current accepted model of cell membrane structure. It was proposed by Seymour Singer and Garth Nicolson in 1972, based on the evidence that falsified the Davson–Danielli model. The fluid-mosaic model describes the cell membrane as a dynamic and heterogeneous structure, composed of a phospholipid bilayer with embedded proteins and other molecules.

The main features of the fluid-mosaic model are:

• The phospholipid bilayer forms the basic framework of the membrane. It is fluid, meaning that the phospholipids can move laterally within their own layer. The fluidity of the membrane depends on the temperature and the composition of the phospholipids (e.g., degree of saturation, presence of cholesterol).
• The proteins are not confined to the outer surface of the membrane, but can be integral or peripheral. Integral proteins span the entire thickness of the membrane and have hydrophobic and hydrophilic regions. Peripheral proteins are attached to either surface of the membrane and have only hydrophilic regions. Some proteins can move within the membrane, while others are anchored to the cytoskeleton or extracellular matrix.
• The proteins have various functions, such as transport, enzymatic activity, signal transduction, cell recognition, and cell adhesion. Some proteins form channels or carriers that allow specific substances to cross the membrane. Others act as receptors that bind to specific molecules and trigger cellular responses. Some proteins serve as markers that identify the cell type or its antigens. Some proteins mediate the interactions between cells or between cells and their environment.
• The membrane also contains other molecules, such as carbohydrates, glycoproteins, glycolipids, and sterols. Carbohydrates are attached to some proteins or lipids and form a glycocalyx that protects the cell and facilitates cell recognition. Glycoproteins and glycolipids are involved in cell signaling and adhesion. Sterols, such as cholesterol, modulate the fluidity and stability of the membrane.

The fluid-mosaic model accounts for the diversity and complexity of cell membranes. It also explains how membranes can adapt to different conditions and perform various functions. The fluid-mosaic model is not a static picture, but a dynamic and flexible representation of the cell membrane structure.

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