Membrane Danielli-Davson model

Although numerous models for the structure of membranes have been proposed, the structural features which are generally accepted at present are rather similar to the original Danielli-Davson model. There is convincing evidence that the structure is dominated by lipid bilayers. The state of order of the hydrocarbon chains is now being studied extensively by many groups (see below). Less is known about the proteins. Besides the proteins that are located on the outside according to the Danielli-Davson model, there are also proteins that are partly buried in the hydro-phobic interior of the lipid layer however, little is known about the lipid-protein interaction. 

Furthermore, monolayer measurements demonstrate the variation of the dipole moment of peripheral proteins under stretching or compression. According to us, this implies the possibility of bimorph flexoelectricity of peripheral proteins (an analogue of the piezoelectricity of a bimorph plate), especially if these are symmetrically adsorbed over the two membrane interfaces, as suggested in the Danielli-Davson model. ... 

The Danielli-Davson model has been modified by Wallach and Zahler (1966) who demonstrated by optical rotatory dispersion and circular dichroism techniques that a substantial amount of membrane protein exists in a helical conformation as opposed to random coil It is envisioned that hydrophilic portions of membrane protein are located on both sides of the membrane and are connected by hydrophobic microtubules. Lenard and Singer (1966) also advanced similar views and suggested that subunits could result from this model which, when assembled in two dimensions, could form a membrane. 

On the basis of electron microscopic studies, Robertson (1967) has modified the Danielli-Davson model, presenting a new model—the unit membrane hypothesis—for all cell membranes. This model was similar to that of Danielli-Davson but with a few significant changes. The number of bimolecular phospholipid layers in Robertson s model was restricted to one, and the membrane was considered to be asymmetrical, with mucopolysaccharide or mucoprotein on the outside and unconjugated protein on the inner part of the membrane (Fig. 2). The... 

Our knowledge of biological membrane ultrastructure has increased considerably over the years as a result of rapid advances in instrumentation. Although there is still controversy over the most correct biological membrane model, the concept of membrane structure presented by Davson and Danielli of a lipid bilayer is perhaps the one best accepted [12,13]. The most current version of that basic model, illustrated in Fig. 7, is referred to as the fluid mosaic model of membrane structure. This model is consistent with what we have learned about the existence of specific ion channels and receptors within and along surface membranes. 

Figure 7.2 Danielli-Davson membrane model. A layer of protein was thought to sandwich a lipid bilayer.

Figure 1 The Danielli and Davson model for cell membrane Until the late 1960 s bilayer models were often perceived to be static structures but studies with a series of biophysical techniques involving X-ray diffraction, nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopy, fluorescence microscopy, electron microscopy and others have revealed the dynamic nature of the cell membranes which have been summarized in the Singer and Nicolson s fluid mosaic model proposed in 1972 (see Figure 2).

Cell Membranes The sequence of models of membrane structure Danielli-Davson Robertson S inger-N icholson The fluid mosaic model of cell membrane structure The interplay between data and models in the development of models of cell structure... 

The time line is provided in the published resourees. It shows that evidence about the movement of lipids within cell membranes did not appear until after the development of the Danielli-Davson and Robertson models. Claire also refers back to the fluid mosaic model at two further points in the lesson during whole class discussions. 

Of the various membrane models which have been proposed, current attention is focused on the mosaic model of Glaser et al. (1970) and Lenard and Singer (1966), which combines the subunit (functional and structural model (Green and Perdue, 1966) with the Danielli-Davson (1934) lipid bilayer-protein model. On the one hand, it provides for consecutive membrane-enzyme interactions and on the other, for the fact that altertions in either membrane lipid or protein can be produced without affecting the other. 

The first membrane model to be widely accepted was that proposed by Danielli and Davson in 1935 [528]. On the basis of the observation that proteins could be adsorbed to oil droplets obtained from mackerel eggs and other research, the two scientists at University College in London proposed the sandwich of lipids model (Fig. 7.2), where a bilayer is covered on both sides by a layer of protein. The model underwent revisions over the years, as more was learned from electron microscopic and X-ray diffraction studies. It was eventually replaced in the 1970s by the current model of the membrane, known as the fluid mosaic model, proposed by Singer and Nicolson [529,530]. In the new model (Fig. 7.3), the lipid bilayer was retained, but the proteins were proposed to be globular and to freely float within the lipid bilayer, some spanning the entire bilayer. 

Another microscopic technique is to freeze the specimen and then fracture it with a knife. A knife cutting through the frozen specimen splits the membrane down the middle, exposing the inside of the bilayer (fig. 17.13a). If the Davson-Danielli model for membrane structure were correct, the two exposed surfaces would be featureless. However, electron micrographs of metallic casts of such samples reveal surfaces studded with particles of various sizes (fig. 17.13(f)- Additional studies indicate that these particles are proteins that are deeply embedded in the membrane. The particles seen on the inner and outer leaflets of the bilayer usually differ in size and distribution because of an asymmetrical disposition of the proteins across the bilayer. 

During the 1960s, various alternatives to the Davson-Danielli model were proposed. Some investigators abandoned the idea of a phospholipid bilayer and suggested instead that membranes consist of aggregates of lipid-protein complexes. However, in 1972, Jon Singer and Garth Nicol-... 

Why does the Davson-Danielli membrane model predict that the exposed inside of the lipid bilayer is featureless ... 

Sodium and potassium ions are vital to the normal functioning of the nerve cell. The ions are separated by the cell membrane with sodium on the outside and potassium on the inside of the resting cell. A model of the basic membrane, which in principle was built as a bilayer according to the well known Davson-Danielli-Robertson scheme but which... 

Protein first came into the model as a result of considerations of interfacial tension. Danielli and Davson (1934/5) pointed out that measured interfacial tension at lipid-water interfaces was much higher than that evident at cell surfaces. Therefore they put forward a new model in which the lipid bilayer was encased in a sandwich of protein, thereby avoiding this problem and accommodating the protein known to be present in membranes. Thirty years later, the basic premise upon which protein was added to the model was shown to be incorrect (Haydon and Taylor, 1963) phospholipids have interfacial... 

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