What evidence from electron microscopy led to the proposal of the Davson-Danielli model?

SouravNovember 7, 2024

Answer

The proposal of the Davson-Danielli model of membrane structure was significantly influenced by evidence obtained from electron microscopy. Here’s how electron microscopy contributed to the development of this model:

Key Evidence from Electron Microscopy

1. Trilamellar Appearance:
   • When biological membranes were examined under a transmission electron microscope, they exhibited a characteristic trilamellar structure. This appearance consists of two dark outer layers and a lighter inner region, which suggested a specific arrangement of membrane components. The dark layers were interpreted as protein layers, while the lighter central layer was attributed to the phospholipid bilayer. This trilamellar view supported the idea that membranes consist of a lipid bilayer sandwiched between protein layers, aligning with the Davson-Danielli concept of a “lipid-protein sandwich” .
2. Interpretation of Electron Density:
   • In electron micrographs, proteins appear darker due to their higher electron density compared to lipids. The observation that membranes showed these dark bands led researchers to propose that proteins coat both sides of the phospholipid bilayer. This interpretation was crucial for supporting the idea that proteins were integral to membrane structure and function, as suggested by Davson and Danielli in their 1935 model .
3. Confirmation of Membrane Composition:
   • The findings from electron microscopy provided direct visual evidence that membranes were not merely lipid bilayers but included significant protein components. This was a departure from earlier models that did not adequately account for the role of proteins in membranes.
4. Influence of Subsequent Studies:
   • The work of J.D. Robertson in the 1950s further reinforced the Davson-Danielli model through his observations using electron microscopy, where he noted that all cellular membranes shared this trilamellar structure. Robertson proposed his own Unit Membrane Model, which aligned closely with the Davson-Danielli model by suggesting that all membranes consist of a phospholipid bilayer coated with proteins on both sides .