Davson-Danielli Model & Singer-Nicoloson Fluid Mosaic Model Video Summary Life occurs in an aqueous environment. Proteins are essential to life because they help maintain solute homeostasis. The phospholipid bi-layer, which is found in cell membranes, is one way a phospholipid arranges itself. The phosphate head, which is hydrophilic, moves toward the water. The hydrocarbon tail is hydrophobic. The Davson-Danielli model is an idea proposed on how cells regulate their environment. The proteins are on the outside of the bi-layer membrane. However, membranes need to allow for solute particles to pass back and forth. When proteins are on the outside, it blocks the solutes from passing through, preventing homeostasis. Therefore, we have to reject this model. In the Singer-Nicholson Fluid Mosaic Model, the integral proteins are embedded in the bi-layer. They have the ability to open up and let particles move back and forth. This model differs from the Davson-Danielli Model because it recognizes that biological systems are open systems that interact with their environments. This is a much more dynamic entity. This model allows for homeostasis, or a balance. The embedded proteins act as gatekeepers. They transport with the concentration gradient. Passive transport and facilitated transport move from a region of high solute concentration to a region of low solute concentration. Neither of these processes require energy because they are working with the concentration gradient. The

In 1925, the original basic model for what a cell membrane consisted of was conceived by Gorter and Grendel. This model consisted of just the phospholipid bilayer and had yet to conceive of protein as a factor in the cell membrane. In 1935, Hugh Davson and James Danielli proposed that there were flanking proteins in the membrane. This was used in an attempt to explain the surface tension that was observed by Danielli in an experiment. The served as the model scientists believed to be correct, until 1972 when Singer and Nicolson proposed the fluid mosaic model. The new model stated that the surface tension could be explained by the phospholipid head groups and the proteins correct place was actually imbedded in the bilayer. These discoveries were far more supported by following experiments, conclusively proving the Singer Nicolson model was the true make up of the cell membrane. –David Litwin, Sophie Toback, Dan Koch, Tim Anastas

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