According to the Davson-Danielli model, what structures were thought to be adjacent to the phospholipid bilayer?

The Davson-Danielli model proposes that the structure of the plasma membrane consists of a phospholipid bilayer sandwiched between layers of proteins. According to this model, the hydrophilic heads of the phospholipids align with the outer layers of protein, creating distinct outer and inner surfaces of the membrane.

The notion that there are layers of proteins on both sides of the phospholipid bilayer suggests that these proteins serve various roles, such as acting as structural components, mediating transport, and serving as receptors. This idea of a membrane's structure was foundational in the early understanding of cell membranes and highlights the importance of proteins in membrane functionality.

In the context of the other options, while integral proteins do play a critical role in membrane structure, they do not represent the complete picture according to this model. Carbohydrates are related to membrane structure but are generally associated with proteins or lipids rather than forming layers by themselves. Peripheral proteins are indeed relevant but not sufficient to encompass the idea of layering as described in the Davson-Danielli model. Hence, the correct understanding of the model emphasizes the presence of protein layers surrounding the phospholipid bilayer.