0.6 The fluid mosaic model - determining the matrix

Singer and Nicolson (1972) described the plasma membrane as a mosaic of proteins and phospholipids in a fluid phospholipid matrix . Proteins, however, form a substantial component of the membrane as Singer and Nicolson knew so what evidence suggested the matrix was lipid rather than protein in structure?

As is true of all good science, these two alternative models of the membrane matrix generated different testable predictions (hypotheses). Previous work had shown that the lipids of membranes behaved as a fluid under physiological conditions (Singer and Nicolson, 1972). Thus, if integral proteins were embedded, unanchored in a fluid matrix, then they would diffuse laterally through membrane surfaces much as dye particles diffuse through water. As a result, proteins would be randomly distributed on the surface of a membrane and, if monitored, would redistribute over time. That is, the distribution of proteins would be random and dynamic in time (Singer and Nicolson, 1972).

In contrast, if proteins formed the matrix in which lipids were dispersed, noncovalent bonds between proteins would inhibit diffusive protein movement and make the membrane relatively ridged. Consequently, proteins would likely be distributed in a regular, not random, pattern across the surface of the membrane and this distribution would be static (unchanging) not dynamic in time (Singer and Nicolson, 1972).

Prior to the 1972 Science publication, Singer's lab had developed a technique to test these predictions focusing, in particular, on the spatial distribution of proteins within the cell membrane at a single moment in time. Which model of the membrane matrix did their results support? Answer the questions below to find out!

1. Imagine you could see the proteins on the surface of a cell membrane sufficiently well to determine how they were distributed (spread). For each model above, sketch the predicted distribution of proteins on the surface of a cell membrane. That is, draw a cell membrane and add proteins to it in a pattern you might expect to see according to each hypothesis. Clearly label your diagrams.

Now let's turn your descriptive or qualitative predictions into measurement based or quantitative predictions.

2. Carefully compare your two drawings illustrating the expected distribution of proteins based on each model of the membrane matrix. What could you measure to enable you to determine numerically whether proteins are randomly or regularly spread across a cell's surface? Why? Please explain.

A reasonable variable to measure would be the distance between any two adjacent proteins. Take a look at your sketches again. On each roughly measure the distances between 50 pairs of adjacent proteins. Measure only uninterrupted distances; those for which the measurement path does not cross another protein. Record the distances in two parallel columns of numbers, one for each sketch corresponding to each model of membrane matrix structure.

3. Compare the two columns of measurements. Do they differ in anyway? If so, how? Please describe the nature of their differences.