Friday, July 16, 2010

How do we know the structure of cell membranes (part 1)

Why would anyone care about the structure of a cell membrane? For nutrients to enter the cell (and for wastes to leave it) they will have to cross the cell membrane. Knowing about the composition and structure of the cell membrane is the first step in understanding how things can pass through it, and the ability of a cell to regulate what enters and leaves is fundamental to a host of important physiological processes: generation and conduction of nerve signals and urine production are two examples.

We all learned that animal cells are surrounded by a membrane, variously referred to as the plasma membrane, the cell membrane, or just the membrane. Anyone who has taken a class in Anatomy and Physiology (or Cell Biology, or even most General Biology courses) has learned about the structure of cell membranes: what they are made of and how the components are arranged. If you remember anything about cell membranes, it’s probably a few key phrases like “lipid bilayer” or “fluid mosaic model”. In a typical A&P class, these details probably are stated matter-of-factly. But why do these phrases capture the essence of cell membrane structure, and where did they come from? How do we know what we think we know about the structure of cell membranes?

The composition of cell membranes

It now seems obvious that cell membranes are composed primarily of lipids; it was not always so apparent. The idea of a lipoidal cell membrane seems to originate from the research of C.E. Overton in the 1890s (De Weer, 2000). Although the details were never published, Overton apparently deduced that lipids were a primary component of cell membranes from his research on the ability of various substances to pass through cell membranes. He found that membranes were more permeable to substances that were lipid soluble (meaning that they dissolved in lipids). In effect, things passed through the membrane by dissolving through it. By 1935, the lipoidal composition of membranes was entrenched enough for two researchers to assert the fact without attribution or reference. They wrote: “There is now a considerable body of evidence supporting the view that living cells are surrounded by a thin film of lipoidal material.” (Danielli and Davson, 1935). Proteins were also known to be associated with membranes. But how were these lipids and proteins arranged? I'll begin to address that in subsequent posts.

Danielli, J.F. and Davson, H. (1935) “A contribution to the theory of permeability of thin films.”
Journal of Cellular and Comparative Physiology 5: 495-507

De Weer, P. (2000) “A century of thinking about cell membranes.” Annual Review of Physiology
62: 919-926

There's also a nice teaching module designed to lead students to the recognition that membranes must be primarily composed of lipids by showing them data from R. Collander (1937) Trans. Faraday Soc. 33:985-990. The data relate the solubility of a solute in oil to how fast it moves into an algal cell:

Deducing the Biochemical Composition of the Cell Membrane - Reasoning from Quantitative Evidence (by Laura Martin) - Created October 15, 2007; Connexions web site. Version 1.2

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