Sistema Endomenbranal

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Membranes—An Introduction
In the second half of the 19th century it became evident that an osmotic barrier separates the inside and the outside of cells (Nägeli and Cramer, 1855; de Vries, 1871, 1884; Pfeffer, 1877). Plant cell protoplasts were permeable to water but not to larger macromolecules like sucrose (de Vries, 1871). Pfeffer was the first to study the osmotic pressure withincells and formulated the idea that the protoplasm of cells is surrounded by a thin layer, which he called the plasma membrane. In fact, Pfeffer proposed that this membrane does not only cover the outer surface of cells but also separates all aqueous environments of different composition from each other. One may therefore consider Pfeffer as the father of membrane theory. The developments in biologyand botany coincided with a rapid development in the theory of thermodynamics of solutions. In particular, based on Pfeffer’s work van’t Hoff found the formal analogy of concentrations of solutes in water and the partial pressures of ideal gases (van’t Hoff, 1887). Ostwald formulated descriptions for the osmotic pressure across semipermeable walls and the related electrical properties (Ostwald,1887, 1890).1

1.1 Overton (1895)

Charles Ernest Overton is a very important figure in the development of a picture of cell membranes. He investigated the osmotic properties of cells and noticed in the late 19th century that the permeation of molecules through membranes is related to their partition coefficient between water and oil (Overton, 1895). Overton’s findings led to the hypothesis thatthe thin membranes surrounding cells have the properties of oil. In his book on anesthesia (Overton, 1901. Jena, Germany. English translation: Studies of Narcosis, Chapman and Hall, 1991, R. Lipnick, Ed., 1991) he called the layers surrounding cells “lipoids” made from lipids and cholesterol. The properties of lipids are described in detail in Chapter 3 and theory of anesthesia is treated in Chapter19.
1) The history of biomembrane research is nicely reviewed in Ling (2001).

Thermal Biophysics of Membranes. Thomas Heimburg Copyright © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-40471-1

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1 Membranes—An Introduction

1.2 Langmuir (1917) and Gorter and Grendel (1925)

Langmuir (1917) developed an apparatus in which molecular layers of lipids were spread atthe air–water interface. With this monolayer trough (see Section 6.7 and Fig. 6.14) the lateral pressure of the monolayer films could be measured. Langmuir proposed that in the molecular film the polar head groups were directed toward the water whereas the hydrophobic hydrocarbons are pointed toward the air phase. Gorter and Grendel (1925) experimentally investigated the surface area of lipids. Forthis purpose they extracted the lipids from red blood cells of man, dog, rabbit, sheep, guinea pig, and goat in acetone. The lipids were spread on a water surface and the area was measured using a Langmuir film balance. From the same blood preparations they measured the surface area of the red blood cells from the microscopic images. They found that the surface area of the monofilms was within errorexactly two times that of the cells. They concluded that cell membranes are made of two opposing thin molecular layers, and they proposed that this double layer is constructed such that two lipid layers form a bilayer with the polar head groups pointing toward the aqueous environment (Fig. 1.1). This is the picture of the lipid membrane we know today. As Robertson (1959) noted later, theattractive simplicity of Gorter’s and Grendel’s pictures is also its greatest weakness since it fails to account for the manifold of functions attributed to cell membranes.

Fig. 1.1 The cell membrane according to Gorter and Grendel (1925). They proposed the lipid bilayer structure.

1.3 Danielli and Davson (1935)

The earliest molecular model for the biomembrane structure including proteins was...