In the study of transmitter mechanisms in the peripheral nervous system and the spinal cord, intracellular recording has been a great advantage. In contrast, studies on central synapses have often used more indirect techniques, often extracellular recording from a cell or a small group of cells. In particular, a large amount of information has been acquired by iontophoretic application of drugs, often with a multibarrel electrode system where the iontophoretic electrodes are coupled to a recording electrode. Because of the difficulties with proper intracellular recording from intact brain preparations, there are relatively few intracellular studies on central transmitter effects. This situation compares unfavourably with the many studies on spinal cord, and, in particular, on peripheral nervous tissue.

‘Tunnelling’ is the metaphorical name given to the process, possible in quantum mechanics, but not in classical mechanics, whereby a particle can disappear from one side of a potential-energy barrier and appear on the other side without having enough kinetic energy to mount the barrier. One can think of this as a manifestation of the wave-nature of particles. The wavelength is larger if a particle is lighter. In particular electrons, being very light compared to atoms, have wavelengths as large or larger than atoms at energies found in the valence shells of molecules. Thus, they easily ooze through and around atoms and molecules. We are also concerned with the tunnelling of heavy particles: nuclei, atoms, molecules.

Membranes are the most common cellular structures in both plants and animals. They are now recognized as being involved in almost all aspects of cellular activity ranging from motility and food entrapment in simple unicellular organisms, to energy transduction, immunorecognition, nerve conduction and biosynthesis in plants and higher organisms. This functional diversity is reflected in the wide variety of lipids and particularly of proteins that compose different membranes. An understanding of the physical principles that govern the molecular organization of membranes is essential for an understanding of their physiological roles since structure and function are much more interdependent in membranes than in, say, simple chemical reactions in solution. We must recognize, however, that the word ‘understanding’ means different things in different disciplines, and nowhere is this more apparent than in this multidisciplinary area where biology, chemistry and physics meet.

The scope of this article is to describe a transmission X-ray microscope, possible biological applications of soft X-ray microscopy and preliminary results.

For soft X-ray microscopy of biological specimens the wavelength range of 1–10 nm is best suited. Microscopy in this wavelength range requires intense X-ray sources as well as high-resolution X-ray lenses. Intense X-radiation is provided by the synchrotron radiation of electron and positron storage rings. Suited X-ray lenses are zone plates.

A theoretical treatment of the contrast mechanism and the radiation damage as well as first experiments yield the following results. Firstly, relatively thick (1–10 μm) biological specimens can be investigated. This means that unsectioned dried and even wet cells and cell organelles in a natural state can be examined. Second, it will be possible to resolve cellular aggregates in live cells with a resolution in the region of ≤10nm.

Protein molecules in solution or in protein crystals are characterized by rather well-defined structures in which α-helical regions, β-pleated sheets, etc., are the key features. Likewise, the double helix of nucleic acids has almost become the trademark of molecular biology as such. By contrast, the structural analysis of lipids has progressed at a relatively slow pace. The early X-ray diffraction studies by V. Luzzati and others firmly established the fact that the lipids in biological membranes are predominantly organized in bilayer structures (Luzzati, 1968). V. Luzzati was also the first to emphasize the liquid-like conformation of the hydrocarbon chains, similar to that of a liquid paraffin, yet with the average orientation of the chains perpendicular to the lipid–water interface. This liquid–crystalline bilayer is generally observed in lipid–water systems at sufficiently high temperature and water content, as well as in intact biological membranes under physiological conditions (Luzzati & Husson, 1962; Luzzati, 1968; Tardieu, Luzzati & Reman, 1973; Engelman, 1971; Shipley, 1973). In combination with thermodynamic and other spectroscopic observations these investigations culminated in the formulation of the fluid mosaic model of biological membranes (cf. Singer, 1971). However, within the limits of this model the exact nature of lipid conformation and dynamics was immaterial, the lipids were simply pictured as circles with two squiggly lines representing the polar head group and the fatty acyl chains, respectively. No attempt was made to incorporate the well-established chemical structure into this picture. Similarly, membrane proteins were visualized as smooth rotational ellipsoids disregarding the possibility that protruding amino acid side-chains and irregularities of the backbone folding may create a rather rugged protein surface.

Protein chains are usually folded into one or several discrete globular units called domains (Schulz & Schirmer, 1979). Levitt & Chothia (1976) have shown that the structures of such domains frequently fall into one of the following three classes; α-proteins which are mainly α-helical, β-proteins which contain antiparallel β-strands and α/β proteins which have a central core consisting of a sheet of strands, most of which are parallel. The connexions between the parallel strands in αβ-proteins frequently contain helices which are packed on both sides of the sheet in a regular way (Chothia, Levitt & Richardson, 1977).

For the cells of an organism to act in the coordinated fashion necessary for complex functioning, they must be able to receive and transmit information. Information transfer is mediated by molecules released by the cells and may be local, as in the case of neurotransmitters, or long range, as in the case of hormones. It is apparent, however, that irrespective of the range of interaction, a cell must be able to distinguish, with a high degree of precision, the signals relevant to it from an enormous flow of background noise.

Molecular recognition at the cell surface is mediated by receptors: cell surface glycoproteins that usually form an integral part of the plasma membrane (see, for example, Cuatrecasas & Greaves, 1978). Typically, receptors bind the ligands they are designed to recognize with affinities of the order of 108 M-1, and they translate that interaction into a sequence of signals that ultimately lead to biological activity.

A bacterium incorporates amino acids into protein with an error frequency close to one in three thousand (Edelman & Gallant, 1977). Nevertheless, the structural differences between related amino acids are so small that it is difficult to see how they can be distinguished from each other with such accuracy (see, for example, Pauling, 1958).

Indeed, the selection of amino acids during protein synthesis is carried out twice: first by the aminoacyl-tRNA synthetases and then by the codon-programmed ribosome. Each of these substrate selections is in fact a double selection. In the case of the synthetase both a particular amino acid and a corresponding cognate tRNA must be chosen to form the aminoacyl-tRNA. On the ribosome, the aminoacyl-tRNA must be matched with a cognate codon, and then the mRNA must be advanced by exactly one codon length to position the next codon in the appropriate ribosome site so that it too can be translated.

Biological membranes are composed of mainly lipids and proteins. The physical properties of the lipids, forming a bilayer structure, are of crucial importance for the living cell, since the plasma membrane is the guardian barrier towards the environment. Thus, the functioning cell needs a highly stable lipid bilayer, which depends on molecular packing and orientation properties of the various membrane components (Wieslander et al. 1980). The spatial arrangement of the membrane proteins incorporated in the lipid matrix plays an essential role for the different chemical processes occurring at or within the membrane. Information about molecular orientation and mobility is therefore necessary for unravelling the functional mechanisms of a biological membrane.

(A) Evolutionary similarities of protein structures Two decades have passed from the time that the three dimensional structure of the first globular protein, sperm whale myoglobin, was decoded (Kendrew et al. 1960). Its structure, which now looks so simple and habitual, then seemed to be unusually complicated. The decoding of the subsequent proteins, lysozyme (Blake et al. 1965), ribonuclease (Kartha, Bello & Harker, 1967), chymotrypsin (Matthews et al. 1967), carboxypeptidase (Lipscomb et al. 1969) redoubled the feeling of amazement and even of some confusion before the extremely complicated, intricate and, above all, absolutely unlike protein structures. Some consolation against this background was the evident and far-reaching similarity between the three-dimensional structures of myoglobin and hemoglobin subunits (Perutz, Kendrew & Watson, 1965) and an analogous similarity between the structures of chymotrypsin and other serine proteases, elastase (Shotton & Watson, 1970) and trypsin (Stroud, Kay & Dickerson, 1972). However this similarity was easily explained by the far-reaching homology between the primary structures of myoglobin and hemoglobin and between the primary structures of serine proteases.

The discovery of nucleosomes as a basic repeating unit of the chromatin structure organizing the major part of eukaryotic DNA greatly catalyzes the expansion of our knowledge on chromatin. Several lines of experimental evidence have led to the formulation of the nucleosome conception: the observation of chains of globular particles in electron micrographs of chromatin (Olins & Olins, 1974); the demonstration that DNA is released as a set of discrete sizes upon digestion of chromatin with endogenous nucleases (Hewish & Burgoyne, 1973); the isolation of discrete nucleoprotein particles upon digestion of chromatin with micrococcal nuclease (Rill & Van Holde, 1973).