A fair beginning has been made in understanding the Mössbauer spectra of many types of haem proteins. The diamagnetic compounds have all the usual difficulties associated with the calculation of any quadrupole spectra, with the added complication of larger molecules and much less crystallographic data. In the ferric paramagnets the large molecules are quite helpful in that they prevent interaction between neighbouring sites and make possible long electron spin-relaxation times. The resulting magnetic Mössbauer spectra contain a large amount of information and, by their complexity, tend to be a guard against ambiguity in interpretation. The theoretical problem of the magnetic hyperfine interaction appears more complex than the quadrupole interaction, but this apparent complexity is only superficial in the sense that it requires more complex mathematical manipulations. The underlying physical problem is much simpler in the magnetic case because it involves interactions only with the relatively few unpaired electrons, and is insensitive to all others and has no lattice contribution. Fortunately the unpaired electrons are those which lie high in energy, and are most likely to be involved in the chemical activity of the enzyme. Most of the results on ferric haems have been fairly satisfactory, and it seems likely that many of the uncertainties will be cleared up by further work. From the physicist's point of view it appears reasonable to expect that the search for better agreement between theory and paramagnetic spectra may be a route to a better understanding of the factors affecting the quadrupole interaction. The integral spin paramagnets are an intermediate case, usually requiring large applied field to reveal their magnetic properties. No high-spin ferrous haems have yet been subjected to detailed magnetic experimental or theoretical investigation, but this should be a fruitful field.

In all life sciences there can be noted a marked trend toward an increased use of physical instrumentation. Calorimeters, which are instruments for measurements of heat quantities or heat effects, offer one approach in this respect.

It has been known since the time of Schultze (1866) that in the vertebrate retina there are two types of photoreceptors, rods and cones, and that they serve different visual functions; rods for scotopic vision, and cones for photopic. The terminology originates from the shape of the outer segments in which the photosensitive pigment molecules are contained. The cone outer segments are conic and taper towards the tips, while the rod outer segments are typically cylindrical. Fig. 1 is a schematic diagram from Brown, Gibbons & Wald (1963) of the ultrastructure of the rod and cone outer segments of the mudpuppy, Necturus, as studied by electron microscopy. Both appear to be made up of a pile of transverse paired membranes. In cones these arise by infolding of the plasma membrane, and in rods they have probably arisen in a similar way, but each pair of membranes is sealed around the edge so as to form a closed double-membrane disc (Sjöstrand, 1961). Because of the universal lamellation within the rod and cone outer segments, it looks as if there were no appreciable intracellular space, but yet Toyoda, Nosaki & Tomita (1969), and Toyoda et al. (1970) were successful in intracellular recording from the outer segments of single rods of the nocturnal gecko and frog.

In a recent review of the transport of salts and water across multicellular secretory tissues in animals (Keynes, 1969), a summary was given of the various types of active transport of ions necessary to explain the experimental observations in a very wide range of tissues, and five basic types of ion pump were discussed. The question of whether plants and animals have any common mechanisms for the transport of salts and water was specifically excluded. The original aim of the present review was to survey the types of ion pump found in plant cells and tissues, and to compare these with those found in animals. Its aims narrowed very considerably in writing. It now reviews ion transport processes in giant algal cells, and tries to assess progress towards understanding the mechanisms involved. It indicates the existence of similar ion transports in higher plant cells, but it does not present a complete review of the experimental work on higher plants.

The code of inheritable information which directs the synthesis of protein molecules is now known. This information is contained in the replicative molecules deoxyribonucleic or ribonucleic acid (Avery, Macleod & McCarty, 1944; Loeb & Zinder, 1961). Translation of the nucleic acid ‘information’ into protein molecules involves the initial transcription of the nucleic acid molecules into transient messenger ribonucleic molecules which are single stranded base paired complements of the more stable ‘information’ nucleic acid molecules. The events leading to the translation of the linear order of nucleic acid information in mRNA to a corresponding linear sequence of amino acids in a protein are largely elucidated at this time and will be the subject of this discussion. Since the knowledge of the genetic code is a result of data obtained from in vitro translational studies which employ the equivalent of mRNA, the genetic code is described as the RNA code rather than its DNA complement. Elucidation of the RNA code has been the focus of research activity for a large number of research laboratories over the past 10–15 years. As a result of these investigations we have a detailed description of the genetic code for life on this planet. This code appears universal in its organization and translational mechanism. Since the interactions of macromolecules necessary for genetic translation are understood with greater precision than the factors responsible for the origin of the Genetic Code, this discussion will deal in more detail with the events in the translation of genetic information.

The problem of how the brain stores and retrieves information is ultimately an experimental one, and its solution will doubtless call for the combined resources of psychology, physiology and molecular biology. But it is also a problem of great theoretical sophistication; and one of the major tasks confronting the brain scientist is the construction of theoretical models which are worthy of, and open to, experimental test. In this review we shall be concerned with the latter aspect of the problem of memory, which has attracted quite a lot of attention in the last few years. It is early yet to judge the relative merits of the various models in any detail; but as we shall see, most of those which have been developed beyond their initial hypotheses have a certain family resemblance, and it seems as if we may now be in possession of the basic ideas which will be needed for the understanding of one of the central problems of memory, namely the mechanism of associative recall.

Only two spectroscopic methods are capable of detecting individual atoms in macromolecular systems, X-ray diffraction in the crystalline state and nuclear magnetic resonance (NMR) in the liquid state. For an enzyme-substrate complex, X-ray diffraction can yield information on the geometric structure at the active site and nuclear magnetic resonance absorption can, in principle, yield information on the electronic structure at the active site and on the conformation of enzyme-substrate complexes. Both types of information are needed for unraveling the mechanism of enzyme catalysis on the molecular level. The exciting successes of X-ray diffraction in delineating active sites are already established; NMR, a comparative latecomer among spectroscopic techniques is just beginning to demonstrate its potentialities. McDonald & Phillips have summarized in their excellent review (1969) the work on direct observation of hydrogen atoms (protons) by NMR spectroscopy which has advanced our knowledge of protein structure. The extensive studies of Jardetzky and his co-workers on the NMR of ribonuclease and its inhibitor complexes has culminated in a suggested mechanism of catalytic action (Roberts et al. 1969).

The first recorded measurements of temperature changes in a muscle during contraction were made more than 120 years ago by means of a series of thermocouples connected to a galvanometer. The technique is the same today. In principle it is that of the adiabatic isoperibol calorimeter (Wadsö, 1970); heat generated by the system under study is shared with a thermal transducer, the electrical output of which is a measure of the consequent change of temperature and may be displayed by one of several devices. Most exponents in this field have to rely on home-produced assemblies and it is the intention of this article to consider the practical details of the separate components and, where possible, to discuss some of the various innovations which different workers have brought to the technique.

Biological systems provide the physical chemist with an abundance of interesting, challenging and significant problems. One example is the problem of the molecular basis of co-operative or allosteric interactions between distant ligand or substrate binding sites in hemoglobin and in enzymes. This problem has been discussed recently in This Journal by Eigen (1968) and by Wyman (1968). Another particularly challenging problem is the molecular organization of biological membranes. Such problems tend to be particularly resistant to solution by the straight-forward application of most spectroscopic techniques, in large part because of the enormous chemical and spectroscopic complexity of biological macromolecules. This spectroscopic complexity has stimulated the use of various ‘probes’ that can be introduced into selected sites in complex systems to provide spectroscopic signals that are comparatively free from interference. The use of heavy metal atoms (‘isomorphous replacement’) in X-ray studies of protein crystals (Green, Ingram & Perutz, 1954), and fluorescent dyes in the study of proteins in solutions (Weber, 1953; Steiner & Edelhoch, 1962) are early examples. Spin labels represent a new member of the family of spectroscopic structural probes. A spin label is a synthetic paramagnetic organic free radical, usually having a molecular structure and/or chemical reactivity that results in its attachment or incorporation at some particular target site in a biological macromolecule, or assemblage of macromolecules (Ohnishi & McConnell, 1965; Stone et al. 1965). This type of probe is being used in our laboratory to study allosteric interactions in proteins, and molecular dynamics and organization in membranes.

A great deal of information about evolutionary events and processes has been inferred from careful studies of fossil records. Other forms of evidence have also contributed greatly to the understanding of evolution. Comparative biochemistry (Florkin, 1949), immunology (Boyden, 1942), protein sequencing (Dayoff, 1969; Anfinsen, 1959), and early DNA studies (McCarthy & Bolton, 1963; Schildkraut, Marmur & Doty, 1961) have for the most part corroborated earlier evolutionary findings, and at the same time provided new understanding of molecular processes in evolution. Of these approaches the comparison of DNA seems most promising since a relatively precise quantitative comparison can be made of all of the genetic material of different species.

It has been known for many years that there are two distinct ways of designing an electron microscope.