The cell theory

The idea that all living creatures are made of cells has changed more than anything else our concept of life, and is still the foundation of modern biology. This great generalization was made possible by the invention of the microscope, but did not come suddenly. It has been the culmination of a collective research which lasted more than two hundred years, and in order to understand it we must be aware of the main problems that had to be solved. 

The invention of the microscope brought about an immense revolution in science. It led to the discovery of an entirely new world of living creatures that are invisible to the naked eye, the so-called micro-organisms. The microscopists of the seventeenth century were the first men who saw bacteria, protozoa, blood cells, spermatozoa and a thousand other animalcula, and gradually realised that the large 

In 1852 Robert Remak explicitely rejected the free-formation idea and concluded that Cells always come from the division of other cells. In 1855 Rudolf Virchow reached the same conclusion by studying a great number of normal and pathological adult tissues, and condensed it with the motto omnis cellula e cellula . The final version of the cell theory is therefore the combination of Schleiden and Schwann s first theory with the conclusion of Remak and Virchow All living creatures are made of cells and of cell products, and cells are always generated by the division of other cells.  

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Microscopists were not entirely idle, however, and after Giovanni Battista Amici (1786-1863) introduced achromatic lenses in France in 1827, the cell theory was not far behind. Henri Dutrochet (1776-1847) had already proposed that animal and plant tissues were constituted of cells, a view reiterated by many, notably Jan Evangelista Purkyne (1787-1869), Johannes Muller (1801 -1858), andjacob Henle (1809-1885). Felix Dujardin... 

Schwann, the cell theory emerged.. cells are organisms and entire animals and plants are aggregates of these organisms arranged according to definite laws (Schwann, 1838). 

Virchow, who was largely responsible for the acceptance of the cell theory, developed microscopy of cells from normal and diseased tissues as a major tool (histopathology) in the clinical armory. He believed the vital functions of the cell, growth, maintenance, and multiplication were discharged by its nucleus the specialised, distinguishing functions were made possible by the extranuclear constituents. In a Sunday evening lecture in Edinburgh in 1868, On the Physical Basis of Life , Thomas Huxley described cells as protoplasmic masses usually... 

The cell theory of life was finally put forward in the early nineteenth century by Matthias Schleiden and Theodor Schwann. Schleiden worked primarily with plant tissue he argued for the central importance of a dark spot—the nucleus—within all cells. Schwann concentrated on animal tissue, in which it was harder to see cells. Nonetheless he discerned that animals were similar to plants in their cellular structure. Schwann concluded that cells or the secretions of cells compose the entire bodies of animals and plants, and that in some way the cells are individual units with a life of their own. He wrote that the question as to the fundamental power of organized bodies resolves itself into that of individual cells. As Schleiden added, Thus the primary question is, what is the origin of this peculiar little organism, the cell ... 

With the advent of the cell theory, embryonic growth was immediately accounted for by a sequence of cell divisions. A fertilised egg becomes 2 cells, and then 4, 8, 16, 32, 64 and so on. With 10 divisions the cell number is about a thousand, with 20 is a million, with 30 is a billion, with 40 is a thousand billion, and so forth. For the... 

The great philosophers of antiquity discussed quite a number of world views, such as the atomic theory, determinism and indeterminism, relativity and evolution, and yet none of them conceived the cell theory, which makes us wonder why. The fact that they did not have the microscope does not seem to be decisive from a conceptual point of view. Even atoms cannot be seen, and yet the atomic theory was explicitely formulated. The problem is therefore the following Why could ancient people think about atoms but not about cells The idea that matter can be divided into particles is suggested by many facts of daily life a house is made of bricks, a desert is made of grains of sand, drops of rain can be turned into a river, and so on. Why not add that organisms are made of micro-organisms ... 

The first version is a mere acknowledgment that cells exist, at least on our planet. The second one states that cells are the fundamental components of all forms of life, including extraterrestrial and artificial life. It states that cells are the logical units of the living world, just as atoms are the units of the physical world. The strong version of the cell theory, in other words, declares that life does not exist without cells, and represents therefore a definition of life itself life is the state of activity of cells and of cellular systems. 

Tn recent years, developing interests in surface energetics and adhesion of liquid-like polymers, or polymer liquids, have prompted both theoretical and experimental work on surface tension. Unlike low molecular weight liquids, polymer liquids have not been extensively studied. Bondi and Simkin (1) mentioned surface tension in their study on high molecular weight liquids. Roe (28) applied both the cell theory of polymer liquids and the hole theory of surface tension of simple liquids to develop an approximate theory of surface tensions of polymer liquids. His approach has met some degree of success. Notably, both Bondi s and Roes work are somewhat related to the cell theory introduced by Prigogine and... 

Because of the high symmetry of the main cavities of sieve A, it might seem reasonable, in the spirit of the cell theory of liquids (5), to assume that the various charged species of the zeolite are distributed with complete spherical symmetry about the center of each cavity. This, of course, leads to zero electric field throughout the cavity and hence to zero contribution from ion-induced dipole and similar forces. The remaining terms, dispersion and repulsion, are assumed to be of the Lennard-Jones form and are taken to be additive. This theory then becomes essentially the same as the cell theory of liquids, except that we permit several molecules per cell. The total P.E. is given as = 299(fi) +... 

Only during 17 and 18 centuries, important foundations were laid in many fields of biology. The 19 century observed the development of very crucial concepts, which include the cell theory by Schleiden and Schwaim, Mendel s study of inheritance and Darwin s theory of evolution. The real push to biochemistry was given in 1828 when total s)mthesis of urea from lead cyanate and ammonia was successfully achieved by Wohler who thus initiated the synthesis of organic compound from inorganic compound. Louis Pasteur,... 

Barker [92-94] has presented a general formulation of the cell theory and we give a brief review of his approach here. We will restrict our discussion to single-component atomic solids and discuss the application to mixtures and nonspherical molecules later. Suppose we have a system of N molecules in the canonical ensemble. The configurational partition function, Eq. (2.205), may be rewritten by breaking the volume into N identical subvolumes or cells so that... 

Up to this point we have focused on the use of a single theory to describe both phases in a calculation of SFE. A somewhat less ambitious approach is to use a theory appropriate to each phase in the calculations. In particular, the cell theory can be used for the solid phase and a liquid state theory (e.g., a hard-sphere equation of state or thermodynamic perturbation theory) used for the fluid phase. This approach turns out to be at least as accurate as either the two-phase cell theory or DFT approaches described above and is often more accurate. Moreover, it has been more successful in the treatment of systems more complex than hard spheres. 

The cell theory plus fluid phase equation of state has been extensively applied by Cottin and Monson [101,108] to all types of solid-fluid phase behavior in hard-sphere mixtures. This approach seems to give the best overall quantitative agreement with the computer simulation results. Cottin and Monson [225] have also used this approach to make an analysis of the relative importance of departures from ideal solution behavior in the solid and fluid phases of hard-sphere mixtures. They showed that for size ratios between 1.0 and 0.7 the solid phase nonideality is much more important and that using the ideal solution approximation in the fluid phase does not change the calculated phase diagrams significantly. 

DFT has been extended to SFE in chain molecules by Yethiraj et al. [250] and applied to the freezing of polyethylene. Now that simulation results are available for molecular models of chain molecules, it will be interesting to investigate the performance of DFT for such model systems. The cell theory has recently been applied to the SFE in the hard chain models considered by Malanoski and Monson [62] with results comparable in accuracy to those achieved for hard spheres and hard dumbbells [251]. 

Although most of the studies of this model have focused on the fluid phase in connection with the theory of electrolyte solutions, its solid-fluid phase behavior has been the subject of two recent computer simulation studies in addition to theoretical studies. Smit et al. [272] and Vega et al. [142] have made MC simulation studies to determine the solid-fluid and solid-solid equilibria in this model. Two solid phases are encountered. At low temperature the substitutionally ordered CsCl structure is stable due to the influence of the coulombic interactions under these conditions. At high temperatures where packing of equal-sized hard spheres determines the stability a substitutionally disordered fee structure is stable. There is a triple point where the fluid and two solid phases coexist in addition to a vapor-liquid-solid triple point. This behavior can be qualitatively described by using the cell theory for the solid phase and perturbation theory for the fluid phase [142]. Predictions from density functional theory [273] are less accurate for this system. 

If valid, such a crude theory of metals should in a certain sense be analogous to the cell theory of ordinary liquids, which also involves two molecular parameters e and cr. It would then be possible to develop along the same lines a theory of metallic solutions. The excess thermodynamic properties of a mixture of two metals A and B would then depend on the differences ( a b) 3-nd (ZoA — > ob)- That this theory is very rough for pure metals is not a limitation for its applicability to mixtures it is indeed well known from the theory of non-electrolyte solutions that the properties of mixtures may be reasonably analyzed from rather crude statistical models. 

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