Guy Brown - The Energy of Life:

Cells were first seen by Robert Hooke in the early days of microscopes. But Hooke had only seen the large woody cells of plants. It was much harder to see animal cells, because they were smaller and their walls (membranes) were almost invisible. So the structure of animal tissue was unclear, and it had mostly been described in terms of fibres and ‘globules’ of unknown function. Schwann benefited from a great improvement in microscope optics, using this to show that not only were cells everywhere in the body, but that they were the body’s organizing principle. All cells in the body were derived from embryonic cells which divided and differentiated to form the hundreds of different types of cell making up the organism. If there was a vital principle in the body, Schwann believed that it had to be located in the cells, because all the essential processes, such as reproduction, growth, and respiration, were located in individual cells. Doubting the possibility of a vital force, Schwann thought all the properties of cells could be explained in terms of physical and chemical forces. He also believed that living processes within cells could be explained in terms of the physical structures and movements of the molecules. This was an important and influential insight which foreshadowed the spectacular explosion of cellular and molecular biology in the twentieth century. Though intensely religious, Schwann argued persuasively that the concept of a vital force was completely unnecessary, denying God’s achievement in originally producing the Universe and its physical forces: these were all that was necessary to create life.

Schwann did not have the whole answer to how cells created life, but he had found important clues in his notion of ‘metabolism’ and his discovery that digestion was partly due to pepsin. Pepsin was thought to be a ‘ferment’, but at the end of the nineteenth century it was found that ferments consisted of single biological molecules, now called ‘enzymes’. Enzymes are the magic molecules inside cells that actually cause the ‘change’ of metabolism. Enzymes are made from protein. They act on the chemicals and structures inside and outside the cell changing them from one form to another. For example, pepsin cuts other proteins into pieces without itself being cut up. Each type of enzyme can cause only one type of change but there are roughly 10,000 different types of enzyme in a cell. These enzymes are the alchemists of the cell. But each enzyme molecule can be regarded as a minute, exquisitely designed, molecular machine. Machines, because they are designed structures, performing specific tasks and transforming things by physically interacting with them; and molecular, because they consist of single molecules. Enzymes and the other molecular machines of the cell are the engines of life.

Enzymes were first discovered within yeast, as the word itself reflects – ‘enzyme’ means ‘in yeast’. Although Schwann and others had shown that fermentation was caused by yeast cells, this discovery was ridiculed by von Liebig and the chemists and replaced by von Liebig’s own nebulous chemical theory. So, the biological theory of fermentation (that it is caused by living cells rather than dead chemicals) had to be re-established later in the century by Louis Pasteur. Pasteur was unable, however, to isolate from yeast cells a ‘ferment’ which could cause the fermentation of grape juice into alcohol, in the absence of live cells. Thus it was unclear whether fermentation was a truly vital process, only occurring within living cells. This was crucial because if the sub-processes of life such as the transformation of chemicals, could not occur in isolation from a living cell, then this implied that there was indeed some vital force involved. In more practical terms, it also meant that science would never penetrate far into the cell, because the individual processes could not be studied in isolation. It was left to Buchner at the very end of the century to at last successfully grind up yeast cells, and isolate something (a bunch of enzymes) that could cause fermentation in the absence of living yeast cells. It is this event that marks the true beginning of Biochemistry, in part because it destroyed the concept of the vital force, but mainly because science had finally broken into the cell and was able to study the processes of life at the molecular level.

Schwann had opposed von Liebig and the other chemists’ views on virtually everything: the role of biology rather than chemistry in digestion, fermentation, putrefaction, metabolism, tissue structure, muscle function and the vital force. The chemists, clearly rattled by this upstart, went onto the attack, writing a satirical article on the views of the ‘biologists’ on fermentation. This article, drafted by Wöhler and made more vitriolic by von Liebig, ridiculed the cell theory of Schwann and others, scathingly describing it in terms of anthropomorphized cells shaped like distilling flasks with big mouths and stomachs, gulping down grape juice and belching out gases and alcohol. Schwann’s credibility was destroyed, he lost his job and was prevented from obtaining another academic post in Germany. He escaped into exile in Belgium, with a post in the Catholic University of Louvain, where his time was filled teaching anatomy. He never did any significant biological research again, keeping his head well below the parapet, and the chemists held the field once again in Germany. However, the experiments and book Schwann had produced in his four years of active research proved immensely influential, eventually leading to the demise of von Liebig’s ascendancy and the transformation of biology. Von Liebig publicly battled on against Pasteur, but after thirty years of denial eventually had to admit that he had been mistaken about the biological basis of fermentation. The stresses of the struggle and eventual defeat may well have contributed to his death soon afterwards. The idea of the vital force died with him, later to be reborn in the transmuted form of ‘Energy’.

We have now learnt our ‘chemistry’. We know life is not created by spirits sucked in from the air to push and pull the body’s levers; but rather an element of air, oxygen, is combined with molecules of food within the cells of the body, producing something then able to animate our bodies and minds. The stage is now set for the discovery of energy itself.

The modern scientific concept of energy was an invention of the mid-nineteenth century. ‘Energy’ is a child of the industrial revolution: its father a thrusting steam engine; its mother, the human body itself, in all its gory physicality; and its ancestors the ethereal spirits of breath and air. The evolution of this concept was aided by an eclectic group of engineers, physicians, mathematicians, physiologists and physicists, with a strong supporting cast of soldiers, sailors and, inevitably, accountants. Today, the scientific concept of energy has a harsh façade of cold forces and austere maths, but its core is much softer and more appealing, reflecting its biological origins in vital forces and wild spirits.

The physical heritage of energy begins with Watt’s invention of the steam engine in the eighteenth century. A steam engine produces work (movement against a force) from heat, something never before possible. The question is how? Is heat somehow converted into work or does the flow of heat from hot to cold drive work as the flow of water in a stream drives a water-mill? Sadi Carnot (1796–1832) thought the latter was true but was only half right. Carnot’s father was a Minister of War in Napoleon’s government and Sadi fought in the defence of Paris in 1814. The total defeat of Napoleon’s armies and France’s ignoble subjugation turned Carnot’s thoughts towards one source of England’s growing power: James Watt’s steam engine. The engine seemed to promise limitless power derived from hot air and steam alone but the elaborate contraptions of the early nineteenth century did not always deliver what was promised. Carnot wanted to improve the efficiency of steam engines but there was still no good theory of how they actually worked. So Carnot produced one, based on Lavoisier’s conception of heat. Lavoisier had disposed of the phlogiston theory of combustion but had replaced it with something rather similar: the caloric theory of heat. According to Lavoisier, heat was a substance, a massless fluid called ‘caloric’, which he considered one of the elements, like oxygen or phosphorus. This caloric theory was mistaken but its legacy still remains in our unit of heat energy: the ‘calorie’. Carnot thought if heat was an indestructible fluid, then steam engines must be driven by the flow of heat from a hot source (the boiler) to a cold sink (the condenser), just as a mill-wheel is impelled by the flow of water. His important insight was that there had to be a large temperature difference to cause the heat to flow and that there was a quantitative relation between this heat flow and the power output of the engine, which could then be used to predict the efficiency of conversion of coal into work.