Life's Ratchet: How Molecular Machines Extract Order from Chaos by Peter M. Hoffmann Page B
Authors: Peter M. Hoffmann
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pointlike particles, interacting through forces depending only on the distance between the particles. This mathematical proof and the expansive view of the new law met with some resistance from his older colleagues. Soon, however, new experiments proved Helmholtz and his fellow energy conservers correct.
Helmholtz was a dedicated mechanist from the beginning. Although he had studied with the influential researcher and teacher Johannes Peter Müller (1801–1858), who was part of the teleomechanist faction, he despised the very idea of invoking a vital force to explain anything. He designed several experiments to prove that vital forces were unnecessary to explain irritability. Most of these experiments, ironically, were to be performed on frog legs, the favorite experimental object of Galvani. Galvani, of course, had presented his frog leg experiments as support for vitalism.
In the first set of experiments, Helmholtz set out to prove that motion in muscles is caused by chemical processes, that is, that animal motion is a physicochemical process and is not related to any mysterious vital force. To prove this, he irritated frog legs several hundred times by passing electrical currents through them, just as Galvani had done. He then made several chemical extracts of the irritated frog legs and compared the extracts with extracts from non-irritated frog legs. He found that if the muscles had been irritated, a water-based extract lost mass and an ethanol-based extract gained an equivalent amount of mass. Clearly, some chemical compound in the muscles had been changed from a water-soluble to an alcohol-soluble form through the action of the muscles. This proved that the motion of the muscles caused a chemical change in the muscles, and Helmholtz concluded that muscles were machines that converted chemical to mechanical energy.
To establish that this energy was purely chemical, he next compared the heat that can be released upon chemical breakdown of food, called the latent heat, with the latent heat of excreted substances in animals. This was Lavoisier’s experiment. However, since the time of Lavoisier, more-refined experiments had improved on Lavoisier’s guinea pig. Helmholtz reviewed these experiments and concluded that the difference in energy between food and excrement accounted well for the observed animal heat. He was able to correct an error introduced by Lavoisier and the famous German chemist Justus von Liebig (1803–1873): Liebig (like Lavoisier before him) believed that the energy expended by an animal was exactly the same as oxidizing (burning) the animal’s food in the oxygen the animal breathes. But the French physicist Pierre Louis Dulong (1785–1838) and the Belgian physicist César-Mansuète Despretz (1798–1863) had shown in their careful repeats of Lavoisier’s experiments that an animal generated about 10 percent more energy than could be accounted for by the oxidation from respiration alone. This left an opening to the vitalists, who could point to the missing 10 percent as the contribution of the vital force. Instead, Helmholtz showed, the missing 10 percent came from the oxygen already contained in food, especially in carbohydrates and sugars. If this additional oxygen was included, food energy perfectly matched animal heat plus energy of the excrements, and no vital force was needed.
One more experiment was needed to completely eliminate the need for the vital force: Helmholtz had to show unequivocally that the energyto move muscles was contained in the chemical energy of the muscles (which they had received from food) and did not come from someplace else. At the time, it was known that a loss of the nervous system led to a cooldown of the body. Therefore, some biologists believed that the nervous system provided a source of vital force or animal heat. Helmholtz devised an ingenious setup to eliminate this last refuge of the vital force. By irritating three selections of tissue—a frog leg with
Helmholtz was a dedicated mechanist from the beginning. Although he had studied with the influential researcher and teacher Johannes Peter Müller (1801–1858), who was part of the teleomechanist faction, he despised the very idea of invoking a vital force to explain anything. He designed several experiments to prove that vital forces were unnecessary to explain irritability. Most of these experiments, ironically, were to be performed on frog legs, the favorite experimental object of Galvani. Galvani, of course, had presented his frog leg experiments as support for vitalism.
In the first set of experiments, Helmholtz set out to prove that motion in muscles is caused by chemical processes, that is, that animal motion is a physicochemical process and is not related to any mysterious vital force. To prove this, he irritated frog legs several hundred times by passing electrical currents through them, just as Galvani had done. He then made several chemical extracts of the irritated frog legs and compared the extracts with extracts from non-irritated frog legs. He found that if the muscles had been irritated, a water-based extract lost mass and an ethanol-based extract gained an equivalent amount of mass. Clearly, some chemical compound in the muscles had been changed from a water-soluble to an alcohol-soluble form through the action of the muscles. This proved that the motion of the muscles caused a chemical change in the muscles, and Helmholtz concluded that muscles were machines that converted chemical to mechanical energy.
To establish that this energy was purely chemical, he next compared the heat that can be released upon chemical breakdown of food, called the latent heat, with the latent heat of excreted substances in animals. This was Lavoisier’s experiment. However, since the time of Lavoisier, more-refined experiments had improved on Lavoisier’s guinea pig. Helmholtz reviewed these experiments and concluded that the difference in energy between food and excrement accounted well for the observed animal heat. He was able to correct an error introduced by Lavoisier and the famous German chemist Justus von Liebig (1803–1873): Liebig (like Lavoisier before him) believed that the energy expended by an animal was exactly the same as oxidizing (burning) the animal’s food in the oxygen the animal breathes. But the French physicist Pierre Louis Dulong (1785–1838) and the Belgian physicist César-Mansuète Despretz (1798–1863) had shown in their careful repeats of Lavoisier’s experiments that an animal generated about 10 percent more energy than could be accounted for by the oxidation from respiration alone. This left an opening to the vitalists, who could point to the missing 10 percent as the contribution of the vital force. Instead, Helmholtz showed, the missing 10 percent came from the oxygen already contained in food, especially in carbohydrates and sugars. If this additional oxygen was included, food energy perfectly matched animal heat plus energy of the excrements, and no vital force was needed.
One more experiment was needed to completely eliminate the need for the vital force: Helmholtz had to show unequivocally that the energyto move muscles was contained in the chemical energy of the muscles (which they had received from food) and did not come from someplace else. At the time, it was known that a loss of the nervous system led to a cooldown of the body. Therefore, some biologists believed that the nervous system provided a source of vital force or animal heat. Helmholtz devised an ingenious setup to eliminate this last refuge of the vital force. By irritating three selections of tissue—a frog leg with
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