Müller I. A history of thermodynamics. The doctrine of energy and entropy (Müller I. A history of thermodynamics. The doctrine of energy and entropy.pdf), страница 70

Heating or cooling was another matter. Lavoisiermaintained that heat was indeed a weightless fluid which he called caloric.Respiratory Quotient309release oxygen, while animals breathe oxygen and give off carbon dioxide.In this manner there is a stable balance. Ingenhousz showed that the plantsneed light in order to build up their tissue. That is why we now call theprocess photosynthesis.Ingenhousz, who was first to discover this grand scheme, is not verymuch known nowadays, but he was a celebrity in his time.

Being aphysician, he became an early expert on inoculation, particularly smallpoxinoculation, and he travelled all over Europe serving the members of royalfamilies with smallpox, as it were, – in small doses!Respiratory QuotientIt was the eminent chemist Berzelius, cf. Chap.

4, who introduced thedistinction of organic and inorganic substances in 1807. The former werethe substances of life, and – in Berzelius’s view – they called for a separatetype of chemistry from the chemistry of elements and of their simplestoichiometric compounds that were the stock-in-trade of his own work andeverybody else’s at the time. There were vague notions that a vis viva, avitalistic force, was involved in living bodies, a spark of life. Berzeliushimself and his followers even conceived of a strict barrier between thechemistries of life and non-life.Seeing and appreciating the difference between rock and lizard, as itwere, one must admit that there is a certain plausibility to the idea and ittook at least half a century to refute it.

This required an improvedknowledge of the life functions, and exact measurements. The first organicprocess to be thoroughly investigated was respiration. Even Lavoisier andHenry Cavendish (1731–1810) had understood that respiration supported akind of combustion in the body of animals by which the oxygen of the airwas partially consumed and converted to carbon-dioxide and water.Obviously therefore, whatever substance, or substances fed the combustionhad to contain carbon and hydrogen. Beyond that, the substances wereunknown chemically, so that no quantitative conclusions could be drawn.However, it stood to reason that, whatever it was that burned had to besupplied to the animal – or man – with the food.Early in the 19th century it became clear upon analysis of the food ofanimals that there were three main typesx carbohydrates x lipidsx proteins.The carbohydrates form the chief components of cereals, and of fruit andvegetables. They are of different types but closely related and, for themoment, we take sugar – more precisely glucose – as their representative.31011 MetabolismThe chemical formula is C6H12O6, so that Gay-Lussac – one of thediscoverers of the thermal equation of state of ideal gases – could assumethat glucose consisted of 6 carbon atoms strung together and a watermolecule attached to each one in the manner of hydrates.

The structure ismore complex, as we know now, see Fig. 11.1, but Gay-Lussac’s conceptled to the misnomer carbohydrate, which is here to stay. Actually, what weeat is not glucose itself, but rather something like starch or other substanceswhich are built up from several or many glucose molecules. The largemolecules are held together by glycoside bonds, having shedded watermolecules in a process that is called condensation – obviously because itproduces liquid water.Again lipids, or fats are of varied types. Their pioneer was MichelEugène Chevreul (1786–1889).

Fats are used in manufacturing soap andas a young man Chevreul was involved in that business. He was able toisolate different insoluble organic acids – also called carbonic acids, or fattyacids – like stearic acid, palmitic acid and oleic acid. Lipids themselvesresult from the carbonic acids by esterification with glycerol C3H8O3,giving off water, i.e. undergoing condensation cf. Fig. 11.1. A typicalrepresentative is oleine C57H104O6, an ingredient of olive oil, or also ofblubber, i.e. whale oil.Fig.

11.1. Left: Two glucose molecules combining by a glycoside bond. Right: Olein.Glycerol combining with oleic acidsRespiratory Quotient311While carbohydrates and lipids contain only carbon, hydrogen andoxygen, the third type of food-stuff – of which egg-white is the best-knownrepresentative – also contains nitrogen, a little sulphur and, sometimes, stillless phosphorus. The molecules are polymers formed from amino-acidswhich are bound together by a peptide link, again a bond formed bycondensation. The detailed structure is too complex and varied to be easilycharacterized. In 1838 Gerardus Johannes Mulder devised a modelmolecule of 88 individual atoms which he hoped might be used to build upother albuminous substances. The word albuminous is derived from albus =white in Latin; it is sometimes used as a generic name for substances likeegg-white.5 More often these substances are called proteins in English,because Mulder called his model molecule Protein, from Greek, meaning offirst importance.

Otherwise the model sank into oblivion; it was too simple.Now, if indeed food was involved in a combustion inside animals, and ifCO2 and H2O were the reaction products, the reactions for carbohydratesand lipids had to obey the stoichiometric formulae16C6 H12O6 O2  CO2 H 2O18052C57 H104O6 O2  5780 CO2 80 H 2O.The volume ratio of exhaled CO2 to inhaled O2 is called the respiratoryquotient, abbreviated as RQ.

Thus the stoichiometric formulae implyRQ = 1RQ = 0.71for the carbohydratefor the lipids,since both CO2 and O2 are ideal gases. The value for proteins lies inbetween, at roughly RQ = 0.8.So, if chemistry is involved in respiration, the RQ should lie between 0.7and 1. And indeed, the chemist Henri Victor Regnault6 put animals in acage and carefully measured the oxygen input and the carbon-dioxideoutput and found the ratio to be right. What is more, if he fed the animals adiet of carbohydrates, the RQ tended to one, while on a fat-rich diet ittended to 0.7. This was later confirmed for a man in a cage by the chemistMax von Pettenkofer (1818–1901) – the founder of scientific hygiene. Allof this provided strong evidence that there was no vis viva involved, at leastnot in respiration.56Actually, in German proteins are called ‘‘Eiweisse” [egg whites].We have met him before in connection with his 700 page-long memoir of carefulmeasurements of vapour properties, cf.

Chap. 3.31211 MetabolismMetabolic RatesSo what about the energy to be gained from food? Was the first lawsatisfied, or did the intervention of a vitalistic force render thermodynamiclaws invalid in the field of nutrition?If sugar and fat and the mix of proteins normally eaten by an animal areburned in a calorimeter they provide heats of reaction as follows7sugarproteinslipids' J4­17.1 ˜ 103°3®23.6 ˜ 10°39.5 ˜ 103¯kJ.gThe question is whether these values are also relevant when food isconsumed by eating.The experimental investigation was infinitely more difficult than thedetermination of the respiratory quotient.

First of all, it requires calorimetricstudies which are notoriously difficult even in the best of circumstances.Secondly, here the feces had to be analysed in order to find out whichproportion of the ingested food remained unconsumed by the body. And aquantitative urine analysis had to determine the urea content, which is thesubstance by which the body gets rid of the nitrogen ingested with proteins.Naturally the RQ was also part of the investigation.The person who did all this carefully was the physiologist Max Rubner(1854–1932). He presented his findings in a report8 in which he came to theconclusion that the law of conservation of energy was maintained innutrition just as punctiliously as in ordinary combustion.

By now scientistswere ready to believe that physical laws govern both: life and non-life.Once this was understood, the distinction between organic and inorganicchemistry began to lose its original meaning. Organic chemistry became thebranch that deals with carbon compounds.The chemical changes that take place in animals and humans are calledmetabolism; from Greek: to rearrange. The metabolic rate may bemeasured in Watt – just like the power of a heat engine. The maximalmetabolic rate that a person can achieve is approximately 700W, but thatcan only be sustained for a few minutes.

So what is the minimum, the basalmetabolic rate?The basal metabolic rate is abbreviated as BMR; it can be achieved by aperson lying down in a comfortably warm room, having fasted for some78We are now back from the mol to ordinary mass units. The use of the mol in organicchemistry with it huge molecules would be totally impractical. Not so, however, for theglucose synthesis and the glucose decomposition, see below.M. Rubner: ‘‘Gesetze des Energieverbrauchs bei der Ernährung.” [Laws of energyconsumption in nutrition] (1902).Digestive Catabolism313time and being mentally relaxed. In that case we measure a BMR of 50Wfor a typical adult man; that is the rate at which we need to feed him to keephim alive. A normally active person may need approximately twice thatamount.

And he or she emit this power as heat, which is why a crowdedroom needs no heating.Digestive CatabolismSo far so good. But the fact remains that there is a lot of difference whenfood is burned in a fire or when it is consumed in the body. And indeed, thedirect reaction between the sugar (say) and oxygen involves so large anactivation energy that it takes an open flame to start it. This is not feasiblein the body, of course. In the body the energy barrier must be bypassed bysuitable catalysers rather than overcome by brute force, i.e. heat in this case.The catalysers were originally called ferments.

Later – when their naturebecame clearer – they were called enzymes; and they are proteins. Readingabout biochemistry, one gets the impression that we do not know muchabout body thermodynamics, when all we do know is that carbohydrates, orlipids, or proteins burn to give CO2 and H2O. The real question is how thebody goes about this, and that makes biochemistry a science of enzymaticcatalysis.