Past exam of the medicine course of the University of Oxford / 1867 / First examination / Anatomy and physiology / 3

‘In the vegetable kingdom solar force is absorbed in the production of our food ; in the animal kingdom it is liberated by the eremacausis of our fat and glands and muscle.’ Odling, Lect. on Animal Chemistry. Explain and illustrate these statements.

The defensible core of William Odling's statement is a transfer of energy: sunlight helps build reduced food material, and its subsequent oxidation can supply animal heat and work. Here solar force means solar energy, and eremacausis means slow oxidation, rather than a special vital agent. The historical wording, from his 1866 lectures, must be qualified where it identifies the fuel with the substance of the working muscle or contrasts plant and animal metabolism too absolutely.^[ref]

In oxygenic photosynthesis, light absorbed by chlorophyll supports conversion of carbon dioxide and water into reduced organic material, with release of oxygen. An overall account, written for one glucose unit, isThis summarizes several stages, rather than describing a single elementary chemical reaction. The evolved oxygen comes from water. The product can furnish starch, other carbohydrates and precursors for fat and protein; the latter additionally requires a source of nitrogen. A human obtains these materials directly from plants or through animals that have eaten plants. Thus a grain meal and the flesh of a grain-fed animal can trace much of their chemical energy to the same original sunlight.^[ref]

The reverse overall balance for complete aerobic utilization of glucose isSimilarly a representative fatty acid, palmitic acid, has the complete oxidation balanceBoth equations conserve each chemical element. The relevant energy difference belongs to the complete set of reactants and products, including oxygen; merely breaking a bond does not supply net energy. In modern terms, controlled cellular respiration couples part of the available chemical free energy to ATP formation. ATP hydrolysis can then support muscle contraction, active transport and biosynthesis; some energy is transferred to heat. The process occurs through enzyme-mediated steps, without a flame, although complete combustion in a calorimeter can help measure the corresponding chemical energy change.

Glands and muscles continually replace constituents, and their proteins can be degraded. Amino acids can contribute carbon skeletons to energy metabolism, while their nitrogen is disposed of largely as urea. But ordinary muscle contraction does not require proportional destruction and rebuilding of the contractile protein at every stroke. Carbohydrates and fats are major fuels for muscle, and a gland can use those fuels to support secretion. The historical picture of burning the organ's own substance is therefore an inadequate general explanation.^[ref]

This correction was already an experimental issue before the exam. Edward Frankland's 1866 discussion compared the mechanical work of climbing with the energy that could be assigned to protein destruction estimated from urinary nitrogen. The latter was insufficient to account for all the work. His combustion measurements also distinguished the heat from burning a sample completely from the smaller amount available when nitrogenous material leaves the animal as still-combustible urea. Measuring external work, urinary products and calorific values gives the hypothesis a quantitative test; it is more informative than simply pointing out that an active animal gets warm.^[ref]

Finally, plants themselves carry out cellular respiration, and not every ecosystem obtains its input from sunlight: chemolithotrophy is an alternative. The plant-animal contrast is useful for following energy through an ordinary food chain, but it is not an exhaustive division of biochemical capabilities. Conservation of energy also requires the bookkeeping formfor an idealized interval in which $E_{\mathrm{food}}$ is the metabolizable food input, with energy lost in excreta already subtracted and other material-energy exchanges neglected. Here $W_{\mathrm{external}}$ is external work, $Q_{\mathrm{outward}}$ is net outward heat transfer, and $\Delta E_{\mathrm{body}}$ includes changes in stored chemical and other energy. The living body transforms energy; it does not create it.