= Solution
\b[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.<Lectures on Animal Chemistry by William Odling>[Lecture IV, discussion of solar energy and animal oxidation; see also the concluding recapitulation]{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, is
$$
6\mathrm{CO_2}+6\mathrm{H_2O}+\text{light energy}
\longrightarrow \mathrm{C_6H_{12}O_6}+6\mathrm{O_2}.
$$
This 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.<Biology 2e by Mary Ann Clark and others/8.1 Overview of Photosynthesis>[Main Structures and Summary of Photosynthesis; photosynthetic products and their passage through food chains]{ref}
The reverse overall balance for complete aerobic utilization of <glucose> is
$$
\mathrm{C_6H_{12}O_6}+6\mathrm{O_2}
\longrightarrow 6\mathrm{CO_2}+6\mathrm{H_2O}+\text{released energy}.
$$
Similarly a representative <fatty acid>, <palmitic acid>, has the complete <oxidation> balance
$$
\mathrm{C_{16}H_{32}O_2}+23\mathrm{O_2}
\longrightarrow16\mathrm{CO_2}+16\mathrm{H_2O}+\text{released energy}.
$$
Both 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.<Anatomy and Physiology 2e by J. Gordon Betts and others/24.4 Protein Metabolism>[Protein as metabolic fuel; amino-acid carbon skeletons and the urea cycle]{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.<On the Source of Muscular Power by Edward Frankland>[pp. 394–395, criticism of tissue-consumption doctrine; pp. 401–406, calorific measurements and comparison with climbing work, 1866]{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 form
$$
\boxed{E_{\rm food}=W_{\rm external}+Q_{\rm outward}+\Delta E_{\rm body}}
$$
for an idealized interval in which $E_{\rm food}$ is the metabolizable food input, with energy lost in excreta already subtracted and other material-energy exchanges neglected. Here $W_{\rm external}$ is external <work>, $Q_{\rm outward}$ is net outward <heat transfer>, and $\Delta E_{\rm body}$ includes changes in stored chemical and other <energy>. The living body transforms <energy>; it does not create it.
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