Bespiration and Cell Energy. 
409 
up energy for use in other parts of the plant. Thus a heap of coal is often 
pointed out as containing an enormous store of energy. Now before coal 
can be used as a source of energy it requires an even greater amount of 
oxygen to combine with it ; that is, 12 tons of coal require about 32 tons 
of oxygen to unite with it in order to burn it up. Does the energy or heat 
given out come from the oxygen or the coal ? It cannot be said to come 
from either. It is from the chemical union of the two, in which process 
heat happens to be given out. Carbon will give out heat when burnt, 
whether it is in the form of coal, charcoal, or diamond. Should it be said 
that the heat given out by burning a diamond has been obtained from the 
sun ? No units of heat can therefore be assigned to the carbon atom or 
any other atom. Atoms and molecules undoubtedly contain heat or energy, 
but this can only be given out by the union of two or more of them, and 
in some cases more heat is actually absorbed into such atoms during the 
chemical union. When oxygen unites with carbon more heat is given out 
than when oxygen unites with hydrogen, as seen in the above equations. 
Atoms and molecules when heated contain more energy, and it is for this 
reason that many substances require heating before chemical union can 
be brought about. No substance, therefore, can be considered as con- 
taining energy, but a substance may represent available energy by virtue 
of its power of combining with other chemical substances, but even then 
it cannot be foretold whether heat will be given out or absorbed. 
The question arises, are carbon-dioxide and water decomposed in the 
leaves before they are reconstructed into a carbohydrate such as sugar, or 
are they elaborated as carbon- dioxide and water? If not, it is difficult to 
imagine for what purpose the energy of the sun's rays is required. Light 
is certainly necessary to all green plants, and it is more than probable 
that it gives the necessary impetus to the protoplasm to perform these 
decompositions. It must be noted that protoplasm has the power of 
performing other decompositions without the presence of light. The first 
product of the leaf metabolism is supposed to be sugar. In any case it is 
probably in the form of sugar that it leaves the cell. In the formation of 
sugar heat is given out as represented in the following equation : — 
6C0, + 6H,0 + 315,000 cal. = CgH^.Og + 60„ 
but allowing that the carbon-dioxide and water have been decomposed, it 
may be convenient to represent the equation thus : — 
60 + 60, -f 6H, -f 60 + 315,000 cal. - CeH^.Og + 60„ 
all the carbon being used up and some of the oxygen liberated. The sun's 
radiant energy is thus used only to bring about certain decompositions. 
In building up the sugar molecule from the elements energy is given out. 
(The energy involved in the transformation of sugar into starch and back 
