The general equation for photosynthesis was:.
Carbon dioxide (CO2) + Water + light energy à sugar (glucose) + oxygen.
And this one for respiration:.
Sugar (glucose) + oxygen à CO2 + water + energy.
Each of these equations is the reverse of the other, and are fundamentally correct in indicating the interdependence of the two life processes. Photosynthesis and respiration are two very complex and they could only be considered reversible in the broadest ecological sense.
Priestly and Ingenhouz overlooked a fact about respiration, they only said that animals respire and make air unfit for life, and did not say anything about plant cells, that left in a dark area will run out of air, and will die also. All living creatures need oxygen and release carbon dioxide in order to live. If Priestly had attempted this experiment, measuring carbon dioxide levels of a plant in a dark area, and then to measure the same kind of plant in a bright area, he would have discovered that the darker of the two has less oxygen and the one from the brighter area has more oxygen.
Since all of the photosynthetic ideas were theory, the scientists have to guess how all of the reaction works, so they use a black box theory, and draw a diagram like this:.
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Since 1900, biologists have studied the box to see what they could get out of it, and have found that they could separate the overall process into two different reactions. One reaction requires light and water, and is called a light reaction, the other require carbon dioxide but do not require light so they are considered a dark reaction. Dark reactions are not restricted solely to darkness but take place in both light and darkness, the term dark reactions means that light is not required.
The light reactions produce hydrogen ions, electrons, and oxygen, which are derived from photolysis, the splitting of water molecules by energy from light. The hydrogen ions and electrons are used in further photosynthetic reactions, but not as independent particles.
