Video Transcript
Van Niel determined that the
equation for photosynthesis in green and purple bacteria was as follows. Carbon dioxide plus hydrogen
sulfide results in glucose plus water plus sulfur. In this reaction, what has happened
to the hydrogen sulfide? (A) It has been used to form
inorganic molecules. (B) It has been hydrolyzed into
oxygen and sulfur. (C) It has been broken down into
hydrogen and sulfur. Or (D) it has been synthesized into
sulfur and water.
By comparing the photosynthesis of
bacteria and plants, Van Niel was able to determine the equation for
photosynthesis. The reactants of this equation are
carbon dioxide and hydrogen sulfide. So, what happens to these
reactants? How are the elements divided up
among the products?
First, what is hydrogen
sulfide? It is a molecule that consists of
one sulfur atom and two hydrogen atoms. We know that the correct answer
must explain where these two elements end up. The products of a reaction that the
hydrogen sulfide is responsible for can only contain the elements which are present
in the hydrogen sulfide itself. We can therefore eliminate (B) and
(D), as both of these contain an element not present in the hydrogen sulfide,
namely, oxygen. The photosynthesis reaction first
breaks hydrogen sulfide into its components. And these combine with the atoms
from carbon dioxide to create the products: sulfur, water, and glucose.
Now, let’s look at answer (A). It has been used to form inorganic
molecules. We know that two of the products,
water and sulfur, are inorganic, meaning they do not contain carbon. However, some of the hydrogens from
hydrogen sulfide end up in glucose as well, which is organic. Therefore, we can eliminate
(A).
So, the correct answer must be
(C). In this reaction, hydrogen sulfide
is broken down into hydrogen and sulfur.