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Question Video: Explaining the Outcome of Hydrogen Sulfide in Photosynthetic Bacteria Biology

Van Niel determined that the equation for photosynthesis in green and purple bacteria was as follows: Carbon dioxide + hydrogen sulfide ⟶ glucose + water + sulfur. In this reaction, what has happened to the hydrogen sulfide?

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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.

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