Van Niel’s work was confirmed by another group of scientists. They carried out a photosynthetic reaction where the plants were supplied with water that contained the oxygen-18 isotope and carbon dioxide that contained the oxygen-16 isotope. Which isotope of oxygen would be present in the oxygen produced by photosynthesis?
So let’s start by summarizing the process of photosynthesis. Using photosynthesis, plants convert carbon dioxide and water into glucose and oxygen. Light energy is required for the reaction to take place. This is absorbed by chlorophyll in the green part of the plant. For example, the leaves. The glucose produced provides nutrition for the plant, while the oxygen produced can be used by organisms for respiration. In the 1930s, van Niel, a Dutch American microbiologist, studied photosynthesis in green and purple sulfur bacteria. These bacteria contain a pigment called bacteriochlorophyll, which is very similar to the chlorophyll founding chloroplasts in plants.
Here, we can see the reaction for photosynthesis in green and purple sulfur bacteria. Carbon dioxide plus hydrogen sulfide in the presence of light energy, which is absorbed by bacteriochlorophyll, produces glucose, water, and sulfur. Let’s compare photosynthesis in green plants and photosynthesis in green and purple sulfur bacteria side by side.
If you look at the word in simple equations for each reaction, you will notice many similarities between the two processes. In both organisms, carbon dioxide is a reactant. The other reactant in each case is a hydrogen-containing compound. In green plants, this is water. And in green and purple sulfur bacteria, this is hydrogen sulfide. In both of the organisms, these reactants are converted into the products using energy from light. Both types of organism produce glucose. In green plants, the other product is oxygen, whereas in green and purple sulfur bacteria, water is produced along with sulfur.
In green plants, it was previously thought that the oxygen produced in photosynthesis came from carbon dioxide. However, van Niel noticed that in green and purple sulfur bacteria, no oxygen was produced despite there being carbon dioxide in the reactants. This led him to suggest that the oxygen produced came from water molecules rather than from carbon dioxide as previously thought. For other scientists to confirm van Niel’s work, they would need to investigate photosynthesis using isotopes of oxygen. By using water and carbon dioxide containing different isotopes of oxygen and then tracking where these oxygen isotopes were found in the products, they were able to confirm van Niel’s work. So let’s look in more detail at the work carried out by this group of scientists.
From the question, we can see that they used two different isotopes of oxygen, oxygen-18 and oxygen-16. But what are isotopes? And what does this actually mean? Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. Oxygen-16 and oxygen-18 both have eight protons in their nucleus, shown here in fuchsia. The number of neutrons in each isotope is different. Oxygen-16 contains eight neutrons in its nucleus, while oxygen-18 contains 10 neutrons in its nucleus. These are shown here in the diagram in orange. Because of their different physical properties, isotopes can be used to tag elements so they can be tracked during chemical reactions.
In the question, we are told that plants were supplied with water that contained the oxygen-18 isotope shown here in orange. The carbon dioxide provided contained oxygen in its most abundant form, the oxygen-16 isotope. Van Niel suggested that the oxygen produced in photosynthesis came from water. We would therefore expect that these scientists would see the oxygen produced also contained the oxygen-18 isotope if it was produced from the splitting of the water molecule in the reactants. So the answer to the question “Which isotope of oxygen would be present in the oxygen produced by photosynthesis?” is oxygen-18.