In this explainer, we will learn how to describe the experiments carried out by Van Neil and Calvin and outline how they increased the understanding of photosynthesis.
Photosynthesis is an incredibly important process, for plants and for all animals on Earth! By converting carbon dioxide and water into glucose and oxygen using light energy, plants both provide themselves with nutrition and respiring organisms with crucial oxygen.
Frederick Blackman was a scientist who conducted research into photosynthesis in plants. He carried out experiments looking at the limiting factors of photosynthesis: light, temperature, and carbon dioxide concentration. He determined through his experiments that there are two main stages of photosynthesis in plants: the light-dependent stage and the light-independent stage. You can see the general equation for photosynthesis in plants below.
Reaction: Photosynthesis in Plants
Here, we will look at some important experiments that helped us understand the reactants, process, and products of the biological reactions involved in photosynthesis.
In the early 1930s, Cornelis Bernardus van Niel, a Dutch-American microbiologist, was studying the process of photosynthesis in green and purple sulfur bacteria. These bacteria proved to be very interesting study specimens. They contain bacteriochlorophyll, which are pigments highly similar to the chlorophyll found in green plants. They inhabit environments like swamps and ponds, where the gas hydrogen sulfide is abundant. It was found to be this gas that they utilize in photosynthesis.
Chlorophyll is a class of green pigments found in the chloroplasts of plants that absorbs the light energy required for photosynthesis.
Bacteriochlorophyll is a class of photosynthetic pigments found in photosynthetic bacteria.
The reaction for photosynthesis in these green and purple sulfur bacteria is given below.
Reaction: Photosynthesis in Green and Purple Sulfur Bacteria
As you may have noticed, this reaction is very similar to the process of photosynthesis in green plants. Both reactions convert carbon dioxide and a hydrogen-containing compound into glucose, and both do so using energy captured from light. A notable difference is that in the green and purple sulfur bacteria, hydrogen sulfide is a reactant as opposed to water, and as a result, sulfur and water are products alongside glucose. In green plants, oxygen is produced rather than sulfur.
It was previously thought that the oxygen produced in photosynthesis came from carbon dioxide. However, upon determining these reactants and products of photosynthesis in green and purple sulfur bacteria, Van Niel proposed that the oxygen released by the plants when they photosynthesize must be released from water.
Example 1: Recalling Van Niel’s Ideas about Photosynthesis in Plants
After his experiments using photosynthetic bacteria, what did Van Niel assume about photosynthesis in green plants?
- The reactions would be completely identical.
- The reactions were highly similar, but instead of carbon dioxide being a reactant, it would be oxygen.
- The reactions would be completely different.
- The reactions were highly similar, but instead of glucose being formed, it would be sucrose.
- The reactions were highly similar, but instead of hydrogen sulfide being broken down, it would be water.
Various different organisms are able to produce their own nutrition by converting inorganic compounds into organic ones. Photosynthesis in plants is a biological process that converts water and carbon dioxide into glucose and oxygen using light energy.
Van Niel was a microbiologist who investigated photosynthesis in green and purple sulfur bacteria. In these reactions, carbon dioxide and hydrogen sulfide were converted into glucose, water, and sulfur. He was able to compare this reaction to that observed in plants. Let’s have a look at the two equations to understand the similarities and differences:
Photosynthesis in green plants:
Photosynthesis in bacteria:
As we can see, the hydrogen-containing reactants differ, but in both, carbon dioxide is a reactant. The products also differ; bacteria produce sulfur, whereas plants produce oxygen. In both reactions, glucose is a product.
We can see that the reactions are not identical, but also not completely different. Looking back at our options we should be able to spot that E is our correct answer: Van Niel assumed that in plants the reactions were highly similar, but instead of hydrogen sulfide being broken down, it would be water.
Van Niel’s idea about how oxygen is produced by photosynthesizing plants was confirmed in 1941 by a group of scientists aiming to verify his work. This group of scientists at the University of California carried out experiments with the green algae “Chlorella”, which is able to photosynthesize using chlorophyll.
The algae were kept in conditions that encouraged photosynthesis. They were provided with carbon dioxide, water, and light. However, the scientists made an important chemical change and used a different isotope of oxygen in the water that they supplied to the algae. Let’s quickly recap what an isotope is and why it is important.
An isotope is an atom of an element that has the same number of protons, but a different number of neutrons. A simple diagram of the two isotopes of oxygen that we are going to discuss, and , are provided below in Figure 1. The isotope has two more neutrons in the nucleus of the atom than the “ordinary” isotope has. Oxygen-18 is “heavier” than oxygen-16, which allows it to be traced through reactions.
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.
So, to investigate Van Niel’s proposed ideas about photosynthesis, the water that these scientists used contained the isotope instead of the commonly occurring isotope found in water, . The carbon dioxide, however, still contained “ordinary” oxygen in the form of isotope . The oxygen gas that was produced in photosynthesis was found to contain the isotope . This provided valuable evidence in support of Van Niel’s idea that the source of oxygen in photosynthesis is indeed water. The symbol equation for this reaction is shown below, and the isotope is indicated by the red oxygen atoms:
To confirm this further, the scientists then switched the isotopes: carbon dioxide now contained and water contained the “ordinary” isotope . They found that in this case, the oxygen gas contained . The symbol equation for this reaction is shown below, and the isotope is indicated by the red oxygen atoms:
Example 2: Outlining the Experiments with Isotopes of Oxygen that Confirmed Van Niel’s Ideas
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?
- Both oxygen-18 and oxygen-16
Photosynthesis is the process by which plants, and some other organisms, produce their own nutrition by converting inorganic molecules into organic ones. They also produce oxygen as a product, which is incredibly important for all living organisms to use in cellular respiration. The basic equations for photosynthesis in plants are provided below:
Van Niel was a microbiologist who studied photosynthesis in some species of green and purple sulfur bacteria and made comparisons to the process in plants. In green and purple sulfur bacteria, the equations for photosynthesis are as follows:
In sulfur bacteria, oxygen is not produced in photosynthesis, but water and sulfur are. To produce the sulfur, the hydrogen-containing reactant hydrogen sulfide is broken down. If we apply this pattern to photosynthesis in plants, we can assume that to produce oxygen, water is broken down.
Van Niel made this assumption, and to confirm this, a group of scientists carried out an experiment in plant photosynthesis using different isotopes of oxygen. Initially, they replaced the commonly found isotope in the water molecules with the isotope and tracked the progress of this isotope. They found that isotope was found in the oxygen produced, and not in the glucose. In the equation below, the isotope is signified by the red oxygen atom:
If we look back at our question, we are being asked which isotope will be present in the oxygen produced if the water contains the oxygen-18 isotope and the carbon dioxide the oxygen-16 isotope. We now know that the oxygen produced in photosynthesis comes from the breakdown of water, so whichever isotope is in the water, it will be present in the oxygen.
Therefore, our correct answer is B: oxygen-18.
As mentioned, photosynthesis occurs in two main stages: the light-dependent stage (or the light-dependent reactions) and the light-independent stage (often referred to as the Calvin cycle). These stages happen sequentially, so the light-dependent stage occurs first and produces the reactants needed for the light-independent stage to be carried out. Therefore, for photosynthesis to occur, the photosynthetic pigments, like chlorophyll, must be exposed to light.
Melvin Calvin was an American biochemist who began studying the process of photosynthesis in 1946. His team also used isotopes, this time of carbon, to determine key products formed by the light-dependent stage.
Calvin and his team took a population of Chlorella algae and placed it into the apparatus as shown in Figure 2. The algae were provided with carbon dioxide containing the isotope , rather than the more commonly occurring . This isotope of carbon is radioactive, and it allowed Calvin to follow the path of carbon atoms through the process. The Chlorella algae was then exposed to a brief flash of light to initiate photosynthesis.
After this brief flash, the algae were dropped into a beaker of hot alcohol below. This will quickly stop any biochemical reactions from proceeding by destroying the protoplasm of the algae cells.
Upon studying the algae after this process, it was found that even with a very, very brief flash of light, a 3-carbon compound was formed. This 3-carbon compound was phosphoglyceraldehyde, also called glyceraldehyde 3-phosphate or triose phosphate.
Key Term: Phosphoglyceraldehyde/PGAL (Glyceraldehyde 3-Phosphate/G3P, Triose Phosphate/TP)
Phosphoglyceraldehyde is a 3-carbon compound produced in the light-independent stage of photosynthesis and is used to form other organic compounds.
Phosphoglyceraldehyde, or PGAL, is a very important compound for plants. Using PGAL, plants can produce a range of other organic compounds crucial for life. These include glucose, which is broken down in cellular respiration to provide organisms with energy, starch, which acts as a store of glucose, proteins, which are important for growth and repair of cells and tissues, and fats, which act as stores of energy and provide insulation.
Melvin Calvin went on to develop the idea of the light-independent stage of photosynthesis. Through repeatedly initiating photosynthesis in algae and stopping it at different stages, he demonstrated that the light-independent stage was composed of several intermediate reactions and compounds, as outlined in Figure 3 below.
For his work, the light-independent stage of photosynthesis was named after him, so you may now hear it called “the Calvin cycle!”
Key Term: NADP (Nicotinamide Adenine Dinucleotide Phosphate)
NADP, or nicotinamide adenine dinucleotide phosphate, is a coenzyme that functions as an electron acceptor and can accept hydrogens to form NADPH, or reduced NADP.
Key Term: ATP (Adenosine Triphosphate)
ATP, or adenosine triphosphate, is the molecule that stores chemical energy in living organisms.
Example 3: Explaining the Uses of PGAL in Plant Cells
Melvin Calvin investigated photosynthesis in algae. He determined that, in the process, a 3-carbon compound was formed. What are the carbon atoms of this compound used for in plant cells?
- To act as cell-signaling molecules
- To synthesize key biological molecules such as glucose, starch, proteins, and fats
- To help synthesize other key elements such as oxygen, hydrogen, and calcium
- To be used as a reactant in chemosynthesis
Photosynthesis in plants and algae is the process by which organisms produce their own nutrition, and it is an incredibly important process for all living things on Earth. If it is not used to produce their food, it produces the oxygen that living organisms need for respiration!
There are two distinct and chronological stages in photosynthesis: the light-dependent stage and the light-independent stage. For photosynthesis to occur at all, the cells of the photosynthetic organism must be exposed to light, even if it is only briefly!
When Melvin Calvin investigated photosynthesis in algae, he exposed the algae to a short burst of light, and then dropped them into hot alcohol. This was to initiate photosynthesis, and then quickly stop it. After repeated experiments, he found that a 3-carbon compound was produced. This 3-carbon compound, known as phosphoglyceraldehyde, or PGAL, is incredibly important for plants. From each molecule produced, one carbon is taken and used to build up other organic compounds.
For instance, glucose is an incredibly important compound as it is readily broken down in cellular respiration to provide cells with energy. Glucose is a 6-carbon compound. One carbon is taken from six molecules of PGAL and combined to form glucose. Glucose can then be stored in an insoluble form such as starch. Proteins and fats are also carbon-containing compounds synthesized using the carbon atoms of PGAL.
Therefore, our correct answer must be B: to synthesize key biological molecules such as glucose, starch, proteins, and fats.
Combining Van Niel’s and Calvin’s—and their teams’—experimental work provides us with valuable information about the process of photosynthesis. We know that the oxygen produced by green plants in photosynthesis comes from the water that they take in. We know that for any part of photosynthesis to occur, the photosynthetic organism must be exposed to light, even if only very briefly. Thanks to Melvin Calvin and his Nobel Prize winning work, we now understand the compounds and reactions involved in the Calvin cycle and how PGAL allows the synthesis of many important organic compounds.
Let’s summarize what has been learned in this explainer.
- In plants, the reaction for photosynthesis is
- Van Niel used the similarities between the process of photosynthesis in plants and green and purple sulfur to propose that the oxygen produced by plants came from water.
- Subsequent experiments using isotopes of oxygen confirmed Van Niel’s proposed ideas.
- Photosynthesis in plants is split into two cycles, the light-dependent stage and the following light-independent stage (also known as the Calvin cycle).
- Melvin Calvin and his team carried out experiments on Chlorella algae to determine the role of phosphoglyceraldehyde in photosynthesis and the intermediate compounds and reactions of the light-independent stage.