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Lesson Video: Photosynthesis Experiments Biology

In this video, we will learn how to describe the experiments carried out by Van Neil and Calvin and outline how they increased the understanding of photosynthesis.

13:47

Video Transcript

In this video, we’ll remind ourselves of some key facts about photosynthesis. We’ll also describe the experiments carried out by van Niel and Calvin and outline how they’ve contributed to our understanding of photosynthesis. Let’s get started.

Like all living organisms, plants need food to keep them alive and healthy. But unlike animals, plants can’t move around to hunt or gather their food. Instead, they carry out photosynthesis. You may recall that photosynthesis is the chemical reaction by which plants convert carbon dioxide and water into glucose and oxygen. We can learn a bit about photosynthesis just from the word. Photo- means light and synthesis means to make, so photosynthesis refers to a process that makes food using light energy.

Frederick Blackman was a scientist who determined that photosynthesis in plants has two main stages: one stage that is completely dependent on light being available and one stage that is independent of light. So even though one stage of photosynthesis can happen in the dark, the overall reaction will not happen unless there is a light source present. Now, let’s have a look at some key experiments, starting with those carried out by Cornelius Bernardus van Niel.

In the 1930s, van Niel was studying photosynthesis in green and purple sulfur bacteria. These bacteria contain pigments called bacteriochlorophyll. You may recognize the chlorophyll part of the word. This is the pigment found in plant cells which captures the light energy the plant needs for photosynthesis. Well, bacteriochlorophyll does the same thing, just in green and purple sulfur bacteria instead of plants.

As we can see from the equation, photosynthesis in these bacteria is a slightly different process to the reaction that happens in plants. They take carbon dioxide and hydrogen sulfide and convert them into glucose, water, and sulfur. This reaction still requires energy captured from a light source, however. To better understand van Niel’s experiments, let’s have a look at the symbol equation for photosynthesis in green and purple sulfur bacteria. As we can see, six molecules of carbon dioxide react with 12 molecules of hydrogen sulfide to produce one molecule of glucose, six molecules of water, and 12 molecules of sulfur.

Let’s compare this to the equation for photosynthesis in plants. In plants, we see that six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen. van Niel noticed a key difference between these two reactions. In bacteria, sulfur is produced instead of oxygen. Previously, scientists believed that the oxygen produced by plants came from the breakdown of carbon dioxide. But because both reactions use carbon dioxide and only one produces oxygen, van Niel realized this could not be the case. He proposed that the oxygen produced in plant photosynthesis is released when water is broken down. In science, one experiment by one scientist is never enough to conclusively prove something. So let’s take a look at some subsequent experiments that helped to confirm van Niel’s theory.

In 1941, a group of scientists at the University of California continued this research into photosynthesis using green algae of the genus Chlorella. Their experiments used different isotopes of oxygen to demonstrate where exactly the oxygen produced in photosynthesis comes from. To understand this, let’s quickly recap what isotopes are. Atoms, like the one shown here, are made up of multiple subatomic particles. These are electrons, which are negatively charged; protons, which are positively charged; and neutrons, which have no charge. Protons and neutrons are found within the central part of the atom, which is called the nucleus. Be careful not to get this confused with the nucleus of a cell. This nucleus does not contain any DNA.

Isotopes are atoms of the same element that have the same number of protons, but a different number of neutrons. The researchers in California investigated photosynthesis using two isotopes of oxygen, oxygen-16 and oxygen-18. Both of these isotopes have eight protons. However, oxygen-16 has eight neutrons, whereas oxygen-18 has 10 neutrons. Here we have the equation for photosynthesis in the Chlorella algae. This reaction is very similar to what we see in plants, apart from the fact that it produces water. Oxygen-16 is the isotope most commonly found in water molecules. But in the first of the Californian experiments, water containing oxygen-18 was used instead in order to investigate van Niel’s ideas.

Let’s call this experiment A and highlight the oxygen-18 present in the water molecules. After photosynthesis had been carried out and the researchers had studied the products, they found that the oxygen-18 isotope was present in the oxygen produced by the reaction. The researchers then carried out a second experiment, experiment B, where this time the carbon dioxide molecules contained oxygen-18 instead of the water. Light energy was applied, and once again the Chlorella underwent photosynthesis. This time, the scientists found that the oxygen-18 was present in the glucose and water produced, but not in the oxygen. They concluded that the oxygen gas produced by photosynthesis is indeed released from the water molecules rather than the molecules of carbon dioxide.

We mentioned before that photosynthesis has two main stages, the light-dependent stage and the light-independent stage. Let’s have a look at an experiment investigating the light-independent aspect of photosynthesis. Melvin Calvin was an American biochemist who was studying photosynthesis in the 1940s. His work also used Chlorella algae as well as isotopes, which this time were isotopes of carbon. This is because carbon dioxide is a key reactant for photosynthesis in Chlorella as you can see from our equation.

Calvin and his team took a population of Chlorella and placed it in the apparatus as shown here. They supplied the algae with carbon dioxide containing the isotope carbon-14 rather than the more commonly occurring carbon-12. A beaker of hot alcohol was placed underneath the Chlorella. The algae were then exposed to a brief flash of light to initiate the process of photosynthesis. Almost immediately, the algae were dropped into the beaker of hot alcohol. This quickly stops anymore biochemical reactions from happening. Following the experiment, Calvin analyzed the Chlorella and discovered that even after a very quick flash of light, a three-carbon compound was formed. This three-carbon compound is known as phosphoglyceraldehyde, or PGAL for short. You may also see it referred to as glyceraldehyde 3-phosphate or triose phosphate.

The PGAL was identified because it contained the carbon-14 isotope, as represented in blue on this diagram. PGAL can be used by organisms to produce a range of other organic compounds. These include glucose, proteins, and fats, which are all important for survival. Calvin continued his work and found that by dropping the algae into the hot alcohol after different lengths of time following the flash of light, he could stop photosynthesis at different stages of the reaction and hence produce different compounds. These include 3-phosphoglycerate, phosphoglyceraldehyde, which we’ve already seen, and ribulose bisphosphate.

The series of reactions that form these compounds make up the light-independent stage of photosynthesis, an outline of which is shown here. This stage is now commonly referred to as the Calvin cycle due to his significant contributions to its discovery. Now we’ve learned all about the experiments that have informed our understanding of photosynthesis, let’s have a go at a couple of practice questions.

After his experiments using photosynthetic bacteria, what did van Niel assume about photosynthesis in green plants? (A) The reactions were highly similar, but instead of glucose being formed, it would be sucrose. (B) The reactions were highly similar, but instead of carbon dioxide being a reactant, it would be oxygen. (C) The reactions were highly similar, but instead of hydrogen sulfide being broken down, it would be water. (D) The reactions would be completely identical. Or (E) the reactions would be completely different.

van Niel was a scientist who studied photosynthesis in green and purple sulfur bacteria. To help us answer this question, let’s have a look at the equation for photosynthesis in these bacteria. During this reaction, carbon dioxide and hydrogen sulfide are converted into glucose, water, and sulfur. The sulfur is produced when hydrogen sulfide is broken down, so the hydrogen atoms can be used to make glucose and water. Now, let’s compare this process to the equation for photosynthesis in green plants. We can see that the equations are very similar. The major difference is that in plants, instead of hydrogen sulfide being a key reactant, water is. And instead of sulfur being produced, oxygen is.

If we apply the same logic as before to the equation for photosynthesis in plants, we can assume that the oxygen produced comes from the breakdown of water. Now, let’s use these equations to eliminate some of the answer options. We can immediately exclude options (D) and (E). The reactions are not completely identical, but neither are they completely different. We can see that in both equations, glucose is a product, so option (A) cannot be correct. And in both cases, carbon dioxide is a reactant, not a product. So finally, we can eliminate option (B).

van Niel applied his knowledge of photosynthesis in green and purple sulfur bacteria to the process in green plants and made the assumption that the oxygen produced would’ve come from water. So the correct answer is (C). The reactions were highly similar, but instead of hydrogen sulfide being broken down, it would be water.

Let’s try another question.

Melvin Calvin investigated photosynthesis in algae. He determined that, in the process, a three-carbon compound was formed. What are the carbon atoms of this compound used for in plant cells? (A) To help synthesize other key elements such as oxygen, hydrogen, and calcium. (B) To be used as a reactant in chemosynthesis. (C) To act as cell signaling molecules. Or (D) to synthesize key biological molecules such as glucose, starch, proteins, and fats.

Melvin Calvin was an American biochemist who studied photosynthesis in the 1940s. It was his work on green algae that led to the development of the Calvin cycle, the sequence of chemical reactions which summarized the light-independent stage of photosynthesis that happens in all photosynthetic organisms, including plants. One of the key compounds that’s made during the carbon cycle is phosphoglyceraldehyde. Phosphoglyceraldehyde, or PGAL for short, is a three-carbon compound as you can see from the diagram of its chemical structure.

Once PGAL has been made, its carbon atoms are used to synthesize other organic compounds, which are vital for the growth and survival of plants. They can synthesize glucose, which is primarily used for cellular respiration to release energy. They can produce starch, which is a molecule used by plants to store energy. They can be used to synthesize proteins, which act as structural components and catalyze chemical reactions. And finally, they can make fats, which are stored in pollen grains and seeds.

We have therefore determined that the correct answer to the question is (D). The carbon atoms of the three-carbon compound that’s formed during photosynthesis are used to synthesize key biological molecules such as glucose, starch, proteins, and fats.

Let’s summarize what we’ve learnt in this video by reviewing the key points. Photosynthesis is the process by which plants convert carbon dioxide and water into glucose and oxygen using light energy. Other organisms, like green and purple sulfur bacteria, can also photosynthesize. The scientist van Niel proposed that the oxygen produced during plant photosynthesis comes from the breakdown of water. And finally, another scientist called Calvin carried out experiments to explain the light-independent stage of photosynthesis.

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