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.
