Video Transcript
In this video, we will begin by
reviewing genes and their expression. We will then learn how
environmental factors can affect gene expression and examine the relationships
between light intensity, chlorophyll synthesis, and the rate of photosynthesis in
plants. Let’s begin by defining some of the
key terms we’ll be using throughout the video.
A gene is a section of DNA that
contains the information needed to produce a certain characteristic or a functional
unit. For example, proteins are a type of
functional unit, and many genes contain the information needed to produce proteins
like enzymes and hormones. Gene expression is the process by
which the information encoded in the gene is used to create or synthesize
proteins. Due to the information they carry,
genes are partly responsible for how an organism looks and behaves. For example, genes code for the
proteins that determine eye color, which can vary considerably among
individuals. And genes can influence a person’s
tendency to be more shy or more social.
However, an organism’s environment
can cause the level of gene expression to vary, and these variations in gene
expression can in turn affect an organism’s appearance or its behavior. A rather extreme example of the
influence of environment on gene expression is seen in sea turtles. The temperature experienced by sea
turtle eggs determines the sex of the embryo inside. This, in turn, determines the sex
ratio of the hatchlings. When the temperatures experienced
by the eggs are warmer, the hatchlings will be primarily female. When temperatures are colder, the
hatchlings will be primarily male. And intermediate temperatures lead
to a sex ratio of approximately 50, 50. This phenomenon occurs because the
temperature experienced during egg incubation changes the expression of genes that
code for various enzymes and sex hormones.
Scientists continue to research the
evolutionary advantage of this temperature-dependent sex determination. In plants, environmental factors
such as carbon dioxide concentration, temperature, and light intensity affect the
expression of genes involved in photosynthesis. You probably already know that
photosynthesis is the process by which plants use light, carbon dioxide, and water
to produce glucose and oxygen. You may also remember that
photosynthesis takes place inside plant organelles called chloroplasts. If we zoom in to get a closer look
at a chloroplast, we can see a structure inside called the thylakoid membrane. And embedded in the thylakoid
membrane is the molecule chlorophyll.
Chlorophyll is a biological
pigment, which is a molecule that absorbs specific wavelengths of light while
reflecting other wavelengths. Chlorophyll absorbs blue and red
wavelengths of light more efficiently than green wavelengths. This means more green light is
available to be reflected by the plant, giving them their characteristic green
appearance. Inside the chloroplasts,
chlorophyll is constantly being broken down, so the plant must continually
synthesize new chlorophyll in order to maintain photosynthesis. In fact, the amount of available
chlorophyll is one of the main factors affecting the rate of photosynthesis. And the environmental factors that
affect gene expression influence the amount of available chlorophyll.
Now let’s take a closer look at how
the environmental factor of light intensity interacts with available chlorophyll to
affect the rate of photosynthesis. This simplified graph has light
intensity on the 𝑥-axis and photosynthetic rate on the 𝑦-axis. Starting at point 𝐴 and moving
right along the 𝑥-axis, we can see that as light intensity increases,
photosynthetic rate also increases. This relationship exists because
light intensity influences chlorophyll synthesis in plants. Under low-light conditions, the
genes involved in chlorophyll synthesis, shown here as blue bars on a strand of DNA,
will be downregulated. This means the plant will produce
fewer of the proteins needed to make chlorophyll and the rate of photosynthesis will
be low.
If the plant remains in these
low-light conditions, it may become yellow. In some cases, the plant may
die. But this process also occurs
naturally, such as when trees lose their leaves in the fall. This scenario of low light and low
photosynthetic rate is represented by point 𝐴 on the graph. Under conditions of high light, the
genes involved in chlorophyll synthesis will be upregulated. The plant will produce plenty of
the proteins needed to make chlorophyll and the rate of photosynthesis will be
high. Plants that are grown under
conditions of high-light intensity are typically greener and healthier than plants
grown in conditions of low light. The scenario of high-light
intensity and high photosynthetic rate is represented by point 𝐵 on the graph.
Increasing light intensity cannot
increase photosynthetic rate indefinitely. And we can see this on the
graph. After point 𝐵, the curve levels
off, or plateaus, and the rate of photosynthesis becomes stable. This occurs because other factors
become limiting. So further increases in chlorophyll
synthesis alone won’t allow the plant to increase its photosynthetic rate. However, changing another
environmental factor, such as increasing the temperature to a more optimal level,
will allow the plant to increase its photosynthetic rate relative to the colder
temperature. But even in this scenario, the
curve eventually plateaus and photosynthetic rate remains stable because neither
light intensity nor temperature are limiting factors.
Now that we’ve seen how light
intensity affects chlorophyll synthesis through gene expression, which in turn
affects photosynthetic rate, let’s try a practice question.
Chlorophyll is required by the
chloroplasts of leaves to capture light. For what process do the
chloroplasts require this light energy?
You may remember that chlorophyll
is the green pigment found within the thylakoid membrane of chloroplasts. A pigment is a molecule that
absorbs specific wavelengths of light and reflects other wavelengths. In the case of chlorophyll, blue
and red wavelengths of light are absorbed more efficiently than green wavelengths of
light. Inside the chloroplasts, this
absorbed light energy is used to break apart water molecules, releasing protons and
electrons that are used to create chemical energy. The chemical energy is then used to
fix the carbon from carbon dioxide into glucose and to release oxygen as a
product. The name for this process is
photosynthesis, and it’s how plants make their own nutrition to survive and
grow.
Therefore, the name of the process
that chloroplasts require to capture light energy is photosynthesis.
Now let’s try a final practice
question.
Three groups of plants belonging to
the same species are exposed to different intensities of light. Group one was exposed to the light
intensity shown for 12 hours, group two for 24 hours, and group three for 36
hours. The graph provided shows the amount
of chlorophyll synthesized by these groups of plants at different light
intensities. What can be concluded from this
graph? (A) For all light intensities, more
chlorophyll is synthesized when the plant is exposed to the light for 12 hours than
when it is exposed to the light for 36 or 24 hours. (B) For all light intensities, more
chlorophyll is synthesized when the plant is exposed to the light for 24 hours than
when it is exposed to the light for 36 or 12 hours. (C) The intensity of light does not
have an effect on how much chlorophyll is synthesized by a plant, but the duration
of exposure does. (D) For all light intensities, more
chlorophyll is synthesized when the plant is exposed to the light for 36 hours than
when it is exposed to the light for 24 or 12 hours.
The graph shows light intensity in
lx on the 𝑥-axis and the amount of chlorophyll being synthesized by the plant in
milligrams per gram on the 𝑦-axis. The three curves represent the
three groups of plants that were exposed to different light intensities: group one
for 12 hours, group two for 24 hours, and group three for 36 hours. We can see that all three groups
had an increase in chlorophyll synthesis as light intensity increased from zero to
5000 lx. And from this we can make a
conclusion that an initial increase in light intensity also increases chlorophyll
synthesis.
We can also see that at zero lx,
group three, the 36-hour group, synthesized more chlorophyll than groups one or
two. And this pattern holds true for
every light intensity. So a second conclusion we can make
is that group three synthesize the most chlorophyll at every light intensity. We now have enough information to
answer the question. So let’s bring back the answer
options one at a time to determine the correct one. “For all light intensities, more
chlorophyll is synthesized when the plant is exposed to the light for 12 hours than
when it is exposed to the light for 36 or 24 hours.” We concluded that group three, the
36-hour group, synthesized the most chlorophyll at all light intensities. So we can rule this answer choice
out.
“For all light intensities, more
chlorophyll is synthesized when the plant is exposed to the light for 24 hours than
when it is exposed to the light for 36 or 12 hours.” As with the previous answer, we can
rule this one out because we know that group three, the 36-hour group, synthesized
the most chlorophyll at all light intensities. “The intensity of light does not
have an effect on how much chlorophyll is synthesized by a plant, but the duration
of exposure does.” Well, our first conclusion was that
an increase in light intensity initially increases chlorophyll synthesis. So we can rule this answer choice
out.
“For all light intensities, more
chlorophyll is synthesized when the plant is exposed to the light for 36 hours than
when it is exposed to the light for 24 or 12 hours.” This answer choice is correct
because it restates our second conclusion about group three.
Now, let’s review some of the key
points from the video. Gene expression leads to the
synthesis of proteins. The level of gene expression can
vary, and this variation is influenced by an organism’s environment. Chloroplasts are the site of
photosynthesis, and they contain the green pigment chlorophyll. In plants, exposure to light
affects the expression of genes involved in chlorophyll synthesis. If light is a limiting factor, a
higher-light intensity will increase chlorophyll synthesis, while lower-light
intensities decrease chlorophyll synthesis. In turn, an increase in chlorophyll
synthesis leads to a higher photosynthetic rate, while a decrease in chlorophyll
synthesis leads to a lower photosynthetic rate. When light is no longer a limiting
factor, the rate of photosynthesis plateaus.
