Lesson Explainer: Environmental Effects on Gene Expression Biology

In this explainer, we will learn how to describe the effect of light intensity on the production and expression of chlorophyll genes.

As autumn starts and the hours of daylight become shorter, the leaves of trees also undergo a change, such that their vibrant green leaves gradually turn red or yellow. While festive and seemingly automatic, this change in the color of leaves is actually directly related to the environment. Genes can determine a large portion of our physical characteristics, but the environment that surrounds us is in part responsible for how our genes are expressed.

Let’s take a closer look at the relationship between gene expression and the environment in plants.

The environment can be many things, like sunlight, our food choices, pollution exposure, oxygen deficiency and UV rays and it even includes the surroundings you were exposed to as a fetus. In plants, the most impactful environmental factors include those that affect photosynthesis, such as sunlight exposure and carbon dioxide concentration.

Photosynthesis is the process by which plants make their own food. In photosynthesis, carbon dioxide enters the plant through the leaf stomata and water is transported up from the roots, and these reactants are converted into sugars, such as glucose and oxygen by sunlight, as shown in Figure 1. The term photosynthesis is derived from the Greek words “photo” meaning “light” and “synthesis” meaning “putting together.” The main sites of photosynthesis are the chloroplasts found in green leaves. There are often over half a million chloroplasts per square millimetre of a leaf!

Definition: Photosynthesis

Photosynthesis is the process by which green plants convert carbon dioxide and water into sugars, such as glucose and oxygen in the presence of sunlight.

Reaction: Photosynthesis

Carbondioxidewaterlightenergysugaroxygen+(+)+

Chloroplasts are specialized organelles largely found in the cells of the leaf. Inside each chloroplast are structures called thylakoids that are arranged in stacks called grana (singular: granum). You can see the anatomy of the leaf, including the chloroplasts, the thylakoids, and grana in Figure 2. Found within the membrane of each of the thylakoids is chlorophyll, the green pigment that gives plants their characteristic green color. A pigment is a molecule that absorbs only a specific wavelength of light but reflects others.

Definition: Chlorophyll

Chlorophyll is a class of green pigments found in the chloroplasts of plants that absorbs the light energy required for photosynthesis.

Chlorophyll is used by all plants to absorb the specific wavelengths of sunlight needed in photosynthesis. The chlorophyll found in the chloroplasts of green plants absorbs all the red and blue wavelengths of light, but it reflects green wavelengths. While leaves contain several other pigments, chlorophyll is the most abundant in plants.

The chloroplasts are where the two stages of photosynthesis, the light-dependent and the light-independent stages, take place. The light-dependent stage converts light energy into the reactants that will be used in the light-independent stage. The light energy needed in the light-dependent stage is captured by chlorophyll.

Example 1: Describing the Function of Chlorophyll

Chlorophyll is required by the chloroplasts of leaves to capture light. For what process do the chloroplasts require this light energy?

  1. Photosynthesis
  2. Respiration
  3. Mitosis
  4. Transcription
  5. Translation

Answer

Chlorophyll is the green pigment used by plants to absorb the specific wavelengths of sunlight to be used as the light energy needed in photosynthesis. A pigment is a molecule that absorbs only a specific wavelength of light but reflects others. Chlorophyll is found in the chloroplasts of plants cells. It absorbs all the red and blue wavelengths of light, but it reflects green wavelengths. This is why plants appear green.

Inside each chloroplast are structures called thylakoids that are arranged in stacks called grana (singular: granum). Within the membrane of each thylakoid is a chlorophyll. The chloroplasts are where the two stages of photosynthesis, which is the process by which plants make their own food, take place. In photosynthesis, carbon dioxide enters the plant through the leaf stomata and is combined with water to be converted into sugars, such as glucose, and oxygen by sunlight.

The equation for photosynthesis is as follows: Carbondioxidewaterlightenergysugaroxygen+(+)+

Photosynthesis takes place in two stages, the light-dependent stage and the light-independent stage. It is in the light-dependent stage of photosynthesis that sunlight is captured by chlorophyll and the energy is used to combine carbon dioxide and water. The products of the light-dependent stage can then be used in the light-independent stage. Without the energy from sunlight, the first stage of photosynthesis would be unable to proceed.

Therefore, chlorophyll is required by the chloroplasts to capture light for the process of photosynthesis.

Chlorophyll is not very stable, so it is continually being broken down inside cells. In order to maintain a sufficient amount of this pigment inside leaves, chlorophyll must be continually synthesized to be used in photosynthesis.

In genetics, proteins are complex biological macromolecules that perform functions and are put together or “synthesized” from genes, which are small sections of DNA. You may remember that the sequence of bases in a section of DNA provides specific instructions on how to make a protein. So, protein synthesis is a process that involves translating certain sections of mRNA, which is made from DNA, into specific proteins.

Definition: Protein

A protein is a complex biological macromolecule that is made up of amino acid monomers and can have a wide variety of forms and functions.

Definition: Gene

A gene is a section of DNA that contains the information needed to produce a functional unit, for example, a protein. It is the functional unit of heredity.

Key Term: Protein Synthesis

Protein synthesis is a process that involves translating the genetic code of mRNA, which is made from DNA, into specific proteins.

The rate of photosynthesis in plants is not always the same. This is because the genes that regulate synthesis of chlorophyll interact with environment factors. There are four key limiting factors in photosynthesis: light intensity, temperature, amount of chlorophyll, and carbon dioxide concentration. These four limiting factors can interact with each other to change the rate of photosynthesis.

For example, light intensity and the amount of chlorophyll are related environmental limiting factors.

The amount of sunlight that a plant is exposed to determines the synthesis of proteins that are expressed or “turned on” to produce chlorophyll. As more chlorophyll is synthesized, more light can be absorbed, which increases the rate of photosynthesis.

Key Term: Gene Expression

Gene expression is the process where the information encoded in a gene is used to create proteins.

To better understand the relationship between light intensity and the rate of photosynthesis, we can graph the relationship as shown below in Figure 3.

Light intensity affects the rate of photosynthesis because of the interaction between chlorophyll and light absorption. Plants synthesize chlorophyll in order to absorb the light required for photosynthesis. So, increasing the light intensity increases the amount of chlorophyll produced, which in turn increases the rate of photosynthesis. This is shown between points A and B on the graph in Figure 3.

However, this increase in the rate of photosynthesis does not continue indefinitely.

When all of the chlorophyll in the leaves is being used to convert the light energy, the rate of photosynthesis can no longer increase. This means light is no longer a limiting factor. This change in the relationship between light intensity and photosynthesis changes the slope of the graph line.

After the light intensity can no longer increase the rate of photosynthesis, the reaction “levels off” or plateaus. When a reaction plateaus, it means that the reaction is stable and will not increase anymore. This is shown as part C on the graph in Figure 3. At this point, another factor will need to change to increase the rate of photosynthesis; for instance, the temperature.

So, in environments with low levels of sunlight, the rate of photosynthesis decreases that it may cause the plant to die. This can happen naturally, like when the seasons change, or it can happen because of a malfunction in gene expression of chlorophyll. Diseases that affect a plant’s ability to synthesize chlorophyll are often lethal to plants.

When there is less sunlight, for example during winter, the need for chlorophyll is much lower, so, less of the chlorophyll needs to be synthesized. As the production of this green pigment is decreased, the plant develops a yellow color, further slowing the rate of photosynthesis. Over time, the decreased synthesis of chlorophyll and the decrease of photosynthesis can lead to death of the plant.

Example 2: Describing the Relationship between Light Intensity and Synthesis of Chlorophyll

Which of the following correctly describes the relationship between light intensity and the synthesis of chlorophyll in a plant?

  1. Prolonged exposure to low light levels, or no light, prevents a plant from synthesizing enough chlorophyll.
  2. The more light a plant is exposed to, the less chlorophyll will be synthesized.
  3. Exposure to light intensity has no effect on the synthesis of chlorophyll, only on how much photosynthesis can be carried out.

Answer

Chlorophyll is the green pigment used by plants to absorb the specific wavelengths of sunlight to be used as the light energy needed in photosynthesis. It helps capture the light energy needed in photosynthesis. However, chlorophyll is continually being broken down inside cells. In order to maintain a sufficient amount of this pigment inside leaves, chlorophyll must be continually synthesized to be used in photosynthesis.

In genetics, proteins are complex biological macromolecules that perform functions and are put together or “synthesized” from genes, which are small sections of DNA. You may remember that the sequence of bases in a section of DNA provides specific instructions on how to make a protein. So, protein synthesis is a process that involves translating the genetic code of mRNA, which is made from DNA, into specific proteins.

The rate at which a plant conducts photosynthesis is affected by environmental factors like light intensity. This is because the genes that regulate synthesis of chlorophyll interact with the environment. The amount of sunlight that a plant is exposed to determines the synthesis of proteins that are expressed to produce chlorophyll.

The more sunlight is available, the more chlorophyll is synthesized and expressed for use in photosynthesis. As more chlorophyll can be made, more light can be absorbed, and the photosynthesis rate becomes faster.

So, the interaction between the environmental factor of light intensity and the gene for chlorophyll synthesis controls the growth and development of the plant. As light intensity increases, so does the rate of photosynthesis.

Therefore, the relationship between light intensity and the synthesis of chlorophyll in a plant is given by option A: prolonged exposure to low light levels, or no light, prevents a plant from synthesizing enough chlorophyll.

Demonstration: Observing the Environmental Effects of Light Intensity on Plant Growth

To see the relationship between chlorophyll synthesis and environmental factors like light exposure, we can perform a small experiment using two plants.

Experimental Steps

First, gather all the items needed for the experiment, which includes two healthy green seedling plants. Make sure the seedlings are identical in every way. To ensure the two seedling plants for our experiment are identical, take seeds from a plant that is self-pollinated. This way, you can ensure that the seedlings are very similar and any differences we observe are due to the environmental effects and not genetic differences in the plant.

Next, we will test the impact of light exposure.

To do this, place one plant under a halogen light bulb to provide constant light to the plant. Place the second seedling plant in a room where there is limited lighting, like a closet or another shady place.

Thirdly, over the next week (7 days), treat the two plants identically, including watering them regularly.

Finally, at the end of the week, measure the changes in the plants.

Results

You should be able to see a change in the color and health of the plants.

Discussion of the Results

The plant that is grown in a well-lit area will be healthy and have very green leaves, much like the plant on the left below. This is because with light exposure, more chlorophyll is able to be synthesized. As more chlorophyll is synthesized, more food is made for the plant, and the healthier the plant grows.

In contrast, the plant grown in a dark room will not be healthy or very green. In fact, the plant grown in the dark is likely to be yellow or even brown. This is because with no light exposure, chlorophyll is not synthesized, and as the amount of chlorophyll is reduced, the less green a plant will appear, much like the plant on the right above. Additionally, the less chlorophyll that is synthesized, the less nutrition is made for the plant, and the sicker the plant grows. Ultimately, without good light exposure, photosynthesis is unable to proceed, and the plant will not be able to make its own nutrition

Example 3: Visualizing the Relationship between Light Intensity and Chlorophyll Synthesis

Three groups of plants belonging to the same species are exposed to different intensities of light. Group 1 was exposed to the light intensities shown for 12 hours, group 2 for 24 hours, and group 3 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?

  1. 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.
  2. 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.
  3. 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.
  4. The intensity of light does not have an effect on how much chlorophyll is synthesized by a plant, but the duration of exposure does.

Answer

Light provides the energy necessary for photosynthesis to occur, so, without light, there is no photosynthesis. Light intensity and amount of chlorophyll are related environmental limiting factors. Plants synthesize chlorophyll in order to absorb the light required for photosynthesis. The amount of light that a plant is exposed to determines the synthesis of proteins that are expressed or “turned on” to produce chlorophyll.

The higher the intensity of the light, the more chlorophyll is synthesized and expressed for use in photosynthesis. This is shown as the linear relationship between rate of photosynthesis and light intensity. So, increasing the light intensity increases the amount of chlorophyll produced, which in turn increases the rate of photosynthesis. However, this increase in the rate of photosynthesis does not continue indefinitely.

When all of the chlorophyll in the leaves is being used to convert the light energy, the rate of photosynthesis can no longer increase. This means light is no longer a limiting factor. This change in the relationship between light intensity and photosynthesis changes the slope of the graph line.

After the light intensity can no longer increase the rate of photosynthesis, the reaction “levels off” or plateaus. When a reaction plateaus, it means that the reaction is stable and will not increase anymore.

Using this information to interpret the lines on the graph, for group 1, where light exposure was for 12 hours, we see a linear increase from 0 to 5‎ ‎000 lx. After 5‎ ‎000 lx, the slope of the group 1 plants is flat, meaning there was no increase in chlorophyll production with light intensities from 5‎ ‎000 lx to 20‎ ‎000 lx.

For group 2, which was exposed to the light intensities shown for 24 hours, we see a linear increase from 0 to 5‎ ‎000 lx. From 5‎ ‎000 lx to 10‎ ‎000 lx, there is still a linear slope in the graph, even if it is not as steep as it was from 0 to 5‎ ‎000 lx. Then, from 10‎ ‎000 lx to 20‎ ‎000 lx, the slope of the group 2 plants is flat, meaning there was no increase in chlorophyll production with these light intensities.

For group 3, which was exposed to the light intensities shown for 36 hours, we see a linear increase from 0 to 20‎ ‎000 lx. There is no plateau in the synthesis of chlorophyll for group 3 plants that were exposed to all the light intensities for 36 hours. This means there was no limitation in the amount of chlorophyll that was synthesized in group 3 plants. The lack of a plateau in the group 3 plants in the rate of chlorophyll synthesis shows that the amount of chlorophyll in group 3 plants is higher than the plants in groups 1 (the 12-hour group) and 2 (the 24-hour group).

Therefore, from the graph, it can be concluded that 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.

Let’s summarize what we have learned in this explainer.

Key Points

  • The genes in plants can be impacted by the environmental factors that affect the synthesis of proteins.
  • Chloroplasts are the site of photosynthesis and contain the green pigment chlorophyll.
  • In plants, exposure to sunlight affects the synthesis of chlorophyll, thus affecting the expression of the green pigment.
  • The environmental factors can affect the rate of photosynthesis.
  • The more intense the light, the more chlorophyll is synthesized, which increases the rates of photosynthesis.
  • On the other hand, the rate of photosynthesis does not increase indefinitely. When light intensity can no longer influence the rate of photosynthesis, the rate of photosynthesis stabilizes or plateaus.
  • When a reaction plateaus, it means that the reaction is stable and will not increase anymore.

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