Pigments of Photosynthesis - Online Tutor, Practice Problems & Exam Prep

Photosynthesis relies on pigments within chloroplasts, primarily chlorophyll a, which absorbs light energy to drive the process. Accessory pigments, like chlorophyll b and carotenoids, absorb different wavelengths, reflecting others, which is why plants appear green. The absorption spectrum illustrates how various pigments absorb light at specific wavelengths, crucial for photosynthesis. Photosystems, complexes of pigments and proteins in thylakoid membranes, play a vital role in capturing light energy, facilitating the light reactions of photosynthesis. Understanding these components is essential for grasping the overall process of photosynthesis.

Pigments of Photosynthesis

In this video, we're going to begin our lesson on pigments of photosystems. But really, we're going to start off focusing on pigments, and we'll define what photosystems are a little bit later in our course in a different video. And so what you need to know now is that in order for plants to harness the light energy that's required to power photosynthesis, chloroplasts, the site of photosynthesis, need to have several different types of pigments. Ultimately, these pigments are really important for driving photosynthesis because they allow for the harnessing of the light energy. What exactly are these pigments? Well, pigments are defined as molecules that have this amazing ability to absorb wavelengths of visible light, which can then be utilized to power photosynthesis. Now, the main pigment that you all should be aware of is chlorophyll a, and chlorophyll is not to be confused with chloroplast. Remember, chloroplasts are organelles that serve as the site of photosynthesis, and chlorophyll, even though it sounds like chloroplast, is not the same thing as a chloroplast. Chlorophyll is once again a type of pigment. In fact, chlorophyll a is the main photosynthetic pigment found within chloroplasts. Chlorophyll a, even though it is the main photosynthetic pigment, it's not the only photosynthetic pigment. Accessory pigments are pretty much all of the other photosynthetic pigments that are not chlorophyll a.

It's important to note that different pigments are going to absorb different wavelengths of light. Also, some wavelengths of light are actually going to be absorbed and utilized to drive photosynthesis, but other wavelengths of light are not going to be absorbed. Instead, these other wavelengths of light will be reflected. Reflected light is really the light that we end up visualizing with our eyes. Some wavelengths of light are absorbed, others are reflected. We visualize the reflected wavelengths of light, but the absorbed wavelengths of light are really the light that is being used to drive photosynthesis, not the reflected light.

Let's take a look at our image down below to get a better understanding of the different types of photosynthetic pigments. Here in this first column, what we're showing you is the main photosynthetic pigment which is, of course, chlorophyll a. Chlorophyll a, as you'll notice, is going to mainly reflect green and blue. You can see that green wavelengths of light are reflected almost the most. The reason we perceive plants to be green is that they reflect green wavelengths of light, and so we end up visualizing those green wavelengths of light. But notice that the other wavelengths of light like yellow and orange, are going to be absorbed by chlorophyll a and, that energy that is absorbed is going to be used to power photosynthesis. Over here in this next column what we have is a different type of chlorophyll, which is chlorophyll b. Chlorophyll b is not as predominant as chlorophyll a, it's found in smaller amounts. Notice it actually reflects mainly yellow and green wavelengths of light. The yellow wavelength of light here is being detected by our eyes because that's the light that's being reflected. Thus, we can perceive chlorophyll b as being yellow in color, but once again it absorbs all of these other wavelengths of light which can be utilized to drive photosynthesis.

Last but not least, over here on the far right, what we have are the carotenoids. The carotenoids with this little carrot prefix, which can remind you of the carrots themselves, mainly reflect orange, red, and yellow wavelengths. You can see the orange wavelength of light being reflected here, and so we can see these as being orange because this is the light that's being reflected. But once again, the other wavelengths of light are being absorbed and utilized. This here really concludes our brief introduction to the pigments of photosystems. As we move forward in our course, we'll continue to talk more about pigments and we'll also introduce photosystems as well. I'll see you all in our next video.

Examples of accessory pigments for photosynthesis are:

a) Chlorophyll b and carotenoids.

b) Chlorophyll a and chlorophyll b.

c) Chlorophyll a and carotenoids.

d) Carotenoids, chlorophyll b and chlorophyll a.

• A. Chlorophyll b and carotenoids.
• B. Chlorophyll a and chlorophyll b.
• C. Chlorophyll a and carotenoids.
• D. Carotenoids, chlorophyll b and chlorophyll a.

Absorption Spectrum of Photosynthesis

In this video, we're briefly going to talk about the absorption spectrum of photosynthesis. An absorption spectrum is really just a graph that shows the light absorption of pigment molecules. Notice that we're showing you a pigment light absorption spectrum, and on the y-axis, we indicate the absorption of light. Low on the y-axis represents a low amount of light being absorbed and high on the y-axis represents a lot of light being absorbed. On the x-axis, we show the particular wavelength of light in nanometers, essentially just showing you the visible wavelength of light here.

It's important to note that on this particular spectrum, we're showing the curves for three different pigments: chlorophyll a in green, chlorophyll b in yellow, and carotenoids in a dark orangish color. You can see the spectra for each of these curves throughout the graph. These curves are just showing the amount of light that's being absorbed at a particular wavelength. You'll notice that wavelengths in specific regions are mainly being absorbed, which is marked on the graph. However, in the middle region from here to here, where there are not many peaks, it indicates that there's not a lot of absorption, so these wavelengths are being reflected.

That's the biggest takeaway here: different pigments are going to absorb and reflect different wavelengths of light, but collectively it's the amount of them and the particular wavelengths of light that are reflected that dictate the colors we end up seeing in the plants. This concludes our brief introduction to an absorption spectrum, and we'll be able to get some practice as we move along in our course. I'll see you all in our next video.

Which of the following pigments does NOT absorb yellow/orange light (650-750nm)?

a) Chlorophyll a.

b) Carotenoids.

c) Chlorophyll b.

d)  Both a & c.

• A. Chlorophyll a.
• B. Carotenoids.
• C. Chlorophyll b.
• D. Both a and c.

Introduction to Photosystems

In this video, we're going to begin our introduction to photosystems. Photosystems are defined as complexes of pigments, proteins, and other molecules found in the thylakoid membrane. Recall from our previous lesson videos that these thylakoids are the green pancake-looking structures in the image of the chloroplast organelle. On the inside of these chloroplast organelles are these green-looking pancake structures called thylakoids. If we zoom into just one of these structures, that is where we find the photosystem. Notice in this image, here, representing the membrane of the thylakoid. Inside the thylakoid, the inside of the green pancake itself, and just above the thylakoid membrane, we have the stroma, which is outside of the green pancake but still inside the chloroplast.

Embedded within the thylakoid membrane is where we find our photosystem, this complex of pigments, proteins, and other molecules. Photosystems are composed of several light-harvesting complexes that surround a reaction center. Notice our light-harvesting complexes color-coded here in this purplish color, which corresponds to the purplish colors seen below. We also have the reaction center up above in orange that matches the reaction center below. On our image below, notice in purple here and on the right, we have the light-harvesting complexes. They surround this central region, which contains the reaction center. The reaction center is highlighted in orange right in the middle, surrounded by light-harvesting complexes as described above.

Within the light-harvesting complex, you'll notice teal circles representing accessory pigments helpful in absorbing photons of light. Photons of light interact with the light-harvesting complex, enabled by these accessory pigments that absorb light. Looking at the reaction center again in the middle, notice it has two green molecules that are chlorophyll a molecules, the predominant pigment found inside chloroplasts. Also, in red, there is the primary electron acceptor, which will be covered more later in our course. This photosystem is crucial for absorbing light, which allows chloroplasts to absorb light due to its accessory pigments and chlorophyll a.

As we move forward in our course, we'll talk more about photosystems and their involvement in what's known as the light reaction. Most plants will have two photosystems involved in performing the light reactions of photosynthesis, which we'll discuss more later. But for now, this is our brief introduction to photosystems, how they are found inside the membranes of thylakoids, these green pancake structures within chloroplasts, and their importance in absorbing light and involvement with the light reactions. We'll be able to apply some of these concepts as we move forward in our course. So I'll see you all in our next video.

The figure shows the absorption spectrum for chlorophyll a and the action spectrum for photosynthesis. Why are they different?

a) Green and yellow wavelengths of light inhibit the absorption of red and blue wavelengths.

b) Oxygen given off during photosynthesis interferes with the absorption of light.

c) Accessory pigments are absorbing light in addition to chlorophyll a which can be used in photosynthesis.

d) Aerobic bacteria take up oxygen, which changes the measurement of the rate of photosynthesis.

• A. Green and yellow wavelengths of light inhibit the absorption of red and blue wavelengths.
• B. Oxygen given off during photosynthesis interferes with the absorption of light.
• C. Accessory pigments are absorbing light in addition to chlorophyll a which can be used in photosynthesis.
• D. Aerobic bacteria take up oxygen, which changes the measurement of the rate of photosynthesis.

Here’s what students ask on this topic:

What are the main pigments involved in photosynthesis?

The main pigments involved in photosynthesis are chlorophyll a, chlorophyll b, and carotenoids. Chlorophyll a is the primary photosynthetic pigment, absorbing light most efficiently in the blue-violet and red parts of the electromagnetic spectrum. Chlorophyll b acts as an accessory pigment, extending the range of light wavelengths a plant can use by absorbing blue and red-orange light. Carotenoids, which include carotenes and xanthophylls, absorb light in the blue and green parts of the spectrum and reflect yellow, orange, and red wavelengths. These pigments work together to capture light energy, which is then used to drive the photosynthetic process.

How do different pigments absorb different wavelengths of light?

Different pigments absorb different wavelengths of light due to their unique molecular structures, which determine the specific wavelengths they can absorb. Chlorophyll a absorbs light primarily in the blue-violet and red regions, while chlorophyll b absorbs light in the blue and red-orange regions. Carotenoids absorb light in the blue and green regions. The absorption spectrum of each pigment shows the specific wavelengths of light that are absorbed, with peaks indicating the wavelengths where absorption is highest. This diversity in absorption allows plants to utilize a broader range of the light spectrum for photosynthesis.

Why do plants appear green?

Plants appear green because chlorophyll a, the main photosynthetic pigment, reflects green wavelengths of light. While chlorophyll a absorbs light most efficiently in the blue-violet and red parts of the spectrum, it reflects green light, which is not absorbed. This reflected green light is what we perceive with our eyes, giving plants their characteristic green color. Accessory pigments like chlorophyll b and carotenoids also contribute to the absorption of light but do not significantly alter the green appearance of plants.

What is the role of photosystems in photosynthesis?

Photosystems are complexes of pigments, proteins, and other molecules located in the thylakoid membranes of chloroplasts. They play a crucial role in capturing light energy and converting it into chemical energy during the light reactions of photosynthesis. Each photosystem consists of a reaction center surrounded by light-harvesting complexes. The reaction center contains chlorophyll a molecules and a primary electron acceptor. When light is absorbed by the pigments in the light-harvesting complexes, the energy is transferred to the reaction center, where it excites electrons. These high-energy electrons are then used to drive the synthesis of ATP and NADPH, which are essential for the Calvin cycle.

What is an absorption spectrum and why is it important in photosynthesis?

An absorption spectrum is a graph that shows the absorption of light by pigment molecules at different wavelengths. It is important in photosynthesis because it illustrates which wavelengths of light are absorbed by the various pigments involved in the process. The absorption spectrum helps us understand how different pigments, such as chlorophyll a, chlorophyll b, and carotenoids, contribute to capturing light energy. By absorbing light at different wavelengths, these pigments maximize the efficiency of photosynthesis, allowing plants to utilize a broader range of the light spectrum for energy conversion.