Photosynthesis and the Role of Pigments PDF

Unit 8: Photosynthesis Lesson 8.1 Photosynthesis and the Role of Pigments Contents Introduction 1 Learning Objectives 2 Warm Up 2 Learn about It! 3...

Unit 8: Photosynthesis Lesson 8.1 Photosynthesis and the Role of Pigments Contents Introduction 1 Learning Objectives 2 Warm Up 2 Learn about It! 3 Photosynthetic Pigments 4 Principal Pigment 5 Chlorophyll a 5 Accessory Pigments 7 Chlorophyll b 7 Carotenoids 9 Phycobilins 10 Key Points 11 Check Your Understanding 13 Challenge Yourself 14 Photo Credits 14 Bibliography 15 Unit 8: Photosynthesis Lesson 8.1 Photosynthesis and the Role of Pigments Introduction Inside our home, some of us love the kitchen the most. The kitchen is where most of the food are prepared. The kitchen serves our comfort food when we are hungry or even when we are stressed about our lives. We all know that plants are autotrophs since they make their own food through the use of sunlight. But how is this possible? Why can animals or even us humans not do the same? Plants have an organelle that can be compared to a kitchen. This organelle helps them convert certain molecules into food by using the energy from sunlight. This food is even available to other organisms that cannot synthesize their own food. In addition, this plant organelle has pigments with different colors. If you are 8.1. Photosynthesis and the Role of Pigments 1 Unit 8: Photosynthesis wondering how some plants do not have green leaves, it is because of these pigments. But these pigments do not just serve the purpose of giving vivid colors to plants. They have their own importance related to food production, which we will know as we progress with our discussion in this lesson. Learning Objectives DepEd Competency In this lesson, you should be able to do the Explain the importance of chlorophyll and other pigments following: (STEM_BIO11/12-IIa-j-3). Explain the functions of photosynthetic pigments. Distinguish principal photosynthetic pigments from accessory photosynthetic pigments. Warm Up Plant Pigment Chromatography 15 minutes Paper chromatography is a technique used to separate a mixture into its component molecules. In chromatography, pigments are dissolved in a solvent, which makes the molecules migrate at different rates because of differences in their solubilities. In this activity, we need to use an organic solvent to separate pigments from chloroplasts. Materials beaker (at least 250 ml) coin ether and acetone mixture (organic solvent) filter paper (best alternative to normal paper strips) leaf paper strip 8.1. Photosynthesis and the Role of Pigments 2 Unit 8: Photosynthesis Procedure 1. Form groups with three members each. You may also be grouped by the teacher depending on the availability of the materials. 2. Prepare a strip of paper that will fit the height of the available beaker. 3. To put the plant pigment on a strip of paper, roll the coin on a leaf with the paper below it for approximately 15 times to make a heavy green line. Perform this close to one end of the paper strip. 4. Put a small amount of mixture of ether and acetone (solvent) in the beaker. 5. Submerge the tip of the paper allowing the line of green pigment to touch the solvent. 6. Wait for a few minutes for the green pigment line to separate in colors. This setup is called a chromatogram. 7. Compare your output with that of your classmates. 8. Answer the guide questions afterward. Guide Questions 1. What did you observe on the green pigment line after submerging it to the solvent? 2. How will you describe the separated colors in the chromatogram? 3. If a different solvent was used, would you expect the same result? Why? Learn about It! Photosynthesis, as you have learned from the previous lessons, is the process in which autotrophs, particularly plants, produce their own food by harnessing the energy from the sun. Plants perform this intricate process in their chloroplasts, and the link between this organelle and the use of solar energy is the photosynthetic pigment that gives distinct color to our plants. How do chlorophyll and other pigments help in the process of photosynthesis in plants? 8.1. Photosynthesis and the Role of Pigments 3 Unit 8: Photosynthesis Photosynthetic Pigments Pigments are organic molecules that selectively absorb light of specific wavelengths. Plants have photosynthetic pigments built in the thylakoid membranes of their chloroplasts. Wavelengths of light that are not absorbed are reflected. This reflected light gives each pigment its characteristic color. For instance, a blue dress can be perceived with its “blue” color because the pigments in the fabric absorb the other colors, leaving only the blue portion of the spectrum to be reflected. The photoreceptors of our eyes then detect these reflected colors. The selective absorption of light by leaves also explains why leaves appear green (as in Fig. 8.1.1) to our eyes. The green color is poorly absorbed by chloroplasts and is thus reflected to the observer. Pigment molecules are a bit like antennas specialized for receiving light energy of only certain wavelengths. The energy captured by the pigments enables the plant to produce organic molecules or “food” from the raw materials present in its environment. Fig. 8.1.1. The characteristic color of plants allows them to become the primary producers in ecosystems by harnessing solar energy to produce high-energy organic molecules. Aside from the primary green color of plants, you might have also seen some plants with yellow or white color in their leaves. In all plants, the green pigment is the primary tool for the photosynthetic machinery. Other plants (as shown in Fig. 8.1.2) are said to have variegated leaves, i.e., both green and non-green portions are present. Apparently, less 8.1. Photosynthesis and the Role of Pigments 4 Unit 8: Photosynthesis green coloration in leaves will also reduce the photosynthetic efficiency of the plant. However, this does not mean that they are completely outcompeted by plants with green leaves. It has been found by Thomas Givnish of the University of Wisconsin that the variegation in leaves serves as a camouflage so they will be less likely eaten by herbivores. Fig. 8.1.2. Variegation in the leaves of some plants reduces their capacity for photosynthetic activity. However, in the wild, this results in an advantage that they become less likely eaten by herbivores. The pigments in plants, which can be classified according to their role during photosynthesis, can be grouped into principal and accessory pigments. The principal pigments contribute to the majority of food production during photosynthesis. The extent of the absorbance of energy by the principal pigments is supplemented by the accessory pigments. Principal Pigment Chlorophyll a Chloroplasts contain several kinds of pigments. We have already learned that photosynthesis in plants and algae occurs in the organelle called chloroplasts (as shown in Fig. 8.1.3). Leaves are the major sites of photosynthesis. Its green color is due to the pigment chlorophyll that can be found in the chloroplasts. This pigment is responsible for converting solar energy into chemical energy stored in organic molecules, particularly sugars, which can be utilized in other metabolic pathways, not just by plants, but by other heterotrophs. 8.1. Photosynthesis and the Role of Pigments 5 Unit 8: Photosynthesis Fig. 8.1.3. The chloroplast is a cell organelle in most autotrophs that can convert solar energy into reusable chemical energy. All chlorophylls consist of a porphyrin ring with a central magnesium core. One of which is the chlorophyll a or chl a (as shown in Fig. 8.1.4.). Being the principal pigment, chlorophyll a is the most common and most important photosynthetic pigment in plants, algae, some protists, and cyanobacteria. This pigment absorbs violet, blue, and red light, and reflects mainly green, so most plants appear green to us. It participates directly in the light reactions during photosynthesis, particularly an important player in the light-harvesting complex of chloroplasts. The porphyrin ring of chlorophyll a, similar to other chlorophylls, is stable. This feature of chlorophyll molecules is essential in the transfer of high-energy electrons. In this ring, the electrons can freely move; thus, the ring can either gain or lose them. This allows the chlorophyll a to transfer the energized electrons to other organic molecules to be stored in the form of chemical energy. This is the premise by which plants can “trap” or “capture” solar energy for food manufacture. 8.1. Photosynthesis and the Role of Pigments 6 Unit 8: Photosynthesis Fig. 8.1.4. The structure of chlorophyll a is similar to the other type of chlorophyll, the chlorophyll b. They differ in only one functional group (in red), CHO being present in chl b. How can one distinguish the accessory pigments from the principal pigment? Accessory Pigments Aside from chlorophyll a, the chloroplast also contains other pigments that are energy-capturing pigment molecules that are necessary for photosynthetic processes. Other pigments do not directly participate in the process of photosynthesis, although they are still of significance to the chloroplast. Each type of pigment absorbs certain wavelengths of light because it is able to absorb a specific amount of energy. The only absorbed light is necessary for photosynthesis. Chlorophyll b Briefly mentioned earlier is another similar pigment, the chlorophyll b, the structure of which is shown in Fig. 8.1.5. This pigment absorbs mainly blue and orange light but reflects olive green. Being an accessory pigment, chlorophyll b does not participate directly in light reactions, although it conveys absorbed energy to chlorophyll a to work in the light reactions. This type of chlorophyll is present only in green algae and higher plants (note that there are other algal groups such as red and brown algae). In addition, a closer look will reveal that chlorophyll a appears bluish-green, while chlorophyll b appears 8.1. Photosynthesis and the Role of Pigments 7 Unit 8: Photosynthesis yellow-green. The green leaves of many plants change color during autumn because they stop synthesizing pigments in preparation for a period of dormancy. The chlorophyll breaks down faster than the other pigments, so the leaves turn red, orange, yellow, or violet as their chlorophyll content declines, and other accessory pigments become visible. Fig. 8.1.5. The molecular structure of chlorophyll b (left) is very much like that of the chlorophyll a. As earlier mentioned, they differ in only one functional group, with a methyl group present in chlorophyll a and aldehyde group in chlorophyll b. Chlorophyll b is also present in higher plants and green algae, such as Volvox (right). Other types of chlorophylls exist, such as chlorophylls c and d (shown in Fig. 8.1.6.). Similarly, these chlorophylls are accessory pigments to other organisms, with chlorophyll a still considered as the universal photosynthetic pigment. Chlorophyll c is the accessory pigment of brown algae, diatoms, and dinoflagellates, whereas chlorophyll d is found only in red algae. Fig. 8.1.6. Accessory pigments chlorophyll c and d are present in brown algae (left, Sargassum sp.) and red algae (right, Gracilaria sp.), respectively. 8.1. Photosynthesis and the Role of Pigments 8 Unit 8: Photosynthesis Carotenoids Chloroplasts also contain pigments called carotenoids, which are various shades of red, yellow, and orange (such as in Fig. 8.1.7). This pigment absorbs mainly violet, blue, and green light. These colors are common in leaves during the season of fall. These yellow-orange hues of longer-lasting carotenoids appear once the chlorophyll breaks down. The energy captured by these pigments cannot be directly used during photosynthesis; they must first be transferred to chlorophyll to be relayed to the rest of the photosynthetic pathway. This pigment is also important in photoprotection. Carotenoids absorb and dissipate excess light energy that would otherwise damage the chlorophyll or interact with oxygen to form reactive oxidative molecules that can damage the cell. We also obtain this pigment when we eat carrots and other vegetables and fruits, which have a photoprotective role in our eyes. Fig. 8.1.7. Carotenoids are evident in autumn leaves (left) in temperate regions and in many of our fruits and vegetables, such as ripe bell peppers (right). Carotenoids can be divided into two groups. The red or orange pigments are the carotenes, whereas the yellow to brown pigments are the xanthophylls. Carotenes include alpha-carotene, beta-carotene, and lycopene. Xanthophyll includes lutein and fucoxanthin. Most carotenoids have antioxidant properties and are important for various aspects of vision, such as beta-carotene and lutein. Lycopene has been found to lower the risk of heart disease. Fucoxanthin, on the other hand, is the pigment that gives brown algae their characteristic color. 8.1. Photosynthesis and the Role of Pigments 9 Unit 8: Photosynthesis Phycobilins Phycobilins are accessory pigments found in red algae and cyanobacteria. Unlike chlorophyll, phycobilins are water-soluble, making them present in the aqueous cytoplasm or stroma of chloroplasts. Two colors of phycobilins exist: the red one (phycoerythrin) is present in red algae, while the blue one (phycocyanin) is present in cyanobacteria (hence called the blue-green algae shown in Fig. 8.1.8). Red algae thrive in deep waters of the seas, and their phycobilins allow them to still perform photosynthesis because of the ability of this pigment to absorb blue light and reflect the red one. Apparently, blue light penetrates the seawater a greater depth than other colors of longer wavelength, such as red. Fig. 8.1.8. Aside from chlorophyll a, the prokaryotic and “alga-like” cyanobacteria also have phycobilins that aids them during photosynthesis. Why do leaves of deciduous trees change color during fall? 8.1. Photosynthesis and the Role of Pigments 10 Unit 8: Photosynthesis Did You Know? Plants are built for photosynthesis. Plants are considered wizards. Their whole structure is built to support photosynthesis. The plant's roots are meant to absorb water, which is then transported by a special vascular tissue called xylem, so it can be accessible in the photosynthetic stem and leaves. Leaves contain a structure called stomata that control gas exchange and limit water loss. Leaves may have a waxy coating to reduce water loss. Some plants have spines to promote water condensation. Key Points ___________________________________________________________________________________________ Photosynthesis is the process in which plants harness solar energy and use it to synthesize high-energy organic compounds in the form of sugars. The process of photosynthesis in plants and algae occurs in the membrane-bound organelle called chloroplasts. Pigments are organic molecules that selectively absorb light of specific wavelengths and are built into the thylakoid membranes of the chloroplasts. Photosynthetic pigments can be classified into primary and accessory pigments based on their contribution to light energy harvesting during photosynthesis. Chlorophyll a is the primary pigment during photosynthesis. It is also the universal pigment in all photosynthetic organisms. It participates directly in light reactions and is the most common green photosynthetic pigment in plants, algae, some protists, and cyanobacteria. This pigment absorbs mainly blue, violet, and red light, thus it appears green to us. Chlorophyll b, an accessory pigment, absorbs mainly blue and orange light but reflects olive green. Although chlorophyll b does not participate directly in light 8.1. Photosynthesis and the Role of Pigments 11 Unit 8: Photosynthesis reactions, it conveys absorbed energy to chlorophyll a to work in the light reactions. Other types of chlorophyll exist, particularly chlorophylls c and d. These accessory pigments are found in brown and red algae, respectively. Carotenoids are accessory pigments consisting of various shades of yellow and orange. These pigments absorb mainly violet, blue, and green light. This pigment is important in photoprotection. These pigments also convey light energy harvest from other bands of the visible light from the sun. Phycobilins are also accessory pigments in red algae and cyanobacteria that either give red or blue coloration. They are especially important for deep-sea red algae as they can utilize the blue light that can penetrate into deeper waters. Aside from the primary and accessory classification of photosynthetic pigments, they can also be classified into chlorophyll, carotenoids, and phycobilins. ____________________________________________________________________________________________ 8.1. Photosynthesis and the Role of Pigments 12 Unit 8: Photosynthesis Check Your Understanding A. Identify the terms being described by the following statements. 1. This biological process primarily takes place in the leaves, and it harnesses solar energy to produce an organic by-product. 2. These organic molecules selectively absorb light of specific wavelengths, and they are built into the thylakoid membranes of chloroplasts. 3. This pigment participates directly in light reactions which absorbs mainly blue, violet, and red colors of visible light. 4. This pigment absorbs mainly blue and orange light but does not participate directly in light reactions, but rather it conveys absorbed energy to chlorophyll a for use in the photosynthetic pathway. 5. These pigments absorb mainly violet, blue, and green light, and they absorb and dissipate excess light energy that would otherwise damage chlorophyll. B. Write true if the statement is correct and false if otherwise. 1. Chlorophyll can only be found in chloroplasts, which are present in most photosynthetic organisms. 2. The green color of the leaves is due to the presence of chlorophyll. 3. Chloroplasts poorly absorb the green color, thus reflecting it to the observer. 4. Chlorophylls a and b are pigments that directly participate during light reaction of photosynthesis. 5. Carotenoids absorb and dissipate yellow and orange hues. 6. Each type of pigment absorbs a specific amount of energy. 7. Pigment molecules are specialized for receiving light energy of only certain wavelengths. 8. All colors absorbed by the pigments are necessary for photosynthesis. 9. Chlorophyll b is a pigment that is important in photoprotection. 10. Chlorophyll a absorbs mainly green color, so it appears green to the observer. 8.1. Photosynthesis and the Role of Pigments 13 Unit 8: Photosynthesis C. Determine the possible consequences should the following events take place. 1. A farmer accidentally treated his plant with a substance that degrades chlorophyll. 2. Chlorophyll degradation fails to occur in preparation for winter. 3. The chloroplast gene that synthesizes chlorophyll a becomes mutated. 4. Plants that normally produce xanthophylls and carotene suddenly failed to synthesize them. 5. Red algae suddenly needed to rely solely on its chlorophyll a. Challenge Yourself Provide brief answers and explanations to the following questions. 1. What could be the effect of stressful conditions such as drought and high temperature on chloroplasts of a plant? 2. Why is chloroplast the major site of photosynthesis? 3. How do carotenoids protect the cell from harmful oxidative molecules? 4. How would you distinguish chlorophyll a, chlorophyll b, and carotenoids? 5. How are pigments important to photosynthetic cells? Photo Credits Great Rosebay (Tabernaemontana corymbosa) (Variegated Leaf) by Mokkie is licensed under CC BY-SA 4.0 via Wikimedia Commons. Mikrofoto.de-volvox-4 by Frank Fox is licensed under CC BY-SA 3.0 DE via Wikimedia Commons. Gracilaria2 by Emoody26 at English Wikipedia is licensed under CC BY 3.0 via Wikimedia Commons. CSIRO ScienceImage 4203 A bluegreen algae species Cylindrospermum sp under magnification by CSIRO is licensed under CC BY 3.0 via Wikimedia Commons. 8.1. Photosynthesis and the Role of Pigments 14 Unit 8: Photosynthesis Bibliography Hoefnagels, Marielle. Biology: The Essentials. 2nd ed. McGraw-Hill Education, 2016. Mader, Sylvia S., and Michael Windelspecht. Biology. 11th ed. McGraw-Hill Education, 2014. Maoka, T. Carotenoids as natural functional pigments. J Nat Med 74, 1–16. 2020. https://link.springer.com/article/10.1007/s11418-019-01364-x#citeas, last accessed on March 19, 2020. Reece, Jane B, Martha R. Taylor, Eric J. Simon, Jean L. Dickey, and Kelly Hogan. Biology Concepts and Connections. 8th ed. Pearson Education South Asia Pte Ltd., 2016. Simon, Eric J., and Jane B. Reece. Campbell Essential Biology. 5th ed. Pearson Education Inc., 2013. Starr, Cecie, Christine A. Evers, and Lisa Starr. Biology Today and Tomorrow. 4th ed. Cengage Learning Asia Pte Ltd, 2014. Timson, J. How plants with patterned leaves compete. The Daily Newsletter, New Scientist. September 1, 1990. https://www.newscientist.com/article/mg12717323-700-science-how-plants-with-pat terned-leaves-compete/, last accessed on March 19, 2020. 8.1. Photosynthesis and the Role of Pigments 15