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Pigments and Photosynthesis LEARNING OBJECTIVES 1 Apply the procedure of paper chromatography in the separation of the photosynthetic pigments of plants and relate the pigments to their photosynthetic ability 2 Discuss how experimentation with the Hill reaction provides evidence of the operation of the light reactions 3 Design and implement an experimental procedure to determine factors that affect the rate of photosynthesis in an organism 4 Analyze and interpret experimental data from the Hill reaction INTRODUCTION Photosynthesis is the process by which light energy is converted to chemical bond energy Autotrophic organisms are responsible for primary production and photosynthesis is the process by which such production is affected In terrestrial communities plants are the most significant photosynthetic organisms In aquatic communities phytoplankton contribute the most to primary production Photosynthesis comprises numerous vital and complex reactions that take place in the cells of organisms capable of carrying out this process The process can be summarized with this chemical equation 6 CO2 12H2O Light Energy C6H12O6 6O2 6H2O The stomata small openings in the epidermis of the plant allow CO2 and H20 to leave the plant and 02 to enter the plant Guard cells surround the stomata and control the opening and closing of the stomata When guard cells take on water they pull apart and allow gas exchange to occur and water vapor to escape See Figure 6 Figure 6 Guard cells purple surrounding stomata green CELLULAR STRUCTURES Before considering the molecules that are involved in the photosynthetic reactions let s examine the cellular structures in which these molecules are contained Most photosynthetic organisms are eukaryotic and the photosynthetic reactions take place in cytoplasmic organelles known as chloroplasts Although photosynthetic eukaryotes such as plants and algae are more common photosynthetic bacteria are also abundant Since bacteria are prokaryotic their cells do not contain organelles but they do have membrane layers organized into thylakoids that perform functions analogous to chloroplasts The chloroplast houses the photosynthetic process in eukaryotic cells In each chloroplast a double membrane surrounds a fluid filled compartment known as the stroma Enzymes electron carriers and numerous other molecules essential to photosynthesis are dissolved in this fluid Also within the stroma are abundant thylakoids Each thylakoid is a tiny membranous vesicle containing photosynthetic pigments and electron carriers These thylakoids are organized in stacks known as grana singular granum Different photosynthetic reactions take place in the grana and the stroma See Figure 7 Figure 7 Leaf cross section and chloroplast https en wikipedia org wiki Chloroplast media File Figure 08 01 05 png https www pathwayz org Tree Plain CROSS SECTION OF A LEAF 5BBASIC 5D PHOTOSYNTHETIC REACTIONS The total process of photosynthesis comprises two major series of reactions These two series are known as the light reactions and the dark reactions or light independent reactions This terminology emphasizes the form of energy required to drive these different reactions The light reactions require light energy in order to take place The dark reactions can and do take place in the dark but they also take place in the light Dark is used to emphasize that these reactions do not require light energy in order to take place but they also take place in the light They do require specific forms of chemical energy that are produced in the light reactions Accordingly the light reactions must begin and produce this appropriate chemical energy before the dark reactions will take place LIGHT REACTIONS AND PHOTOSYNTHETIC PIGMENTS The light reactions take place in the membranes of chloroplast thylakoids These reactions are possible because the embedded pigment molecules absorb the energy of sunlight Each pigment is capable of absorbing specific wavelengths of light energy in the visible spectrum Photosynthetic pigments are classified functionally as primary and accessory The primary pigment is that molecule that is capable of giving up an excited electron such that light energy is converted to chemical bond energy In eukaryotes and some bacteria the single primary pigment is a type of chlorophyll designated as chlorophyll a In other groups of photosynthetic bacteria the primary pigment is a bacteriochlorophyll that performs a similar function When these pigment molecules absorb sufficient energy of the appropriate wavelengths they release high energy electrons that drive the light reactions There are several accessory pigments in both eukaryotic and prokaryotic cells Whereas chlorophyll a is a green pigment the accessory pigments may also be green chlorophyll b or they may appear blue green yellow orange or brown The familiar yellow and orange of hues autumn foliage are produced by those accessory pigments known as carotenoids These pigments absorb wavelengths other than those absorbed by chlorophyll a and thus increase the organism s total capacity for light absorption You should be able to identify these pigments in the chromatogram you will he doing today Through a process known as electron resonance accessory pigments transfer their absorbed energy to the primary pigment This transferred energy excites the primary pigment chlorophyll a which then gives up a high energy electron By both direct absorption of light and transferred energy the chlorophyll a is capable of driving the light reactions As the light reactions take place in the thylakoids of the chloroplast they produce chemical energy in the forms of NADPH and ATP that drive the dark reactions in the stroma See a summary of the light reactions below Figure 8 Figure 8 Light reactions https en wikipedia org wiki Light dependent reactions DARK REACTIONS AND FORMATION OF GLUCOSE These dark reactions are commonly called the Calvin cycle This name refers to the physiologist Melvin Calvin who first described these reactions in the 1940s and 1950s The function of the Calvin cycle is to produce glucose the carbohydrate that incorporates energy initially harvested from sunlight The glucose molecules will be used in many ways in the cells For example glucose is the starting point for the synthesis of numerous other sugars that are involved in metabolism Hundreds of glucose molecules may be bonded to one another to form carbohydrate polymers such as starch that serve as

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NCSU BIO 183 - Pigments and Photosynthesis

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