Photosynthesis PDF

Dr. GHERAISSA N. Module: Plant Physiology CHAPTER III: Photosynthesis In Chapter III, "Photosynthesis," we dissect the intricacies of light and dark phases, unraveling processes within chloroplasts, photophosphorylation, and t...

Dr. GHERAISSA N. Module: Plant Physiology CHAPTER III: Photosynthesis In Chapter III, "Photosynthesis," we dissect the intricacies of light and dark phases, unraveling processes within chloroplasts, photophosphorylation, and the Calvin Cycle. The chapter concludes with an assessment and a comparative analysis of photosynthesis in C3, C4, and CAM plants, providing a comprehensive overview of plant energy conversion. Dr. GHERAISSA N. Module: Plant Physiology INTRODUCTION Carbon nutrition is the process through which the green plant obtains the materials necessary for its survival, as the green plant is considered autotrophic (able to synthesize its own materials necessary for growth). The green plant is transformed according to the visual synthesis process into: 1) Chemical energy represented by the energy-rich compound adenosine triphosphate (ATP). 2) Reductive power in the form of the coenzyme nicotinamide Adenine Dinucleotide Phosphate Reductase (NADPH++H+). These two compounds lead the reactions leading to the conversion of carbon dioxide (CO2) into carbohydrates, which represent a source of energy in the cell and constitute raw materials for the synthesis of proteins, fats and plant compounds. The process of photosynthesis occurs in an organ called the photosynthetic apparatus, the chloroplast, equipped with complex layers of membranes and pigments. These two compounds lead the reactions that lead to the conversion of carbon dioxide into carbohydrates, which represent a source of energy in the cell and constitute the raw materials for the synthesis of proteins, fats, and other plant compounds. The process of photosynthesis occurs in a device called the photosynthetic apparatus, “Chloroplast”. Chloroplasts are equipped with complex layers of membranes and pigments. CHAPTER III: Photosynthesis 25 Dr. GHERAISSA N. Module: Plant Physiology Core Concepts - Chloroplasts are organelles with a diameter between 4-6 micrometers and a thickness of 1-3 micrometers. Their number varies according to the cell and the physiological state. It is also a factory for producing oxygen and organic matter in the cells of high-end plants. Chloroplasts are surrounded by two membranes; outer and inner membranes. Inside, it contains discoidal cysts or vesicles called thylakoids. Figure 1: Chloroplasts. - Thylakoids are membrane-bound compartments inside chloroplasts, and from the inside they contain a liquid called the filling, which contains most of the photosynthetic enzymes in addition to starch granules, nucleic acids, and ribosomes so that the synthesis can synthesize some of its protein components. The vesicles can stack on top of each other in areas of the cyst, forming grana granules, or they can continue parallel within the lining, forming a cytoplasmic lamina. The thylakoid membrane is characterized by the presence of protein complexes similar to normal membrane components, known as photosystems, electron carriers, and ATP-synthesizing enzymes.  The photosystem is a set of proteins and pigments - does not contain chlorophyll - and is exposed to thylakoid membranes, cyanobacteria and chloroplasts in vegetable cells. There are two types of photovoltaic systems:  The first photosystem (PSI), which is rich in the chlorophyll a molecule and is symbolized by the symbol (P700), contains cartenoids and a smaller amount of chlorophyll b. Its optimal light absorption degree is at 700 nm.  The second photosystem (PSII) also contains a special chlorophyll a molecule, symbolized by the symbol (P680) because its optimum light absorption degree is at 680 nm. CHAPTER III: Photosynthesis 26 Dr. GHERAISSA N. Module: Plant Physiology Each of these two systems contains a number of pigments ranging from 200 to 300 pigment molecules, which work together as sensing points for light energy. When the unit of light (photon) is absorbed by the first chlorophyll molecule, the unit of light is transferred by these chromosomes one after the other through wave resonance until it reaches the special chlorophyll molecule in the system, which is either (P700) or (P680), which is located in the reaction center of the photosystem. Which are called traps, and as a result, high-energy electrons are released from the chlorophyll molecule stimulated by a photovoltaic unit.  Electron carriers in photosynthesis are protein complexes that transfer electrons, collectively called the electron transfer chain, which consists of plastoquinone Pq, cytochrome Cyt, plastocyanin Pc, ferredoxin Fd, and ferredoxin reductase Fdr.  ATP-synthesizing enzymes are comet-shaped proteins consisting of a base, a neck, and a spherical head. The globules protrude toward the basic substance. They work to synthesize ATP, and that is why they are called ATP-synthetase. - Photosynthetic pigments are biological compounds found in chloroplasts or photosynthetic bacteria, and they are the most important compounds in converting light energy into chemical energy, as they efficiently absorb light at 400 to 700 nm for the process of photosynthesis.  There are three types of photosynthetic pigments: 1) Chlorophylls, which are green pigments in plants, are one of the most important pigments active in the process of photosynthesis. We can distinguish nine types, namely chlorophylls A, B, C, D, E, Bacteriochlorophyll A and B, and Chlorobium chlorophylls 660 and 660. Figure 2: Structure and Reactions of Chlorophyll Chlorophyll A and B are considered the most well-known and dominant ones. They are found in all autotrophic organisms except autotrophic bacteria, as chlorophyll B is not found in blue- CHAPTER III: Photosynthesis 27 Dr. GHERAISSA N. Module: Plant Physiology green and red-brown algae. Chlorophyll A is bluish-green in color, while chlorophyll B is yellowish-green. Chlorophylls C, D, and E are found in algae (Mycophytes), while “Chlorbium chlorophylls” are found in autotrophic bacteria.  Chemical structure of chlorophyll The chlorophyll molecule is composed of a porphyrin ring (a ring consisting of tetrapyrrole C4H4) containing a magnesium atom in its middle. A chain of phytol alcohol extends from one of the rings, which is linked by an ester bond to the carboxyl group of the seventh carbon atom in the porphyrin ring. Phytol alcohol consists of a long hydrophobic chain containing... On one double bond, the overall formula of chlorophyll is C55H72MgN4O5. Figure 3: Structural formula of a and b type chlorophylls. The difference between chlorophyll A and B is concentrated in the chemical composition and also absorption spectra, where: - Chlorophyll A strongly absorbs blue radiation 430 nm and red radiation 663 nm. CHAPTER III: Photosynthesis 28 Dr. GHERAISSA N. Module: Plant Physiology - Chlorophyll B strongly absorbs blue radiation 430 nm and red radiation 663 nm. 2) Carotenoids These pigments are found associated with chloroplasts within chloroplasts in green plants, photosynthetic bacteria, and Mycophytes. Carotenoids include three types: carotenes, xanthophylls, and lycopene. They have an auxiliary role in the process of photosynthesis, which is: - Absorption and transfer of energy to chlorophyll A. - Protection of chlorophyll from photo-oxidation. Figure 4: The photosystem contains Carotenoids and Chlorophylls. 3) The phycobilins are especially efficient at absorbing red, orange, yellow, and green light, wavelengths that are not well absorbed by chlorophyll a. Organisms growing in shallow waters tend to contain phycobilins that can capture yellow/red light, while those at greater depth often contain more of the phycobilins that can capture green light, which is relatively more abundant there. CHAPTER III: Photosynthesis 29 Dr. GHERAISSA N. Module: Plant Physiology I. The mechanism of photosynthesis Photosynthesis is a vital process through which light energy is converted into chemical energy potential within the molecules of organic matter according to the following equation: 6CO2 + 6H2O → C6H12O6 + 6O2 Many scientists have contributed to discovering the truth about the reactions that take place during the process of photosynthesis, and the best of them was the scientist Blackman in 1905, when he arrived with evidence that the process of photosynthesis is not only a photochemical reaction, but also includes a biochemical reaction. 1. The Light Phase of Photosynthesis: The light phase of photosynthesis, also known as the light-dependent reactions or the photochemical phase, takes place in the thylakoid membrane of the chloroplasts. This phase involves capturing light energy and converting it into chemical energy in the form of ATP and NADPH. This stage is directly responsible for: a) Photooxidation of water b) NADP+ coenzyme reductase c) Phosphorylation There are two main processes within the light phase: cyclic photophosphorylation and non- cyclic photophosphorylation. Figure 4: The photosystem unit absorbs light, where pigment molecules absorb it and transfer it to the reaction center from which electrons are released. CHAPTER III: Photosynthesis 30 Dr. GHERAISSA N. Module: Plant Physiology 1.1. Non-cyclic Photophosphorylation: - Location: Thylakoid membrane. - Process: Non-cyclic photophosphorylation involves the transfer of excited electrons from chlorophyll to both the electron transport chain and NADP+. Water molecules are split in a process called photolysis, releasing electrons, protons, and oxygen. Electrons are then transferred through the electron transport chain, ultimately reducing NADP+ to form NADPH. - Outcome: The primary outcomes of non-cyclic photophosphorylation are the production of ATP and NADPH. Oxygen, a byproduct of photolysis, is released into the atmosphere. Figure 5: Non-cyclic Photophosphorylation chart. 1.2. Cyclic Photophosphorylation: - Location: Thylakoid membrane. - Process: In cyclic photophosphorylation, the excited electrons from the chlorophyll molecule are not transferred to NADP+ but are instead cycled back to the chlorophyll molecule itself. As a result, these electrons re-enter the photosystem and continue to go through the electron transport chain, releasing energy in the process. - Outcome: The primary outcome of cyclic photophosphorylation is the production of ATP. This cyclic process does not result in the formation of reduced NADPH. CHAPTER III: Photosynthesis 31 Dr. GHERAISSA N. Module: Plant Physiology Figure 6: Cyclic Photophosphorylation chart. 2. The Dark Phase (Calvin Cycle): The dark phase, also known as the Calvin Cycle, is the second stage of photosynthesis. It takes place in the stroma of the chloroplasts and is independent of light. The Calvin Cycle is a series of biochemical reactions that utilize the ATP and NADPH produced in the light phase to convert carbon dioxide (CO2) into glucose. 1. Carbon Fixation: The Calvin Cycle starts with the fixation of carbon dioxide. This is achieved through the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO), which combines carbon dioxide with a five-carbon sugar, ribulose bisphosphate, to form two molecules of 3-phosphoglycerate (3-PGA). 2. Reduction: ATP and NADPH, generated in the light phase, are utilized to convert 3-PGA into glyceraldehyde-3-phosphate (G3P). This involves a series of reduction reactions. 3. Regeneration of RuBP: G3P molecules are used to regenerate the initial five-carbon sugar, ribulose bisphosphate (RuBP), ensuring the continuation of the Calvin Cycle. This step consumes additional ATP. 4. Formation of Glucose: The remaining G3P molecules go on to form glucose and other carbohydrates. Several turns of the Calvin Cycle are needed to produce one molecule of glucose. CHAPTER III: Photosynthesis 32 Dr. GHERAISSA N. Module: Plant Physiology Figure 7: Calvin cycle chart.  Carbon fixation pathways in photosynthesis The method of fixing carbon dioxide varies according to the anatomical and physiological structure of the leaf and the climate in which the plant grows. There are three types of plants that differ from each other in the way of fixing carbon dioxide, which are as follows:  The first path: In plants the three-carbon C3: It is called the Calvin cycle. The first result after fixing carbon dioxide is “3-phosphoglyceric acid” and the first acceptor is “the hexa-ribose sugar-5-1 diphosphate.” Examples of some plants in this path: Rice, Wheat, and Soybeans  The second path: In four-carbon plants C4: it is called the Hatch and Slack cycle. The first result after fixing carbon dioxide is “oxalochallic acid,” and the first acceptor is “phosphoenolpyruvic acid.” Example of some plants of this path: Sugarcane and Corn  The second method: In Crassulacean acid metabolism (CAM) (succulent/succulent): The first results after carbon dioxide fixation are “oxaloacetic acid” and the first receptor is “phosphoenolpyruvic acid”. CHAPTER III: Photosynthesis 33 Dr. GHERAISSA N. Module: Plant Physiology An example of some plants in this path: Pineapple and Aloe vera, whose leaves are characterized by increased thickness of the dermis on the skin, small size of the leaves, and sunken stomata covered by hairs. ◄ The difference between CAM plants and C4 plants in fixing carbon dioxide is where “the place of the process” and its “time”. Table 1: The difference between C4 plants and CAM plants is through CO2 fixation. Differences C4 CAM CO2 fixation site In carrier beam cells In the cells of the mesophyll layer CO2 fixation During the opening of the At night, the stomata open for CO2 to times stomata during the day enter, and it is only fixed during the day Figure 8: The difference between CAM plants, three-carbon plants, C3, and four-carbon plants C4. CHAPTER III: Photosynthesis 34 Dr. GHERAISSA N. Module: Plant Physiology  Factors affecting the rate of photosynthesis External factors include: - Light intensity: The rate of photosynthesis increases with increasing light intensity until it reaches the optimum level, at which point this rate remains constant no matter how much light intensity increases. This indicates that the pigments have reached the saturation point. - Water: A lack of water has a detrimental effect on the rate of photosynthesis due to a decrease in the water content of the cell and the closure of pores. - Temperature: affects enzymatic activity and metabolic reactions. - Oxygen concentration: A decrease in oxygen in chloroplasts leads to an increase in the rate of photosynthesis. - Carbon dioxide concentration: The rate of photosynthesis increases with the increase in the concentration of carbon dioxide in the atmosphere until it reaches the optimal rate for the photosynthesis process. Internal factors include: - Chlorophyll content: If it is absent, photosynthesis does not occur. - Accumulation of photosynthesis products: Increasing the concentration of sugars and their accumulation in the leaf reduces the rate of photosynthesis. - Mineral salts: They act as regulatory aids that affect the rate of photosynthesis. - Anatomical structure: The speed at which carbon dioxide gas enters the leaf tissues, the amount of light that enters the cells, and the speed at which the products of photosynthesis move out of the cells depend to a large extent on the anatomical structure of the leaf and thus on the speed of photosynthesis. Figure 9: Factor Affecting Photosynthesis. CHAPTER III: Photosynthesis 35