Chapter_7_Photosynthesis_16_11-1.pdf

# Overview of photosynthesis Photosynthesis is the fundamental process by which light energy is converted into chemical energy stored in organic molecules [4](#page=4). ### 1.1 Definition and core concept Photosynthesis is the synthesis of organic compounds from simple inorganic materials like water ($H_2O$) and carbon dioxide ($CO_2$) in the presence of light energy. Organisms that perform photosynthesis harness energy from sunlight [2](#page=2) [4](#page=4). ### 1.2 The process in plants Plants use sunlight to convert carbon dioxide and water into chemical energy stored in sugar and other organic molecules. These sugars are then utilized by the plant for its own energy needs and for building its structure, such as its trunk, branches, and leaves. A significant outcome of this process is the generation of surplus sugars that can sustain other organisms incapable of carrying out photosynthesis [2](#page=2). ### 1.3 Organisms and locations of photosynthesis Photosynthesis occurs in various organisms and locations: * **Green plants** [4](#page=4). * **Multicellular algae** [4](#page=4). * **Unicellular protists** [4](#page=4). * **Cyanobacteria** [4](#page=4). * **Purple sulfur bacteria** [4](#page=4). In eukaryotic organisms like plants and algae, photosynthesis takes place within specialized organelles called **chloroplasts** [4](#page=4). ### 1.4 Global distribution Photosynthetic activity is globally distributed, encompassing both oceanic phytoplankton and terrestrial vegetation. Regions of high photosynthetic activity on land and in the ocean are indicated by blue and green coloring, respectively [8](#page=8). ### 1.5 Importance of photosynthesis Photosynthesis plays a critical role in sustaining life on Earth for several key reasons: 1. **Atmospheric regulation:** It provides the oxygen ($O_2$) that most organisms, including humans, breathe and removes carbon dioxide ($CO_2$) from the air [7](#page=7). 2. **Building blocks for plants:** It produces essential structural components for plants, such as cellulose and lignin [7](#page=7). 3. **Energy provision:** It generates the energy (in the form of ATP) required for various biochemical changes within organisms [7](#page=7). 4. **Foundation of food chains:** It provides the primary source of food for the entire food chain [7](#page=7). > **Tip:** Understanding the interconnectedness of photosynthesis with oxygen production, carbon cycling, and the global food web is crucial for grasping its ecological significance. > **Example:** The oxygen we inhale and the food we eat, whether plant-based or from animals that consume plants, ultimately originate from the process of photosynthesis. --- # Chloroplasts and pigments in photosynthesis Chloroplasts are the cellular organelles responsible for photosynthesis, and pigments within them capture light energy to convert it into chemical energy [6](#page=6) [9](#page=9). ### 2.1 Chloroplasts: The sites of photosynthesis Chloroplasts are a type of plastid found in green plants that carry out photosynthesis. They are characterized by their internal structure, which is crucial for their function. Photosynthesis within the chloroplast occurs in two main stages [10](#page=10) [9](#page=9): * **Light-dependent reactions:** These reactions take place in the grana of the chloroplasts and require a continuous supply of light [9](#page=9). * **Light-independent reactions:** These reactions occur in the stroma of the chloroplasts and do not require light [9](#page=9). ### 2.2 Pigments: Light-absorbing molecules Pigments are molecules embedded within the thylakoid membranes of chloroplasts that absorb specific wavelengths of light. They do not absorb all light; instead, they absorb some wavelengths and reflect others. The color we perceive from a plant is due to the wavelengths of light that are reflected, not absorbed, by the pigments. Chloroplasts contain various types of pigments, each with different absorption characteristics [12](#page=12) [13](#page=13). #### 2.2.1 Chlorophyll Chlorophyll is the primary pigment involved in photosynthesis and is found in all plants, algae, and cyanobacteria. It plays a direct role in the light-dependent reactions [17](#page=17). * **Absorption spectrum:** Chlorophyll absorbs light most strongly in the blue and red regions of the spectrum and poorly in the green region, which is why leaves appear green [13](#page=13) [16](#page=16). * **Types of chlorophyll:** There are two main types of chlorophyll molecules: chlorophyll \textit{a} and chlorophyll \textit{b}. A third type, chlorophyll \textit{d}, is found in dinoflagellates [15](#page=15). * **Function:** Chlorophyll captures energy from sunlight and converts it into chemical energy in organic compounds. It also participates in the light reactions and can accept energy from accessory pigments [17](#page=17) [6](#page=6). > **Tip:** The magnesium atom at the center of chlorophyll is essential for its function [15](#page=15). #### 2.2.2 Accessory pigments Accessory pigments are also present in plants and help to broaden the spectrum of light that can be absorbed for photosynthesis. These pigments are often masked by the high concentration of chlorophyll but become visible when chlorophyll levels decrease, such as in autumn [17](#page=17) [21](#page=21). * **Carotenoids:** These include carotenes, which are red pigments (e.g., found in papayas and carrots) and xanthophylls [21](#page=21). * **Phycobilins:** Though not explicitly detailed in the provided pages concerning accessory pigments, they are known pigment classes in photosynthetic organisms. * **Anthocyanins:** These pigments give colors like blue and purple, for example, in mulberries and grapes [21](#page=21). > **Example:** In autumn, as chlorophyll production decreases, the yellow and orange colors of carotenoids and the red colors of anthocyanins become visible in leaves, leading to the characteristic fall foliage [21](#page=21). ### 2.3 Light absorption and pigment excitation The process of light absorption by chlorophyll involves several steps: 1. **Photon absorption:** When a chlorophyll molecule absorbs a photon, an electron is boosted from its ground state to a higher energy level, entering an excited state [19](#page=19). 2. **Energy transfer:** This energized electron has more potential energy [19](#page=19). 3. **Energy dissipation:** If the illuminated molecule is isolated, the excited electron quickly returns to its ground state. The excess energy is then released as heat and fluorescence (light). In the context of photosynthesis, this energy is typically transferred to other molecules in the reaction center rather than being lost as heat or fluorescence [19](#page=19). ### 2.4 Autumn color change The phenomenon of autumn leaf color change occurs in deciduous trees and shrubs as their normally green leaves transform into reds, yellows, and various intermediate shades [20](#page=20). * **Mechanism:** In late summer, the veins that transport fluids into and out of the leaf begin to close off, leading to a decrease in chlorophyll production [20](#page=20). * **Pigment revelation:** As chlorophyll concentrations decline, the colors of accessory pigments, which were previously masked, are revealed [21](#page=21). --- # Light-dependent and light-independent reactions Photosynthesis, the process by which green plants convert light energy into chemical energy, is divided into two main stages: the light-dependent reactions and the light-independent reactions (Calvin Cycle) [23](#page=23). ### 3.1 Light-dependent reactions The light-dependent reactions occur within the thylakoid membranes of chloroplasts and require a continuous supply of light [24](#page=24) [9](#page=9). #### 3.1.1 Overview and purpose During these reactions, light energy is absorbed by chlorophyll molecules, initiating a process that splits water molecules. This splitting of water yields hydrogen ions ($H^+$), oxygen gas ($O_2$) as a byproduct, and electrons (-). The absorbed light energy drives the transfer of these electrons and $H^+$ from water to $NADP^+$ and generates ATP. Ultimately, the light-dependent reactions produce energy-carrying molecules: ATP and $NADP H$ [24](#page=24) [26](#page=26) [27](#page=27). #### 3.1.2 Electron transport and ATP synthesis Photons are captured by light-harvesting antenna complexes of Photosystem II (PSII) by chlorophyll. When a chlorophyll molecule at the core of the Photosystem II reaction center gains sufficient excitation energy, an electron is transferred to the primary electron-acceptor molecule, pheophytin [28](#page=28). These electrons are then shuttled through an electron transport chain. This chain functions to generate a chemiosmotic potential across the thylakoid membrane. An ATP synthase enzyme utilizes this chemiosmotic potential to synthesize ATP during photophosphorylation [29](#page=29). The electron then enters a chlorophyll molecule in Photosystem I (PSI). Light absorbed by the photosystem excites this electron again. As a second electron carrier accepts the electron, it is passed down a series of electron acceptors with progressively lower energies. The energy created by these electron acceptors is used to move hydrogen ions across the thylakoid membrane into the lumen [30](#page=30) [31](#page=31). Finally, the electron is used to reduce the coenzyme $NADP^+$, which has functions in the light-independent reactions [31](#page=31). > **Tip:** The light-dependent reactions are often referred to as the "photo" part of photosynthesis because they directly utilize light energy to convert water and $NADP^+$ into ATP and $NADP H$. #### 3.1.3 Water splitting The light-dependent reactions begin with the splitting of water molecules. This process is represented by the following equation [26](#page=26): $H_2O \rightarrow 2 H^+ + 1/2 O_2 + 2^-$ [24](#page=24). ### 3.2 Light-independent reactions (Calvin Cycle) The light-independent reactions, also known as the Calvin Cycle, take place in the stroma of chloroplasts. These reactions do not directly require light but depend on the products of the light-dependent reactions (ATP and $NADP H$) [32](#page=32) [9](#page=9). #### 3.2.1 Overview and purpose The Calvin Cycle assembles sugar molecules using carbon dioxide ($CO_2$) and $NADP H$. The incorporation of carbon from $CO_2$ into organic compounds is termed carbon fixation. Carbon enters the cycle as $CO_2$ and leaves as sugar. The cycle expends ATP as an energy source and consumes $NADP H$ as a reducing power to add high-energy electrons for sugar synthesis [32](#page=32) [33](#page=33). > **Tip:** The light-independent reactions are often referred to as the "synthesis" part of photosynthesis because they use the energy from the light-dependent reactions to build organic molecules. #### 3.2.2 Stages of the Calvin Cycle The Calvin Cycle consists of three main phases [33](#page=33): ##### 3.2.2.1 Phase 1: Carbon fixation 1. Carbon dioxide diffuses into the stroma of the chloroplast [35](#page=35). 2. The enzyme Ribulose Bisphosphate Carboxylase (also known as RubisCO) catalyzes the combination of $CO_2$ with a five-carbon sugar called Ribulose Biphosphate (RuBP) [35](#page=35). 3. This initial six-carbon compound is unstable and quickly splits into two molecules of a three-carbon sugar, 3-phosphoglycerate (3-PGA) [35](#page=35). ##### 3.2.2.2 Phase 2: Reduction 4. The two three-carbon sugar molecules are then rearranged to form Glyceraldehyde-3-phosphate (G3P) [36](#page=36). 5. One molecule of G3P will proceed to form glucose or other molecules required by the plant [34](#page=34) [36](#page=36). > **Example:** Three-carbon chains from the C3 cycle join to form the six-carbon molecule, glucose ($C_6H_{12}O_6$), or other required organic molecules [34](#page=34). ##### 3.2.2.3 Phase 3: Regeneration 6. The other molecule of G3P is used to regenerate the five-carbon sugar, Ribulose Biphosphate (RuBP). This regeneration ensures the cycle can continue to fix more carbon dioxide [37](#page=37). --- # Types of photosynthesis and adaptations This section explores the three distinct biochemical types of photosynthesis: C3, C4, and CAM, detailing their carbon fixation mechanisms, stomatal regulation, and adaptations to different environmental conditions [39](#page=39). ### 4.1 C3 photosynthesis C3 photosynthesis, also known as the Calvin Cycle, is characterized by the initial formation of a three-carbon sugar as the first stable organic molecule. Common examples of C3 plants include grasses, oak trees, maple trees, and rose bushes [40](#page=40). #### 4.1.1 Stomatal regulation and water loss C3 plants typically keep their stomata open during the day to allow carbon dioxide (CO2) influx and oxygen (O2) efflux, closing them at night. However, on hot, dry days, this open stomatal regulation leads to significant water loss through transpiration, which can reduce photosynthetic rates [42](#page=42). #### 4.1.2 Photorespiration When stomata are closed on hot, dry days, CO2 cannot enter the leaf, and O2 cannot exit. The high internal O2 concentration then competes with CO2 in the Calvin Cycle, binding with RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase). This process, known as photorespiration, effectively shuts down the Calvin Cycle because the initial five-carbon sugar is not available to form sugars, and it breaks down into water and CO2. When O2 combines with RuBisCO, it initiates a breakdown of the five-carbon sugar, leading to the release of CO2 and inhibiting sugar production [44](#page=44) [45](#page=45). > **Tip:** Photorespiration is an inefficient process that wastes energy and fixed carbon, particularly under conditions of high O2 and low CO2 concentrations within the leaf [44](#page=44) [45](#page=45). ### 4.2 Alternative pathways and adaptations Plants have evolved alternative photosynthetic pathways to cope with diverse environmental conditions, particularly in hot and dry climates, to combat water loss, photorespiration, and the need for CO2 entry into the leaf [46](#page=46). ### 4.3 C4 photosynthesis C4 photosynthesis is an adaptation found in plants like corn, sugarcane, and sunflowers. These plants help mitigate water loss by keeping their stomata partially closed during the day and partially closed at night. This partial stomatal opening minimizes water loss [47](#page=47). #### 4.3.1 Carbon fixation mechanism In C4 plants, CO2 is initially fixed into a temporary storage molecule that has four carbons. This is why it's called the C4 pathway, as the first stable compound formed from CO2 fixation is a four-carbon compound. The process involves [47](#page=47): 1. CO2 is fixed into phosphoenolpyruvate (PEP) to form oxaloacetate (which is then converted to malate) by PEP carboxylase (PEPC) [50](#page=50). 2. Malate helps transport the CO2 into inner cells where it meets RuBisCO [50](#page=50). 3. RuBisCO then meets CO2, away from O2, thereby preventing wasteful photorespiration [50](#page=50). #### 4.3.2 Kranz anatomy C4 plants exhibit Kranz anatomy, characterized by two layers of photosynthetic cells surrounding the vascular bundles. This specialized structure allows for more efficient photosynthesis, resulting in up to 50% higher rates compared to common plants. Consequently, C4 plants grow at a faster rate in regions with high light intensity and temperature [49](#page=49). #### 4.3.3 Advantages C4 plants photosynthesize faster than C3 plants under high light intensity and high temperatures. The CO2 is delivered directly to RuBisCO, preventing it from binding with oxygen and thus avoiding photorespiration. The four-carbon compound is transported to bundle-sheath cells where the Calvin Cycle takes place. Here, the four-carbon compound releases CO2 to run the Calvin Cycle [48](#page=48) [51](#page=51). ### 4.4 CAM photosynthesis Crassulacean Acid Metabolism (CAM) is a photosynthetic pathway found in plants like jade plants, succulents, pineapple, agave, and orchids. It is an adaptation to arid conditions, allowing these plants to survive in environments that would otherwise be too dry [52](#page=52) [53](#page=53). #### 4.4.1 Stomatal regulation and water conservation CAM plants keep their stomata closed during the day to reduce water loss through transpiration. They open their stomata at night to collect CO2. This nocturnal CO2 uptake is a significant adaptation, as it allows them to reduce water loss by up to 97% compared to C3 plants [53](#page=53). #### 4.4.2 Carbon fixation and storage CAM plants store the collected CO2 as an inorganic acid, specifically malic acid, during the night. During the day, this organic acid releases carbon dioxide directly to the Calvin Cycle for photosynthesis [52](#page=52). #### 4.4.3 Growth and water requirements CAM plants grow very slowly but require less water than C3 or C4 plants [52](#page=52). ### 4.5 Comparison of photosynthetic types | Feature | C3 Plants | C4 Plants | CAM Plants | | :----------------- | :----------------------------------------------------------------------------------- | :------------------------------------------------------------------------------------------------ | :------------------------------------------------------------------------------------------------------ | | Stomata | Open (day), closed (night) | Partially closed (day), partially closed (night) | Closed (day), open (night) | [57](#page=57). | Carbon fixation | Three-carbon compound | Four-carbon compound | Temporarily stored as a 4-carbon organic acid | [57](#page=57). | Water Loss | Significant water loss due to transpiration; trouble with photorespiration | Less water loss than C3 plants; photorespiration is not a problem | Grow very slowly and no problems with photorespiration | [57](#page=57). | Photosynthetic Rate | Moderate | Higher than C3 under high light and temperature | Slow growth rate | [51](#page=51) [52](#page=52). | Environmental Niche | Wide range, but less competitive in hot, dry climates | Hot, dry, high light environments | Arid and semi-arid environments | | Initial CO2 Fixer | RuBisCO | PEP carboxylase (PEP), then RuBisCO in bundle sheath cells | PEP carboxylase (PEP) at night, then RuBisCO during the day | | First stable product | 3-phosphoglycerate (a 3-carbon compound) | Oxaloacetate or malate (4-carbon compounds) | Malic acid (stored overnight) | [40](#page=40) [47](#page=47) [52](#page=52). --- # Factors affecting the rate of photosynthesis This section explores the law of limiting factors and how temperature, light intensity, and carbon dioxide concentration impact the speed of photosynthesis [59](#page=59). ### 5.1 The law of limiting factors The law of limiting factors states that for a chemical process dependent on a number of essential conditions, the rate of reaction is limited by the factor that is closest to its minimum value. Photosynthesis is influenced by several conditions, including temperature, light intensity, and carbon dioxide concentration [59](#page=59). ### 5.2 Temperature Photosynthesis is regulated by enzymes, which are sensitive to temperature changes. As temperature increases, the rate of reaction initially rises because reactants gain kinetic energy, leading to more frequent collisions. However, beyond a certain optimal temperature, the rate of photosynthesis decreases as essential enzymes begin to denature [60](#page=60). > **Tip:** Understanding the optimal temperature range for photosynthesis is crucial for agricultural applications and understanding plant distribution in different climates. ### 5.3 Light intensity Light is absorbed by chlorophyll, which converts radiant energy into chemical energy in the form of ATP. As light intensity increases, the rate of photosynthesis also increases because more chlorophyll molecules are photo-activated. At a certain light intensity, the photosynthetic rate will plateau because all available chlorophyll molecules are saturated with light. Different wavelengths of light can affect the rate of photosynthesis differently; for instance, green light is largely reflected [61](#page=61). > **Example:** In dense forests, the lower canopy plants may experience light limitation, while plants in open fields might be limited by other factors once light saturation is reached. ### 5.4 Carbon dioxide concentration Carbon dioxide is involved in the fixation of carbon atoms to form organic molecules. As the concentration of carbon dioxide increases, the rate of photosynthesis rises because more organic molecules are produced. However, at a certain concentration of $CO_2$, the photosynthetic rate will plateau as the enzymes responsible for carbon fixation become saturated [62](#page=62). > **Tip:** While ambient $CO_2$ levels are a factor, greenhouse growers often supplement $CO_2$ to enhance plant growth and yield, demonstrating the impact of this factor. --- ## Common mistakes to avoid - Review all topics thoroughly before exams - Pay attention to formulas and key definitions - Practice with examples provided in each section - Don't memorize without understanding the underlying concepts

| Term | Definition | |---|---| | Photosynthesis | The process by which green plants and some other organisms use sunlight to synthesize foods with the help of chlorophyll pigment. It converts light energy into chemical energy, stored in organic compounds. | | Chloroplast | An organelle found in plant and algal cells that conducts photosynthesis. It contains chlorophyll and other pigments that capture light energy. | | Chlorophyll | The primary green pigment found in plants and algae, responsible for absorbing light energy during photosynthesis. It absorbs light most strongly in the blue and red portions of the electromagnetic spectrum. | | Pigments | Molecules that absorb specific wavelengths of light. In photosynthesis, pigments like chlorophyll and carotenoids capture light energy necessary for the process. | | Light-dependent reactions | The first stage of photosynthesis, which occurs in the thylakoid membranes of chloroplasts. It uses light energy to split water, releasing oxygen, and producing ATP and NADPH. | | Light-independent reactions (Calvin Cycle) | The second stage of photosynthesis, also known as the Calvin Cycle, which takes place in the stroma of chloroplasts. It uses the ATP and NADPH generated during the light-dependent reactions to fix carbon dioxide into glucose. | | Stroma | The fluid-filled space within a chloroplast, surrounding the grana. It is the site of the light-independent reactions (Calvin Cycle). | | Grana | Stacks of flattened sacs called thylakoids within the chloroplast. The light-dependent reactions of photosynthesis occur within the thylakoid membranes. | | Thylakoid membranes | Internal membrane system within chloroplasts where the light-dependent reactions of photosynthesis occur. They contain chlorophyll and other light-absorbing pigments. | | Accessory pigments | Pigments, such as carotenoids and xanthophylls, that absorb light at different wavelengths than chlorophyll and transfer energy to chlorophyll. They broaden the spectrum of light that can be used for photosynthesis. | | Photorespiration | A process in plants where RuBisCO binds to oxygen instead of carbon dioxide, leading to a reduction in photosynthetic efficiency. It occurs when stomata are closed and oxygen concentration inside the leaf rises. | | C3 photosynthesis | The most common type of photosynthesis where the first stable organic product formed is a three-carbon compound (3-PGA). It is performed by plants like grasses and trees. | | C4 photosynthesis | A type of photosynthesis that has an initial carbon fixation step that produces a four-carbon compound, allowing for more efficient photosynthesis in hot and dry environments. It involves Kranz anatomy. | | CAM (Crassulacean Acid Metabolism) | A photosynthetic pathway adapted to arid conditions where stomata open at night to fix CO2 into organic acids, which are then used during the day for photosynthesis. This conserves water. | | Stomata | Small pores on the surface of leaves that regulate gas exchange (carbon dioxide intake and oxygen release) and transpiration. Their opening and closing are crucial for photosynthesis and water balance. | | Calvin Cycle | The series of biochemical reactions in photosynthesis in which carbon dioxide is fixed and reduced to form carbohydrate. It utilizes ATP and NADPH produced during the light-dependent reactions. | | RuBisCO | Ribulose-1,5-bisphosphate carboxylase/oxygenase, an enzyme that catalyzes the first step of carbon fixation in the Calvin Cycle. It can also bind to oxygen, leading to photorespiration. | | ATP (Adenosine Triphosphate) | A molecule that serves as the primary energy currency of the cell. It is produced during photosynthesis and used to power various cellular processes, including the Calvin Cycle. | | NADPH (Nicotinamide Adenine Dinucleotide Phosphate) | An electron carrier molecule that stores and transfers energy during photosynthesis. It is produced during the light-dependent reactions and used as a reducing agent in the Calvin Cycle. | | Carbon fixation | The process by which inorganic carbon (from carbon dioxide) is incorporated into organic molecules. This is a key step in photosynthesis. | | Limiting factors | Environmental conditions that restrict the rate of a biological process, such as photosynthesis. Key limiting factors include temperature, light intensity, and carbon dioxide concentration. | | Kranz anatomy | A specialized leaf anatomy found in C4 plants, characterized by two distinct types of photosynthetic cells: mesophyll cells and bundle sheath cells. This arrangement enhances carbon dioxide concentration around RuBisCO. |