2nd Quarter General Biology Handout PDF

General Biology 1 2nd Quarter ADP (Adenosine Diphosphate) and ATP (Adenosine Triphosphate) are essential molecules in cellular energy transfer, serving as the primary energy currency in cells. ATP (Adenosine Triphosphate): ATP consists of an adenosine molecule (adenine and ribose) bonded to...

General Biology 1 2nd Quarter ADP (Adenosine Diphosphate) and ATP (Adenosine Triphosphate) are essential molecules in cellular energy transfer, serving as the primary energy currency in cells. ATP (Adenosine Triphosphate): ATP consists of an adenosine molecule (adenine and ribose) bonded to three phosphate groups. It is a high-energy molecule because the bonds between its phosphate groups, especially the bond between the second and third phosphates, store a significant amount of energy. ADP (Adenosine Diphosphate): ADP is similar in structure to ATP but has only two phosphate groups instead of three. ADP has less energy than ATP since it is missing one of the high-energy phosphate bonds. How They Function: Energy Storage and Release: Cells store energy by adding a third phosphate group to ADP, creating ATP. This process, known as phosphorylation, requires energy input and occurs in cellular respiration and photosynthesis. When energy is needed, ATP is broken down back into ADP and a free phosphate group, releasing energy that can be used by the cell for various processes, like muscle contractions, cell division, and active transport. ATP-ADP Cycle: This cycle is a constant, ongoing process within cells. ATP is continuously synthesized from ADP as cells capture and store energy, and it’s continuously broken down to release energy where needed. This efficient cycle helps cells meet their energy needs dynamically. Role in Cellular Processes: ATP powers almost every cellular activity that requires energy, including synthesizing molecules, transporting substances across cell membranes, and signaling within cells. The regeneration of ATP from ADP is crucial for cellular function because it ensures that the cell can maintain its energy balance and support continuous metabolic activity, which are essential for the survival and proper functioning of the cell. 1. Energy Balance in the Cell: ATP is the primary energy carrier in the cell, providing energy for almost every cellular process, including protein synthesis, active transport across membranes, cell division, and muscle contraction. Since ATP is rapidly consumed during these processes, it must be constantly regenerated from ADP to prevent the cell from running out of energy. Without a reliable supply of ATP, these vital processes would stop, leading to cellular dysfunction or death. By converting ADP back into ATP, the cell maintains a steady energy supply to balance energy demand and production. 2. Support for Continuous Metabolic Activity: Metabolic reactions, whether catabolic (breaking down molecules) or anabolic (building molecules), require energy. ATP provides this energy by releasing a phosphate group, which powers cellular activities. During metabolism, ATP is continuously used and needs to be replenished. For instance: Catabolic reactions (like glycolysis and the Krebs cycle) break down molecules to release energy, which is used to regenerate ATP. Anabolic reactions (like protein and DNA synthesis) require energy input, which is supplied by ATP. Without the continuous regeneration of ATP, cells wouldn't be able to sustain these processes, causing metabolic imbalance and hindering the cell’s ability to grow, repair, or respond to its environment. Alongside ATP, several other key molecules are produced during the Krebs cycle (also known as the citric acid cycle or TCA cycle). These molecules are crucial for energy production and cellular function. The main products of one full turn of the Krebs cycle (per one molecule of glucose) are: 1|Page NADH (Nicotinamide Adenine Dinucleotide): For each turn of the cycle, three NADH molecules are produced. NADH is an important electron carrier, which will be used later in the electron transport chain (ETC) to produce more ATP through oxidative phosphorylation. FADH₂ (Flavin Adenine Dinucleotide): One molecule of FADH₂ is produced per cycle. Like NADH, FADH₂ also carries high-energy electrons to the electron transport chain, where it contributes to ATP synthesis. Carbon Dioxide (CO₂): Two molecules of CO₂ are released for each turn of the Krebs cycle. This carbon dioxide is a byproduct of the decarboxylation reactions in the cycle and is eventually exhaled as a waste product in animals. ATP (or GTP in some organisms): One molecule of ATP (or GTP in some cases) is directly produced per turn of the cycle through substrate-level phosphorylation. This ATP is used for immediate cellular energy needs. Overall, from one glucose molecule (which undergoes two rounds of the Krebs cycle, as one glucose molecule is broken down into two pyruvate molecules): 2 ATP (or GTP) 6 NADH 2 FADH₂ 4 CO₂ These products (especially NADH and FADH₂) carry high-energy electrons to the electron transport chain where they are used to generate a large amount of ATP through oxidative phosphorylation, making the Krebs cycle a crucial step in cellular respiration. The enzyme primarily responsible for producing ATP in cells is ATP synthase. This enzyme plays a key role in the process of chemiosmosis, which occurs during both cellular respiration (in the mitochondria) and photosynthesis (in the chloroplasts). In cellular respiration, specifically during the electron transport chain in the mitochondria, ATP synthase uses the proton gradient created by the movement of electrons to drive the conversion of ADP (adenosine diphosphate) and inorganic phosphate into ATP. As protons (H⁺) flow through ATP synthase, it spins and catalyzes the synthesis of ATP. In photosynthesis, a similar process occurs in the thylakoid membrane of the chloroplasts during the light- dependent reactions. ATP synthase uses the proton gradient generated by the electron transport chain to produce ATP, which is then used in the Calvin cycle to synthesize sugars. Thus, ATP synthase is the central enzyme in ATP production in both cellular respiration and photosynthesis. Role of Chemiosmosis in ATP Synthesis: Proton Gradient Creation: During oxidative phosphorylation in the electron transport chain (ETC), electrons from NADH and FADH₂ are transferred through a series of protein complexes embedded in the inner mitochondrial membrane. As electrons move through the ETC, they release energy that is used by certain protein complexes (Complex I, III, and IV) to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space. This creates a proton gradient (high concentration of protons in the intermembrane space and low concentration in the matrix). Establishment of an Electrochemical Gradient: The pumping of protons creates both a chemical gradient (higher concentration of H⁺ ions outside the matrix) and an electrical gradient (positive charge on the outside relative to the matrix). This electrochemical gradient is often referred to as the proton motive force (PMF). Proton Flow through ATP Synthase: The protons that have accumulated in the intermembrane space flow back into the mitochondrial matrix through an enzyme called ATP synthase. ATP synthase is a protein complex that spans the inner mitochondrial membrane. As protons flow through ATP synthase, 2|Page  the energy from this flow drives the enzyme to catalyze the phosphorylation of ADP (adenosine diphosphate) to ATP (adenosine triphosphate). ATP Production: The flow of protons through ATP synthase generates the energy needed to bond a third phosphate group to ADP, forming ATP. This process of generating ATP from ADP is called phosphorylation, and the energy driving this reaction is provided by the proton gradient and the flow of protons back into the matrix, facilitated by chemiosmosis. Chemiosmosis is the mechanism by which the flow of protons across the inner mitochondrial membrane, driven by the electron transport chain, powers ATP synthase to generate ATP. This process is essential for producing the majority of ATP during cellular respiration, particularly in aerobic conditions. GLYCOLYSIS, KREBS CYCLE, ELECTRON TRANSPORT CHAIN Glycolysis is the first step in the breakdown of glucose, and it prepares glucose for further breakdown by splitting it into two smaller molecules and capturing some energy. 1. Glucose Phosphorylation: Glucose (a 6-carbon sugar) enters the cell and is first phosphorylated (a phosphate group is added) by ATP to form glucose-6-phosphate. This step "traps" the glucose inside the cell since the phosphorylated form cannot easily leave the cell membrane. 2. Energy Investment: The process begins with an energy investment phase, where 2 molecules of ATP are used to phosphorylate glucose and then convert it into an intermediate, fructose-1,6-bisphosphate. This is necessary to prepare glucose for the next steps and to destabilize the molecule for splitting. 3. Cleaving the 6-Carbon Sugar: The 6-carbon sugar, fructose-1,6-bisphosphate, is then split into two 3-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P). Although DHAP can be converted into G3P, only G3P proceeds through the remainder of glycolysis. 4. Energy Harvesting: Through a series of enzymatic steps, G3P is further processed, leading to the production of NADH (electron carrier) and ATP (direct energy). In the process, the 3-carbon molecule is gradually oxidized and phosphorylated. 5. End Product – Pyruvate: Finally, two molecules of pyruvate (a 3-carbon compound) are produced. This is the critical point where glycolysis ends, and the pyruvate can then be further metabolized in the mitochondria (through the Krebs cycle or anaerobic processes like fermentation, depending on oxygen availability). Why Glycolysis Prepares Glucose for Further Breakdown: Breaking Glucose into Pyruvate: Glycolysis essentially breaks glucose into smaller 3-carbon units (pyruvate) that can enter the mitochondria for further breakdown. Generating Energy: Glycolysis generates small amounts of ATP and NADH, which can be used immediately or later in oxidative phosphorylation (if oxygen is available). Regenerating NAD⁺: It also helps regenerate NAD⁺, which is required for further breakdown in subsequent metabolic pathways. Glycolysis produces a small amount of ATP and NADH, whereas the Krebs cycle produces a larger amount of ATP, NADH, FADH₂, and CO₂. Glycolysis occurs in the cytoplasm and involves the breakdown of one molecule of glucose (a 6-carbon sugar) into two molecules of pyruvate (3 carbon each). This process produces: 2 ATP (net gain), 2 NADH, 2 Pyruvate molecules (which will enter the mitochondria for the Krebs cycle) 3|Page  The Krebs cycle takes place in the mitochondria and involves the complete oxidation of pyruvate (from glycolysis) into carbon dioxide. For each pyruvate, the cycle produces: 1 ATP (through substrate-level phosphorylation), 3 NADH, 1 FADH₂, 2 CO₂ molecules (released as waste products) Since two pyruvate molecules are produced from one glucose molecule, the Krebs cycle operates twice for each glucose molecule, doubling the outputs. Role of NAD⁺ in the Krebs Cycle: NAD⁺ plays a crucial role in the Krebs cycle by acting as an electron acceptor. During the cycle, NAD⁺ is reduced to NADH in several steps (specifically in the dehydrogenation reactions where hydrogen atoms are removed from substrates). This includes reactions like the conversion of isocitrate to alpha-ketoglutarate and malate to oxaloacetate. The NADH produced carries high-energy electrons to the electron transport chain (ETC), where it helps generate ATP through oxidative phosphorylation. Moreover, NAD⁺ is essential for the continuation of the Krebs cycle. A decrease in its availability would disrupt the cycle, leading to a halt in energy production via this pathway. On the other hand, NADH and FADH₂ are vital for cellular respiration because they: Carry high-energy electrons from earlier metabolic processes (glycolysis and the Krebs cycle) to the electron transport chain. Their electrons fuel the production of ATP, the primary energy source for the cell. Their oxidation (losing electrons) regenerates NAD⁺ and FAD, enabling the continuation of energy extraction processes. The Krebs cycle (also known as the citric acid cycle or TCA cycle) occurs in the mitochondria, specifically within the mitochondrial matrix. The mitochondria are often referred to as the "powerhouses" of the cell because they are the site of most ATP production. The mitochondrial matrix is the innermost part of the mitochondria, where the enzymes of the Krebs cycle are located. These enzymes catalyze the reactions that break down pyruvate (produced from glucose during glycolysis) into carbon dioxide, NADH, FADH₂, and ATP (or GTP). Electron Transport Chain (ETC) The electron transport chain (ETC) is the final stage of cellular respiration and occurs in the inner mitochondrial membrane. Its primary role is to produce a large amount of ATP through a process called oxidative phosphorylation. Steps of the Electron Transport Chain: Electron Donors (NADH and FADH₂): The ETC starts with the electrons from NADH and FADH₂ (produced during earlier stages of cellular respiration like glycolysis and the Krebs cycle). NADH donates its electrons to Complex I of the ETC, while FADH₂ donates electrons to Complex II. These electrons are high-energy and are transferred along the chain through various protein complexes. Proton Gradient (Proton Motive Force): As electrons are passed through the complexes, protons (H⁺ ions) are pumped from the mitochondrial matrix into the intermembrane space. This creates a proton gradient across the inner mitochondrial membrane, where there is a high concentration of protons in the intermembrane space and a low concentration in the matrix. This gradient is essential for the next step in ATP production. ATP Synthesis via Chemiosmosis: The protons (H⁺ ions) that have accumulated in the intermembrane space flow back into the matrix through a protein complex called ATP synthase. This process is known as chemiosmosis. As protons flow through ATP synthase, the enzyme uses the energy from this flow to synthesize ATP from ADP and inorganic phosphate. Formation of Water: At the end of the electron transport chain, the electrons combine with protons (H⁺) and oxygen molecules to form water. This is the final electron acceptor in the chain, ensuring that the electrons are fully used and preventing the chain from backing up. 4|Page Overall Results of the Electron Transport Chain: ATP Production: The main outcome of the ETC is the production of a large number of ATP molecules. Each NADH produces about 3 ATP, and each FADH₂ produces about 2 ATP through oxidative phosphorylation. Water Formation: Oxygen serves as the final electron acceptor and combines with electrons and protons to form water. This is why oxygen is essential for aerobic respiration. Proton Gradient: The proton gradient created by the pumping of protons into the intermembrane space drives ATP synthesis through chemiosmosis. PHOTOSYNTHESIS The flow of electrons through Photosystem II (PSII) and Photosystem I (PSI) is a crucial part of the light- dependent reactions of photosynthesis, but each system plays a different role in the process. In Photosystem II, light energy is absorbed by chlorophyll, which excites electrons, raising them to a higher energy state. These high-energy electrons are then passed through a series of electron carriers in the electron transport chain (ETC). One of the key actions of PSII is that it uses the energy from light to split water molecules (a process known as photolysis), releasing oxygen and providing protons (H⁺) for the proton gradient needed in ATP synthesis. The electrons produced from the splitting of water replenish the excited electrons in PSII, ensuring the cycle continues. In contrast, Photosystem I primarily functions to facilitate the reduction of NADP⁺ to NADPH. Electrons, now lower in energy after passing through PSII and the ETC, enter PSI. Here, light energy re-excites these electrons, which are then passed to ferredoxin, and ultimately to NADP⁺, reducing it to NADPH. This NADPH is used later in the Calvin cycle for carbon fixation. Thus, PSII primarily focuses on using light energy to split water and generate electrons for the electron transport chain, while PSI uses light energy to produce NADPH, a key energy carrier for the light-independent reactions of photosynthesis. Both systems work together to create the energy needed for the plant to synthesize glucose and other essential molecules. LIGHT-DEPENDENT AND LIGHT-INDEPENDENT REACTIONS IN PHOTOSYNTHESIS The relationship between light-dependent and light-independent reactions in photosynthesis is integral to the overall process of converting light energy into chemical energy. In the light-dependent reactions, which take place in the thylakoid membranes of the chloroplasts, light energy is absorbed by pigments like chlorophyll. This energy excites electrons, which are passed through an electron transport chain, leading to the production of ATP and NADPH, two crucial energy carriers. Additionally, water molecules are split during these reactions, releasing oxygen as a byproduct. These ATP and NADPH molecules are then used in the light-independent reactions, also known as the Calvin- Benson cycle, which occurs in the stroma of the chloroplast. In the Calvin cycle, the energy stored in ATP and NADPH is used to fix carbon dioxide into organic molecules, ultimately producing sugars like glucose. The ATP provides the energy for the reactions, while NADPH supplies the reducing power necessary to convert carbon dioxide into a usable form of carbon for the plant. In summary, the light-dependent reactions produce the energy carriers (ATP and NADPH) that fuel the Calvin cycle. This relationship ensures a continuous flow of energy, enabling the plant to convert light into stored chemical energy, which can be used for growth, metabolism, and other cellular functions. When the light intensity is decreased, it affects the light-dependent reactions of photosynthesis, which are responsible for producing the energy carriers ATP and NADPH. These molecules are essential for driving the Calvin cycle, the part of photosynthesis where carbon dioxide is fixed into sugars. In the light-dependent reactions, light energy is used to split water molecules, releasing oxygen, and generating ATP and NADPH through processes like the electron transport chain and chemiosmosis. With lower light intensity, the rate of these light-dependent reactions decreases, leading to reduced production of ATP and NADPH. Since these 5|Page energy carriers are required in the Calvin cycle to convert carbon dioxide into organic molecules (like glucose), a reduction in their availability causes the Calvin cycle to slow down. Without adequate ATP and NADPH, the plant cannot efficiently fix carbon into sugars, thus slowing down overall sugar synthesis and reducing the plant's energy reserves. Over time, this could limit the plant's growth and metabolic processes, as it becomes less capable of producing the carbohydrates needed for its functions. The balanced chemical equations for photosynthesis and cellular respiration. 1. Photosynthesis: The process by which plants, algae, and some bacteria convert light energy, carbon dioxide, and water into glucose (a form of chemical energy) and oxygen. Balanced Chemical Equation: CO2 + 6H2O C6H12O6 + 6O2 Explanation: Reactants: The inputs to photosynthesis are carbon dioxide (CO₂) from the air, water (H₂O) from the soil, and light energy (usually from the sun). Products: The products of photosynthesis are glucose (C₆H₁₂O₆), which is used as a source of chemical energy for the plant, and oxygen (O₂), which is released into the atmosphere as a byproduct. This process occurs in the chloroplasts of plant cells, where chlorophyll (the green pigment) absorbs light energy and converts it into chemical energy. This energy is used to form glucose from carbon dioxide and water, primarily in the Calvin cycle. 2. Cellular Respiration: The process by which cells break down glucose in the presence of oxygen to produce ATP (energy), carbon dioxide, and water. Balanced Chemical Equation: C6H12O6 + 6O2 6CO2 + 6H2O + ATP (energy) Explanation: Reactants: The reactants of cellular respiration are glucose (C₆H₁₂O₆) and oxygen (O₂). Glucose comes from food (or, in the case of plants, from the photosynthesis process), and oxygen is taken from the air. Products: The products are carbon dioxide (CO₂), water (H₂O), and ATP, which is the primary energy source for the cell. Cellular respiration occurs in two stages: glycolysis (in the cytoplasm) and aerobic respiration (in the mitochondria), which involves the Krebs cycle and the electron transport chain. In this process, glucose is oxidized to produce ATP, which powers the cell's functions. Relationship Between Photosynthesis and Cellular Respiration: Photosynthesis and cellular respiration are complementary processes. Photosynthesis stores energy by producing glucose and oxygen from light energy, while cellular respiration releases energy from glucose and oxygen, producing ATP for the cell's activities and releasing carbon dioxide and water as byproducts. The oxygen produced by photosynthesis is used by organisms during cellular respiration, and the carbon dioxide produced in cellular respiration is used by plants in photosynthesis. Thus, they form a cyclic relationship: photosynthesis captures energy and creates glucose, and cellular respiration breaks down glucose to release energy for the cell's use. PLANT PIGMENTS The compounds listed—Carotenoids, Xanthophyll, Chlorophyll, Anthocyanins, and Phycobilins—are all pigments that play crucial roles in photosynthesis and plant coloration. 1. Carotenoids: Carotenoids are a group of pigments that are yellow, orange, or red in color. They are found in the chloroplasts and help capture light energy for photosynthesis. Additionally, carotenoids protect plant cells from damage by 6|Page absorbing excess light and acting as antioxidants. Common carotenoids include beta-carotene, which is responsible for the orange color in carrots, and lutein, which is found in leaves. Role in Photosynthesis: Carotenoids absorb light in the blue-green and violet range and transfer that energy to chlorophyll for use in the light-dependent reactions. 2. Xanthophyll: Xanthophylls are a subclass of carotenoids that are yellow pigments. Unlike carotenoids, which mainly absorb light for photosynthesis, xanthophylls play a key role in protecting the plant by helping to dissipate excess energy as heat and prevent photodamage. Role in Photosynthesis: Xanthophylls work in the light-harvesting complex, absorbing light and contributing to the overall efficiency of photosynthesis, particularly in protecting the plant under high light conditions. 3. Chlorophyll: Chlorophyll is the primary pigment in plants responsible for capturing light energy during photosynthesis. It is green in color because it absorbs mostly blue and red light and reflects green light. There are two main types of chlorophyll: chlorophyll a (which plays a direct role in the light reactions) and chlorophyll b (which assists by capturing additional light and transferring it to chlorophyll a). Role in Photosynthesis: Chlorophyll absorbs light energy, which is then used in the light-dependent reactions to produce ATP and NADPH, essential for the Calvin cycle and overall sugar synthesis in plants. Chlorophyll is green in color because it primarily absorbs light in the blue and red wavelengths of the visible light spectrum but reflects and transmits green light. This reflection and transmission of green light is what gives chlorophyll its characteristic green color. Absorption of Light: Chlorophyll is a photosynthetic pigment found in plants, algae, and some bacteria. It absorbs light energy needed for photosynthesis. There are two main types of chlorophyll in plants: chlorophyll a and chlorophyll b. Both types absorb light most efficiently in the blue (around 430–450 nm) and red (around 640–680 nm) parts of the light spectrum. These wavelengths carry the energy that the plant uses for photosynthesis. Reflection of Green Light: Chlorophyll reflects light at the green (500–570 nm) wavelength, which is why plants appear green to our eyes. This green light is not absorbed by chlorophyll because the pigment's molecular structure is more efficient at capturing light in other parts of the spectrum. Instead, the green light is either reflected off the plant's surface or transmitted through the plant. Significance in Photosynthesis: The primary function of chlorophyll is to absorb light energy and convert it into chemical energy during photosynthesis. By absorbing light in the blue and red parts of the spectrum, chlorophyll maximizes its energy absorption while minimizing the energy absorbed from the less useful green light. 4. Anthocyanins: Anthocyanins are pigments responsible for red, blue, and purple colors in plants, found in flowers, fruits, and leaves. They do not play a direct role in photosynthesis but are involved in protecting the plant from UV radiation and oxidative stress. They also contribute to attracting pollinators and seed dispersers. Role in Photosynthesis: While not directly involved in light absorption for photosynthesis, anthocyanins help reduce oxidative damage in plants and may enhance photosynthetic efficiency in some cases by absorbing light in certain wavelengths. Anthocyanins are important for plant survival beyond their role in providing color because they serve several crucial functions that help plants adapt to their environment and enhance their overall health. While they are responsible for the red, blue, and purple colors seen in flowers, fruits, and leaves, their functions extend well beyond mere pigmentation: Anthocyanins contribute to plant survival by providing protection from UV light, acting as antioxidants, helping in pollination and seed dispersal, enhancing cold and stress tolerance, and serving 7|Page as part of the plant's defense strategy. These multiple functions highlight their essential role in a plant's ability to thrive in a variety of environments. 5. Phycobilins: Phycobilins are a group of water-soluble pigments found in cyanobacteria and red algae. They are blue (phycocyanin) or red (phycoerythrin) pigments that are important for light absorption, particularly in aquatic environments where light wavelengths differ from those on land. Role in Photosynthesis: Phycobilins absorb light in the orange and red parts of the spectrum and transfer the energy to chlorophyll for photosynthesis. They are crucial in environments with low light, such as underwater, where they help the organisms efficiently capture available light. FERMENTATION While fermentation does create an anaerobic (oxygen-free) environment, which helps inhibit certain aerobic (oxygen-requiring) pathogens, the ethanol itself is not the primary factor in creating this environment. Instead, it's the activity of the microorganisms (such as lactic acid bacteria in yogurt) that generates the conditions needed to prevent spoilage and pathogenic bacteria growth. An organism would rely on fermentation instead of aerobic respiration in situations where oxygen is either absent or limited. FERMENTATION VS. AEROBIC RESPIRATION: Aerobic respiration requires oxygen to produce ATP (energy) efficiently, yielding about 36-38 ATP molecules per glucose molecule. Fermentation occurs when oxygen isn’t available. This process is less efficient, producing only 2 ATP molecules per glucose molecule, but it allows cells to generate energy even in low-oxygen environments. 1. Anaerobic Environments: In places where oxygen is scarce, such as deep in soil, stagnant water, or in environments like the intestines of animals, organisms must rely on fermentation to survive. 2. Strenuous Activity in Muscles: For animals (including humans), when muscles work intensely (e.g., during heavy exercise), oxygen delivery to muscles may not keep up with the demand. Muscles temporarily switch to lactic acid fermentation to produce energy until oxygen levels are sufficient again. 3. Single-Celled Organisms: Some organisms, like yeast and certain bacteria, primarily use fermentation for energy because they thrive in oxygen-poor environments or lack the structures needed for aerobic respiration. Aerobic respiration is considered more efficient than fermentation because it produces a significantly higher amount of ATP, the molecule that powers cellular processes. In aerobic respiration, cells use oxygen to completely break down glucose, resulting in carbon dioxide, water, and up to 36-38 ATP molecules per glucose molecule. This full breakdown occurs in several stages, including glycolysis, the Krebs cycle, and the electron transport chain, where oxygen acts as the final electron acceptor, allowing for the maximum energy release from glucose. In contrast, fermentation only partially breaks down glucose when oxygen is absent or limited, producing just 2 ATP molecules per glucose. This lower yield makes fermentation much less efficient for energy production. Because of its efficiency, aerobic respiration is the preferred pathway for energy generation in organisms that have access to oxygen, allowing them to sustain more energy-demanding processes, maintain complex structures, and support active lifestyles. Fermentation, though helpful in oxygen-poor conditions, does not support these high energy demands as effectively as aerobic respiration. The main disadvantage of fermentation is that it generates significantly less energy than aerobic respiration. While aerobic respiration can produce up to 36-38 ATP molecules per glucose molecule, fermentation yields only 2 ATP molecules. This substantial difference in energy production is due to the fact that fermentation only partially breaks down glucose, converting it into byproducts like lactic acid or ethanol, depending on the 8|Page organism. Without oxygen, fermentation cannot utilize the full breakdown pathways that aerobic respiration does, especially the Krebs cycle and the electron transport chain, which are responsible for producing most of the ATP in aerobic respiration. As a result, organisms relying on fermentation have limited energy, making it harder to sustain energy-intensive activities and complex cellular functions. This inefficiency in energy production is why fermentation is typically used as a short-term solution or in environments where oxygen is unavailable, as it provides only the minimum energy required for basic survival. If a microorganism is placed in an anaerobic chamber, a metabolic product that would indicate successful anaerobic respiration is lactic acid or ethanol (depending on the type of microorganism), along with carbon dioxide. Absence of Oxygen: In an anaerobic chamber, there is no oxygen available, so the microorganism cannot undergo aerobic respiration. Instead, it shifts to anaerobic processes to produce energy. Anaerobic Respiration and Fermentation Products: Depending on the type of microorganism, different end products are generated through anaerobic respiration: Lactic Acid: Certain bacteria (and human muscle cells under low oxygen) produce lactic acid during anaerobic respiration. Ethanol and Carbon Dioxide: Yeast and some bacteria produce ethanol and carbon dioxide in the absence of oxygen. Indicators of Anaerobic Respiration: The presence of lactic acid or ethanol, along with carbon dioxide, signals that the microorganism is metabolizing glucose anaerobically, confirming successful anaerobic respiration. These products are key indicators of fermentation, the common anaerobic pathway, and confirm that the microorganism is generating ATP without oxygen. If a yeast fermentation experiment produces ethanol and carbon dioxide, it can be concluded that the experiment was conducted under anaerobic conditions—meaning no oxygen was available. Fermentation Pathway: Yeast performs fermentation when oxygen is absent. Without oxygen, yeast cells switch from aerobic respiration to anaerobic respiration, using fermentation to break down glucose and produce energy. The byproducts of this process are ethanol and carbon dioxide. Anaerobic Environment: The production of ethanol and carbon dioxide specifically indicates that the environment lacked oxygen. In the presence of oxygen, yeast would use aerobic respiration, which fully breaks down glucose into carbon dioxide and water, with no ethanol produced. Confirmation of Anaerobic Fermentation: Since ethanol and carbon dioxide are products exclusive to anaerobic fermentation in yeast, their presence confirms that the yeast was in an oxygen-free environment, relying on fermentation for ATP production. In anaerobic respiration, organisms can use alternative electron acceptors instead of oxygen to complete the energy production process. Some common alternative electron acceptors include: Nitrate (NO₃⁻): Many bacteria, such as E. coli, use nitrate as an electron acceptor, reducing it to nitrogen gas (N₂) or nitrite (NO₂⁻). This process, known as denitrification, is common in soil and aquatic environments where oxygen is limited. Sulfate (SO₄²⁻): Certain bacteria, especially sulfate-reducing bacteria like Desulfovibrio, use sulfate as an electron acceptor, reducing it to hydrogen sulfide (H₂S). This process often occurs in oxygen-poor, sulfate-rich environments, such as deep-sea hydrothermal vents and anaerobic sediments. Carbon Dioxide (CO₂): Some microorganisms, like methanogens (a type of archaea), use carbon dioxide as an electron acceptor and reduce it to methane (CH₄). This process, called methanogenesis, is common in oxygen- free environments such as wetlands, rice paddies, and the digestive tracts of ruminants. 9|Page CALVIN CYCLE The Calvin cycle is the process in photosynthesis where plants convert carbon dioxide (CO₂) into glucose and other sugars. It takes place in the stroma of the chloroplasts and does not require light, making it part of the light-independent reactions. The cycle involves three main stages: Carbon Fixation: CO₂ is attached to a 5-carbon molecule called ribulose bisphosphate (RuBP), forming a 6-carbon compound that quickly breaks down into two 3-carbon molecules, called 3-phosphoglycerate (3-PGA). Reduction: ATP and NADPH (produced during the light-dependent reactions) are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar. Some of the G3P molecules exit the cycle to form glucose and other carbohydrates. Regeneration of RuBP: The remaining G3P molecules are used, with the help of ATP, to regenerate RuBP, allowing the cycle to continue. In summary, the Calvin cycle captures carbon from CO₂ and uses energy from ATP and NADPH to produce sugars, which are then used by the plant for growth, energy, and metabolism. In the Calvin cycle, each cycle to fix carbon and produce G3P (glyceraldehyde-3-phosphate) requires both ATP and NADPH. To produce one G3P molecule (half a glucose molecule), the Calvin cycle requires 18 ATPs and 12 NADPHs. Since 6 NADPHs are used, this implies half the cycle is complete, meaning it would require 18 ATPs to process the entire Calvin cycle fully when 6 NADPHs are used. The typical estimate for ATP production from one glucose molecule through aerobic respiration is 32 ATP. This value includes the ATP produced during glycolysis, the Krebs cycle, and oxidative phosphorylation: Glycolysis produces 2 ATP. Krebs cycle produces 2 ATP. Oxidative phosphorylation (electron transport chain) produces approximately 28 ATP. These combined give a total of about 32 ATP molecules, though exact numbers can vary slightly depending on cell type. In anaerobic respiration (fermentation), each glucose molecule only produces 2 ATP molecules. Therefore, if a cell processes 10 glucose molecules anaerobically, the total ATP produced would be: 10 glucose × 2 ATP per glucose = 20 ATP If a plant's stomata remain closed during the day, it would significantly affect the Calvin cycle and overall photosynthesis. The effect of Stomata Closure will decrease Carbon Dioxide Uptake, so when the stomata are closed, carbon dioxide cannot enter the leaf in sufficient amounts. This reduces the amount of CO₂ available for the Calvin cycle to fix into an organic molecule (glucose, for example). Also, reduced Calvin Cycle Activity because the Calvin cycle relies on CO₂ to produce glucose (and other sugars), which are essential for the plant's energy and growth. Without adequate CO₂, the Calvin cycle cannot run effectively, leading to reduced sugar synthesis: Since the Calvin cycle cannot fix enough carbon, the plant will produce less glucose and other sugars. And the last but not the least, energy deficit wherein the plant may not be able to produce sufficient ATP and NADPH for its metabolic needs, as these molecules are also produced during the light-dependent reactions of photosynthesis, which require the Calvin cycle to continue functioning efficiently. 10 | P a g e