Cellular Respiration: Glycolysis PDF

1. Role of Coenzymes: Coenzymes like NAD and FAD are essential in cellular respiration. They cycle between a loaded (reduced) and unloaded (oxidized) state, facilitating energy transfer. The primary energy carrier in cells is ATP, produced via substrate-level phosphorylation (dire...

1. Role of Coenzymes: Coenzymes like NAD and FAD are essential in cellular respiration. They cycle between a loaded (reduced) and unloaded (oxidized) state, facilitating energy transfer. The primary energy carrier in cells is ATP, produced via substrate-level phosphorylation (direct transfer of phosphate from a substrate to ADP). Hydrogen carriers (e.g., NAD and FAD) indirectly help generate ATP by transferring high-energy electrons to the electron transport chain (ETC) during oxidative phosphorylation. 2. Redox Reactions: Redox reactions involve the transfer of electrons (or hydrogen atoms) between molecules. In these reactions: ○ Oxidation is the loss of electrons or hydrogen atoms (or gain of oxygen). ○ Reduction is the gain of electrons or hydrogen atoms (or loss of oxygen). ○ The key mnemonic for redox reactions is OIL RIG (Oxidation Is Loss, Reduction Is Gain). 3. Hydrogen Carriers (NAD and FAD): NAD (Nicotinamide Adenine Dinucleotide) is a major hydrogen carrier. It is reduced to NADH when it picks up electrons and protons (hydrogen atoms). FAD (Flavin Adenine Dinucleotide) is a less common hydrogen carrier, reduced to FADH2. These carriers act like taxis, transporting hydrogen atoms (electrons and protons) to the electron transport chainin the mitochondria. 4. Oxidation and ATP Production: During cellular respiration, organic compounds (e.g., glucose) are broken down, releasing hydrogen atoms(protons and electrons). The electrons are transferred to the electron transport chain (ETC), where they are used to produce ATP via oxidative phosphorylation. This process requires oxygen, which is the final electron acceptor in the ETC, forming water (H2O). 5. Oxidative Phosphorylation: The energy from the high-energy electrons (carried by NADH and FADH2) is used to pump protons across the mitochondrial membrane, creating a proton gradient.  The protons flow back into the mitochondrial matrix via ATP synthase, driving the production of ATP. Summary: NAD and FAD are hydrogen carriers that transport electrons and protons to the electron transport chain in mitochondria, where oxidative phosphorylation occurs. Redox reactions (oxidation and reduction) are crucial in transferring electrons, and this process ultimately drives the synthesis of ATP using the energy from hydrogen atoms. The process is aerobic, requiring oxygen to accept electrons at the end of the electron transport chain, ensuring efficient ATP production. 1. Glycolysis Overview: Glycolysis is the process by which glucose (6C) is broken down into two molecules of pyruvate (3C) in the cytosol of the cell. It is the first step in cellular respiration and can occur with or without oxygen (anaerobic or aerobic conditions). 2. Key Steps in Glycolysis: Phosphorylation: Glucose is phosphorylated by 2 ATP molecules to form hexose bisphosphate. This makes glucose more reactive and prevents it from diffusing out of the cell. Lysis: The hexose bisphosphate (6C) is split into two triose phosphates (3C), also known as glyceraldehyde-3-phosphate (G3P). Oxidation: Each of the 3C sugars (G3P) undergoes oxidation, where hydrogen atoms are removed and transferred to NAD+, forming NADH. This results in 2 NADH molecules being produced. ATP Formation: The energy released from the sugar intermediates is used to generate 4 ATP via substrate-level phosphorylation. However, 2 ATP are consumed in the early stages, so the net gain is 2 ATP. 3. Net Results of Glycolysis: 2 molecules of pyruvate (3C) are produced from 1 molecule of glucose (6C). 2 NADH molecules are produced by the reduction of NAD+. A net gain of 2 ATP (4 ATP produced, but 2 ATP were used in the initial steps). 4. Significance of Glycolysis: Glycolysis is an anaerobic process, meaning it does not require oxygen, and it provides a quick source of ATP in the absence of oxygen. The production of NADH is crucial as it carries electrons to the electron transport chain for further ATP production (in aerobic conditions). The breakdown of glucose is stepwise, involving multiple enzymes, reducing the activation energy needed and controlling the release of energy. Summary: Glycolysis converts glucose (6C) into 2 pyruvate (3C) molecules, producing 2 ATP and 2 NADH in the process. The process involves key steps: phosphorylation, lysis, oxidation, and ATP formation, with enzymes facilitating each step. The net gain of glycolysis is 2 ATP and the generation of 2 NADH, making it an essential metabolic pathway for energy production, especially in the absence of oxygen. 1. Role of NAD in Anaerobic Respiration: Glycolysis produces NADH by reducing NAD+ (from glucose breakdown), but in the absence of oxygen, NADH cannot be oxidized in the mitochondria via the electron transport chain. For glycolysis to continue, NAD+ needs to be regenerated, as it is required to keep glycolysis running and producing ATP. 2. Fermentation to Regenerate NAD: Fermentation is a metabolic process that converts pyruvate into another carbon compound (lactate in animals, ethanol in plants/yeast) and oxidizes NADH back to NAD+. This allows glycolysis to continue producing ATP in the absence of oxygen by maintaining a steady supply of NAD+. Fermentation is essential for anaerobic respiration, as it enables ATP production without the need for oxygen. 3. Lactic Acid Fermentation in Animals: In animals, pyruvate is converted to lactate (lactic acid) through lactic acid fermentation.  This reaction is reversible, meaning when oxygen becomes available, lactate can be converted back into pyruvate to enter the aerobic respiration pathway (Krebs cycle and electron transport chain). 4. Ethanol Fermentation in Yeast/Plants: In yeast and plants, pyruvate is converted to ethanol and carbon dioxide in alcoholic fermentation. This reaction is irreversible, meaning pyruvate cannot be reformed under anaerobic conditions, and the process leads to the production of ethanol. 5. Applications of Fermentation: Muscle Contractions: In animals, anaerobic fermentation allows muscles to generate energy even when oxygen is limited (e.g., during intense exercise), leading to lactic acid buildup. Food Production: Fermentation has practical applications such as: ○ Baking: The carbon dioxide produced by yeast causes dough to rise (leavening). ○ Alcoholic Beverages: Yeast fermentation produces ethanol, which is the intoxicating agent in alcohol. ○ Dairy: Lactic acid produced by fermentation is used to modify milk proteins to make yogurt and cheese. 6. Biofuel Production: Fermentation is also used to produce biofuels like bioethanol, a renewable source of chemical energy, which can be used as an alternative to fossil fuels. Summary: Anaerobic respiration relies on fermentation to regenerate NAD+, allowing glycolysis to continue and produce ATP without oxygen. Lactic acid fermentation in animals produces lactate, which is reversible when oxygen returns, while alcoholic fermentation in yeast produces ethanol and carbon dioxide, which is irreversible. Fermentation has various practical applications, including muscle energy, food production (bread, alcohol, dairy), and biofuel creation. 1. Overview of the Link Reaction: The link reaction connects the anaerobic process of glycolysis (which breaks down glucose into pyruvate) with the aerobic processes in the mitochondria (Krebs cycle and electron transport chain). It occurs in the mitochondrial matrix and involves the transformation of pyruvate into acetyl CoA. 2. Steps of the Link Reaction: Pyruvate Transport: Pyruvate (produced from glycolysis in the cytosol) is transported into the mitochondrial matrix by specific carrier proteins. Decarboxylation: Pyruvate loses a carbon atom, forming carbon dioxide (CO2). This is known as decarboxylation. Oxidation: The remaining 2-carbon compound undergoes oxidation, where it loses hydrogen atoms. NAD+ is reduced to NADH during this process. Formation of Acetyl CoA: The 2-carbon acetyl group formed from the oxidation is attached to coenzyme A, resulting in the production of acetyl CoA. 3. Key Outputs of the Link Reaction (per glucose molecule): 2 molecules of acetyl CoA (one for each pyruvate, as glycolysis produces two pyruvate molecules per glucose). 2 NADH molecules (one from each pyruvate). 2 CO2 molecules (one from each pyruvate). 4. Significance of the Link Reaction: It links glycolysis (which occurs in the cytosol) with the Krebs cycle (which takes place in the mitochondria). The acetyl CoA produced in the link reaction enters the Krebs cycle, where it will be further oxidized for ATP production. The NADH produced will carry electrons to the electron transport chain, contributing to the production of ATP via oxidative phosphorylation. Summary: The link reaction is essential for transitioning from anaerobic glycolysis to aerobic respiration.  It involves the transport of pyruvate into the mitochondria, its decarboxylation into CO2, oxidation to form NADH, and the production of acetyl CoA, which enters the Krebs cycle. For each glucose molecule, the link reaction produces 2 acetyl CoA, 2 NADH, and 2 CO2, which are critical for further ATP production in aerobic respiration. 1. Overview of the Krebs Cycle: The Krebs cycle (also called the citric acid cycle or tricarboxylic acid cycle) occurs in the mitochondrial matrixand is the second stage of aerobic respiration. It begins with the transfer of the acetyl group from acetyl CoA to a 4-carbon molecule called oxaloacetate, forming a 6-carbon compound called citrate. 2. Key Reactions in the Krebs Cycle: Acetyl CoA donates its acetyl group to oxaloacetate, forming citrate (6C). Through a series of reactions, the 6C citrate is oxidized and broken down, releasing two molecules of carbon dioxide (CO2) through decarboxylation. Coenzyme A is released and can return to the link reaction to combine with another acetyl group. 3. Oxidation and Reduction: During the cycle, four oxidation reactions occur, during which hydrogen carriers (NAD+ and FAD) are reduced to form: ○ 3 NADH (from three oxidation steps) ○ 1 FADH2 (from one oxidation step) 4. ATP Production: One ATP is produced directly by substrate-level phosphorylation during the cycle. 5. Per Glucose Molecule: Since glycolysis produces two pyruvate molecules, the Krebs cycle occurs twice per glucose. For each glucose molecule, the cycle produces: ○ 2 ATP ○ 4 CO2 (2 per cycle)  ○ 8 hydrogen carriers (6 NADH and 2 FADH2) 6. Importance of the Krebs Cycle: The cycle generates hydrogen carriers (NADH and FADH2), which will carry electrons to the electron transport chain, where a large amount of ATP will be generated via oxidative phosphorylation. The CO2 released in the cycle is a waste product, exhaled by organisms. Summary: The Krebs cycle processes acetyl CoA (from glucose breakdown) to produce ATP, CO2, and reduced hydrogen carriers (NADH, FADH2). Acetyl CoA combines with oxaloacetate to form citrate, which undergoes oxidation and decarboxylation to regenerate oxaloacetate, completing the cycle. Key outputs per glucose molecule: 2 ATP, 4 CO2, and 8 hydrogen carriers (NADH x6, FADH2 x2). The NADH and FADH2 are essential for ATP production in the electron transport chain. 1. Location and Structure: The electron transport chain (ETC) occurs on the inner mitochondrial membrane, which is folded into cristaeto increase surface area for the chain’s activity. 2. Oxidative Phosphorylation: Oxidative phosphorylation refers to the process of using the energy from the oxidation of reduced hydrogen carriers (NADH and FADH2) to produce ATP. 3. The Role of Hydrogen Carriers: NADH and FADH2 are oxidised, releasing high-energy electrons and protons (H⁺). The electrons are transferred to the electron transport chain (ETC), a series of carrier proteins embedded in the inner mitochondrial membrane. 4. Proton Gradient (Proton Motive Force):  As electrons move through the ETC, they lose energy, which is used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space. This creates an electrochemical gradient (proton motive force), which is essential for ATP synthesis. 5. Chemiosmosis: The proton gradient drives chemiosmosis, where H⁺ ions flow back into the matrix through the enzyme ATP synthase. This flow triggers the rotation of ATP synthase, catalysing the conversion of ADP and inorganic phosphate (Pi)into ATP. 6. Role of Oxygen: Oxygen acts as the final electron acceptor in the ETC. It combines with electrons and protons (H⁺) to form water. This is essential because it prevents the chain from becoming blocked by accumulated electrons. Oxygen's removal of protons from the matrix helps maintain the proton gradient. 7. In the Absence of Oxygen: Without oxygen, the electron transport chain stalls because the de-energised electrons cannot be removed. As a result, ATP production stops. Summary: The electron transport chain (ETC) is where NADH and FADH2 release energy by donating electrons. Protons are pumped into the intermembrane space, creating an electrochemical gradient (proton motive force). ATP synthase uses this gradient to produce ATP via chemiosmosis. Oxygen is crucial as the final electron acceptor, forming water and maintaining the flow of electrons. Without oxygen, ATP production halts. 1. Mitochondrial Function:  Mitochondria are the energy producers of the cell, synthesising ATP via aerobic respiration. This occurs in all eukaryotic cells, whereas certain prokaryotes use their cell membrane for aerobic respiration. 2. Endosymbiotic Origin: Mitochondria are believed to have originated from independent prokaryotes that were engulfed by an ancestral eukaryotic cell through endosymbiosis. 3. Mitochondrial Structure: Double Membrane Structure: ○ The mitochondrion has an outer membrane and an inner membrane due to its evolutionary history as a prokaryote. Outer Membrane: ○ The outer membrane contains transport proteins to move materials between the cytosol and the mitochondrion. Inner Membrane: ○ The inner membrane contains the electron transport chain and ATP synthase, essential for oxidative phosphorylation (ATP production). Cristae: ○ The inner membrane is folded into cristae (increased surface area), which maximises the efficiency of the electron transport chain and ATP synthesis. Intermembrane Space: ○ The space between the outer and inner membranes helps accumulate protons (H⁺), creating an electrochemical gradient necessary for ATP synthesis. Matrix: ○ The matrix contains enzymes and a suitable pH environment for the Krebs cycle (part of aerobic respiration). 4. Autonomous Features: Mitochondria have their own DNA (circular and naked) and ribosomes (70S), which reflect their prokaryotic origins. They are also susceptible to certain antibiotics that target prokaryotic machinery. Summary:  Mitochondria produce ATP through aerobic respiration in all eukaryotic cells. They originated from independent prokaryotes via endosymbiosis. Their structure is adapted for ATP production: ○ Outer membrane for material transport. ○ Inner membrane with electron transport chains and ATP synthase. ○ Cristae increase surface area for better energy production. ○ Matrix for the Krebs cycle and other metabolic processes. Mitochondria contain their own DNA and ribosomes, supporting their prokaryotic origin. 1. Carbohydrates as Respiratory Substrates: Carbohydrates (like glucose) are the most commonly used respiratory substrates because they are easier to digest and transport. Carbohydrates are broken down into monosaccharides (e.g., glucose), which can be directly used in glycolysis to produce ATP. 2. Lipids as Respiratory Substrates: Lipids (mainly fats) are a long-term energy source and are easier to store in the body because they are non-polar, meaning they have less osmotic effect compared to carbohydrates. Lipids are broken down into fatty acid chains, which are then converted into 2C compounds (acetyl CoA) and used in the Krebs cycle, bypassing glycolysis. Lipids produce more energy per gram than carbohydrates because their carbon chains have less oxygen and more oxidizable hydrogen and carbon. Summary: Carbohydrates are quick and easy to use for energy because they can be processed through glycolysis. Lipids provide a long-term energy source, store more efficiently, and release more energy per gram, but they are metabolized aerobically and bypass glycolysis.