Cellular Respiration PDF

CHAPTECR IV: Cellular Respiration Cellular respiration may be described as a set of metabolic reactions and processes that take place in the cells of organisms to convert chemical energy from nutrients into ATP, and then release waste products. Cellular respiration is a vital process that occurs in...

CHAPTECR IV: Cellular Respiration Cellular respiration may be described as a set of metabolic reactions and processes that take place in the cells of organisms to convert chemical energy from nutrients into ATP, and then release waste products. Cellular respiration is a vital process that occurs in the cells of all living organisms. Within the realm of cellular respiration, three distinct pathways govern the energy dynamics of living organisms: Photorespiration, Fermentation, and Aerobic Respiration. Each method showcases the diverse strategies employed by cells to extract and utilize energy from different sources and under varying conditions. 1. Aerobic Respiration Cellular respiration or aerobic respiration can be defined as a set of reactions and metabolic processes that occur in the cells of living organisms to convert chemical energy from nutrients into ATP, and then release waste. Cellular respiration is a vital process that can all be summarized in this chemical equation: C6H12O6+6O2⟶6CO2+6H2O There are three stages to aerobic respiration: glycolysis, the Krebs cycle, and the Oxidative phosphorylation. Figure 1: Cellular respiration takes place in the stages shown here. 2 Aerobic Respiration takes place in the stages shown here. The process begins with Glycolysis. In this first step, a molecule of glucose, which has six carbon atoms, is split into two three- carbon molecules. The three-carbon molecule is called pyruvate. Pyruvate is oxidized and converted into Acetyl CoA. These two steps occur in the cytoplasm of the cell. Acetyl CoA enters into the matrix of mitochondria, where it is fully oxidized into Carbon Dioxide via the Krebs cycle. Finally, During the process of oxidative phosphorylation, the electrons extracted from food move down the electron transport chain in the inner membrane of the mitochondrion. As the electrons move down the ETC and finally to oxygen, they lose energy. This energy is used to phosphorylate ADP to make ATP. 1) Glycolysis The first stage of cellular respiration is glycolysis. This process is shown in the top box in Figure 1 showing a 6-carbon molecule being broken down into two 3-carbon pyruvate molecules. ATP is produced in this process which takes place in the cytosol of the cytoplasm. The word glycolysis means “glucose splitting,” which is exactly what happens in this stage. Enzymes split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as shown in Figure 2. Glucose is first split into glyceraldehyde 3-phosphate (a molecule containing 3 carbons and a phosphate group) this process uses 2 ATP. Next, each glyceraldehyde 3-phosphate is converted into pyruvate (a 3- carbon molecule) this produces two 4 ATP and 2 NADH. Figure 2: In glycolysis, a glucose molecule is converted into two pyruvate molecules. The second stage of Glycolysis is “Transformation of Pyruvate into Acetyl-CoA”. In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into mitochondria, which are sites of cellular respiration. If oxygen is available, aerobic respiration 3 will go forward. In mitochondria, pyruvate will be transformed into a two-carbon acetyl group (by removing a molecule of carbon dioxide) that will be picked up by a carrier compound called coenzyme A (CoA), which is made from vitamin B5. The resulting compound is called acetyl CoA and its production is frequently called the oxidation or the Transformation of Pyruvate (see Figure 3). Figure 3: Pyruvate is converted into acetyl-CoA. Acetyl CoA can be used in a variety of ways by the cell, but its major function is to deliver the acetyl group derived from pyruvate to the next pathway step, the Citric Acid Cycle (Krebs Cycle). 2) The Krebs Cycle (Citric Acid Cycle) The second stage of cellular respiration, the Krebs cycle, takes place in the matrix (The mitochondrial matrix is the space included in the inner mitochondrial membrane) as shown in Figure 4. Figure 4: The structure of a mitochondrion is defined by an inner and outer membrane. The citric acid cycle is a series of biochemical reactions to release the energy stored in nutrients through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins. 4 In the Citric Acid Cycle, the acetyl group from acetyl CoA is attached to a four-carbon oxaloacetate molecule to form a six-carbon citrate molecule. Through a series of steps, citrate is oxidized, releasing two carbon dioxide molecules for each acetyl group fed into the cycle. In the process, three NAD+ molecules are reduced to NADH, one FAD molecule is reduced to FADH2, and one ATP or GTP (depending on the cell type) is produced (by substrate-level phosphorylation). Because the final product of the citric acid cycle is also the first reactant, the cycle runs continuously in the presence of sufficient reactants. Figure 5: The Citric Acid Cycle. 3) Oxidation in the Respiratory Chain Oxidative phosphorylation is the final stage of aerobic cellular respiration. There are two substages of oxidative phosphorylation, Electron transport chain and Chemiosmosis. In 5 these stages, energy from NADH and FADH2, which result from the previous stages of cellular respiration, is used to create ATP. This third stage takes place on the inner membrane. Figure 6: Oxidative Phosphorylation.  Electron Transport Chain (ETC): In this stage, high-energy electrons from NADH and FADH2 traverse a series of molecules in the inner membrane of the mitochondrion, forming three complexes. As electrons move through these complexes, energy is utilized to pump hydrogen ions (H+) from the matrix to the intermembrane space, creating an electrochemical gradient. This gradient drives ATP synthesis. The final ETC protein transfers electrons to oxygen, reducing it to water in the mitochondrial matrix.  Chemiosmosis: Hydrogen ions pumped across the inner membrane generate an electrochemical gradient, with a higher concentration in the intermembrane space. This gradient propels the flow of ions back into the matrix through ATP synthase, a protein complex acting as a channel. This ion movement constitutes chemiosmosis. ATP synthase, functioning as an enzyme, catalyzes ATP synthesis from ADP and inorganic phosphate using energy derived from the hydrogen ion flow. Ultimately, low-energy electrons combine with oxygen in the matrix, forming water after passing through the electron transport chain. 6   Assessment of Aerobic Respiration Aerobic respiration in plants is a vital process that links cellular energy production to internal and external conditions. Evaluating its effectiveness involves considering various factors that impact plant metabolism and overall health.  External Conditions:  Temperature: Investigate the impact of temperature on plant respiration. Optimal temperatures facilitate enzymatic reactions, while extremes may hinder metabolic processes.  Oxygen Availability: Ensure an adequate supply of oxygen to plant cells. Oxygen deficiency can limit the efficiency of the electron transport chain, affecting ATP synthesis.  Light Intensity: Consider the role of light in influencing plant metabolism. Light availability affects the rate of photosynthesis, indirectly influencing the availability of respiratory substrates.  Water Availability: Assess water availability, as water is essential for various metabolic processes. Water stress can disrupt cellular activities, impacting aerobic respiration.  Internal Conditions:  Mitochondrial Activity: Assess the efficiency of mitochondrial function, as it plays a central role in aerobic respiration. Healthy mitochondria contribute to effective energy production.  Availability of Substrates: Evaluate the presence of substrates like glucose and oxygen. An ample supply ensures the continuous flow of reactants into the aerobic respiration pathway.  Cellular Health: Examine the overall cellular health, as compromised cell integrity can impede the transport of substances across membranes, affecting the respiration process.  Enzyme Activity: Monitor the activity of enzymes involved in glycolysis, the Krebs cycle, and oxidative phosphorylation. Enzyme efficiency directly influences the rate of aerobic respiration. 7 2. Fermentation (Anaerobic Cellular Respiration) Some organisms can respire in the absence of oxygen. This process is called fermentation or anaerobic respiration. To solve this problem, organisms use fermentation to reoxidize the NADH, thus allowing glycolysis to continue (Figure 7 and 9). There are two types of fermentation: Alcoholic fermentation, and Lactic acid fermentation. 1 Alcoholic fermentation The cells of roots in water logged soil respire by alcoholic fermentation because of lack of oxygen by converting pyruvic acid into ethyl alcohol and CO2. Many species of yeast (Saccharomyces) also respire anaerobically. Figure 7: In alcoholic fermentation, pyruvate is converted to acetaldehyde, and CO2 is released. NADH is used to reduce acetaldehyde to ethanol, again regenerating NAD+ for glycolysis. This process takes place in two steps: Pyruvate decarboxylase and Alcohol dehydrogenase. 8 Figure 8: The reaction resulting in alcohol fermentation is shown. Industrial uses of alcoholic fermentation: - In bakeries, it is used for preparing bread, cakes, biscuits. - In producing vinegar and in tanning, curing of leather. - Ethanol is used to make gasohol (a fuel that is used for cars in Brazil). 2 Lactic acid fermentation Lactic acid fermentation is a metabolic process by which glucose or other six-carbon sugars (also, disaccharides of six-carbon sugars, e.g. sucrose or lactose) are converted into cellular energy and the metabolite lactate, which is lactic acid in solution. 9 Figure 7: In lactic acid fermentation, NADH is used to reduce pyruvate to lactic acid, thus regenerating NAD+ to keep glycolysis operating. 3. Photorespiration Photorespiration is a metabolic process in plants that occurs in the chloroplasts during photosynthesis, particularly under conditions of high oxygen concentration and low carbon dioxide levels. In photorespiration, oxygen is taken up by the plant cells, and carbon dioxide is released, leading to the consumption of energy rather than its production. This process involves the oxygenation of ribulose-1,5-bisphosphate, a key molecule in the Calvin cycle of photosynthesis, and subsequent metabolic reactions that ultimately result in the release of carbon dioxide. Photorespiration is considered a wasteful and energetically costly side reaction, and its occurrence is influenced by environmental factors such as temperature, light intensity, and the plant's water status. 10 Figure 8: Représentation simplifiée de la photorespiration et du Calvin cycle. 11