Chapter 6: How Cells Harvest Chemical Energy PDF

**Chapter 6-How Cells Harvest Chemical Energy** **6.1 Photosynthesis and cellular respiration provide energy for life** Life requires energy, which is provided by photosynthesis and cellular respiration. In most ecosystems, energy comes from the sun. Photosynthesis uses sunlight to convert carbon...

**Chapter 6-How Cells Harvest Chemical Energy** **6.1 Photosynthesis and cellular respiration provide energy for life** Life requires energy, which is provided by photosynthesis and cellular respiration. In most ecosystems, energy comes from the sun. Photosynthesis uses sunlight to convert carbon dioxide and water into organic molecules and oxygen. Cellular respiration consumes oxygen to break down organic molecules into carbon dioxide and water, capturing energy as ATP. Photosynthesis occurs in some prokaryotes and in the chloroplasts of plants and algae, while cellular respiration occurs in many prokaryotes and in the mitochondria of almost all eukaryotes, including plants, animals, fungi, and protists. In energy conversions, some energy is lost as heat. Life on Earth is powered by the sun, with energy flowing one way through ecosystems. Matter, however, is recycled. Carbon dioxide and water released by cellular respiration are converted through photosynthesis into sugar and oxygen, which are then used in respiration. These processes illustrate the theme of energy and matter. What is misleading about the following statement? "Plant cells perform photosynthesis, and animal cells perform cellular respiration." The statement implies that cellular respiration does not occur in plant cells. In fact, almost all eukaryotic cells use cellular respiration to obtain energy for their cellular work. **Terms to Know** **cellular respiration**-The aerobic harvesting of energy from food molecules; the energy-releasing chemical breakdown of food molecules, such as glucose, and the storage of potential energy in a form that cells can use to perform work; involves glycolysis, pyruvate oxidation and the citric acid cycle, and oxidative phosphorylation (the electron transport chain and chemiosmosis). **Photosynthesis**-The process by which plants, algae, and some protists and prokaryotes convert light energy to chemical energy that is stored in sugars made from carbon dioxide and water. **6.2 Breathing supplies 0~2~ for use in cellular respiration and removes C0~2~** Respiration, often synonymous with \"breathing,\" involves the exchange of gases: an organism takes in oxygen (O₂) from its environment and releases carbon dioxide (CO₂) as a waste product. Breathing and cellular respiration are closely related. When a person breathes, their lungs take up O₂ and pass it to the bloodstream, which carries it to muscle cells. There, O₂ is used in cellular respiration to produce ATP, powering the muscle cells. During cellular respiration, inhaled oxygen atoms become part of water in the cells, while CO₂, originating from glucose, is produced as a waste product. The bloodstream carries CO₂ from the cells to the lungs, where it is exhaled. Thus, breathing and cellular respiration are tightly linked. Are the oxygen atoms a runner exhales the same oxygen atoms she inhaled from the environment? No. The oxygen atoms inhaled become part of water, which may become part of the runner's urine. The oxygen atoms in exhaled CO~2~ originate from glucose (or other food molecules). **6.3 Cellular respiration banks energy in ATP molecules** Cellular respiration is the process by which cells generate ATP, the energy currency for cellular work. Breathing and eating provide the necessary reactants for this process. Glucose is the primary fuel, but other organic molecules like fats, proteins, and complex carbohydrates can also be used. During cellular respiration, the atoms in glucose (C6H12O6) and oxygen (O2) are rearranged to form carbon dioxide (CO2) and water (H2O). This exergonic process releases the chemical energy stored in glucose, with some of it being stored in ATP and the rest released as heat. The process involves multiple steps, as indicated by the series of arrows in Figure 6.3. Cellular respiration captures about 34% of the energy stored in glucose, which is more efficient than the average automobile engine that converts only about 25% of gasoline energy into mechanical energy. The heat released during cellular respiration helps maintain a constant body temperature of 37°C (98.6°F), which is crucial for the survival of many animals. The energy needs of a cell are immense. Without the ability to regenerate ATP through cellular respiration, you would consume nearly your entire body weight in ATP each day. This highlights the significant energy demands of the human body and sets the stage for understanding the energy requirements of various activities. **6.5 Cells capture energy from electrons \"falling\" from organic fuels to oxygen** During cellular respiration, electrons are transferred from glucose or other organic fuels to oxygen, releasing energy. Oxygen strongly attracts electrons, causing them to lose potential energy when they move to oxygen. Unlike the rapid energy release when burning sugar, cellular respiration is a controlled process, releasing energy in small amounts that can be stored in ATP. The transfer of electrons from one molecule to another is called an oxidation-reduction (redox) reaction. In a redox reaction, oxidation refers to the loss of electrons, while reduction refers to the gain of electrons. Remembering \"OIL RIG\" (Oxidation Is Loss, Reduction Is Gain) can be helpful. Oxidation and reduction always occur together, as electron transfer requires both a donor and an acceptor. In cellular respiration, glucose (C~6~H~12~O~6~) loses hydrogen atoms and is oxidized to CO~2~, while O~2~ gains hydrogen atoms and is reduced to H~2~O. This exergonic process releases energy, which is harnessed by the cell to produce ATP. **NADH and Electron Transport Chains** An important player in oxidizing glucose is the coenzyme NAD+, which accepts electrons and becomes reduced to NADH. NAD+ (nicotinamide adenine dinucleotide) is an organic molecule that shuttles electrons in redox reactions. An enzyme called dehydrogenase strips two hydrogen atoms from an organic fuel molecule, transferring two electrons and one proton to NAD+, reducing it to NADH. The other proton is released into the surrounding solution. The other proton is lost because NAD+ only needs one proton to balance the two electrons it accepts, and the remaining proton is released to maintain the overall charge balance and contribute to the proton gradient. Using the energy staircase analogy, the transfer of electrons from an organic molecule to NAD+ is just the beginning. NADH delivers these electrons to the top of a chain of carrier molecules, most of which are proteins. At the bottom of the staircase, an oxygen atom accepts the electrons, picks up two H+, and becomes reduced to water. The one H+ proton rejoins the other. These carrier molecules form an electron transport chain within the inner membrane of a mitochondrion. Through a series of redox reactions, electrons are passed from one carrier to another, releasing energy that is used to produce ATP. **Terms to Know** **redox reaction**-short for reduction-oxidation reaction; a chemical reaction in which one substance loses electrons (oxidation) and added to another (reduction). **electron carrier***-*An electron carrier is a molecule that transports electrons during cellular respiration and other metabolic processes. These carriers, such as NADH, play a crucial role in transferring electrons from one molecule to another, facilitating the release of energy that is ultimately used to produce ATP, the cell\'s energy currency. **NADH***-*(nicotinamide adenine dinucleotide) is the reduced form of NAD+. It acts as an electron carrier in cellular respiration, transporting electrons to the electron transport chain where ATP is produced. NADH is crucial for the energy production process in cells. --------------------------------------------------------------------------------------------------------------------------------------------------------------------- --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- **Oxidation**-the loss of electrons from a substance involved in a redox reaction, always accompanied by reduction. **Dehydrogenation***-*Dehydrogenation is a chemical reaction that involves the removal of hydrogen atoms from a molecule. In cellular respiration, dehydrogenase enzymes facilitate this process by stripping hydrogen atoms from organic fuel molecules, such as glucose, and transferring the electrons and protons to electron carriers like NAD+, forming NADH. **electron transport chains**-a series of electron carrier molecules that move electrons during a series of redox reactions that release energy used for ATP. Located in the inner membranes of mitochondria, the thylakoid membranes of chloroplasts, and the plasma membranes of prokaryotes. **Reduction**-the gain of electrons by a substance involved in a redox reaction, always accompanies oxidation. **NAD+**: a coenzyme that acts as an electron carrier in cellular respiration. It helps transfer electrons from one molecule to another, facilitating the production of ATP, the cell\'s energy currency. **6.6 Overview: Cellular respiration occurs in three main stages** Cellular respiration consists of a series of chemical reactions divided into three main stages. In eukaryotic cells, these stages occur in specific locations, while in prokaryotic cells using aerobic respiration, the steps occur in the cytosol, and the electron transport chain is built into the plasma membrane. **Stage 1**: Glycolysis occurs in the cytosol of the cell. It initiates cellular respiration by breaking down glucose into two molecules of a three-carbon compound called pyruvate. **Stage 2**: Pyruvate oxidation and the citric acid cycle occur within the mitochondria. These processes complete the breakdown of glucose to carbon dioxide, which is exhaled. While a small amount of ATP is produced during glycolysis and the citric acid cycle, their main function is to supply electrons for the third stage of respiration. **Stage 3**: Oxidative phosphorylation, the process involves electron transport and chemiosmosis. NADH and FADH~2~ transport electrons to electron transport chains in the inner mitochondrial membrane. Most ATP in cellular respiration is produced by oxidative phosphorylation, which uses energy from redox reactions in the electron transport chain to generate ATP. Finally, electrons are passed to oxygen, reducing it to H~2~O. Chemiosmosis is the process by which the energy stored in a proton gradient is used to drive the synthesis of ATP. The electron transport chain couples to ATP synthesis by pumping hydrogen ions (H+) across the inner mitochondrial membrane, creating a concentration gradient. In chemiosmosis, the potential energy of this gradient is used to produce ATP. Of the three main stages of cellular respiration, which one does not take place in the mitochondria? Stage 1, glycolysis, occurs in the cytosol. **6.7 Stage 1: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate** Glycolysis, meaning \"splitting of sugar,\" involves breaking down one glucose molecule into two pyruvate molecules. This process includes nine enzyme-catalyzed reactions, resulting in the oxidation of glucose and the reduction of NAD+ to NADH. The net gain is two molecules of ATP. ATP is formed in glycolysis by substrate-level phosphorylation, where an enzyme transfers a phosphate group from a substrate molecule to ADP, forming ATP. Some ATP is also generated by substrate-level phosphorylation in the citric acid cycle, but the majority of ATP in cellular respiration is produced through the electron transport chain in stage 3. The breakdown of glucose to pyruvate releases energy, stored in ATP and NADH. The cell can use ATP immediately, but NADH\'s energy requires electrons to pass down the electron transport chain. Pyruvate still holds about 90% of the energy from glucose and will be further oxidized in stage 2, producing more NADH for stage 3. So, in summary, glycolysis splits glucose into two pyruvate molecules, produces ATP through substrate-level phosphorylation, and reduces NAD+ to NADH, resulting in a net gain of two ATP molecules. For each glucose molecule processed, what are the net molecular products of glycolysis? Two molecules of pyruvate, two molecules of ATP, and two molecules of NADH **Terms to Know** **Glycolysis**-the process of breaking down glucose into two molecules of pyruvate, producing a small amount of energy in the form of ATP and NADH. It occurs in the cytosol of the cell and is the first step in cellular respiration. **citric acid cycle***-*also known as the Krebs cycle, is a series of chemical reactions in the mitochondria that break down acetyl CoA to produce energy carriers like ATP, NADH, and FADH~2~. These carriers then transfer energy to the electron transport chain to generate more ATP. **oxidative phosphorylation**-Oxidative phosphorylation is the final stage of cellular respiration, where ATP is produced using energy from electrons transferred through the electron transport chain in the mitochondria. ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- **substrate level phosphorylation**-the formation of ATP by an enzyme directly transferring a phosphate group to ADP from an organic molecule (e.g., one of the intermediates in glycolysis or the citric acid cycle) **6.8 Multiple reactions in glycolysis split glucose into two molecules** Figure 6.8 shows simplified structures for the organic compounds in the nine chemical reactions of glycolysis. Each chemical step feeds into the next, with the product of one reaction serving as the reactant for the next. Compounds formed between an initial reactant and a final product are called intermediates. Each chemical step is catalyzed by a specific enzyme (not shown in the figure). The steps of glycolysis can be grouped into two main phases. **Energy Investment Phase** The energy investment phase consumes energy. In this phase, two molecules of ATP are used to add a phosphate group to each glucose molecule, which is then split into two small sugars. **Steps 1--3**: Glucose is energized using ATP. Three chemical reactions convert glucose into an energized intermediate. The cell uses 2 ATP (one at step 1 and one at step 3) to make a more reactive molecule. **Step 4**: The six-carbon intermediate splits into two three-carbon intermediates called glyceraldehyde 3-phosphate (G3P). Each G3P molecule enters the next phase, so the energy payoff steps happen twice per glucose molecule. **Energy Payoff Phase** The energy payoff phase occurs after glucose is split into two G3P molecules. In **step 5**, a redox reaction generates NADH by transferring hydrogen atoms, oxidizing G3P, and reducing NAD+ to NADH. This reaction also attaches a phosphate group to the substrate. During steps **6--9**, ATP and pyruvate are produced, completing glycolysis. Two molecules of pyruvate and four ATP are produced in this phase. Water is produced as a by-product in step 8. Overall, two NADH and four ATP are generated per glucose molecule, with a net gain of two ATP after accounting for the two ATP used in the first phase. The two ATP molecules from glycolysis account for only about 6% of the energy a cell can harvest from glucose. Some organisms, like yeasts and certain bacteria, can meet their energy needs with ATP from glycolysis alone. Muscle cells may use this anaerobic ATP production temporarily when lacking sufficient oxygen. However, most cells and organisms have higher energy demands, and the stages of cellular respiration following glycolysis release much more energy. Simply put: - **Energy Investment Phase**: - In the first phase of glycolysis, two ATP molecules are used to phosphorylate glucose and convert it into two G3P molecules. - **Energy Payoff Phase**: - In the second phase, each G3P molecule is further processed to produce two ATP molecules and one NADH molecule. Since there are two G3P molecules, this results in a total of four ATP molecules and two NADH molecules. So, the overall process of glycolysis produces four ATP molecules, but since two ATP molecules were used in the energy investment phase, the net gain is two ATP molecules per glucose molecule. Additionally, two NADH molecules are produced, which can be used in oxidative phosphorylation to generate more ATP. During glycolysis, how many total substrate-level phosphorylation reactions occur per molecule of glucose? Four occur for each molecule of glucose: two during step 6 and two during step 9. **6.9 Stage 2: The citric acid cycle completes the energy-yielding oxidation of organic molecules** As pyruvate is produced at the end of glycolysis, it is transported from the cytosol into a mitochondrion, where the citric acid cycle and oxidative phosphorylation occur. For each glucose molecule that enters glycolysis, two molecules of pyruvate are produced. **Pyruvate Oxidation** Each pyruvate undergoes redox reactions to produce acetyl CoA and NADH. This process involves three steps: 1. Pyruvate loses a carbon and two oxygens as CO~2~. 2. The remaining two-carbon compound is oxidized, and NAD+ is reduced to NADH. 3. Coenzyme A joins with the two-carbon group to form acetyl CoA, which is ready to enter the citric acid cycle. **Citric Acid Cycle** In the citric acid cycle, each acetyl CoA goes through a series of reactions to produce: 1. Two CO~2~ molecules 2. One ATP molecule 3. Three NADH molecules 4. One FADH~2~ molecule Since two acetyl CoA molecules come from one glucose molecule, the cycle runs twice, doubling these outputs. Most of the energy is stored in NADH and FADH~2~, which then transfer their high-energy electrons to the electron transport chain. **6.10 The multiple reactions of the citric acid cycle finish off the dismantling of glucose** Here\'s a summary of the six major steps of the citric acid cycle as described in Figure 6.10 (step 1): - The cycle begins with enzymes stripping the CoA portion from acetyl CoA. - The remaining two-carbon group combines with the four-carbon molecule oxaloacetate. - This reaction produces the six-carbon molecule citrate (the ionized form of citric acid). In steps 2) and 3), NADH, ATP, and CO~2~ are produced during redox reactions. These reactions harvest energy by removing hydrogen atoms from citrate and alpha-ketoglutarate, creating NADH. In two instances, an intermediate compound loses a CO~2~ molecule. Energy is also harvested by converting ADP to ATP through substrate-level phosphorylation. By the end of step 3, a four-carbon compound called succinate is formed. In the final steps, more redox reactions occur, producing FADH~2~ and NADH. Succinate is oxidized, and FAD is reduced to FADH~2~. Fumarate is converted to malate, which is then oxidized, reducing NAD+ to NADH. The citric acid cycle completes one turn with the regeneration of oxaloacetate, ready to start the next cycle by accepting an acetyl group from acetyl CoA. Here\'s a simplified summary of the citric acid cycle: - **Step 1**: Enzymes remove the CoA portion from acetyl CoA, and the remaining two-carbon group combines with oxaloacetate to form citrate. - **Steps 2 and 3**: Redox reactions produce NADH, ATP, and CO2 by removing hydrogen atoms from citrate and alpha-ketoglutarate. An intermediate compound loses CO2 twice, and ADP is converted to ATP. Succinate is formed by the end of step 3. - **Final Steps**: More redox reactions occur, producing FADH2 and NADH. Succinate is oxidized to fumarate, which is then converted to malate. Malate is oxidized to form NADH, and oxaloacetate is regenerated to start the next cycle. Overall, the cycle produces NADH, FADH2, ATP, and CO2, with oxaloacetate being regenerated to continue the cycle. Each turn of the citric acid cycle generates one ATP molecule, 3 NADH molecules, and 1 FADH2 molecule. Here\'s why: - **NADH Production**: During the citric acid cycle, three molecules of NAD+ are reduced to NADH. This happens in three different steps of the cycle. - **FADH~2~ Production**: One molecule of FAD is reduced to FADH2 during the cycle. - **ATP Production**: One molecule of ATP is produced through substrate-level phosphorylation. **Mnemonic: \"Citrus Is A Substrate For Making Oxes\"** - **C**: Citrate (Step 1: Formation of citrate) - **I**: Intermediate (Step 2: Redox reactions producing NADH, ATP, and CO2) - **A**: Alpha-ketoglutarate (Step 3: Formation of alpha-ketoglutarate and succinate) - **S**: Succinate (Step 4: Formation of succinate) - **F**: Fumarate (Step 6: Formation of fumarate) - **M**: Malate (Step 7: Formation of malate) - **O**: Oxaloacetate (Step 8: Regeneration of oxaloacetate) What is the total number of NADH and FADH~2~ molecules generated during the complete breakdown of one glucose molecule to six molecules of CO~2~? (Hint: Combine the outputs from stages 1 and 2.) 10 NADH: 2 from glycolysis, 2 from the oxidation of pyruvate, and 6 from the citric acid cycle; and 2 FADH~2~ from the citric acid cycle. (Did you remember to double the output after the sugar-splitting step of glycolysis?) **6.11 VISUALIZING THE CONCEPT** **Stage 3: Most ATP production occurs by oxidative phosphorylation** The image is a detailed diagram of the process of oxidative phosphorylation in the mitochondria, specifically focusing on the electron transport chain and chemiosmosis. It shows the inner mitochondrial membrane with various protein complexes (labeled I to IV), mobile electron carriers, and ATP synthase. The diagram illustrates how electrons are transferred through the complexes, leading to the pumping of protons (H⁺) across the membrane, creating a proton gradient. This gradient drives the synthesis of ATP as protons flow back through ATP synthase. The image also highlights the role of oxygen as the final electron acceptor, forming water. Almost 90% of the ATP in cellular respiration is produced in Stage 3, known as oxidative phosphorylation. This stage involves electron transport and chemiosmosis. During this process, electrons carried by NADH and FADH~2~ from the earlier stages are utilized to generate ATP. Electrons from NADH and FADH~2~ are transferred through the electron transport chain in the inner mitochondrial membrane. These electrons move down an \"energy staircase\" to oxygen, the final electron acceptor. The two H+ ions that combine with oxygen at the end of the electron transport chain come from the H+ solutes in the mitochondrial matrix. The energy released from these transfers actively transports H+ into the intermembrane space. During chemiosmosis, the resulting concentration gradient drives H+ through ATP synthase, which phosphorylates ADP to ATP. This stage is the major energy payoff that started with a glucose molecule. What effect would an absence of oxygen (O~2~) have on the process of oxidative phosphorylation? Without oxygen to "pull" electrons down the electron transport chain, the energy stored in NADH and FADH~2~ could not be harnessed for ATP synthesis. **6.13 Review: Each molecule of glucose yields many molecules of ATP** Starting with one molecule of glucose: 1. **Glycolysis**: In the cytosol, glucose is oxidized to two molecules of pyruvate, producing 2 NADH and a net of 2 ATP by substrate-level phosphorylation. 2. **Oxidation of Pyruvate**: In the mitochondrion, 2 pyruvate molecules yield 2 NADH and 2 acetyl CoA. 3. **Citric Acid Cycle**: The 2 acetyl CoA enter the cycle, producing 6 NADH, 2 FADH~2~, and 2 ATP by substrate-level phosphorylation. Glucose is now completely oxidized to CO~2~. 4. **Electron Transport Chain**: NADH and FADH~2~ deliver electrons to the chain, which are passed to O~2~, forming H~2~O. The energy released pumps H+ into the intermembrane space. 5. **Chemiosmosis**: The H+ gradient drives ATP synthase to produce about 28 ATP molecules by oxidative phosphorylation. This process captures energy from glucose oxidation to produce ATP, giving rise to the name oxidative phosphorylation. The total yield of ATP molecules per glucose is about 32. However, this number isn\'t exact because: - NADH from glycolysis can transfer electrons to either NAD+ or FAD. - FADH~2~ contributes less to the H+ gradient and generates less ATP. - Some energy from the H+ gradient may be used for other work besides ATP production. Most ATP is generated through oxidative phosphorylation, which requires oxygen as the final electron acceptor. Without oxygen, electron transport and ATP production stop. However, some cells can still oxidize organic fuel and produce ATP without oxygen. Explain where O~2~ is used, and CO~2~ is produced in cellular respiration. O~2~ accepts electrons at the end of the electron transport chain. CO~2~ is released during the oxidation of intermediate compounds in pyruvate oxidation and the citric acid cycle.