CHAPTER 7 Cellular Respiration

• Cellular respiration is the process by which organisms convert energy from glucose into usable ATP molecules.
  This is the fundamental metabolic pathway that transfers chemical energy from organic molecules into the form of ATP that cells can use for work and maintenance.
• Cellular respiration functions similarly to how power plants convert thermal energy into electrical energy.
  The analogy illustrates that both processes transform one form of energy into a more usable form that can be easily distributed and applied.

Source: Chapter 7 Introduction, page 1

• Cells capture energy released during glucose breakdown and use it to drive the synthesis of ATP, the molecule that directly powers most cellular work.
  Rather than releasing all glucose energy at once as heat, cells couple glucose oxidation to ATP synthesis through a series of controlled redox reactions, allowing them to store chemical energy in a form they can readily use.
• NAD+ and FAD are electron carriers that accept electrons and hydrogen ions during catabolic reactions, becoming reduced to NADH and FADH2.
  These carrier molecules shuttle high-energy electrons from glucose breakdown pathways to the electron transport chain, where the energy stored in those electrons drives ATP synthesis.
• The transfer of electrons between molecules is coupled to the transfer of energy, allowing cells to capture and use the energy released during oxidation-reduction reactions.
  When one molecule is oxidized and loses electrons, another is reduced and gains electrons; the energy released during this electron transfer is the source that powers ATP synthesis.
• ATP functions as the cell's primary energy currency, transferring energy from catabolic pathways to the anabolic reactions that build cellular structures and carry out cellular functions.
  The high-energy phosphate bonds in ATP are broken to release energy on demand, and ATP is continuously regenerated from ADP to meet the cell's energy needs.

Source: Section 7.1, pages 1-6

• Glycolysis is a metabolic pathway that breaks down glucose into two pyruvate molecules through ten enzyme-catalyzed reactions occurring in the cell cytoplasm.
  This ten-step process converts the six-carbon glucose molecule into two three-carbon pyruvate molecules and is the first stage of cellular respiration.
• Glycolysis is divided into two phases: the energy investment phase consumes two ATP molecules, while the energy payoff phase produces four ATP and two NADH molecules.
  The net result is a gain of two ATP and two NADH per glucose molecule, since two ATP are used in the early steps and four are generated later.
• The first regulatory step of glycolysis is the phosphorylation of glucose to glucose-6-phosphate by hexokinase, which traps glucose in the cell and commits it to metabolism.
  This irreversible reaction marks the initial commitment of glucose to glycolysis and is the rate-limiting step for glucose entry into the pathway.
• Glycolysis does not require oxygen and can proceed under both aerobic and anaerobic conditions.
  This makes glycolysis the only part of glucose catabolism that can function without oxygen, providing ATP even in anaerobic environments.
• Each glucose molecule generates enough energy and reducing power during glycolysis to allow for the production of substantially more ATP molecules in subsequent stages of cellular respiration.
  The NADH produced during glycolysis carries electrons to later stages (the electron transport chain) where most cellular ATP is generated.

Source: Section 7.2, pages 6-9

• Pyruvate undergoes oxidative decarboxylation by the pyruvate dehydrogenase complex to form acetyl-CoA, releasing CO2 and generating NADH.
  This irreversible reaction occurs in the mitochondrial matrix and is the critical link between glycolysis and the citric acid cycle. The acetyl group is attached to coenzyme A for entry into the cycle.
• The citric acid cycle is a circular metabolic pathway that accepts acetyl-CoA and oxidizes it completely to CO2 while extracting energy in the form of electron carriers and ATP.
  Operating in the mitochondrial matrix, the cycle regenerates its starting molecule (oxaloacetate) with each turn, allowing continuous processing of acetyl units.
• The citric acid cycle generates three NADH, one FADH2, and one GTP (or ATP) per turn through the oxidation of each acetyl-CoA molecule.
  These electron carriers and the direct ATP product capture the chemical energy remaining after pyruvate oxidation, with NADH and FADH2 later used in oxidative phosphorylation.
• The cycle includes decarboxylation steps that release the two carbon atoms from the acetyl group as CO2, representing the complete oxidation of glucose.
  Two CO2 molecules are released per acetyl-CoA during the oxidation of isocitrate and alpha-ketoglutarate, fulfilling the cycle's role in total glucose combustion.

Source: Section 7.3, pages 9-12

• Oxidative phosphorylation is the process by which ATP is generated through the movement of electrons along the electron transport chain and the energy released driving chemiosmosis across the inner mitochondrial membrane.
  This process accounts for approximately 90 percent of the ATP generated during cellular respiration, making it the dominant mechanism for energy production in aerobic cells. The process couples electron transfer to proton pumping and ATP synthesis.
• The electron transport chain is a series of protein complexes embedded in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, the final electron acceptor.
  As electrons move from one complex to the next, they lose energy that is used to pump protons across the membrane. NADH donates electrons at a higher energy level than FADH2, so NADH typically results in more ATP production per molecule.
• Chemiosmosis is the process by which a proton gradient across the inner mitochondrial membrane drives ATP synthesis as protons flow back through ATP synthase.
  The electron transport chain pumps protons from the mitochondrial matrix into the intermembrane space, creating a concentration gradient. This gradient provides the driving force for the phosphorylation of ADP to ATP as protons flow through ATP synthase down their concentration gradient.
• ATP synthase is a protein channel complex that catalyzes ATP synthesis by coupling the movement of protons down their gradient to the phosphorylation of ADP to ATP.
  The protein functions like a molecular turbine, with the flow of protons through its channel providing rotational energy that drives the chemical reaction linking ADP and inorganic phosphate together to form ATP.

Source: Section 7.4, pages 12-16

• Fermentation is an anaerobic pathway that regenerates NAD+ from NADH so glycolysis can continue producing ATP when oxygen is unavailable.
  Without oxygen, cells cannot use the electron transport chain to recycle NAD+, so fermentation allows glycolysis to proceed by oxidizing NADH back to NAD+, maintaining the NAD+ pool needed for continued glucose breakdown and ATP production.
• Lactic acid fermentation converts pyruvate to lactate in muscle cells and reduces lactate concentration during recovery when oxygen becomes available.
  In lactic acid fermentation, pyruvate is reduced to lactate by the enzyme lactate dehydrogenase, regenerating NAD+. The lactate is later converted back to pyruvate and fully oxidized when oxygen is restored.
• Alcoholic fermentation converts pyruvate to ethanol and carbon dioxide in some microorganisms and plants, regenerating NAD+ in the process.
  Pyruvate is first decarboxylated to acetaldehyde, then reduced to ethanol by alcohol dehydrogenase, which regenerates NAD+ for continued glycolysis.
• Fermentation produces only two ATP per glucose from glycolysis, far less than the 30-32 ATP yielded by complete aerobic respiration.
  Because fermentation does not proceed through the citric acid cycle or electron transport chain, it generates ATP only from the substrate-level phosphorylation occurring in glycolysis.
• Fermentation allows cells to maintain ATP production during hypoxic or anaerobic conditions when oxidative phosphorylation is not possible.
  Though inefficient compared to aerobic respiration, fermentation sustains basic cellular functions by ensuring a steady supply of ATP during periods when oxygen is limited or absent.

Source: Section 7.5, pages 16-19

• Glucose, amino acids, and fatty acids all feed their carbon skeletons and electrons into a single converged pathway centered on the citric acid cycle.
  The central metabolic hub processes energy carriers from three major nutrient classes, allowing the cell to extract ATP from any macronutrient source through a unified system.
• Carbohydrates enter cellular respiration through glycolysis, which converts glucose to pyruvate and then acetyl CoA for the citric acid cycle.
  Glucose degradation is the most direct route into the core energy extraction pathway, making carbohydrates the primary fuel source.
• Proteins are broken down to amino acids, which undergo deamination to yield carbon skeletons that enter glycolysis or the citric acid cycle at various points.
  The carbon backbone of amino acids, once separated from nitrogen-containing groups, integrates with existing metabolic pathways and becomes energy substrate.
• Lipids are hydrolyzed to glycerol and fatty acids, which are converted to acetyl CoA through beta-oxidation and feed directly into the citric acid cycle.
  Fatty acids generate multiple acetyl CoA molecules per molecule of lipid, making them highly efficient energy sources.
• Metabolic pathways are bidirectional - intermediates can be diverted for biosynthesis as well as energy extraction, allowing the same molecules to build other compounds or generate ATP.
  Gluconeogenesis, lipogenesis, and amino acid synthesis use intermediates from glycolysis and the citric acid cycle, linking catabolic and anabolic processes in an integrated metabolic network.

Source: Section 7.6, pages 19-21

• Cellular respiration must be regulated to maintain balanced ATP production and prevent wasteful overproduction of energy and intermediates.
  Without controls, metabolic reactions would reach equilibrium and stall. The cell needs to direct intermediates to biosynthetic pathways for amino acids, proteins, glycogen, lipids, and nucleic acids while balancing energy production.
• Glucose transport into cells is regulated through different forms of GLUT (glucose transporter) proteins that control passage into specific tissues.
  GLUT4, for example, is stored in vesicles and moves to the plasma membrane in response to insulin binding, allowing glucose uptake to be adjusted based on cellular needs.
• Allosteric control of key enzymes by ATP, ADP, AMP, NAD+, and NADH provides feedback mechanisms that increase or decrease enzyme activity based on energy status.
  These allosteric effectors bind to regulatory sites on enzymes and alter their structure, changing substrate affinity and reaction rates. When the effector concentration decreases, it diffuses away and control is relaxed.
• Hexokinase, the first enzyme in glycolysis, is inhibited when glucose-6-phosphate accumulates, preventing glucose from entering the respiration pathway if it is not needed.
  Since hexokinase catalyzes phosphorylation of glucose, its inhibition allows unphosphorylated glucose to diffuse out of the cell and prevents commitment to the glycolytic pathway.
• Phosphofructokinase is the primary regulated enzyme in glycolysis and is inhibited by high ATP, citrate, or low pH, linking pathway control to energy status and metabolic conditions.
  High citrate signals that the citric acid cycle is blocked or producing enough intermediates. Acidic pH from fermentation products indicates anaerobic conditions. These allosteric effects decrease enzyme activity to slow glycolysis.
• Pyruvate kinase, which catalyzes the final step of glycolysis, is regulated by ATP (negative allosteric effect), phosphorylation state, and fructose-1,6-bisphosphate levels.
  When ATP is abundant, pyruvate kinase is inhibited, slowing pyruvate production. Phosphorylation by kinase inactivates it; dephosphorylation by phosphatase reactivates it, providing multiple control points.
• Pyruvate dehydrogenase, which converts pyruvate to acetyl CoA, is regulated by ATP and NADH levels and by phosphorylation, slowing the reaction when energy carriers accumulate.
  High levels of ATP or NADH indicate sufficient energy supply, so the enzyme is phosphorylated into an inactive form. A phosphatase reactivates it when energy is needed.
• In the citric acid cycle, isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase are the primary control points, both inhibited by high ATP and NADH levels.
  When adequate energy carriers are present, these enzymes slow the cycle. When ADP rises (indicating energy need), the rate increases. Alpha-ketoglutarate dehydrogenase is also inhibited by its product succinyl CoA.
• The electron transport chain rate is controlled by the ADP to ATP concentration ratio, independent of direct feedback inhibition of its enzymes.
  High ADP concentration (from ATP consumption) accelerates electron transport. As ATP builds up and ADP decreases, the transport chain slows, adjusting ATP production to cellular demand.
• Feedback control mechanisms work as long as the regulatory molecule remains bound to the enzyme; when its concentration decreases, it diffuses away and control is relaxed.
  This allows the cell to respond dynamically to changing energy demands without permanent alterations to enzyme structure or availability.

Source: Section 7.7, pages 21-24

• ATP is the cell's energy currency that stores energy briefly and transports it within the cell to support endergonic reactions.
  ATP has the structure of an RNA nucleotide with three phosphates; as it is used for energy, one or two phosphate groups are detached to produce ADP or AMP. Energy from glucose catabolism is used to regenerate ATP through substrate-level phosphorylation and oxidative phosphorylation via chemiosmosis.
• Glycolysis is a cytoplasmic pathway that breaks down glucose into two three-carbon pyruvate molecules, yielding a net gain of two ATP and two NADH.
  The first half of glycolysis invests two ATP to prepare the six-carbon glucose for cleavage; the second half extracts energy, producing four ATP and two NADH through substrate-level phosphorylation and electron transfer to NAD+.
• Pyruvate is converted to acetyl CoA, which enters the citric acid cycle where remaining glucose carbons are oxidized and electrons are transferred to NADH and FADH2.
  During pyruvate oxidation, carbon dioxide and high-energy electrons are removed; the citric acid cycle itself produces one GTP or ATP per turn through substrate-level phosphorylation and generates NADH and FADH2 for subsequent ATP production.
• The electron transport chain uses oxygen as the final electron acceptor and is embedded in the inner mitochondrial membrane, with electrons losing energy through a series of redox reactions.
  Energy released at three points in the electron transport chain is used to establish a proton gradient across the membrane; this gradient drives chemiosmosis through ATP synthase to generate approximately 90 percent of cellular ATP. NADH and FADH2 donate electrons that ultimately reduce oxygen to water.
• Fermentation regenerates NAD+ when oxygen is unavailable, allowing glycolysis to continue without producing additional ATP.
  In fermentation, NADH is oxidized using an alternative organic or inorganic electron acceptor instead of oxygen; this process does not use the electron transport chain, so the potential ATP production from NADH is not realized.
• Carbohydrate, protein, and lipid metabolism pathways interconnect through shared intermediates in glycolysis, pyruvate oxidation, and the citric acid cycle.
  Simple sugars enter at glycolysis; amino acids connect through pyruvate and acetyl CoA; and lipids are synthesized from glycerol-3-phosphate and acetyl groups derived from these central pathways.
• Cellular respiration is primarily regulated through negative feedback control of specific enzymes in response to levels of ATP, ADP, AMP, NAD+, and FAD.
  Glucose entry into the cell is controlled by membrane transport proteins, while enzyme activity throughout glycolysis, the citric acid cycle, and the electron transport chain responds to the energy status of the cell, with high ATP levels inhibiting further respiration.
• Phosphorylation is the addition of a high-energy phosphate group to a compound, and dephosphorylation is the removal of a phosphate group.
  These processes are central to energy transfer in cellular respiration, with substrate-level phosphorylation producing ATP directly from chemical reactions and oxidative phosphorylation using the proton gradient to drive ATP synthesis.
• Redox reactions are the fundamental chemical process in cellular respiration, coupling oxidation and reduction reactions to transfer electrons and energy.
  Throughout glycolysis, pyruvate oxidation, the citric acid cycle, and the electron transport chain, electrons are transferred between molecules, allowing energy extraction from glucose to be captured in high-energy electron carriers and ATP.

Source: Chapter Summary, pages 25-26

• ATP is the universal energy currency used by cells to power metabolic processes.
  This fundamental principle underpins why cells break down glucose through respiration rather than using energy directly from carbohydrate bonds, as ATP provides a standardized, controllable energy source for diverse cellular needs.
• The citric acid cycle produces three NADH molecules per turn, which carry electrons to the electron transport chain for ATP generation.
  These NADH molecules are critical reducing agents that transfer high-energy electrons, accounting for a large portion of the ATP ultimately synthesized during oxidative phosphorylation.
• Chemiosmosis depends on the movement of hydrogen ions across the inner mitochondrial membrane to create a proton gradient that drives ATP synthesis.
  This electrochemical gradient, established by electron transport chain activity, is the direct mechanism coupling electron transfer to ATP production.
• Pyruvate conversion to acetyl CoA involves removal of carbon dioxide, linking glycolysis to the citric acid cycle.
  This oxidative decarboxylation step is essential for extracting the remaining energy from the pyruvate molecule and generating additional NADH for the electron transport chain.
• Lactic acid fermentation is the anaerobic pathway available to animal skeletal muscles when oxygen is limiting.
  This process regenerates NAD+ from NADH, allowing glycolysis to continue and providing emergency ATP production during intense exercise when aerobic respiration cannot meet energy demands.
• High levels of ADP increase the activity of specific enzymes in cellular respiration, serving as a positive feedback signal for energy production.
  This allosteric regulation allows cells to sense low ATP status and upregulate catabolic pathways accordingly.
• Phosphofructokinase exerts the greatest control over glycolysis among the glycolytic enzymes.
  This enzyme catalyzes a committed, highly regulated step early in the pathway, making it the primary control point for regulating glucose breakdown.
• Cyanide poisoning blocks cytochrome c oxidase in the electron transport chain, preventing electron transfer and halting both ATP synthesis and the proton gradient.
  This explains why cyanide is lethal - cells cannot generate ATP despite glucose availability, and tissues with high energy demands like the brain fail rapidly.
• Dinitrophenol uncouples oxidative phosphorylation by allowing protons to leak across the inner mitochondrial membrane without generating ATP.
  This dissipates the proton gradient as heat rather than capturing it for ATP synthesis, which is why it was used as a weight-loss drug but proved dangerous due to uncontrollable heat generation.
• Tremetol poisoning prevents lactate metabolism, causing symptoms that worsen after exercise because lactate accumulates when aerobic metabolism is blocked.
  Exercise increases lactate production through anaerobic respiration, and if lactate cannot be metabolized, toxic levels accumulate causing vomiting, abdominal pain, and tremors.

Source: Pages 26-28, Visual Connection Questions and Review Questions