Module 8 - Cellular Respiration Lecture Notes

Overview: Life Is Work

• Living cells need energy from external sources to perform tasks.

• Energy enters ecosystems as sunlight and exits as heat; chemical elements are recycled.

• Photosynthesis produces oxygen and organic molecules, which are used by eukaryotes (including plants and algae) in the mitochondria for cellular respiration.

• Cells extract chemical energy from organic molecules to regenerate ATP.

• Respiration involves glycolysis, the citric acid cycle, and oxidative phosphorylation.

• Fermentation is a simpler pathway related to glycolysis.

Concept 9.1: Catabolic Pathways Yield Energy by Oxidizing Organic Fuels

• Catabolic pathways release energy from complex organic molecules via electron transfer.

• Organic compounds store potential energy in the bonds between their atoms.

• Enzymes degrade energy-rich organic molecules into simpler, lower-energy waste products.

• Released energy does work, with the rest dissipated as heat.

• Fermentation partially degrades sugars without oxygen.

• Aerobic respiration uses oxygen to break down organic molecules, common in eukaryotes and many prokaryotes.

• Anaerobic respiration is similar but uses other compounds instead of oxygen, found in some prokaryotes.

• Cellular respiration usually refers to the aerobic process.

• Aerobic respiration is analogous to gasoline combustion in an engine, producing carbon dioxide and water.

• The overall catabolic process: organic compounds + O2 → CO2 + H_2O + energy (ATP + heat).

• Glucose catabolism: C6H{12}O6 + 6O2 → 6CO2 + 6H2O + energy (ATP + heat).

• The catabolism of glucose is exergonic, with \Delta G = -686 kcal per mole of glucose.

• Energy released is used to produce ATP.

Redox Reactions Release Energy

• Catabolic pathways transfer electrons, releasing energy to synthesize ATP.

• Oxidation-reduction (redox) reactions involve electron transfer.

• Oxidation: loss of electrons.

• Reduction: gain of electrons (reduces positive charge).

• Example: Na + Cl → Na^+ + Cl^− (Na is oxidized, Cl is reduced).

• General form: Xe^− + Y → X + Ye^−.

  • X is the reducing agent (electron donor).

  • Y is the oxidizing agent (electron acceptor).

• Redox reactions require both a donor and an acceptor.

• Electron transfer can be a change in electron sharing in covalent bonds.

• Combustion of methane: nonpolar C—H and O=O bonds become polar C=O and O—H bonds.

• Electrons move away from carbon and closer to oxygen, oxidizing methane.

• Oxygen is electronegative and a potent oxidizing agent.

• Energy is needed to remove an electron from an atom; more for electronegative atoms.

• Electrons lose potential energy when moving to more electronegative atoms.

• Redox reactions, like methane burning, release chemical energy.

Organic Fuel Oxidation During Cellular Respiration

• Respiration oxidizes glucose (and other food molecules), a redox process.

• Glucose is oxidized, oxygen is reduced, releasing energy.

• Hydrogen-rich organic molecules are excellent fuels; their bonds contain “hilltop” electrons.

• Energy is released as electrons transfer from glucose to oxygen.

• Carbohydrates and fats are reservoirs of electrons associated with hydrogen.

• Activation energy prevents immediate combination with O_2.

• Igniting glucose releases 686 kcal (2,870 kJ) of heat per mole.

• Enzymes lower activation energy for stepwise oxidation.

Stepwise Electron Fall via NAD+ and the Electron Transport Chain

• Cellular respiration doesn't oxidize glucose in a single step.

• Glucose is broken down in steps, each catalyzed by an enzyme.

• Electrons are stripped from glucose and transferred to the coenzyme NAD+ (nicotinamide adenine dinucleotide).

• NAD+ cycles between oxidized (NAD+) and reduced (NADH) states, acting as an oxidizing agent.

• Dehydrogenase enzymes remove two hydrogen atoms from glucose, oxidizing it, and pass two electrons and one proton to NAD+.

• NAD+ is reduced to NADH, neutralizing its charge.

• NADH carries stored energy to synthesize ATP.

• Cellular respiration uses an electron transport chain to release energy in steps, avoiding explosive heat release.

• The electron transport chain is in the inner mitochondrial membrane (eukaryotes) or plasma membrane (prokaryotes).

• NADH delivers electrons to the chain's higher-energy end; oxygen captures them at the lower-energy end, forming water.

• Electron transfer from NADH to oxygen is exergonic (-53 kcal/mol).

• Electrons are passed to increasingly electronegative molecules until they reduce oxygen.

• Electrons from glucose, carried by NAD+, fall down the chain to oxygen.

• Overall route: glucose → NADH → electron transport chain → oxygen.

Stages of Cellular Respiration

• Respiration has three stages: glycolysis, the citric acid cycle, and oxidative phosphorylation.

• Cellular respiration typically refers to stages 2 and 3.

• Glycolysis in the cytosol breaks down glucose into two pyruvate molecules.

• In eukaryotes, pyruvate enters the mitochondrion and is oxidized to acetyl CoA.

• Redox reactions in glycolysis and the citric acid cycle transfer electrons to NAD+, forming NADH.

• The electron transport chain accepts electrons from NADH and FADH2.

• Electrons move down the chain, combining with oxygen and hydrogen ions to form water.

• Energy released is used to make ATP via oxidative phosphorylation.

• In eukaryotes, the inner mitochondrial membrane is the site of electron transport and chemiosmosis.

• In prokaryotes, these processes occur in the plasma membrane.

• Oxidative phosphorylation produces almost 90% of the ATP.

• Substrate-level phosphorylation directly forms ATP during glycolysis and the citric acid cycle.

• Each glucose molecule yields up to 32 ATP (each with 7.3 kcal/mol of free energy).

Glycolysis

• Glycolysis splits glucose (a six-carbon sugar) into two three-carbon sugars, which are then oxidized to form two pyruvate molecules.

• Each step in glycolysis is catalyzed by a specific enzyme.

• There are two phases to glycolysis:

  1. Energy Investment Phase: The cell spends ATP.

  2. Energy Payoff Phase: The investment is repaid with interest. ATP is produced by substrate-level phosphorylation, and NAD+ is reduced to NADH.

• The net yield from glycolysis is 2 ATP and 2 NADH per glucose.

• No carbon is released as CO_2 during glycolysis.

• Glycolysis can occur with or without O_2.

• If O_2 is present, pyruvate and NADH's chemical energy can be extracted by pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.

Citric Acid Cycle

• Over three-quarters of glucose's original energy remains in two pyruvate molecules.

• With O_2 present, pyruvate enters the mitochondrion, where the citric acid cycle completes the oxidation of organic fuel to carbon dioxide.

• In prokaryotic cells, this process occurs in the cytosol.

• Pyruvate is converted to acetyl coenzyme A (acetyl CoA) inside the mitochondrion.

• This step links glycolysis and the citric acid cycle, catalyzed by a multi-enzyme complex.

  1. A carboxyl group is removed as CO_2.

  2. The remaining two-carbon fragment is oxidized to form acetate, transferring electrons to NAD+ to form NADH.

  3. Acetate combines with coenzyme A to form acetyl CoA.

• Acetyl CoA has high potential energy due to the CoA group.

• Acetyl CoA feeds its acetyl group into the citric acid cycle.

• The citric acid cycle oxidizes organic fuel derived from pyruvate.

• Three CO_2 molecules are released, including one from the conversion of pyruvate to acetyl CoA.

• The cycle generates one ATP per turn by substrate-level phosphorylation.

• Most chemical energy is transferred to NAD+ and FAD during redox reactions.

• Reduced coenzymes (NADH and FADH2) transfer high-energy electrons to the electron transport chain.

• The citric acid cycle has eight steps, each catalyzed by a specific enzyme.

• The acetyl group of acetyl CoA combines with oxaloacetate, forming citrate.

• The next seven steps decompose citrate back to oxaloacetate, thus making it a cycle.

• For each acetyl group, 3 NAD+ are reduced to NADH.

• Electrons are transferred to FAD to become FADH2.

• The citric acid cycle forms an ATP molecule by substrate-level phosphorylation.

• The output from this step is the only ATP generated directly by the citric acid cycle.

• The total yield per glucose is 6 NADHs, 2 FADHs, and the equivalent of 2 ATPs.

• Most of the ATP results from oxidative phosphorylation, when NADH and FADH2 relay electrons to the electron transport chain.

Oxidative Phosphorylation

• Only 4 of 38 ATP produced by respiration are from substrate-level phosphorylation.

• NADH and FADH2 link glycolysis and the citric acid cycle to oxidative phosphorylation.

• The electron transport chain is in the cristae of the mitochondrion.

• In prokaryotes, the chain is in the plasma membrane.

• Cristae increase surface area for the electron transport chain.

• Most components are proteins in multi-protein complexes (I–IV) with prosthetic groups.

• Electrons drop in free energy as they pass down the chain.

• Electron carriers alternate between reduced and oxidized states.

• NADH transfers electrons to a flavoprotein, then to an iron-sulfur protein, and then to ubiquinone.

• Ubiquinone is a mobile, hydrophobic molecule.

• Cytochromes are proteins with heme groups that accept and donate electrons.

• The last cytochrome, cyt a3, passes electrons to oxygen, which also picks up hydrogen ions to form water.

• FADH2 electrons have lower energy and enter the chain at a lower level.

• The electron transport chain generates no ATP directly but breaks the large free-energy drop into smaller steps.

Chemiosmosis

• ATP synthase makes ATP from ADP and inorganic phosphate.

• ATP synthase uses the energy of an ion gradient to power ATP synthesis.

• The power source is a difference in H+ concentration across the inner mitochondrial membrane, also known as a pH gradient.

• Chemiosmosis: energy stored in a hydrogen ion gradient across a membrane is used to drive cellular work, such as ATP synthesis.

• ATP synthase is a multisubunit complex with four main parts.

• The flow of H+ through ATP synthase catalyzes ATP production.

• The electron transport chain establishes the H+ gradient by pumping H+ across the membrane.

• ATP synthase is the only place where H+ can diffuse back to the matrix.

• The electron transport chain pumps protons using the exergonic flow of electrons.

• Electron transfers cause H+ to be taken up and released into the solution.

• The H+ gradient is the proton-motive force.

• Chemiosmosis is an energy-coupling mechanism using the H+ gradient to drive cellular work.

• In mitochondria, redox reactions form the gradient, and ATP synthesis is the work performed.

• In chloroplasts, light drives both electron flow and H+ gradient formation.

• Prokaryotes generate H+ gradients for ATP synthesis, nutrient transport, and flagella rotation.

ATP Production Accounting

• Energy flow: glucose → NADH → electron transport chain → proton-motive force → ATP.

• Four ATP are produced by substrate-level phosphorylation.

• Many more ATP are generated by oxidative phosphorylation.

• Each NADH generates a maximum of 3 ATP.

• The ratio of NADH to ATP is not a whole number.

  • One NADH results in 10 H+ being transported across the inner mitochondrial membrane.

  • 4 H+ must reenter the mitochondrial matrix via ATP synthase to generate 1 ATP.

  • Therefore, 1 NADH generates enough proton-motive force for the synthesis of 2.5 ATP.

• ATP yield varies based on electron shuttle type.

• Mitochondrial inner membrane is impermeable to NADH.

• Electrons are passed to NAD+ or FAD in the mitochondrial matrix.

  • If passed to FAD (brain cells), 2 ATP result per NADH.

  • If passed to NAD+ (liver and heart cells), 3 ATP result per NADH.

• The proton-motive force drives mitochondrial uptake of pyruvate.

• One glucose molecule can generate a maximum of 28 ATP by oxidative phosphorylation plus 4 ATP by substrate-level phosphorylation, giving a total yield of about 32 ATP (or 30 ATP with a less efficient shuttle).

Efficiency of Respiration

• Complete glucose oxidation releases 686 kcal/mol.

• ADP phosphorylation requires at least 7.3 kcal/mol.

• Efficiency: (7.3 kcal/mol * 32 ATP/glucose) / 686 kcal/mol = 0.34 (34%).

• The actual percentage is probably higher because ΔG is lower under cellular conditions.

• The rest of the energy is lost as heat, which maintains body temperature.

• Cellular respiration is efficient compared to a car engine (25%).

Hibernating Animals

• Hibernating mammals have reduced metabolism and body temperature.

• Brown fat cells are packed with mitochondria.

• Uncoupling protein allows protons to flow back down their concentration gradient without generating ATP, producing heat.

• This prevents ATP buildup from shutting down cellular respiration.

Fermentation and Anaerobic Respiration

• Cells can oxidize organic fuel and generate ATP without oxygen via fermentation and anaerobic respiration.

• Anaerobic respiration uses an electron transport chain but not oxygen as the final electron acceptor.

• Sulfate-reducing bacteria use sulfate ions (SO42-) to produce H2S.

• Fermentation oxidizes organic fuel and generates ATP without oxygen or an electron transport chain.

• Glycolysis oxidizes glucose to two pyruvate molecules, with NAD+ as the oxidizing agent, producing 2 ATP (net) by substrate-level phosphorylation.

• Glycolysis generates 2 ATP with or without oxygen.

• Fermentation relies on substrate-level phosphorylation.

• Glycolysis continues if there's NAD+ to accept electrons.

• Under aerobic conditions, NADH transfers electrons to the electron transfer chain, regenerating NAD+.

Fermentation Pathways

• Fermentation pathways recycle NAD+ by transferring electrons from NADH to pyruvate or its derivatives.

• Alcohol fermentation converts pyruvate to ethanol in two steps.

  • Pyruvate is converted to acetaldehyde by removing CO_2.

  • Acetaldehyde is reduced by NADH to ethanol, regenerating NAD+.

  • Yeast uses alcohol fermentation in brewing, baking, and winemaking.

• Lactic acid fermentation reduces pyruvate directly by NADH to form lactate without releasing CO_2.

  • This is used to make cheese and yogurt.

• Human muscle cells switch to lactic acid fermentation during strenuous exercise when O_2 is scarce.

  • Lactate was thought to cause muscle fatigue, but potassium ions (K+) may be the cause; lactate may enhance muscle performance.

  • Excess lactate is converted back to pyruvate by liver cells.

Fermentation vs. Cellular Respiration

• Fermentation, anaerobic respiration, and aerobic respiration are alternative ATP-producing pathways.

• All three use glycolysis to oxidize sugars to pyruvate, producing 2 ATP by substrate-level phosphorylation.

• NAD+ is the oxidizing agent.

• The key difference is how NADH is oxidized back to NAD+.

  • In fermentation, the final electron acceptor is an organic molecule.

  • In cellular respiration, electrons from NADH go to an electron transport chain to a final electron acceptor.

  • In aerobic respiration, oxygen is the final electron acceptor; in anaerobic respiration, it's another molecule less electronegative than oxygen.

• Electron transport regenerates NAD+ and drives oxidative phosphorylation.

• Respiration yields much more ATP than fermentation.

• Aerobic respiration yields up to 16 times as much ATP per glucose molecule as fermentation (32 vs. 2).

Organisms and Pathways

• Obligate anaerobes use only fermentation or anaerobic respiration and cannot survive in oxygen.

• Vertebrate brain cells can only use aerobic oxidation of pyruvate.

• Yeast and many bacteria are facultative anaerobes, using either fermentation or respiration.

• Human muscle cells can behave as facultative anaerobes.

• Pyruvate is a fork in the metabolic road.

  • Under aerobic conditions, pyruvate is converted to acetyl CoA and enters the citric acid cycle.

  • Under anaerobic conditions, lactic acid fermentation occurs, and pyruvate serves as an electron acceptor.

• Facultative anaerobes consume sugar faster when fermenting to produce the same amount of ATP.

Evolutionary Basis of Glycolysis

• Ancient prokaryotes likely used glycolysis before oxygen was present.

• The oldest bacterial fossils (3.5 billion years old) predate appreciable oxygen accumulation (2.7 billion years ago).

• Cyanobacteria produced oxygen via photosynthesis.

• The first prokaryotes may have generated ATP exclusively from glycolysis.

• Glycolysis is a ubiquitous pathway in the cytosol, suggesting that it evolved very early.

Metabolic Pathways

• Glycolysis and the citric acid cycle connect to catabolic and anabolic pathways.

• Glycolysis accepts various carbohydrates.

• Polysaccharides are hydrolyzed to glucose monomers.

• Disaccharides provide glucose and other monosaccharides.

• Proteins and fats also enter respiratory pathways.

• Proteins are digested to amino acids.

• Amino acids are deaminated, and their carbon skeletons are modified to intermediates of glycolysis and the citric acid cycle.

• Fats are digested to glycerol and fatty acids.

• Glycerol is converted to glyceraldehyde-3-phosphate.

• Fatty acids are split into two-carbon fragments via beta oxidation, entering the citric acid cycle as acetyl CoA.

• NADH and FADH2 are also generated during beta oxidation, leading to further ATP production.

• A gram of fat generates twice as much ATP as a gram of carbohydrate.

Anabolic Pathways

• Food provides carbon skeletons for molecule synthesis.

• Organic monomers are used directly.

• Intermediaries in glycolysis and the citric acid cycle serve as precursors.

• Human cells synthesize about half the 20 amino acids.

• Glucose is synthesized from pyruvate; fatty acids are synthesized from acetyl CoA.

• Anabolic pathways consume ATP.

• Glycolysis and the citric acid cycle are metabolic interchanges.

• Excess carbohydrates and proteins are converted to fats.

Feedback Mechanisms

• Supply and demand regulate metabolism.

• Excess amino acids inhibit diversion from the citric acid cycle to synthesis pathways.

• The end product inhibits an early enzyme, preventing diversion of key intermediates.

• Catabolism speeds up when ATP levels drop.

• Respiration slows down with plenty of ATP.

• Control of catabolism regulates enzymes at strategic points.

• Phosphofructokinase is the pacemaker of respiration, catalyzing an irreversible step.

• Phosphofructokinase is allosterically inhibited by ATP and stimulated by AMP.

• Citrate inhibits phosphofructokinase, synchronizing glycolysis and the citric acid cycle.

• Glycolysis speeds up when citric acid cycle intermediates are diverted.

• Metabolic balance is augmented by controlling other key enzymes.

• Cells are thrifty, expedient, and responsive in metabolism.

• Cellular respiration is central to energy flow and chemical cycling in ecosystems.

• Cellular respiration releases energy stored in food by photosynthesis.