Chapter 9: Cellular Respiration and Fermentation Notes

• Living cells need energy from external sources for tasks. This energy comes from the sun, stored in organic molecules.

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

• Photosynthesis produces oxygen and organic molecules used in cellular respiration.

• Respiration uses oxygen to break down fuel, generating ATP. Waste products are carbon dioxide and water.

Catabolic Pathways and ATP Production

• Catabolic pathways release stored energy by breaking down complex molecules.

• Electron transfer from food molecules (e.g., glucose) is vital.

• Organic compounds store potential energy in their bonds.

• Exergonic reactions can act as fuels; enzymes degrade complex molecules into simpler waste with less energy.

• Some energy is used for work, the rest as heat.

• Fermentation: Partial sugar/organic fuel degradation without oxygen.

• Aerobic respiration: Consumes oxygen with organic fuel (common in eukaryotes/many prokaryotes).

• Anaerobic respiration: Harvests chemical energy without oxygen, using other substances (in some prokaryotes).

• Cellular respiration: Includes both aerobic and anaerobic processes.

Cellular Respiration vs. Aerobic Respiration

• Cellular respiration: Aerobic and anaerobic processes.

• Originally, synonym for aerobic respiration, related to organismal respiration.

• Often refers to the aerobic process.

• Aerobic respiration: Like gasoline combustion, fuel (food) mixes with oxygen, producing carbon dioxide and water.

• Overall: Organic compounds + Oxygen \rightarrow Carbon dioxide + Water + Energy

• Carbohydrates, fats, and proteins: Can be processed as fuel.

• Starch: Storage polysaccharide, broken into glucose (C6H{12}O_6), major carbohydrate source.

• Cellular respiration tracks glucose degradation: C6H{12}O6 + 6 O2 \rightarrow 6 CO2 + 6 H2O + Energy (ATP + heat)

• Exergonic breakdown: \Delta G = -686 kcal/mol or -2,870 kJ/mol

Catabolism and ATP

• Catabolic pathways don't directly perform cellular work.

• Catabolism linked to work by ATP.

• Cells regenerate ATP from ADP and Pi to keep working.

• Cellular respiration regenerates ATP through oxidation and reduction.

Redox Reactions

• Catabolic pathways decompose fuels via electron transfer, releasing energy for ATP synthesis.

The Principle of Redox

• Redox reactions: Transfer of one or more electrons (e^-)

• During redox: One reactant loses electrons (oxidation), another gains (reduction).

• Oxidation: Loss of electrons.

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

• Ex: Sodium (Na) and chlorine (Cl) form table salt:

  • Na \rightarrow Na^+ + e^- (Sodium oxidized; loses electron).

  • Cl + e^- \rightarrow Cl^- (Chlorine reduced; gains electron).

• Generalized redox reaction:

  • X_{e^-} \rightarrow X + e^- (Oxidation).

  • Y + e^- \rightarrow Y_{e^-} (Reduction).

• Reducing agent: Electron donor (reduces Y).

• Oxidizing agent: Electron acceptor (oxidizes X_{e^-}).

• Oxidation/reduction: Always occur together.

• Some redox reactions: Change electron sharing in covalent bonds.

• Methane combustion: Electrons shared unequally.

• In methane (CH_4): Electrons shared nearly equally.

• Methane reacts with \O2 to form \CO2: Electrons shared less equally.

• Carbon atom partially "lost" electrons: Methane is oxidized.

• In reaction with methane: \O2 bonds to H in \H2O, electrons spend more time near oxygen.

• Each O atom partially “gained” electrons: \O_2 reduced.

• Oxygen: Highly electronegative, powerful oxidizing agent.

• Energy must be added to pull an electron away from an atom.

• Electrons lose potential energy: Shifting from less to more electronegative atom.

• Redox reactions: Moving electrons closer to oxygen release chemical energy, e.g., methane combustion.

• Combustion (oxidation) of methane releases chemical energy.

Oxidation of Organic Fuel Molecules During Cellular Respiration

• Gasoline combustion in engines is a redox reaction.

• In respiration, glucose and food molecules are oxidized.

• Summary equation: C6H{12}O6 + 6 O2 \rightarrow 6 CO2 + 6 H2O + Energy

• Glucose oxidized, \O_2 reduced.

• Electrons lose potential energy, releasing energy.

• Organic molecules with abundant hydrogen are excellent fuels.

• Hydrogen atoms transferred from glucose to oxygen in \O_2.

• Respiration: Glucose oxidation transfers electrons to lower energy, liberating energy for ATP synthesis.

• Fuels with C-H bonds oxidized into products with C-O bonds.

Stepwise Energy Harvest via NAD^+ and the Electron Transport Chain

• Energy released all at once cannot be harnessed efficiently.

• Cellular respiration: Breaks down glucose in enzyme-catalyzed steps.

• Electrons stripped from glucose, travel with a proton (as hydrogen atom).

• Hydrogen atoms not transferred directly to \O_2, passed to coenzyme nicotinamide adenine dinucleotide (NAD^+).

• NAD^+: Versatile electron carrier, cycles between oxidized (NAD^+)

• Enzyme delivers 2 electrons and 1 proton to \NAD^+, forming NADH.

• Other proton released as \H^+: NAD^+ + 2H \rightarrow NADH + H^+

• \NAD^+: Neutralized when reduced to NADH.

• NADH: Stored energy, tapped to make ATP when electrons fall from NADH to \O_2.

• Electrons from glucose, stored in NADH, reach oxygen via electron transport chain.

• Hydrogen and oxygen reaction releases energy.

• In cellular respiration: Hydrogen from organic molecules reacts with oxygen via electron transport chain.

• Electron transport chain: Molecules in inner mitochondrial membrane (eukaryotes) or plasma membrane (prokaryotes).

• Electrons from glucose shuttled by NADH to the “top” of chain.

• At the “bottom”, \O_2 captures electrons and \H^+, forming water.

• Electron transfer from NADH to oxygen: Exergonic, \Delta G = -53 kcal/mol (-222 kJ/mol).

• Electrons cascade down chain, losing energy, until they reach oxygen.

• Each carrier has greater affinity for electrons than its neighbor.

• \O_2 has greatest affinity at the bottom.

• Electrons transferred from glucose to \NAD^+, reducing it to NADH, fall to electronegative oxygen atom from \O_2.

Stages of Cellular Respiration

• Harvesting energy from glucose: Three stages:

  1. Glycolysis: Glucose to two pyruvate molecules (cytosol).

  2. Pyruvate oxidation and Citric Acid Cycle: Pyruvate to acetyl CoA, glucose breakdown to carbon dioxide (mitochondrion in eukaryotes).

  3. Oxidative Phosphorylation: Chemiosmosis couples electron transport to ATP synthesis (inner mitochondrial membrane).

• Cellular respiration: Stages 2 and 3 together.

• Glycolysis and citric acid cycle: Redox reactions.

• dehydrogenases: Transfer electrons from substrates to \NAD^+ or FAD, forming NADH or \FADH_2.

• In stage 3: Electron transport chain accepts electrons from NADH or \FADH_2, passes them down chain.

• At chain's end: Electrons combine with \O_2 and \H^+, forming water.

• Energy released: Used to make ATP from ADP.

• Oxidative phosphorylation: Powered by redox reactions of electron transport chain.

• In eukaryotes: Inner mitochondrial membrane is the site of electron transport and chemiosmosis, making up oxidative phosphorylation.

• Oxidative phosphorylation: Accounts for almost 90% of ATP generated by respiration.

• Substrate-level phosphorylation: ATP formed directly in glycolysis and citric acid cycle reactions.

• Substrate-level phosphorylation: Enzyme transfers phosphate group from substrate to ADP.

• Cells make up to 32 ATP molecules per glucose degraded to \CO2 and \H2O by respiration.

Glycolysis

• Glycolysis: "Sugar splitting".

• Glucose (six-carbon sugar): Splits into two three-carbon sugars.

• Smaller sugars are oxidized, rearranged to form two pyruvate molecules.

• Glycolysis: Two phases:

  • Energy investment phase: Cell spends ATP.

  • Energy payoff phase: ATP produced by substrate-level phosphorylation, \NAD^+ reduced to NADH.

• Net energy yield: 2 ATP + 2 NADH per glucose molecule.

• Carbon from glucose accounted for in pyruvate; no carbon released as \CO_2.

• Glycolysis occurs whether or not \O_2 is present.

• If \O_2 present: Chemical energy in pyruvate and NADH extracted by pyruvate oxidation, citric acid cycle, and oxidative phosphorylation.

Oxidation of Pyruvate to Acetyl CoA

• Pyruvate converted to acetyl coenzyme A (acetyl CoA).

• Multienzyme complex carries out this step:

  1. Pyruvate's carboxyl group ($\-COO^−$): Oxidized and given off as \CO_2.

  2. Two-carbon fragment: Oxidized, electrons transferred to \NAD^+, storing energy in NADH.

  3. Coenzyme A (CoA): Attached to two-carbon intermediate, forming acetyl CoA.

• Acetyl CoA: High potential energy, transfers acetyl group in citric acid cycle, highly exergonic.

The Citric Acid Cycle

• The citric acid cycle oxidizes organic fuel from pyruvate.

• Per pyruvate molecule:

  • 1 ATP produced by substrate-level phosphorylation.

  • Chemical energy transferred to \NAD^+ and FAD during redox reactions.

  • Reduced coenzymes (NADH and \FADH_2) shuttle electrons into electron transport chain.

• Citric acid cycle: Tricarboxylic acid cycle or Krebs cycle.

• Cycle has eight steps, each enzyme-catalyzed.

• Per cycle turn: Two carbons enter as acetyl group, two leave as \CO_2 molecules.

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

• Citrate is converted back to oxaloacetate, regenerating it for another turn.

• Per acetyl group entering cycle:

  • 3 \NAD^+ reduced to NADH.

  • Electrons transferred to FAD, becoming \FADH_2.

  • Guanosine triphosphate (GTP) produced by substrate-level phosphorylation.

Oxidative Phosphorylation

• Glycolysis and citric acid cycle: 4 ATP molecules per glucose (substrate-level phosphorylation).

• NADH (and \FADH_2): Account for most energy from glucose.

• Section covers electron transport chain and its coupling to ATP synthesis

The Pathway of Electron Transport

• Electron transport chain: Molecules in inner membrane of mitochondrion (eukaryotes).

• (In prokaryotes: molecules reside in plasma membrane.)

• Most components: Proteins in multiprotein complexes I through IV.

• Tightly bound: Prosthetic groups (nonprotein components).

• During electron transport: Electron carriers alternate between reduced and oxidized states.

• Each chain component: Reduced when accepting electrons, returning to oxidized when donating.

• Electrons from glucose carried by NADH during glycolysis and citric acid cycle are transferred from NADH to molecule of the electron transport chain in complex I.

• Molecule is a flavoprotein with flavin mononucleotide (FMN).

• Flavoprotein returns to oxidized form donating electrons to iron-sulfur protein (Fe • S).

• Iron-sulfur protein passes electrons to ubiquinone (Q).

• Remaining electron carriers past ubiquinone are cytochromes.

• Last cytochrome (cyt a3) passes electrons to oxygen (in \O_2).

Chemiosmosis: Energy-Coupling Mechanism

• Hydrogen ions tend to move back across the membrane.

• Passage of H^+ through ATP synthase uses H^+ flow to drive ADP phosphorylation

• Energy is transformed to a proton-motive force, gradient of H^+ across the membrane.

• Chemiosmosis: Energy stored in the form of an \H^+ gradient, drive cellular work.

• Energy for gradient formation: Exergonic redox reactions along the electron transport chain

ATP Production

• Bookkeeping of ATP during respiration: Glucose --> NADH --> electron transport chain --> proton-motive force --> ATP.

• 4 ATP produced directly by substrate-level phosphorylation plus many ATP generated by oxidative phosphorylation.

• Each NADH produces enough proton to generate a maximum of about 3 ATP, and the citric acid supplies electrons to the chain, responsible for 1.5 ATP.

• Efficiency of respiration: Around 34%.

• Mammals (hibernating): Use thermogenesis to reduce efficiency and generate heat.

Fermentation and Anaerobic Respiration

• Cellular pathways for ATP production:

  • Fermentation: Recycles \NAD^+ from NADH, allowing glycolysis to continue.

  • Anaerobic: Uses electron transport chain with final electron acceptor not being \O_2.

  • Aerobic Respiration: Electrons are transported by NADH to an electron transport chain, which regenerates the \NAD^+.

• Three alternative cellular pathways for producing ATP by harvesting the chemical energy of food.

• All three: Use glycolysis to oxidize glucose and other fuels to pyruvate, \NAD^+ is the oxidizing agent.

• Obligate Anaerobes: They carry out only fermentation, or anaerobic respiration.

• Facultative Anaerobes: species which make enough ATP, using either fermentation or respiration.

• Aerobic conditions: Oxidation continues via Krebs and Aerobic Respiration.

• Anaerobic Conditions: Recycles Krebs cycle, serving as electron acceptor.

• For the evolutionary significance of Glycolysis, ancient Glycolysis evolved before \O_2.

Metabolic Pathways in Cellular Respiration

• Free glucose molecules are not common, we obtain calories from fats, proteins, and carbohydrates.

• Glycolysis: Accepts carbohydrates for catabolism; digestion of disaccharides and hydrolyzing of starch can fuel for respiration.

• Proteins used to build new proteins, amino acids present in excess are converted to intermediates of glycolysis and the citric acid cycle and must be removed, (deamination).

• Fats digested to glycerol and fatty acids, the glycerol is converted to glyceraldehyde 3-phosphate, and a metabolic sequence (beta oxidation) breaks the fatty acids down to two-carbon fragments and their high level of energy compared to carbohydrates.

Metabolic Feedback

• Anabolic pathway off from the intermediate of the citric acid cycle, or end product-Feedback inhibition.

• Control Based Mainly By strategic Enzymes.

• Strategic step: Phosphofructokinase catalyzes step 3 of glycolysis.

• Phosphofructokinase: Allosteric enzyme inhibited by ATP/Citrate, and stimulated