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Biology 101

Chapter 9 - Cellular Respiration

Life is Work
  • Living cells require energy from outside sources to do work.
  • The work of the cell includes:
    • Assembling polymers
    • Membrane transport
    • Movement
    • Reproduction
  • Animals obtain energy to perform this work by feeding on other animals or photosynthetic organisms.
Energy Flow
  • Energy flows into an ecosystem as sunlight and leaves as heat.
  • Chemical elements essential to life are recycled within ecosystems.
  • Photosynthesis generates O₂ and organic molecules, which are used in cellular respiration.
  • Cells use chemical energy stored in organic molecules to generate ATP, which powers cellular work.
Catabolic Pathways and ATP Production
  • Organic compounds possess potential energy due to the arrangement of electrons.
  • Catabolic pathways release stored energy by breaking down complex molecules.
  • Electron transfer is crucial in these pathways, central to cellular respiration.
Types of Catabolic Pathways
  • Fermentation: Partial degradation of sugars occurring without O₂.
  • Aerobic respiration: Consumes organic molecules and O₂, yielding ATP.
  • Anaerobic respiration: Similar to aerobic respiration but consumes compounds other than O₂.
  • Breakdown of organic molecules is exergonic.
  • Cellular respiration includes aerobic and anaerobic respiration, usually referring to aerobic respiration.
  • Balanced equation for cellular respiration with glucose:
    C<em>6H</em>12O<em>6+6O</em>2→6CO<em>2+6H</em>2O+Energyext(ATP+heat)C<em>6H</em>{12}O<em>6 + 6 O</em>2 → 6 CO<em>2 + 6 H</em>2O + Energy ext{ (ATP + heat)}
Redox Reactions
  • How do catabolic pathways yield energy?
    • The transfer of electrons during chemical reactions releases energy stored in organic molecules.
    • This energy is used to synthesize ATP.
  • Oxidation-reduction reactions (redox reactions): Chemical reactions that transfer electrons between reactants.
    • Oxidation: Loss of electrons (substance is oxidized).
    • Reduction: Gain of electrons (substance is reduced; positive charge is decreased).
Components of Redox Reactions
  • The electron donor is called the reducing agent.
  • The electron receptor is called the oxidizing agent.
  • Some redox reactions do not transfer electrons but change the sharing of electrons in covalent bonds (e.g., methane and O₂).
Oxidation of Organic Molecules During Cellular Respiration
  • Redox reactions include combustion of fuels like methane in gas stoves and gasoline in engines.
  • During cellular respiration:
    • The fuel (e.g., glucose) is oxidized.
    • O₂ is reduced.
  • Hydrogen-rich organic molecules (carbohydrates and fats) are excellent sources of high-energy electrons.
  • Energy is released as electrons associated with hydrogen ions are transferred to O₂, moving to a lower energy state.
NAD+ and the Electron Transport Chain
  • Direct release of energy from food is inefficient.
  • Cellular respiration involves breaking down glucose and other organic molecules in a series of steps, with each step catalyzed by an enzyme.
  • Electrons are stripped from glucose at key steps and are usually first transferred to NAD+ (nicotinamide adenine dinucleotide), a coenzyme derived from niacin.
  • NAD+ functions as an oxidizing agent during cellular respiration.
  • Each NADH (the reduced form of NAD+) represents stored energy that is utilized to synthesize ATP.
Mechanisms of Redox Reaction and ATP Production
  • Reactions summarized:
    • 2 e + 2 H^+
      ightarrow NAD^+
      ightarrow NADH
  • Interpretation of reaction: H ions and electrons are transferred.
NAD+ and the Electron Transport Chain Continued
  • NADH passes electrons to the electron transport chain (ETC).
  • Unlike an uncontrolled reaction, electrons move through the ETC in a series of steps managed by controlled reactions.
  • O₂ pulls electrons down the chain, releasing energy used for ATP regeneration:
    extElectrontravelflow:glucose→extNADH→extETC→extoxygenext{Electron travel flow: glucose} → ext{NADH} → ext{ETC} → ext{oxygen}
Key Concepts in Free Energy and Cellular Respiration
  • Free energy: The amount of energy available to do work, and its management is crucial in cellular respiration.
  • Explosive versus controlled release of energy illustrated with:
    • Uncontrolled reaction: H2 + 1/2 O2
      ightarrow ext{explosive release of energy}
    • Controlled reaction (cellular respiration): 2 H^+ + 2 e^-
      ightarrow ext{releases energy in manageable amounts}
Synthesizing ATP
  • ATP is a renewable resource.
  • ATP transfers its terminal phosphate group to a molecule or enzyme, releasing energy to do work (called phosphorylation).
  • After releasing the terminal phosphate group, ATP becomes ADP; regeneration involves reattaching a phosphate group using the energy derived from glucose via cellular respiration.
Stages of Cellular Respiration
  • The harvesting of energy from glucose consists of three stages:
    1. Glycolysis: Breaks down glucose into two molecules of pyruvate.
    2. Citric Acid Cycle: Completes the breakdown of glucose.
    3. Oxidative Phosphorylation: Accounts for most ATP synthesis.
Analogy for Cellular Respiration
  • Glucose is like a $100 bill: valuable but hard to use for small everyday transactions.
  • ATP is likened to a $10 bill: easy to spend.
  • Cellular respiration converts the large denomination of energy (glucose) into the many smaller units of ATP.
Glycolysis Overview
  • Glycolysis: Occurs in the cytosol and initiates glucose degradation into 2 molecules of pyruvate.
    • After glycolysis, pyruvate enters the mitochondrion where it undergoes oxidation to form acetyl CoA, which then enters the citric acid cycle.
    • The citric acid cycle completes glucose breakdown to CO₂ and feeds electrons to the ETC.
  • Oxidative phosphorylation generates nearly 90% of ATP; powered by redox reactions.
Glycolysis Phases
  • Glycolysis occurs in two phases:
    1. Energy Investment Phase: 2 ATP are used.
    2. Energy Payoff Phase: Net gain of ATP and NADH.
  • Glycolysis operates with or without O₂.
Energy Investment Phase Details
  • Start: Glucose is phosphorylated (2 ATP used).
  • Key products after transformations:
    • 2 molecules of glyceraldehyde-3-phosphate (G3P).
Energy Payoff Phase Details
  • Produces:
    • 4 ATP formed (net gain of 2)
    • 2 NADH
    • 2 Pyruvate
    • 2 H₂O
Oxidation of Pyruvate
  • In the presence of O₂, pyruvate enters the mitochondrion for further oxidation.
  • Pyruvate must convert into acetyl coenzyme A (acetyl CoA), connecting glycolysis to the citric acid cycle through a multienzyme complex.
The Citric Acid Cycle (Krebs Cycle)
  • Completes the breakdown of pyruvate to CO₂.
  • Generates:
    • 1 ATP
    • 3 NADH
    • 1 FADH₂ per turn.
  • Cycle process: Acetyl CoA combines with oxaloacetate to create citrate; through eight steps, it returns to oxaloacetate.
Total Electron Carrier Production in Citric Cycle
  • Each glucose molecule yields:
    • 2 molecules of Acetyl CoA enter the cycle.
    • Total yield per glucose:
    • 6 NADH
    • 2 FADH₂
    • 2 ATP
  • Initial ATP generation is small; most ATP results from oxidative phosphorylation where NADH and FADH₂ feed electrons to the ETC.
Oxidative Phosphorylation and Chemiosmosis
  • Following glycolysis and citric cycle, NADH and FADH₂ donate electrons to the ETC for ATP synthesis.
  • The ETC is located in the inner membrane of the mitochondrion, primarily comprising protein complexes.
  • Electrons drop in free energy down the chain and ultimately combine with O₂ to form H₂O.
  • Key Fact: The ETC does not generate ATP directly; it releases energy in manageable steps.
Chemiosmosis Mechanism
  • The energy released from the ETC is used to pump H⁺ ions from the mitochondrial matrix to the intermembrane space.
  • H⁺ moves down its gradient through ATP synthase, powering ATP formation from ADP (chemiosmosis).
  • Process described as the proton-motive force, indicating the energy from the H⁺ gradient driving ATP synthesis.
ATP Synthase Functionality
  • ATP synthase operates like a molecular turbine.
  • H⁺ ions flow down their concentration gradient, passing through a stator and into the rotor's binding sites, causing it to spin.
  • This motion transforms ADP and inorganic phosphate into ATP through several catalytic sites in the knob structure.
Summary of ATP Production Sequence
  • Energy flow during cellular respiration:
    extglucose→extNADH→extETC→extproton−motiveforce→extATPext{glucose} → ext{NADH} → ext{ETC} → ext{proton-motive force} → ext{ATP}
  • Approximately 34% of energy from glucose is stored as ATP, yielding about 32 ATP, with the remainder lost as heat (entropy).
Fermentation and Anaerobic Respiration
  • Cellular respiration typically relies on O₂ to operate the electron transport chain.
  • Without O₂, glycolysis couples with anaerobic respiration or fermentation for ATP production.
  • Anaerobic respiration may use an electron transport chain with an alternative final electron acceptor (e.g., sulfate), while fermentation uses substrate-level phosphorylation.
Types of Fermentation
  • Two primary types are:
    1. Alcohol Fermentation: Converts pyruvate to ethanol (two steps: CO₂ release, NAD⁺ and ethanol production).
    2. Lactic Acid Fermentation: Reduces pyruvate by NADH, generating NAD⁺ and lactate (no CO₂ released). Used by human muscle cells when O₂ is low.
Comparison of Fermentation, Anaerobic and Aerobic Respiration
  • All methods utilize glycolysis (net gain of 2 ATP).
  • NAD⁺ acts as the oxidizing agent in glycolysis for each process.
  • Different mechanisms exist for oxidizing NADH back to NAD⁺:
    • In fermentation: Organic molecules (e.g., pyruvate) serve as final electron acceptors.
    • In cellular respiration: Electrons are sent to the ETC.
  • Cellular respiration produces significantly more ATP (32 ATP/glucose) compared to fermentation (2 ATP/glucose).
Obligate vs. Facultative Anaerobes
  • Obligate anaerobes: Conduct fermentation or anaerobic respiration; cannot survive in the presence of O₂.
  • Facultative anaerobes: Can use fermentation or cellular respiration for energy; decisions made regarding metabolic pathways based on O₂ availability.
Evolutionary Significance of Glycolysis
  • Glycolysis is one of the oldest energy-producing pathways, likely utilized by early prokaryotes before O₂ became abundant in the atmosphere.
  • Earliest bacteria fossils are approximately 3.5 billion years old; oxygen began accumulating about 2.7 billion years ago due to cyanobacteria.
  • Glycolysis, occurring in the cytosol, does not require membrane-bound organelles present in eukaryotes, allowing it a broad utilization.
Connections Beyond Glucose in Glycolysis and Citric Acid Cycle
  • Glycolysis doesn't rely solely on glucose. It can accept other carbohydrates including starch and glycogen.
  • Proteins and fatty acids can also be oxidized:
    • Fatty acids are broken down via beta-oxidation and enter the citric acid cycle.
    • Oxidizing 1 gram of fat yields more ATP than 1 gram of carbohydrates.
Regulation of Cellular Respiration
  • Feedback regulation occurs in cellular respiration:
    • When ATP levels drop, respiration speeds up.
    • When ATP is abundant, respiration slows down.
    • Enzyme activity control at various strategic points regulates catabolism.
Chapter 9 Wrap-Up
  • Cells utilize glucose and other organic molecules to produce ATP through fermentation (absence of O₂) or cellular respiration (presence of O₂).
  • Aerobic respiration involves three stages: glycolysis, citric acid cycle, and oxidative phosphorylation.
  • Glycolysis and the citric acid cycle produce limited ATP but are critical for generating electron carriers that drive the majority of ATP production via chemiosmosis.
  • Aerobic respiration yields significantly more ATP compared to fermentation (approximately 16 times).
  • Other nutrients like proteins and fatty acids have dedicated metabolic pathways entering similar cycles at various points.
  • Feedback regulation ensures efficient production of ATP.