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Cellular Respiration: Harvesting Chemical Energy – Study Notes

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Cellular Respiration: Harvesting Chemical Energy

Overview: Life Is Work

All living cells require a constant supply of energy to perform essential functions. This energy is derived from the breakdown of organic molecules, primarily through cellular respiration.

  • Energy flow in ecosystems:

    • Energy enters as sunlight, captured by photosynthesis in chloroplasts to produce organic molecules and O2.

    • Cellular respiration in mitochondria uses these molecules to produce ATP and release heat.

Concept 9.1 – Catabolic Pathways and Energy

Catabolic Pathways

Catabolic pathways break down complex molecules into simpler ones, releasing energy that cells can use for work.

  • Catabolism: Breakdown of complex molecules; energy release.

  • Fermentation: Partial degradation of sugars without oxygen; yields ATP via glycolysis.

  • Cellular respiration:

    • Most efficient ATP-yielding process.

    • Uses oxygen to oxidize organic molecules (e.g., glucose).

    • Food + O2 → CO2 + H2O + energy.

Energy and Redox Reactions

Energy from food is released via oxidation-reduction (redox) reactions, which involve the transfer of electrons.

  • Oxidation: Loss of electrons.

  • Reduction: Gain of electrons.

  • Example: Na + Cl → Na+ + Cl-

  • The more electrons shift toward oxygen, the more energy is released.

Cellular Respiration Equation

The overall chemical equation for cellular respiration is:

NAD+ and the Electron Transport Chain

NAD+ as Electron Carrier

NAD+ is a key electron carrier that traps energy from glucose oxidation.

  • Dehydrogenases: Remove 2 H atoms (2 e- + 2 H+) from fuel, transferring 2 e- + 1 H+ to NAD+ → NADH.

  • NADH carries high-energy electrons to the electron transport chain (ETC).

  • ETC releases energy stepwise, producing ATP instead of explosive heat.

The Electron Transport Chain (ETC)

Location and Function

The ETC is located in the inner mitochondrial membrane and is essential for ATP synthesis.

  • Electrons from NADH/FADH2 are passed through proteins to oxygen (final acceptor), forming water.

  • Energy released is used to pump H+ ions, driving ATP synthesis.

  • Electron flow: NADH → ETC → O2 → H2O

The Stages of Cellular Respiration

Cellular respiration occurs in three main stages, each contributing to the breakdown of glucose and production of ATP.

  1. Glycolysis (cytoplasm)

    • Breaks glucose into 2 pyruvate.

    • Produces 2 ATP and 2 NADH.

  2. Citric Acid Cycle (Krebs Cycle) (mitochondrial matrix)

    • Completes oxidation of glucose to CO2.

    • Produces 2 ATP, 6 NADH, 2 FADH2.

  3. Oxidative Phosphorylation (inner mitochondrial membrane)

    • Includes ETC + chemiosmosis.

    • Produces ~90% of ATP (32–34 ATP).

Total ATP yield: ~36–38 ATP per glucose molecule.

Concept 9.2 – Glycolysis: Splitting Glucose

Glycolysis is the first step in cellular respiration, occurring in the cytosol and does not require oxygen.

  • Two phases:

    • Energy investment: Uses 2 ATP.

    • Energy payoff: Produces 4 ATP (net gain 2 ATP), 2 NADH, and 2 pyruvate.

  • If oxygen is present: pyruvate enters mitochondria → acetyl CoA.

  • If not: fermentation occurs.

Concept 9.3 – Citric Acid Cycle (Krebs Cycle)

Prep Step

Pyruvate is converted to acetyl CoA, linking glycolysis to the Krebs cycle. This step releases CO2 and produces NADH.

Cycle Details

  • Occurs in mitochondrial matrix.

  • Each turn produces:

    • 2 CO2

    • 3 NADH

    • 1 FADH2

    • 1 ATP (substrate-level phosphorylation)

  • Per glucose: 2 turns → total 4 CO2, 6 NADH, 2 FADH2, 2 ATP.

Concept 9.4 – Oxidative Phosphorylation & Chemiosmosis

Electron Transport Chain

  • Series of protein complexes (I–IV) in the inner mitochondrial membrane.

  • Electrons from NADH/FADH2 pass through, ending at oxygen to make water.

  • Electron transfer drives H+ pumping into intermembrane space.

Chemiosmosis

  • ATP synthase uses H+ gradient (proton motive force) to synthesize ATP.

  • Flow of H+ through ATP synthase → ATP production.

  • Called oxidative phosphorylation because it couples electron transport to ATP synthesis.

Concept 9.5 – Fermentation: ATP Without Oxygen

Fermentation is an anaerobic process that allows cells to produce ATP without oxygen by regenerating NAD+ for glycolysis.

  • Alcohol fermentation: Pyruvate → ethanol + CO2 (e.g., yeast).

  • Lactic acid fermentation: Pyruvate → lactate (e.g., muscle cells).

  • Both regenerate NAD+, allowing glycolysis to continue producing ATP.

  • ATP yield: Only 2 ATP per glucose (from glycolysis).

Comparison Table

Process

Oxygen Used?

Final Electron Acceptor

ATP per Glucose

Aerobic Respiration

Yes

O2

36–38

Anaerobic Respiration

No

Other (e.g., Fe3+, NO3-)

Variable

Fermentation

No

Organic molecule

2

Concept 9.6 – Connections to Other Pathways

Catabolism Versatility

  • Other molecules (fats, proteins) can enter respiration:

  • Fats: Broken into glycerol (→ glycolysis) + fatty acids (→ acetyl CoA).

  • Proteins: Amino acids → intermediates of glycolysis or Krebs cycle.

Anabolism

  • Small molecules from glycolysis/Krebs cycle used to build new compounds.

Regulation

  • Controlled by feedback inhibition:

  • Example: Phosphofructokinase (PFK) in glycolysis.

  • Inhibited by ATP and citrate; activated by AMP.

Summary of Steps

  • Glycolysis (cytoplasm)

  • Citric Acid Cycle (mitochondria)

  • Oxidative Phosphorylation (inner mitochondrial membrane)

  • Fermentation provides ATP without oxygen.

  • Regulation maintains energy balance in cells.

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