Chapter 9: Cellular Respiration and Fermentation

Concept 9.1: Catabolic Pathways Yield Energy by Oxidizing Organic Fuels

Cellular respiration is a series of metabolic pathways that break down organic molecules to release energy, primarily in the form of ATP. This process involves oxidation-reduction reactions and occurs in both prokaryotic and eukaryotic cells.

• Cellular Respiration Equation: The overall reaction for aerobic cellular respiration is:

• Oxidation and Reduction: Oxidation is the loss of electrons from a molecule, while reduction is the gain of electrons. These reactions are coupled in energy exchanges.

• NAD+ in Cellular Respiration: Nicotinamide adenine dinucleotide (NAD+) acts as an electron carrier, accepting electrons during redox reactions and becoming NADH.

• Substrate-Level vs. Oxidative Phosphorylation:

  • Substrate-level phosphorylation directly transfers a phosphate group to ADP from a substrate.

  • Oxidative phosphorylation uses energy from electrons transferred through the electron transport chain to power ATP synthesis via chemiosmosis.

• Electron Transport Chain: A series of protein complexes in the inner mitochondrial membrane that transfer electrons and pump protons to create a gradient for ATP production.

• Stages of Cellular Respiration:

  1. Glycolysis

  2. Pyruvate oxidation

  3. Citric acid cycle (Krebs cycle)

  4. Oxidative phosphorylation (electron transport chain and chemiosmosis)

  Location: In eukaryotes, glycolysis occurs in the cytosol, while the remaining steps occur in the mitochondria.

Concept 9.2: Glycolysis Harvests Chemical Energy by Oxidizing Glucose to Pyruvate

Glycolysis is the first stage of cellular respiration, where glucose is split into two molecules of pyruvate, producing ATP and NADH. This process occurs in the cytosol and does not require oxygen.

• Phases of Glycolysis:

  1. Energy investment phase: 2 ATP are used to phosphorylate glucose and its intermediates.

  2. Energy payoff phase: 4 ATP and 2 NADH are produced, resulting in a net gain of 2 ATP per glucose.

• Summary Equation:

• ATP Requirement: ATP is required to initiate glycolysis by phosphorylating glucose.

• Location: Glycolysis occurs in the cytosol of the cell.

Concept 9.3: After Pyruvate is Oxidized, the Citric Acid Cycle Completes the Energy-Yielding Oxidation of Organic Molecules

After glycolysis, pyruvate is transported into the mitochondria and converted to acetyl CoA, which enters the citric acid cycle. This cycle completes the breakdown of glucose, generating electron carriers and ATP.

• Pyruvate Oxidation: Pyruvate is oxidized to acetyl CoA, producing NADH and releasing CO2.

• Citric Acid Cycle: Acetyl CoA enters the cycle, which consists of eight steps catalyzed by specific enzymes. Each turn of the cycle produces:

  • 3 NADH

  • 1 FADH2

  • 1 ATP (or GTP)

  • 2 CO2

• Electron Carriers: NADH and FADH2 carry electrons to the electron transport chain.

• ATP Production: ATP is produced by substrate-level phosphorylation in the cycle.

Concept 9.4: During Oxidative Phosphorylation, Chemiosmosis Couples Electron Transport to ATP Synthesis

Oxidative phosphorylation is the final stage of cellular respiration, where the electron transport chain and chemiosmosis generate the majority of ATP.

• Electron Transport Chain: Electrons from NADH and FADH2 are transferred through protein complexes, releasing energy used to pump protons across the inner mitochondrial membrane.

• Proton Gradient: The movement of protons creates an electrochemical gradient (proton-motive force).

• ATP Synthase: Protons flow back into the mitochondrial matrix through ATP synthase, driving the synthesis of ATP from ADP and inorganic phosphate.

• ATP Yield: The complete oxidation of one glucose molecule yields approximately 30-32 ATP. ATP Ledger Table:

  Process	ATP Produced (per glucose)
  Glycolysis	2
  Citric Acid Cycle	2
  Oxidative Phosphorylation	26-28
  Total	30-32

• Inputs and Outputs: The process uses NADH, FADH2, glucose, acetyl CoA, pyruvate, O2, CO2, H+ gradients, and ATP synthase.

Concept 9.5: Fermentation and Anaerobic Respiration Enable Cells to Produce ATP Without Oxygen

When oxygen is unavailable, cells can generate ATP through fermentation or anaerobic respiration. These processes allow glycolysis to continue by regenerating NAD+.

• Fermentation Pathways: Cells use fermentation to obtain energy from glucose in the absence of oxygen. Two main types are:

  • Alcohol fermentation: Pyruvate is converted to ethanol and CO2 (e.g., yeast).

  • Lactic acid fermentation: Pyruvate is reduced to lactate (e.g., muscle cells, some bacteria).

• Comparison Table: Fermentation vs. Cellular Respiration

  Feature	Fermentation	Cellular Respiration
  Oxygen Required	No	Yes
  ATP Yield (per glucose)	2	30-32
  End Products	Lactate or Ethanol + CO2	CO2 + H2O
  Electron Carrier Regeneration	NAD+ regenerated by reduction of pyruvate	NAD+ regenerated by electron transport chain

• Fate of Pyruvate: In alcohol fermentation, pyruvate is converted to ethanol; in lactic acid fermentation, it is converted to lactate.

• Comparison: Fermentation produces less ATP and does not require oxygen, while cellular respiration is more efficient and requires oxygen.