Cellular Respiration & Fermentation: Mechanisms of ATP Production in Cells

Cellular Respiration & Fermentation

Overview of Cellular Respiration

Cellular respiration is a series of metabolic processes by which cells extract energy from organic molecules, primarily glucose, to produce ATP. This process occurs in both eukaryotic and prokaryotic cells and involves several distinct stages.

• ATP (Adenosine Triphosphate): The main energy currency of the cell, produced during cellular respiration.

• Stages of Cellular Respiration:

  1. Glycolysis

  2. Pyruvate Oxidation

  3. Citric Acid Cycle (Krebs Cycle)

  4. Electron Transport Chain & Chemiosmosis

• Fermentation: An alternative pathway for ATP production when oxygen is absent.

Glycolysis

Glycolysis is the first step in the breakdown of glucose, occurring in the cytosol of both eukaryotes and prokaryotes.

• Location: Cytosol

• Inputs: 1 Glucose, 2 NAD+, 2 ADP + 2 Pi

• Outputs: 2 Pyruvate, 2 NADH, 2 ATP

• Key Point: Glycolysis does not require oxygen (anaerobic process).

Pyruvate Oxidation

Pyruvate produced in glycolysis is transported into the mitochondria (in eukaryotes) and converted to acetyl CoA by the pyruvate dehydrogenase complex.

• Location: Mitochondrial matrix (eukaryotes), cytosol (prokaryotes)

• Enzyme: Pyruvate dehydrogenase

• Reaction: Pyruvate + NAD+ + CoA → Acetyl CoA + NADH + CO2

• Key Point: This step links glycolysis to the citric acid cycle.

Other Ways to Make Acetyl CoA

Acetyl CoA can also be produced from the breakdown of fatty acids (via β-oxidation) and amino acids.

• Fatty Acid β-Oxidation: Fatty acids are broken down into acetyl CoA, FADH2, and NADH.

• Amino Acid Catabolism: Certain amino acids can be converted into intermediates that enter the citric acid cycle.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the oxidation of carbon atoms from food, generating high-energy electron carriers and ATP.

• Location: Mitochondrial matrix (eukaryotes), cytosol (prokaryotes)

• Inputs: Acetyl CoA, NAD+, FAD, GDP/ADP

• Outputs (per glucose): 6 NADH, 2 FADH2, 2 ATP (or GTP), 4 CO2

• Key Point: Cycle regenerates oxaloacetate and produces electron carriers for the electron transport chain.

Electron Transport Chain & Chemiosmosis

The electron transport chain (ETC) uses electrons from NADH and FADH2 to pump protons across the inner mitochondrial membrane, creating an electrochemical gradient.

• Location: Inner mitochondrial membrane

• Process: Electrons are transferred through complexes I-IV, pumping H+ into the intermembrane space.

• Proton-Motive Force: The gradient of H+ ions drives ATP synthesis via ATP synthase.

• Chemiosmosis: The flow of H+ through ATP synthase produces ATP.

• Equation: $\text{ADP} + \text{P}_i \xrightarrow{\text{ATP synthase}} \text{ATP}$

ATP Yield from Glucose Oxidation

The complete oxidation of one glucose molecule yields a significant amount of ATP, with the majority produced during oxidative phosphorylation.

Catabolic Process	Products Formed	Net ATP Produced Per Molecule of Glucose
Glycolysis	2 NADH, 2 ATP, 2 pyruvate	2 ATP, 3-5 ATP (from NADH)
Pyruvate Oxidation	2 NADH, 2 acetyl CoA	5 ATP (from NADH)
Citric Acid Cycle	6 NADH, 2 FADH2, 2 GTP	15 ATP (from NADH), 3 ATP (from FADH2), 2 ATP (from GTP)
Total		~30 ATP (eukaryotes), ~32 ATP (prokaryotes)

Additional info: NADH produced in the cytosol yields fewer ATP molecules than NADH produced in the mitochondrial matrix due to transport costs.

Fermentation

Fermentation is an anaerobic process that allows cells to regenerate NAD+ and produce ATP when oxygen is unavailable.

• Lactic Acid Fermentation: Occurs in animals; pyruvate is reduced to lactate.

  • Equation: $\text{Glucose} + 2 \text{ADP} + 2 \text{P}_i \rightarrow 2 \text{Lactate} + 2 \text{ATP}$

• Alcohol Fermentation: Occurs in yeast and some bacteria; pyruvate is converted to ethanol and CO2.

  • Equation: $\text{Glucose} + 2 \text{ADP} + 2 \text{P}_i \rightarrow 2 \text{Ethanol} + 2 \text{CO}_2 + 2 \text{ATP}$

• Efficiency: Fermentation produces only 2 ATP per glucose, compared to about 30 ATP in cellular respiration.

• Facultative Anaerobes: Some organisms can switch between fermentation and aerobic respiration depending on oxygen availability.

Comparison: Cellular Respiration vs. Fermentation

Cellular respiration is much more efficient than fermentation in terms of ATP yield.

• Cellular Respiration: Requires oxygen, produces ~30 ATP per glucose.

• Fermentation: Does not require oxygen, produces only 2 ATP per glucose.

• Organism Adaptation: Use fermentation only if electron acceptor (oxygen) is not available.

Key Terms and Concepts

• ATP Synthase: Enzyme that synthesizes ATP using the proton-motive force.

• Proton-Motive Force: The combined effect of a proton gradient and membrane potential across the inner mitochondrial membrane.

• Oxidative Phosphorylation: ATP production linked to the transfer of electrons through the ETC and chemiosmosis.

• Substrate-Level Phosphorylation: Direct synthesis of ATP during glycolysis and the citric acid cycle.