Cellular Respiration and Fermentation

Overview of Cellular Respiration

Cellular respiration is a series of metabolic pathways that convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. It is essential for the survival of most organisms and occurs in both aerobic (with oxygen) and anaerobic (without oxygen) conditions.

• Aerobic respiration uses oxygen as the final electron acceptor and yields the most ATP.

• Anaerobic respiration and fermentation occur when oxygen is absent, producing less ATP.

• ATP is the universal energy currency of the cell.

Redox Reactions in Cellular Respiration

Redox (oxidation-reduction) reactions are fundamental to cellular respiration, involving the transfer of electrons between molecules.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance.

• Reducing agent: Electron donor.

• Oxidizing agent: Electron acceptor.

During cellular respiration, glucose is oxidized to carbon dioxide, and oxygen is reduced to water.

Cellular Respiration Equation:

Structure of the Mitochondrion

The mitochondrion is the site of aerobic respiration in eukaryotic cells. It has a double membrane, with the inner membrane folded into cristae, where the electron transport chain (ETC) and ATP synthesis occur.

Major Stages of Cellular Respiration

1. Glycolysis

Glycolysis is the first step in the breakdown of glucose, occurring in the cytoplasm. It splits one glucose molecule into two pyruvate molecules, producing a small amount of ATP and NADH.

• Occurs in the cytoplasm of all cells.

• Does not require oxygen (anaerobic process).

• Net products per glucose: 2 ATP (net), 2 NADH, 2 pyruvate.

Key Phases:

• Energy Investment Phase: 2 ATP are used to phosphorylate glucose and its intermediates.

• Energy Payoff Phase: 4 ATP (gross) and 2 NADH are produced, resulting in a net gain of 2 ATP.

2. Pyruvate Oxidation and Citric Acid Cycle (Krebs Cycle)

After glycolysis, pyruvate is transported into the mitochondria and converted to acetyl-CoA, which enters the citric acid cycle. This cycle completes the oxidation of glucose derivatives, generating ATP, NADH, FADH2, and CO2.

• Occurs in the mitochondrial matrix.

• Each glucose yields two turns of the cycle (one per pyruvate).

• Products per glucose: 2 ATP, 6 NADH, 2 FADH2, 4 CO2.

Key Steps:

• Acetyl-CoA combines with oxaloacetate to form citrate.

• Series of enzyme-catalyzed reactions regenerate oxaloacetate.

• Electrons are transferred to NAD+ and FAD, forming NADH and FADH2.

3. Electron Transport Chain (ETC) and Oxidative Phosphorylation

The ETC is a series of protein complexes in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through the chain, releasing energy used to pump protons (H+) and create a gradient. Oxygen is the final electron acceptor, forming water.

• ETC consists of four main complexes (I-IV).

• Proton gradient drives ATP synthesis via ATP synthase (chemiosmosis).

• Most ATP is generated in this stage (about 28 ATP per glucose).

Key Terms:

• Oxidative phosphorylation: ATP production powered by redox reactions in the ETC.

• Substrate-level phosphorylation: Direct transfer of phosphate to ADP during glycolysis and the citric acid cycle.

Overall ATP Yield: Cellular respiration yields 32–38 ATP per glucose molecule, depending on the cell type and conditions.

Fermentation and Anaerobic Respiration

Fermentation

Fermentation allows cells to produce ATP without oxygen by regenerating NAD+ for glycolysis. It does not involve the citric acid cycle or ETC and yields only 2 ATP per glucose.

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

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

• Main purpose: Regenerate NAD+ so glycolysis can continue.

Net ATP from fermentation: 2 ATP per glucose (from glycolysis only).

Aerobic vs. Anaerobic Respiration

• Aerobic respiration: Uses O2 as the final electron acceptor, yields high ATP.

• Anaerobic respiration: Uses other molecules (e.g., sulfate, nitrate) as final electron acceptors, yields less ATP.

• Fermentation: No ETC, no additional ATP beyond glycolysis.

Connections to Other Metabolic Pathways

Catabolism of Fats and Proteins

When glucose is scarce, cells can metabolize fats and proteins for energy. Fatty acids undergo beta oxidation to produce acetyl-CoA, NADH, and FADH2, which enter the citric acid cycle and ETC. Proteins are broken into amino acids, which are deaminated and converted into intermediates of cellular respiration.

• Beta oxidation: Fatty acids → acetyl-CoA + NADH + FADH2

• Protein catabolism: Amino acids → intermediates + NH3 (waste)

Additional info: The integration of metabolic pathways allows cells to adapt to varying nutrient availability and energy demands.