Cellular Respiration and Fermentation

Overview

Cellular respiration and fermentation are essential metabolic pathways that allow cells to extract energy from organic molecules, primarily glucose. These processes involve a series of biochemical reactions that convert the chemical energy stored in glucose into adenosine triphosphate (ATP), the cell's main energy currency.

Photosynthesis and Glucose Metabolism

Photosynthesis: Energy Capture

• Photosynthesis is the process by which plants, algae, and some bacteria capture energy from sunlight and store it in the chemical bonds of glucose.

• Overall equation:

• Glucose produced is used as a fuel for cellular respiration in both plants and animals.

Glucose Catabolism: Cellular Respiration

• Cellular respiration is the process of breaking down glucose in the presence of oxygen to release energy, carbon dioxide, and water.

• Overall equation:

• Only about 40% of the energy in glucose is captured as ATP; the rest is lost as heat.

• The complete breakdown of glucose occurs in four main steps:

  1. Glycolysis

  2. Pyruvate Oxidation

  3. Krebs Cycle (Citric Acid Cycle)

  4. Electron Transport Chain and ATP Synthase

Glycolysis

Introduction to Glycolysis

• Glycolysis is the oldest and most universal biochemical pathway, occurring in the cytoplasm of all living cells.

• It does not require oxygen (anaerobic process).

Steps of Glycolysis

• Breaks down one glucose molecule (6 carbons) into two pyruvate molecules (3 carbons each).

• Two main phases:

  1. Energy Investment Phase: Uses 2 ATP to activate glucose.

  2. Energy Payoff Phase: Produces 4 ATP (net gain of 2 ATP) and 2 NADH (high-energy electron carriers).

Summary equation:

Fate of Pyruvate: Fermentation and Aerobic Respiration

Fermentation (Anaerobic Pathways)

When oxygen is not available, cells regenerate NAD+ through fermentation, allowing glycolysis to continue producing ATP.

• Pyruvate accepts electrons from NADH, regenerating NAD+.

• Two main types of fermentation:

Lactic Acid Fermentation

• Occurs in some bacteria (e.g., yogurt, cheese production) and in human muscle cells under low oxygen conditions.

• Equation:

• Lactate can be converted back to pyruvate in the liver when oxygen is available (Cori cycle), but this process requires ATP.

Alcoholic Fermentation

• Occurs in yeast and some types of bacteria.

• Equation:

• Used in bread making (CO2 causes dough to rise) and alcoholic beverage production.

Cellular Respiration (Aerobic Pathway)

Introduction

• In the presence of oxygen, pyruvate is fully oxidized to CO2, generating much more ATP.

• Oxygen acts as the final electron acceptor in the electron transport chain.

• Major stages: Glycolysis, Pyruvate Oxidation, Krebs Cycle, and Oxidative Phosphorylation.

Mitochondria: The Site of Aerobic Respiration

• Mitochondria are double-membraned organelles.

• Key compartments:

  • Intermembrane space: Between the outer and inner membranes.

  • Matrix: Inside the inner membrane; contains enzymes for the Krebs cycle.

• ATP is produced by enzymes in the matrix and by the movement of H+ through ATP synthase in the inner membrane.

Pyruvate Oxidation

• Each pyruvate (3C) is transported into the mitochondrion and converted to acetyl-CoA (2C).

• One CO2 is released per pyruvate.

• NADH is produced per pyruvate.

• Equation:

Krebs Cycle (Citric Acid Cycle)

• Acetyl-CoA enters the cycle; occurs in the mitochondrial matrix.

• For each acetyl-CoA (cycle runs twice per glucose):

  • 2 CO2 produced

  • 1 ATP produced

  • 3 NADH produced

  • 1 FADH2 produced

Oxidative Phosphorylation

• Consists of the Electron Transport Chain (ETC) and Chemiosmosis.

Electron Transport Chain (ETC)

• NADH and FADH2 donate electrons to the ETC, a series of protein complexes in the inner mitochondrial membrane.

• Energy from electrons is used to pump H+ into the intermembrane space, creating a proton gradient (potential energy).

• O2 is the final electron acceptor, forming water:

ATP Production (Chemiosmosis)

• H+ flows back into the matrix through ATP synthase, driving the synthesis of ATP from ADP and Pi.

• This process produces 32-34 ATP per glucose molecule.

Alternative Substrates for ATP Synthesis

Use of Non-Carbohydrates

• Triglycerides (Fats):

  • Broken down into fatty acids and glycerol.

  • Glycerol can be converted to pyruvate or glucose.

  • Fatty acids are converted to acetyl-CoA via beta-oxidation.

• Proteins:

  • Amino acids are deaminated (removal of the amine group).

  • Deaminated amino acids can enter glycolysis, be converted to pyruvate, or enter the Krebs cycle.

Summary Table: Pathways of Glucose Catabolism

Key Terms

• ATP (Adenosine Triphosphate): The main energy carrier in cells.

• NADH/FADH2: Electron carriers that transport high-energy electrons to the electron transport chain.

• Fermentation: Anaerobic process that regenerates NAD+ for glycolysis.

• Oxidative Phosphorylation: Production of ATP using energy derived from the redox reactions of the electron transport chain.

• Chemiosmosis: The movement of ions across a semipermeable membrane, down their electrochemical gradient, to drive ATP synthesis.

Example: During intense exercise, human muscle cells switch to lactic acid fermentation when oxygen is scarce, allowing ATP production to continue, though less efficiently than aerobic respiration.

Additional info: The theoretical maximum ATP yield per glucose is 36-38, but actual yield is often lower due to losses and variations in shuttle mechanisms.