Cellular Respiration and Fermentation

Overview of Cellular Respiration and Fermentation

Cells require energy to perform essential functions, and this energy is primarily supplied by ATP, which is generated through metabolic pathways such as cellular respiration and fermentation. These processes involve the breakdown of organic molecules, especially glucose, to release energy in a controlled manner.

• Cellular Respiration: The most efficient catabolic pathway, involving the complete degradation of sugars. It can be aerobic (using oxygen) or anaerobic (using other electron acceptors).

• Fermentation: A less efficient, anaerobic process that partially degrades sugars without using oxygen as the final electron acceptor.

• Example: When a deer eats grass, its cells break down the sugars from the grass to generate ATP for metabolism.

Redox Reactions: The Chemistry of Energy Release

Understanding Redox Reactions

Redox (reduction-oxidation) reactions are central to energy transfer in biological systems. These reactions involve the transfer of electrons between molecules, releasing energy that can be harnessed for cellular work.

• Oxidation: Loss of electrons by a molecule ("LEO says GER": Lose Electrons Oxidized).

• Reduction: Gain of electrons by a molecule ("Gains Electrons Reduced").

• Reducing Agent: The electron donor in a redox reaction.

• Oxidizing Agent: The electron acceptor in a redox reaction.

• Key Molecule: NAD+ (oxidized form) acts as an electron acceptor, becoming NADH (reduced form), which stores energy for ATP synthesis.

• Electron Transport Chain: NADH donates electrons to the electron transport chain, where energy is released in steps and used to make ATP.

Stages of Cellular Respiration

Main Stages and Their Locations

Cellular respiration in eukaryotes occurs in both the cytosol and mitochondria, while in prokaryotes, all steps occur in the cytoplasm.

• Glycolysis: Occurs in the cytosol; breaks down glucose into two pyruvate molecules.

• Pyruvate Oxidation: In eukaryotes, pyruvate is transported into the mitochondrion and converted to acetyl CoA.

• Citric Acid Cycle (Krebs Cycle): Acetyl CoA enters the cycle, which occurs in the mitochondrial matrix.

• Oxidative Phosphorylation: Includes the electron transport chain and chemiosmosis; generates most of the ATP.

Glycolysis: Sugar Splitting

• Process: Glucose (6C) is split into two pyruvate (3C each).

• Phases:

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

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

• Oxygen Requirement: Glycolysis occurs with or without oxygen.

• Example Calculation: For 2 glucose: 4 ATP, 4 NADH, and 4 pyruvate are produced.

Pyruvate Oxidation

• Transport: Pyruvate enters the mitochondrion via a transport protein.

• Conversion: Pyruvate is converted to acetyl CoA by the enzyme pyruvate dehydrogenase, linking glycolysis to the citric acid cycle.

Citric Acid Cycle (Krebs Cycle)

• Main Function: Completes the breakdown of glucose by oxidizing acetyl CoA to CO2.

• Key Steps: Acetyl CoA combines with oxaloacetate to form citrate, which is processed through the cycle.

• Products per Turn: 1 ATP, 3 NADH, 1 FADH2 per acetyl CoA.

• Products per Glucose: 2 ATP, 6 NADH, 2 FADH2 (since each glucose yields 2 acetyl CoA).

• Energy Storage: Most energy is stored in NADH and FADH2.

Oxidative Phosphorylation

• Electron Transport Chain (ETC): Located in the inner mitochondrial membrane; electrons from NADH and FADH2 are transferred through a series of proteins, ultimately reducing oxygen to water.

• Energy Release: Electrons lose free energy as they move through the chain, which is used to pump protons (H+) across the membrane.

• Chemiosmosis: The proton gradient (proton-motive force) drives protons back through ATP synthase, catalyzing the phosphorylation of ADP to ATP.

Fermentation and Anaerobic Respiration

Anaerobic Respiration

• Definition: Similar to aerobic respiration, but uses a final electron acceptor other than oxygen (e.g., hydrogen sulfide).

• Organisms: Common in bacteria that live in oxygen-poor environments.

Fermentation

• Definition: An anaerobic process that allows ATP production by glycolysis when oxygen is scarce, by regenerating NAD+.

• Process: Glycolysis plus reactions that transfer electrons from NADH to pyruvate or its derivatives.

• Types of Fermentation:

  • Alcohol Fermentation: Pyruvate is converted to ethanol and CO2; used by yeast in brewing and baking.

  • Lactic Acid Fermentation: Pyruvate is reduced to lactate; used by some bacteria, fungi, and human muscle cells during intense exercise.

Comparison of Respiration and Fermentation

The Versatility of Catabolism

Alternative Fuels and Anabolic Pathways

Cells can metabolize a variety of organic molecules, not just glucose, to generate energy or build new molecules.

• Proteins: Broken down to amino acids, which can be deaminated and funneled into glycolysis or the citric acid cycle. Byproducts can be toxic and must be managed.

• Fats: Digested to glycerol (enters glycolysis) and fatty acids (broken down by beta oxidation to acetyl CoA for the citric acid cycle). Fats are energy-rich due to their structure.

• Anabolic Pathways: Intermediates from glycolysis and the citric acid cycle can be used to synthesize other needed organic molecules.

Key Equations

• Overall Equation for Aerobic Cellular Respiration:

• ATP Yield (Aerobic Respiration):

• Alcohol Fermentation:

• Lactic Acid Fermentation: