Section 5: Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions that occur within living organisms to maintain life. These reactions transform matter and energy, following the laws of thermodynamics, and are essential for growth, reproduction, and cellular maintenance.

• Metabolism: The sum of all chemical reactions in an organism, including both anabolic (building up) and catabolic (breaking down) pathways.

• Catabolic pathways: Break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).

• Anabolic pathways: Build complex molecules from simpler ones, consuming energy (e.g., protein synthesis).

• Energy of life: Organisms transform energy to perform work, such as movement, synthesis, and transport.

Key Concepts:

• Metabolism is governed by the laws of thermodynamics.

• The free-energy change (ΔG) of a reaction determines if it occurs spontaneously.

• ATP (adenosine triphosphate) powers cellular work by coupling exergonic (energy-releasing) and endergonic (energy-consuming) reactions.

• Enzymes catalyze metabolic reactions by lowering activation energy barriers.

Fermentation and Anaerobic Respiration

ATP Production Without Oxygen

Cells can generate ATP in the absence of oxygen through two main processes: fermentation and anaerobic respiration. These pathways are crucial for survival in anaerobic environments or during periods of intense activity when oxygen is scarce.

• Anaerobic respiration: Utilizes an electron transport chain with a final electron acceptor other than oxygen (e.g., sulfate SO42−), producing ATP in certain prokaryotes.

• Fermentation: An extension of glycolysis that generates ATP by substrate-level phosphorylation and regenerates NAD+ by transferring electrons from NADH to an organic acceptor. It does not use an electron transport chain.

Key Differences:

• Anaerobic respiration is similar to aerobic respiration but uses a different final electron acceptor.

• Fermentation does not involve an electron transport chain and yields less ATP.

Types of Fermentation

Fermentation allows glycolysis to continue by regenerating NAD+ in the absence of oxygen. There are two main types:

• Alcohol fermentation:

  • Pyruvate is converted to acetaldehyde, releasing CO2.

  • Acetaldehyde is reduced by NADH to ethanol, regenerating NAD+.

  • Occurs in yeast and is used in the production of beer, wine, and bread.

• Lactic acid fermentation:

  • Pyruvate is reduced directly by NADH to form lactate (ionized form of lactic acid), regenerating NAD+.

  • No CO2 is released.

  • Occurs in some bacteria and in muscle cells during strenuous activity.

Summary Table: Comparison of Fermentation and Anaerobic Respiration

Facultative and Obligate Anaerobes

• Obligate anaerobes: Can only survive using fermentation or anaerobic respiration; oxygen is toxic to them (e.g., some bacteria causing gangrene).

• Facultative anaerobes: Can switch between fermentation and aerobic respiration depending on oxygen availability (e.g., E. coli, yeast, muscle cells).

• Obligate aerobes: Require oxygen for survival (e.g., brain cells).

Key Equations

• Alcohol fermentation:

• Lactic acid fermentation:

Examples and Applications

• Alcohol fermentation is used in brewing and baking industries.

• Lactic acid fermentation occurs in muscle cells during intense exercise, leading to temporary muscle fatigue.

Concept Check

• What is the final electron acceptor for NADH during fermentation? An organic molecule (pyruvate or acetaldehyde).

• What is the final electron acceptor for NADH during aerobic respiration? Oxygen (O2).

• How would a yeast cell's glucose consumption change if moved from aerobic to anaerobic conditions, but ATP production rate remained constant? The cell would need to consume glucose at a much higher rate under anaerobic conditions, since fermentation yields much less ATP per glucose molecule than aerobic respiration.