Metabolic Pathways: Respiration and Fermentation

Introduction to Metabolic Pathways

Metabolic pathways are sequences of chemical reactions occurring within a cell that enable the transformation of molecules and the transfer of energy. Two major metabolic pathways in cells are cellular respiration and fermentation, which are essential for producing ATP, the cell's energy currency.

• Cellular respiration is a process that converts energy stored in organic molecules (such as glucose) into ATP.

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

• These pathways are central to energy flow in biological systems and are closely linked to photosynthesis.

Chapter 9 Roadmap: How Cells Make ATP

Overview of ATP Production

This chapter explores how cells generate ATP from sugars and other high-energy compounds, both in the presence and absence of oxygen.

• With oxygen (aerobic respiration): Glycolysis, pyruvate oxidation, citric acid cycle, electron transport, and chemiosmosis.

• Without oxygen (anaerobic conditions): Glycolysis followed by fermentation.

Key Sections:

• 9.1: How cells produce ATP when oxygen is present

• 9.2: Glycolysis

• 9.3: Pyruvate oxidation

• 9.4: Citric acid cycle

• 9.5: Electron transport and chemiosmosis

• 9.6: Fermentation

Connections: Respiration and Photosynthesis

Energy Flow in Biological Systems

Photosynthesis and cellular respiration are interconnected processes that manage energy conversion and storage in living organisms.

• Photosynthesis: Converts light energy into chemical energy stored in glucose and other organic molecules.

• Cellular respiration: Releases energy from glucose to produce ATP for cellular activities.

• Energy stored in starch, glycogen, and fats (synthesized from glucose) is used during cellular respiration and fermentation.

Example: Athletes use cellular respiration for sustained energy during long races, while fermentation provides quick energy in short sprints when oxygen is limited.

Cellular Respiration: Overview and Equation

General Reaction and Energy Conversion

Cellular respiration is a series of metabolic reactions that convert the energy in organic molecules into ATP. The process involves the oxidation of glucose and the reduction of oxygen.

• General equation:

• Glucose is oxidized to carbon dioxide.

• Oxygen is reduced to water.

• Energy is released and captured in the form of ATP.

Combustion vs. Controlled Combustion:

• Combustion releases all energy as heat instantly.

• Controlled combustion (cellular respiration) releases energy gradually, allowing cells to store it as ATP.

Key Concepts in Cellular Respiration

Redox Reactions and Electron Carriers

Energy transfer in respiration occurs via redox reactions, which involve the transfer of electrons between molecules.

• Oxidation: Loss of electrons (OIL: Oxidation Is Loss).

• Reduction: Gain of electrons (RIG: Reduction Is Gain).

• NADH and FADH2 are electron carriers that transport electrons during respiration.

Example: In glycolysis and the citric acid cycle, NAD+ is reduced to NADH as it picks up electrons.

ATP Production Mechanisms

ATP is produced in cellular respiration by two main mechanisms:

• Substrate-level phosphorylation: Direct transfer of a phosphate group from a high-energy substrate to ADP, forming ATP.

• Oxidative phosphorylation: ATP is synthesized from ADP and inorganic phosphate using energy from a proton gradient, driven by electron transport and ATP synthase.

Stages of Aerobic Cellular Respiration

Major Steps and Their Functions

• Glycolysis: Partial oxidation of glucose to form pyruvate; generates ATP and NADH.

• Pyruvate oxidation: Conversion of pyruvate to acetyl-CoA, producing NADH and CO2.

• Citric acid cycle: Completes the oxidation of acetyl-CoA to CO2; generates NADH, FADH2, and ATP.

• Electron transport and chemiosmosis: Electrons from NADH and FADH2 are transferred through the electron transport chain, creating a proton gradient that drives ATP synthesis.

Fermentation

ATP Production Without Oxygen

Fermentation allows cells to produce ATP in the absence of oxygen by regenerating NAD+ from NADH, enabling glycolysis to continue.

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

• Alcohol fermentation: Pyruvate is converted to ethanol and CO2 (common in yeast).

Example: Muscle cells switch to lactic acid fermentation during intense exercise when oxygen is scarce.

Comparison Table: Cellular Respiration vs. Fermentation

Additional info: The "RNA world" hypothesis suggests that early life may have used RNA for both genetic information and catalysis, which could have influenced the evolution of metabolic pathways.