Metabolic Pathways: Respiration and Fermentation

Introduction

Metabolic pathways are sequences of chemical reactions occurring within a cell, crucial for energy production and cellular function. This chapter focuses on cellular respiration and fermentation, highlighting how cells extract energy from organic molecules, primarily glucose.

• Cellular respiration is a process by which cells convert biochemical energy from nutrients into ATP, releasing waste products.

• Fermentation is an alternative pathway for energy production in the absence of oxygen.

• The RNA world hypothesis suggests that self-replicating RNA molecules may have been precursors to life.

Aerobic Cellular Respiration: The Big Picture

Overview and Chemical Equation

Aerobic respiration is the most efficient way for cells to harvest energy from glucose. It requires oxygen and produces carbon dioxide, water, and energy (ATP).

• Overall chemical equation:

• Glucose becomes oxidized (loses electrons).

• Oxygen becomes reduced (gains electrons).

• Energy released is captured in the form of ATP.

Stages of Cellular Respiration

Cellular respiration consists of four main stages:

1. Glycolysis (cytosol): Glucose is split into two molecules of pyruvate, producing ATP and NADH.

2. Pyruvate Processing (mitochondrial matrix): Pyruvate is converted to acetyl-CoA, producing NADH and releasing CO2.

3. Citric Acid Cycle (mitochondrial matrix): Acetyl-CoA is oxidized, generating ATP, NADH, FADH2, and CO2.

4. Electron Transport Chain & Oxidative Phosphorylation (inner mitochondrial membrane): Electrons from NADH and FADH2 are transferred to oxygen, driving ATP synthesis.

Glycolysis

Overview

Glycolysis is the first step in cellular respiration, occurring in the cytosol of both prokaryotic and eukaryotic cells. It involves the partial oxidation of glucose to form pyruvate and generates ATP and NADH.

• Glucose (6-carbon sugar) is converted into two molecules of pyruvate (3-carbon compound).

• ATP and NADH are produced as energy carriers.

Phases of Glycolysis

Energy Investment Phase (Steps 1-5)

The first five steps of glycolysis require an input of energy (ATP) to phosphorylate glucose and its intermediates.

• 2 ATP molecules are consumed to activate glucose.

• Key intermediates: Glucose-6-phosphate, Fructose-6-phosphate, Fructose-1,6-bisphosphate.

• Each step is catalyzed by a specific enzyme.

Energy Payoff Phase (Steps 6-10)

The last five steps generate energy by producing ATP and NADH.

• 4 ATP molecules are produced (net gain: 2 ATP).

• 2 NADH molecules are produced.

• End product: 2 pyruvate molecules.

Enzyme Functions in Glycolysis

• Kinase: Adds a phosphate group to a substrate (phosphorylation).

• Isomerase: Converts a molecule into its isomer.

• Dehydrogenase: Removes hydrogen atoms (oxidation).

• Mutase: Rearranges functional groups within a molecule.

Glycolysis Summary Table

Pyruvate Processing (Pyruvate Oxidation)

Overview

Pyruvate produced from glycolysis is transported into the mitochondrial matrix (in eukaryotes) and converted into acetyl-CoA by the pyruvate dehydrogenase complex.

• Each pyruvate (2 per glucose) is oxidized, producing one NADH and one CO2.

• Acetyl-CoA enters the citric acid cycle.

Pyruvate Processing Summary Table

The Citric Acid Cycle (Krebs Cycle)

Overview

The citric acid cycle completes the oxidation of glucose derivatives, generating high-energy electron carriers and releasing CO2.

• Occurs in the mitochondrial matrix (eukaryotes) or cytosol (prokaryotes).

• Each acetyl-CoA is oxidized, producing NADH, FADH2, ATP, and CO2.

Citric Acid Cycle Summary Table

Electron Transport Chain & Oxidative Phosphorylation

Overview

The electron transport chain (ETC) uses electrons from NADH and FADH2 to create a proton gradient across the inner mitochondrial membrane, driving ATP synthesis via oxidative phosphorylation.

• Oxygen acts as the final electron acceptor, forming water.

• Most ATP is produced in this stage.

Summary of ATP Yield from Cellular Respiration

Total ATP yield per glucose: ~30-32 ATP

Key Terms and Concepts

• ATP (Adenosine Triphosphate): The primary energy currency of the cell.

• NADH and FADH2: Electron carriers that store energy for the electron transport chain.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Substrate-level phosphorylation: Direct synthesis of ATP during glycolysis and the citric acid cycle.

• Oxidative phosphorylation: ATP synthesis driven by the electron transport chain.

Example: Cellular Respiration in Muscle Cells

During intense exercise, muscle cells rely on glycolysis for rapid ATP production. If oxygen is limited, fermentation may occur, producing lactate instead of proceeding through the citric acid cycle and electron transport chain.

Additional info: The notes infer the location of each process (cytosol vs. mitochondria), the role of enzymes, and the overall ATP yield, which are standard academic details for a self-contained study guide.