Metabolic Pathways and ATP Production: Glycolysis, Citric Acid Cycle, and Electron Transport Chain

Chapter 18: Metabolic Pathways and ATP Production

Overview of Cellular Respiration

Cellular respiration is the process by which cells convert nutrients into energy (ATP). It involves a series of metabolic pathways, including glycolysis, the citric acid cycle, and the electron transport chain. Oxygen plays a critical role in maximizing ATP yield.

Glycolysis

• Definition: Glycolysis is the metabolic pathway that breaks down glucose (a six-carbon sugar) into two molecules of pyruvate (three carbons each).

• Location: Cytoplasm of the cell.

• Key Steps:

  • Glucose is phosphorylated and split into two three-carbon molecules.

  • ATP is consumed in early steps, but more ATP is produced later.

  • NAD+ is reduced to NADH.

• Net ATP Yield: 2 ATP per glucose molecule (substrate-level phosphorylation).

• Products: 2 pyruvate, 2 ATP, 2 NADH.

Citric Acid Cycle (Krebs Cycle)

• Definition: The citric acid cycle is a series of enzyme-catalyzed reactions that oxidize acetyl-CoA to CO2 and generate high-energy electron carriers.

• Location: Mitochondrial matrix.

• Key Steps:

  • Acetyl-CoA combines with oxaloacetate to form citrate.

  • Through a series of reactions, citrate is converted back to oxaloacetate.

  • CO2 is released, and NAD+ and FAD are reduced to NADH and FADH2.

• Net ATP Yield: 2 ATP per glucose (one per cycle turn).

• Products: 6 NADH, 2 FADH2, 2 ATP, 4 CO2 (per glucose).

Electron Transport Chain (ETC) and Oxidative Phosphorylation

• Definition: The ETC is a series of protein complexes in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, generating a proton gradient used to produce ATP.

• Location: Inner mitochondrial membrane.

• Key Steps:

  • Electrons from NADH and FADH2 are passed through complexes I-IV.

  • Protons are pumped into the intermembrane space, creating an electrochemical gradient.

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

  • ATP synthase uses the proton gradient to synthesize ATP from ADP and Pi.

• Net ATP Yield: About 28-34 ATP per glucose.

Aerobic vs. Anaerobic ATP Yield

Pathway	Oxygen Required?	ATP Yield per Glucose
Aerobic Respiration	Yes	~32-38 ATP
Anaerobic Respiration (Fermentation)	No	2 ATP

Importance of Oxygen

• Oxygen as Final Electron Acceptor: Oxygen is essential for the electron transport chain to function. Without oxygen, electrons cannot be transferred efficiently, and the chain stops.

• Efficient ATP Production: The majority of ATP is produced during oxidative phosphorylation, which requires oxygen. Without it, cells rely on glycolysis alone, yielding much less ATP.

Key Equations

• Overall Equation for Aerobic Respiration: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

• ATP Yield Summary: $\text{Glycolysis: } 2\, \text{ATP}$ $\text{Citric Acid Cycle: } 2\, \text{ATP}$ $\text{Electron Transport Chain: } 28-34\, \text{ATP}$

Example: Muscle Cells Under Anaerobic Conditions

• During intense exercise, oxygen supply may be insufficient.

• Cells switch to anaerobic glycolysis, producing lactic acid and only 2 ATP per glucose.

• This is much less efficient than aerobic respiration.