ATP-ADP Energy Cycle Overview

Overview

This lecture explains the ATP-ADP energy cycle, detailing how ATP is produced, used, and recycled in cells, alongside the mechanisms and significance of both substrate-level and oxidative phosphorylation.

ATP-ADP Energy Cycle Steps

• Cells produce ATP in mitochondria.
• ATP is transported to the cytoplasm.
• ATP couples with endergonic (energy-requiring) reactions, becoming ADP and inorganic phosphate with energy release.
• ADP and inorganic phosphate are recycled back into ATP.

ATP Formation: Phosphorylation

• Phosphorylation converts ADP and inorganic phosphate into ATP, requiring energy (endergonic reaction).
• Two main types: oxidative phosphorylation (main source, 90% of ATP) and substrate-level phosphorylation (minor source, 10% of ATP).
• Oxidative phosphorylation occurs in mitochondria; hence, mitochondria are called the cell’s powerhouse.
• Substrate-level phosphorylation typically takes place in the cytoplasm during glycolysis or the Krebs cycle.

Substrate-Level Phosphorylation Details

• Direct transfer of phosphate from a substrate to ADP occurs via an enzyme.
• Example 1: Phosphoenolpyruvate donates phosphate to ADP via pyruvate kinase, forming ATP.
• Example 2: 1,3-Bisphosphoglycerate donates phosphate to ADP, becoming 3-phosphoglyceric acid and ATP.
• Glycolysis creates four ATP molecules by substrate-level phosphorylation.

Oxidative Phosphorylation

• Protons and electrons form a gradient that drives ATP synthase to convert ADP to ATP (electron transport chain).
• Citric acid cycle produces NADH and FADH2, transferring electrons through transmembrane proteins to generate ATP.

ATP Hydrolysis

• Hydrolysis of ATP to ADP releases energy (exergonic reaction).
• ATP structure: ribose sugar, adenine base, and three phosphate groups (negative 4 charge).
• Water molecules break a phosphate bond, forming ADP and inorganic phosphate.
• Released energy varies by cell type, concentrations of ATP, ADP, phosphate, and magnesium ions.

Role of Magnesium in ATP Cycle

• Magnesium stabilizes ATP and ADP, preventing unwanted interactions in the cytoplasm.
• ATP binds Mg²⁺ after leaving mitochondria; after hydrolysis, Mg²⁺ is released and stored in the cytoplasm.
• ADP returns to mitochondria, recombines with Mg²⁺ and phosphate, and is converted back to ATP by ATP synthase.

Energy from ATP Hydrolysis

• Standard free energy change (ΔG°’) is –28 to –34 kJ/mol, but in cells like human erythrocytes, it can reach –52 kJ/mol.
• Differences in energy are due to cellular conditions and concentrations of reactants.

Key Terms & Definitions

• ATP (Adenosine Triphosphate) — The primary energy carrier in cells.
• ADP (Adenosine Diphosphate) — Product of ATP hydrolysis, recyclable into ATP.
• Phosphorylation — Addition of a phosphate group to a molecule.
• Substrate-Level Phosphorylation — Direct phosphate transfer from a substrate to ADP.
• Oxidative Phosphorylation — ATP production using energy from the electron transport chain.
• Hydrolysis — Reaction involving water that breaks chemical bonds, e.g., ATP to ADP.
• Endergonic Reaction — Energy-consuming reaction.
• Exergonic Reaction — Energy-releasing reaction.
• ATP Synthase — Enzyme that converts ADP and phosphate into ATP.
• Proton Motive Force — Gradient used to drive ATP synthesis.

Action Items / Next Steps

• Review glycolysis and Krebs cycle processes for substrate-level phosphorylation.
• Await the next lecture for details on the electron transport chain.