ATP Synthesis During Cellular Respiration

Overview

• ATP production during cellular respiration occurs via two main pathways:
  1. Substrate-level phosphorylation
  2. Oxidative phosphorylation
• Key Molecules involved: ATP, ADP, NADH, FADH o Electron transport chain and its role in ATP synthesis

Substrate-Level Phosphorylation

• Occurs during cellular respiration
• Involves direct phosphorylation of ADP to ATP using a high-energy substrate
• ATP acts as an intermediate metabolite in the oxidation of glucose
• Requires an addition of a phosphate group and may lead to molecule alteration (e.g., gaining a hydroxyl group)

Oxidative Phosphorylation

• Occurs in mitochondria
• Requires oxygen (O2)
• Involves electron transport chain (ETC)
• Final electron acceptor: Oxygen (reduced to water)
• Electron carriers: NADH and FADH2
  • NADH formed from glycolysis (pyruvate oxidation)
  • FADH2 formed from Krebs cycle
• Proton gradient drives ATP synthase enzyme to produce ATP
• ATP Yield: Much more ATP compared to substrate-level phosphorylation

Electron Transport Chain (ETC)

• Location: Mitochondria inner membrane
• Composed of four protein complexes:
  1. Complex I
  2. Complex II
  3. Complex III
  4. Complex IV
• Additional: ATP synthase (Complex V)
• Process involves the transfer of electrons from NADH and FADH2 through complexes, driving proton pumping across the membrane
• Proton movement creates a gradient utilized by ATP synthase to generate ATP

ATP Calculation via Experiments

• Accurate ATP yield estimation requires controlled experiments
• ATP production measured using mitochondria exposed to controlled NADH and FADH2 amounts
• Typical yields determined experimentally:
  • 1 NADH = 3 ATP
  • 1 FADH2 = 2 ATP
• Upper limit: 30-38 ATP per glucose molecule
• Yield variability based on cellular conditions and experimental settings

Practical Calculations for ATP Yield

• Experimentally determined ATP yields suggested a general ratio of protons to ATP: ~4 protons per ATP
• NADH: Yields ~2.5 ATP
• FADH2: Yields ~1.5 ATP
• Total ATP estimates for various stages:
  • Glycolysis: 2 ATP, 2 NADH (~5 ATP)
  • Pyruvate oxidation: 2 NADH (~5 ATP)
  • Krebs cycle: 2 ATP, 6 NADH (~15 ATP), 2 FADH2 (~3 ATP)
  • Overall: Roughly 30-32 ATP per glucose
• Variation sources: Transport efficiency, alternative pathways depending on cellular state

Summary

• ATP Synthesis is a critical process for cellular function, varying based on conditions
• Effective yield of ATP may vary but generally falls within a certain range (30-32 ATP per glucose)
• The exact number is influenced by numerous variables including the method of measurement and the type of cell being examined.