7.1 Oxidation-Reduction Reactions

Electron transfer in redox reactions is the heart of many biological processes. These reactions involve the exchange of electrons between molecules, with one losing electrons (oxidation) and another gaining them (reduction).

From energy production to cellular damage, redox reactions play a crucial role in life. Understanding how to balance these reactions and their biological implications is key to grasping the chemical foundations of living systems.

Electron Transfer in Redox Reactions

Oxidation and reduction definitions

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• Oxidation involves loss of electrons and increase in oxidation number (Fe2+ → Fe3+ + e-)
• Reduction entails gain of electrons and decrease in oxidation number (Cu2+ + 2e- → Cu)
• Redox reactions occur simultaneously transferring electrons between species (Zn + Cu2+ → Zn2+ + Cu)

Oxidizing and reducing agents

• Oxidizing agent accepts electrons and gets reduced (O2, Cl2, H2O2)
• Reducing agent donates electrons and gets oxidized (Na, H2, CO)

Balancing redox reactions

1. Separate reaction into two half-reactions
2. Balance atoms other than H and O
3. Balance oxygen atoms by adding H2O
4. Balance hydrogen atoms by adding H+
5. Balance charges by adding electrons
6. Multiply half-reactions to equalize electron transfer
7. Combine half-reactions and cancel common terms

• Acidic solutions use H+ and H2O
• Basic solutions use OH- instead of H+

Redox reactions in biology

• Energy production through cellular respiration oxidizes glucose and reduces oxygen
• Photosynthesis oxidizes water and reduces carbon dioxide
• Electron transport chain in mitochondria and chloroplasts generates proton gradient for ATP synthesis
• Oxidative stress accumulates reactive oxygen species damaging cellular components (DNA, proteins)
• Antioxidants neutralize free radicals protecting against oxidative damage (vitamin C, glutathione)
• Metabolic pathways involve redox reactions in glycolysis, citric acid cycle, and fatty acid oxidation
• Oxidoreductases catalyze redox reactions using cofactors like NAD+ and FAD as electron carriers