Problem 96 - Chemistry Matter and Change

Question

Electrochemical Biological Reactions Standard reduction potentials for some important biological reactions are given in Table \(20.2 .\) The strongest oxidizing agent generally available in biological systems is molecular oxygen. Consider the oxidation of reduced nicotinamide-adenine-dinucleotide \((N A D H)\) by molecular oxygen. The reaction is the following. $$2 \mathrm{NADH}+2 \mathrm{H}^{+}+\mathrm{O}_{2} \rightarrow 2 \mathrm{NAD}^{+}+2 \mathrm{H}_{2} \mathrm{O}$$ Write the two half-reactions that take place in this reaction.

Step-by-Step Solution

Answer

Oxidation: 2 NADH → 2 NAD^+ + 2 electrons + 2 H^+; Reduction: O_2 + 4 electrons + 4 H^+ → 2 H_2O.

Step 1: Identify Reactants and Products

The given reaction shows the oxidation of reduced nicotinamide-adenine-dinucleotide (NADH) by molecular oxygen (O_2). The reactants are 2 NADH, 2 H^+, and O_2, while the products are 2 NAD^+ and 2 H_2O.

Step 2: Write the Oxidation Half-Reaction

In the oxidation half-reaction, NADH is oxidized to NAD^+. The half-reaction can be written as: 2 NADH → 2 NAD^+ + 2 electrons + 2 H^+. This half-reaction involves the loss of electrons from NADH.

Step 3: Write the Reduction Half-Reaction

In the reduction half-reaction, molecular oxygen is reduced to water. The half-reaction is: O_2 + 4 electrons + 4 H^+ → 2 H_2O. This half-reaction involves the gain of electrons by O_2.

Key Concepts

Oxidation-Reduction

Oxidation-reduction reactions, also known as redox reactions, are fundamental to understanding many chemical processes, including those vital to life. In these reactions, one substance loses electrons (is oxidized), while another gains electrons (is reduced). This transfer of electrons allows for the transfer of energy, which is key in many biological processes. In the given exercise, the oxidation-reduction reaction involves nicotinamide-adenine-dinucleotide (NADH) and oxygen (O\(_2\)). NADH, the reducing agent, loses electrons and protons, thus getting oxidized to NAD\(^+\). Conversely, O\(_2\), the oxidizing agent, gains these electrons and protons, reducing to form water (H\(_2\)O). The balance between oxidation and reduction ensures that the number of electrons lost equals the number of electrons gained, highlighting the conservation of charge within the reaction.

Half-Reactions

Half-reactions are a way to illustrate the individual oxidation and reduction processes that occur in a redox reaction. They provide insight into electron flow and help us understand the specific roles each reactant plays. In the oxidation half-reaction, the process involving NADH can be written as:

• 2 NADH → 2 NAD\(^+\) + 2 electrons + 2 H\(^+\)

This shows how NADH loses electrons and protons, being oxidized to NAD\(^+\).The reduction half-reaction for the oxygen can be expressed as:

• O\(_2\) + 4 electrons + 4 H\(^+\) → 2 H\(_2\)O

Here, oxygen gains electrons and protons to form water.Half-reactions are essential in studying electrochemistry as they allow clearer tracking of electron movement—a key step in predicting the direction and spontaneity of redox reactions.

Standard Reduction Potential

The standard reduction potential (E\(^{\circ}\)) measures the tendency of a chemical species to gain electrons and be reduced. It comes into play when predicting the feasibility of redox reactions under standard conditions (25°C, 1 M concentration for solutions, 1 atm pressure for gases).For a given half-reaction, a larger (more positive) standard reduction potential indicates a greater tendency for the species to acquire electrons and undergo reduction. Conversely, a smaller (more negative) value suggests a lesser tendency. In biological contexts, such as the given reaction with NADH and O\(_2\), these potentials help determine which species will act as the oxidizing or reducing agent. Oxygen, with a high positive standard reduction potential, acts as a powerful oxidizing agent, while NADH, with its lower potential, donates electrons readily. Understanding these potentials provides critical insight into cellular respiration and energy production in living organisms.

Biological Systems

Electrochemical reactions are not limited to a lab—our bodies are full of them! Biological systems, like cells, rely on redox reactions to sustain life. One prime example is cellular respiration, where cells convert glucose and oxygen into energy, CO\(_2\), and water. The given exercise reaction reflects part of the electron transport chain, essential for ATP production, the body’s energy currency.Key characteristics of biological redox reactions include:

• Enzyme-mediated steps for efficiency and regulation
• Use of coenzymes, like NAD\(^+\) and FAD, which shuttle electrons
• Compartmentalization within cell organelles for optimal reaction conditions

Biological systems are expertly designed to harness the energy from redox reactions, ensuring living organisms maintain the energy balance necessary for survival and reproduction.