Question

In which of the following reactions is the reactant oxidized? (A) \(\mathrm{FAD} \rightarrow \mathrm{FADH}_{2}\) (B) \(\mathrm{NAD}^{+} \rightarrow \mathrm{NADH}\) (C) \(\mathrm{NADPH} \rightarrow \mathrm{NADP}^{+}\) (D) \(\mathrm{ADP} \rightarrow \mathrm{ATP}\)

Short answer

Reaction (C), \(\text{NADPH} \rightarrow \text{NADP}^+\), involves the reactant being oxidized.

Step-by-step solution

Understand Oxidation and Reduction

Oxidation is the loss of electrons, while reduction is the gain of electrons. In chemical notation, when a reactant loses hydrogen atoms or gains oxygen atoms, it is typically oxidized.

Analyze Reaction (A)

In the reaction \(\text{FAD} \rightarrow \text{FADH}_2\), FAD is gaining hydrogen atoms, meaning it is gaining electrons, which indicates a reduction process.

Analyze Reaction (B)

In the reaction \(\text{NAD}^+ \rightarrow \text{NADH}\), NAD^+ is gaining hydrogen atoms, which means it is gaining electrons, indicating a reduction process.

Analyze Reaction (C)

In the reaction \(\text{NADPH} \rightarrow \text{NADP}^+\), NADPH is losing hydrogen atoms, meaning it is losing electrons, which indicates an oxidation process.

Analyze Reaction (D)

In the reaction \(\text{ADP} \rightarrow \text{ATP}\), ADP is gaining a phosphate group and energy, which does not directly imply oxidation or reduction but rather phosphorylation.

Identify the Oxidized Reactant

Among all given reactions, reaction (C) \(\text{NADPH} \rightarrow \text{NADP}^+\) involves the loss of hydrogen atoms (and electrons), indicating that NADPH is oxidized.

Key concepts

oxidation

Oxidation, in biochemical terms, involves the loss of electrons from a molecule. You can often identify oxidation by spotting a loss of hydrogen atoms or a gain of oxygen atoms. Think about oxidation as the process that makes a molecule more positive (losing negatively charged electrons). It's essential in energy production and metabolism. For example, when glucose is oxidized in the body, it releases energy that cells can use.
The overall idea is:

• Oxidation = Loss of electrons
• Oxidation = Loss of hydrogen atoms
• Oxidation = Gain of oxygen atoms

Recognizing these changes will help you identify oxidation in chemical reactions.

reduction

Reduction, the opposite of oxidation, is when a molecule gains electrons. When you see a molecule gain hydrogen atoms or lose oxygen atoms, it's getting reduced. Reduction makes a molecule more negative because it's gaining the negatively charged electrons.
Think of reduction as filling up an empty space with electrons (or hydrogen). Reduction is also key to energy storage in cells.

• Reduction = Gain of electrons
• Reduction = Gain of hydrogen atoms
• Reduction = Loss of oxygen atoms

By understanding reduction, you can better follow how cells manage energy in processes like photosynthesis and respiration.

NADPH

NADPH is a crucial coenzyme in living cells that assists in reduction-oxidation (redox) reactions. It stands for nicotinamide adenine dinucleotide phosphate. This molecule acts as an electron carrier, meaning it helps transfer electrons from one reaction to another. NADPH is particularly important in anabolic reactions, which build large molecules from smaller ones.
NADPH is mainly used in:

• Fatty acid synthesis, where it provides the necessary electrons for building fat molecules
• Aiding in the antioxidant defense by regenerating reduced glutathione
• Driving biosynthetic reactions in the cell

Knowing the role of NADPH helps you understand how cells maintain balance and build new components.

electron transfer

Electron transfer is a core concept in both chemistry and biochemistry. It refers to the movement of electrons from one molecule to another. This movement is key in processes like cellular respiration and photosynthesis. During electron transfer, one molecule (donor) loses electrons (gets oxidized), while another molecule (acceptor) gains electrons (gets reduced).

• Electron donor = Oxidized
• Electron acceptor = Reduced

These transfers are essential for producing ATP, the energy currency of the cell. Understanding electron transfer helps clarify how cells gain and use energy efficiently.

hydrogen atom loss

Loss of hydrogen atoms is often a sign of oxidation. When a molecule loses a hydrogen atom, it also loses an electron, because hydrogen consists of one proton and one electron. This concept helps to identify reactions where oxidation occurs. For instance, in the reaction \(\text{NADPH} \rightarrow \text{NADP}^+\), NADPH loses hydrogen atoms and gets oxidized.
In biochemical reactions, tracking the movement of hydrogen can reveal crucial details about the redox state of molecules involved. Hydrogen atom loss is an indicator of how molecules interact and change in metabolic pathways.