Question

The source of energy that directly drives the synthesis of ATP during oxidative phosphorylation is the a. oxidation of NADH. b. oxidation of glucose. c. oxidation of pyruvate. d. \(\mathrm{H}^{+}\) electrochemical gradient. e. reduction of \(\mathrm{O}_{2}\)

Short answer

The source of energy that directly drives the synthesis of ATP during oxidative phosphorylation is the \(\mathrm{H}^{+}\) electrochemical gradient.

Step-by-step solution

Understanding Oxidative Phosphorylation

Oxidative phosphorylation is a metabolic pathway that uses energy released by the oxidation of nutrients to produce ATP. The major driving force comes from the proton gradient across the inner mitochondrial membrane, created by the electron transport chain (ETC).

Analyzing The Given Options

We have options mentioning oxidation of NADH, glucose, and pyruvate, an \(\mathrm{H}^{+}\) electrochemical gradient, and the reduction of \(\mathrm{O}_{2}\). The oxidation of NADH takes place but the energy produced enters into the electron transport chain enhancing the proton gradient rather than directly converting into ATP. The oxidation of glucose and pyruvate also takes place at earlier stages of cellular respiration and contribute indirectly towards the proton gradient. The reduction of \(\mathrm{O}_{2}\) is a step in the electron transport chain but it is not the main driving force for ATP synthesis.

Selecting The Correct Answer

Given these options, the direct energy source driving ATP synthesis in oxidative phosphorylation is the \(\mathrm{H}^{+}\) electrochemical gradient, also known as proton motive force. The energy stored in this gradient due to the movement of \(\mathrm{H}^{+}\) ions across the membrane is utilized to convert ADP into ATP. So the correct answer is option (d).

Key concepts

ATP synthesis

ATP synthesis is a crucial process in cells that provides energy for various functions. It mainly occurs in the mitochondria during oxidative phosphorylation. Here, ATP, or adenosine triphosphate, is created from ADP (adenosine diphosphate) and inorganic phosphate. ATP is often referred to as the energy currency of the cell. Cells use ATP to perform work, such as muscle contraction or chemical synthesis.

• ADP + Pi → ATP

The process of ATP synthesis is powered by a proton gradient. This gradient is established by the electron transport chain, which results in the movement of protons across the mitochondrial membrane. The energy from this gradient is captured by ATP synthase, an enzyme that facilitates the addition of a phosphate group to ADP, thus creating ATP. This mechanism ensures the cells can store energy efficiently and use it when needed.

Electron transport chain

The electron transport chain (ETC) is a series of protein complexes and small molecules embedded in the inner mitochondrial membrane. Its primary function is to transfer electrons derived from nutrients like glucose in a series of redox reactions. When electrons pass through the ETC, their energy is used to pump protons from the mitochondrial matrix into the intermembrane space. This creates a higher concentration of protons outside the inner membrane, contributing to the proton gradient.

• Complex I receives electrons from NADH.
• Complex II receives electrons from FADH2.
• Both Complexes III and IV facilitate further electron transfer.

This electron flow concludes at oxygen, which acts as the final electron acceptor. This crucial step produces water and ensures the continuation of the ETC. The energy from these electron transfers is essential to maintaining the proton gradient that powers ATP synthesis.

Proton gradient

The proton gradient is a differential in proton concentration across the inner mitochondrial membrane. This gradient is established by the electron transport chain, which pumps protons from the mitochondrial matrix to the intermembrane space, creating an electrochemical gradient. The gradient consists of two components:

• A difference in concentration of protons (proton gradient).
• A charge difference across the membrane (electrical gradient).

Together, these components generate the proton motive force, which is crucial for ATP synthesis. Protons tend to flow back into the matrix through ATP synthase, a protein complex that uses the energy of this flow to synthesize ATP from ADP and inorganic phosphate. Without this gradient, ATP production would be severely limited, impacting the cell's ability to perform necessary functions.

Cellular respiration

Cellular respiration is a multi-step process that converts biochemical energy from nutrients into ATP, which cells can use for energy. It consists of several stages: glycolysis, the citric acid cycle, and oxidative phosphorylation.

• In glycolysis, glucose breaks down into pyruvate, yielding small amounts of ATP.
• The citric acid cycle processes pyruvate into electron carriers like NADH and FADH2.
• These carriers supply electrons to the electron transport chain in oxidative phosphorylation.

Oxidative phosphorylation is where most ATP is produced. The electron transport chain aids in creating a proton gradient, which is vital for ATP synthesis. Each stage of cellular respiration is interconnected. The efficiency of one stage affects the others as they strive to maximize ATP output from the energy originally stored in nutrients. Understanding cellular respiration is key to understanding how cells harness energy to function and maintain life.