Test: Oxidative Phosphorylation - 1 - MCAT MCQ

NADH (nicotinamide adenine dinucleotide) and FADH₂ (flavin adenine dinucleotide) are the products of glucose oxidation that are essential for oxidative phosphorylation. During the breakdown of glucose through processes like glycolysis, pyruvate oxidation, and the citric acid cycle, NADH and FADH₂ are produced as high-energy electron carriers.

These electron carriers then donate their electrons to the electron transport chain, a series of protein complexes located in the inner mitochondrial membrane. As electrons are passed along the electron transport chain, a proton gradient is generated across the membrane.

This proton gradient drives the synthesis of ATP (adenosine triphosphate) through the action of ATP synthase. NADH donates its electrons at complex I of the electron transport chain, while FADH₂ donates its electrons at complex II. The flow of electrons through the electron transport chain results in the pumping of protons across the membrane, which creates the proton gradient necessary for ATP synthesis.

The increased levels of hydrogen ions (protons) in the intermembrane space of the mitochondria have a direct effect on ATP production during oxidative phosphorylation. As electrons flow through the electron transport chain in the inner mitochondrial membrane, protons are pumped from the mitochondrial matrix to the intermembrane space, creating a concentration gradient of protons.

This concentration gradient drives the process of chemiosmosis, where the protons flow back into the mitochondrial matrix through ATP synthase, an enzyme complex embedded in the inner mitochondrial membrane. The movement of protons through ATP synthase powers the synthesis of ATP from ADP (adenosine diphosphate) and inorganic phosphate.

Therefore, an increased concentration of hydrogen ions in the intermembrane space leads to a steeper proton gradient, which enhances the flow of protons through ATP synthase. This increased flow of protons results in an upregulation of ATP production, as more ATP molecules are generated from ADP and inorganic phosphate.

During the electron transport chain, protons (hydrogen ions) are pumped from the mitochondrial matrix to the intermembrane space. This process creates a higher concentration of protons in the intermembrane space, leading to an increase in acidity or a decrease in pH. The electron transport chain transfers electrons from electron carriers, such as NADH and FADH₂, to a series of protein complexes in the inner mitochondrial membrane. As electrons move through these complexes, protons are actively transported across the membrane, contributing to the buildup of protons in the intermembrane space.

Chemiosmosis, on the other hand, refers to the movement of protons back into the mitochondrial matrix through ATP synthase. This movement occurs due to the electrochemical gradient established by the electron transport chain. As protons flow through ATP synthase, ATP is synthesized. The transfer of protons from the intermembrane space to the matrix reduces the concentration of protons in the intermembrane space, leading to an increase in pH or a decrease in acidity.

When the mitochondrial membrane becomes more permeable to hydrogen ions, it disrupts the normal proton gradient across the membrane. As a result, there is a leakage of protons from the intermembrane space into the mitochondrial matrix. This leakage leads to a decrease in the proton gradient, which affects the function of ATP synthase and reduces ATP production.

Furthermore, the influx of protons into the mitochondrial matrix can also lead to an increase in the concentration of inorganic phosphate (Pi) in the matrix. This is because the leakage of protons can disrupt the electrochemical gradient that drives the import of phosphate into the matrix, resulting in an accumulation of inorganic phosphate.

Cyanide blocks the final step of the electron transport chain, which is the transfer of electrons to oxygen (O₂) by cytochrome C oxidase. By binding to the Fe-Cu center, cyanide prevents the reduction of oxygen, which is essential for the formation of water (H₂O). As a result, the electron transport chain is disrupted, and oxygen cannot be utilized in the process of oxidative phosphorylation to produce ATP.

This inhibition of oxygen reduction by cyanide leads to a severe impairment of cellular respiration and the inability to generate energy in the form of ATP. The lack of ATP production can have widespread consequences on various cellular functions and can ultimately be life-threatening.

NADH is a key electron carrier in the electron transport chain, and it plays a critical role in the generation of ATP through oxidative phosphorylation. NADH transfers electrons to the electron transport chain, which creates a proton gradient across the inner mitochondrial membrane. This proton gradient is then utilized by ATP synthase to produce ATP.

If the production of NADH is inhibited, there would be a reduced supply of electrons entering the electron transport chain. As a result, the generation of the proton gradient would be compromised, leading to a decrease in ATP production. Since NADH is a major contributor to the electron flow and proton gradient generation, its inhibition would have the greatest negative impact on ATP production during oxidative phosphorylation.

Cytochrome c is responsible for shuttling electrons between complex III and complex IV in the electron transport chain. It is a small protein that carries electrons from complex III to complex IV, facilitating the final step of ATP synthesis.

Cyanide inhibits oxidative phosphorylation by blocking the flow of electrons in the electron transport chain. It binds to complex IV, preventing the transfer of electrons to oxygen and ultimately halting ATP synthesis.

Complex IV, also known as cytochrome c oxidase, directly transfers electrons to oxygen. It is the final complex of the electron transport chain and facilitates the reduction of oxygen to water.

The energy required for ATP synthesis during oxidative phosphorylation is derived from the electron transport chain. The electron transport chain is responsible for the transfer of electrons from NADH and FADH2 to oxygen, which generates a proton gradient that drives ATP synthesis.