Question

The inner mitochondrial membrane contains a transporter for A. NADH. B. acetyl CoA. C. GTP. D. ATP. E. NADPH.

Short answer

Answer: ATP

Step-by-step solution

Recall the role of the mitochondria

Mitochondria are known as the "powerhouses" of the cell and are responsible for producing ATP, the primary source of energy for cellular processes. The process of producing ATP occurs during cellular respiration, including glycolysis, the citric acid cycle (Krebs cycle), and the oxidative phosphorylation (electron transport chain).

Identify key molecules in the process

During this process, some important molecules that participate and facilitate energy production include NADH, FADH2, and, of course, ATP.

Consider the role of the inner mitochondrial membrane

The inner mitochondrial membrane is the site where the oxidative phosphorylation, mainly the electron transport chain (ETC) and ATP synthesis, occurs. During this process, NADH and FADH2 are oxidized, and their electrons are transported through a series of protein complexes in the membrane.

Identify the molecule transported by the inner mitochondrial membrane

The primary role of the inner mitochondrial membrane is to facilitate energy production by the electron transport chain and ATP synthesis. Therefore, the answer would be D. ATP. The ATP molecule is transported across the inner mitochondrial membrane through a specific transporter called ATP/ADP translocase.

Key concepts

Mitochondrial ATP Production

The mitochondria are often dubbed as the 'powerhouses' of the cell, and this is largely because they are the main site of adenosine triphosphate (ATP) production. ATP is the fundamental currency of energy in biological systems, driving a multitude of cellular processes. Mitochondrial ATP production is primarily the result of cellular respiration, a multi-stage process including glycolysis in the cytosol, the citric acid cycle within the mitochondrial matrix, and finally, oxidative phosphorylation along the inner mitochondrial membrane.

The mitochondria are unique because they have two membranes, with the inner membrane playing a crucial role in ATP synthesis. This membrane is impermeable to most molecules, which is essential for establishing the proton gradient used by the ATP synthase enzyme to generate ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi). Specifically, the ATP/ADP translocase is the transporter that enables the exchange of ATP produced inside the mitochondria with ADP from the cell's cytosol, ensuring continuous ATP supply for the cell's needs.

Electron Transport Chain

The electron transport chain (ETC) is a series of protein complexes located in the inner mitochondrial membrane. Its main function is to facilitate the transfer of electrons from electron donors like NADH and FADH2 to electron acceptors such as oxygen. During this journey, the electrons travel through four complexes (I-IV), each playing a different, yet equally important, role in the transport process.

As electrons move through these complexes, protons (H+) are pumped from the mitochondrial matrix into the intermembrane space, creating an electrochemical gradient, known as the proton motive force. This gradient is the driving force behind ATP production. A key concept to understand is how the energy from electron transfer is converted to mechanical energy in the form of this gradient and then used to synthesize ATP.

Key Players of the ETC

• Complex I (NADH dehydrogenase): Accepts electrons from NADH originated from the citric acid cycle.
• Complex II (succinate dehydrogenase): Accepts electrons from FADH2.
• Complex III (cytochrome b-c1 complex): Transfers electrons to cytochrome c.
• Complex IV (cytochrome c oxidase): Completes the electron transfer to oxygen, forming water.

ATP synthesis is linked to this electron transfer process, and the concept of the ETC is vital for understanding how mitochondria harness energy from food sources to power cellular activities.

Oxidative Phosphorylation

Oxidative phosphorylation is the final stage of cellular respiration and takes place in the inner mitochondrial membrane. It is a two-part process involving the electron transport chain and chemiosmosis. The term 'oxidative' refers to the involvement of oxygen in the process as the final electron acceptor in the ETC, and 'phosphorylation' denotes the addition of a phosphate group to ADP to form ATP.

The essence of oxidative phosphorylation lies in ATP synthase's ability to use the established proton gradient as an energy source to catalyze the phosphorylation of ADP. ATP synthase is a complex enzyme that operates like a molecular turbine. As protons flow back into the mitochondrial matrix through ATP synthase, their movement drives the enzymatic activity that results in ATP production.

The Chemiosmotic Theory

• The proton motive force is pivotal because it provides the necessary energy for ATP synthesis.
• ATP synthase conducts protons down their gradient, coupling this exergonic flow to the endergonic production of ATP.

Understanding oxidative phosphorylation is essential, as it accounts for most of the ATP generated by cells. Any impairment in this process can lead to serious energy deficits and is implicated in various diseases, underlining the critical nature of this metabolic pathway.