Question

The following series of questions deal with the transport of molecules during cellular respiration.

a. Pyruvate enters the mitochondrion through a passive transporter via facilitated diffusion. How does the cell ensure the transport or pyruvate into the mitochondrion?

b. By what mechanism does CO₂ leave the mitochondrion? How do you know?

c. By what mechanism does ATP leave the mitochondrion?

Answer 1

Answer a: Pyruvate is a critical intermediate that can be used in a variety of anabolic and catabolic processes. Most of pyruvate is metabolized in mitochondria. Mitochondria has two membranes outer membrane and inner membrane. While mitochondrial outer membrane has many protein-based pores that are big enough to allow the passage of ions and molecules such as pyruvate that enters the mitochondrion through a passive transporter via facilitated diffusion, the mitochondrial enzymes that metabolize pyruvate, however, are physically separated from cytosol by impermeable inner mitochondrial membrane (IMM). Thus, the mitochondrial enzymes rely on a membrane transport system to shuttle pyruvate across IMM. Current understanding is that two mitochondrial pyruvate carriers (MPCs) proteins, MPC1 and MPC2, form a hetero-oligomeric complex in the inner mitochondrial membrane to facilitate pyruvate transport, however, the mechanism by which MPCs carry pyruvate across the membrane is unclear.

Answer b: The evidence regarding the transport of CO2 from mitochondria is stil obscure. However, since solubility of CO2 in both water and lipid are more than an order of magnitude greater than those of O2, it is plausible that CO2 is transported across cellular as well as mitochondrial membrane via different mechanisms from O2. More recently, different aquaporins (APQs) with varied affinities such as AQP1, AQP4,AQP5 and APQ9  have been implicated as responsible for transport of carbondioxide out of mitochondria. Having said that, with the limited understanding on the mitochondrial gas channels, it would be a long way to go in complete understanding of the pathway.

Answer c:A monomeric protein known as mitochondria ADP/ATP carrier fascilates the transport of ADT?ATP in anout of mitochondria. It has all of the functional elements of a transporter; a single central substrate binding site and two networks on either side of the membrane that provide alternating access to the binding site. The tranport of ATP outside of mitochondria occur via binding to membrane using salt brdge leading to dissolution of matrix and faciltation of movement of ADP/ATP outside. The salt bridges act by lowering the energy barrier for disruption and formation of these network matrices and opening and closing the carrier to either side of the membrane in an alternating way.