The mitochondrial inner membrane is impermeable to ADP and ATP. These nucleotides must therefore be transported across the membrane by specific carrier proteins. Two such proteins have been identified: the adenine nucleotide translocator (ANT) and the ADP/ATP translocator (AAC).

The ANT is a homodimeric protein that transports ADP and ATP in a 1:1 exchange. The AAC is a heterotrimeric protein that transports ADP and ATP in a 4:1 exchange. Both proteins are driven by the proton gradient across the mitochondrial inner membrane.

When the proton gradient is high, the ANT transports ADP into the mitochondria and ATP out of the mitochondria. This helps to maintain the high ATP levels that are required for mitochondrial function. When the proton gradient is low, the AAC transports ATP into the mitochondria and ADP out of the mitochondria. This helps to prevent the mitochondria from becoming depleted of ATP.

The ANT and AAC are essential for the proper functioning of mitochondria. They play a critical role in maintaining the ATP levels that are required for mitochondrial function.