Final answer:
Autophagy is the mechanism used for degrading obsolete parts of a cell, such as a defective mitochondrion. Lysosomes play a key role in this process by containing enzymes that break down and digest the damaged organelles.
Step-by-step explanation:
The mechanism used for degrading obsolete parts of a cell, such as a defective mitochondrion, is called autophagy. Autophagy is a process in which a cell digests its own structures, including damaged organelles. Lysosomes, which contain enzymes that break down cellular components, play a key role in autophagy. When a defective mitochondrion is detected, it is engulfed by a vesicle that fuses with a lysosome, where it is broken down and digested.