Final answer:
In apoptosis, the mitochondrion releases cytochrome C to initiate a cascade of events that activate caspases, which ultimately cause cell death.
Step-by-step explanation:
In the intrinsic mechanism of apoptosis, the organelle responsible for causing programmed cell death is the mitochondrion. This process begins when pro-apoptotic factors like Bax and Bak create channels in the outer mitochondrial membrane, allowing the release of cytochrome C into the cytosol.
Once in the cytosol, cytochrome C binds to adaptor proteins, which then aggregate and recruit procaspase. This binding induces a conformational change in procaspase, activating caspases which are proteolytic enzymes that start the cellular digestion or auto-digestion process, leading to the cell's demise.