Cellular respiration primarily occurs in the mitochondria, not the nucleus, of both plant and animal cells.

Here's why:

* Mitochondria are the powerhouses of the cell: They are responsible for breaking down glucose (sugar) to produce ATP, the cell's energy currency.

* The nucleus is the control center: It contains the cell's DNA and directs cellular activities, but doesn't directly participate in energy production.

While the nucleus is crucial for cellular function, it's the mitochondria that play the starring role in cellular respiration.