Which organelle is described as the powerhouse of the cell?

Mitochondria earn the nickname because they run the final and most productive stage of energy release. Sugar broken down in the cell's fluid yields only a little usable energy; inside the mitochondrion, those products are oxidised completely using oxygen, and the energy is captured in ATP, the molecule cells spend on nearly everything they do. The organelle has two membranes, and the inner one is thrown into deep folds that pack in a huge surface area for the protein machinery that does this work. Mitochondria also carry a small circular genome of their own and reproduce by dividing, evidence for the widely accepted idea that they descend from free-living bacteria absorbed by an ancestral cell. In humans they are inherited almost entirely from the mother, since the egg supplies the cytoplasm, which makes mitochondrial DNA useful for tracing maternal ancestry. Cells with heavy energy demands, like heart muscle, contain thousands of them.

Plant cells contain chloroplasts as well, which have a similar bacterial origin story.