These parts of cells provide energy for the cell.

The mitochondria are organelles within an animal cell that provide energy for the rest of the cell's functions through the creation of the energy molecule ATP by the chemical reactions occurring in the Krebs Cycle.  In plant cells, which do not contain mitochondria, the energy for cellular functions is created by the chloroplasts, which, like a mini solar cell, convert sunlight into useful energy by the creation of sugar molecules through the process of Photosynthesis.