What is the mitochondria and what is its role within the cell?

The mitochondria is an organelle found in most cells of eukaryotic organisms, excluding some specific cells such as red blood cells. It has a double membrane and is variable in size (but usually between 0.75um and 3um in diameter). 

A mitochondrion has a very specific structure of an outer mitochondrial membrane, an intermembrane space, an inner mitochondrial membrane, the cristae and the matrix. The characteristic structure is essential for carrying out it's function.

The role of the mitochondria is to act as the 'powerhouse of the cell' it is responsible for generating ATP or Adenine Triphospate through the process of respiration. ATP is incredibly important for the cell because it is a form of chemical energy which is used as 'universal currency' within the cell to power countless reactions and processes such as the Na+/K+ pump. 

Other important functions which the mitochondria perform include maintaining cell growth, controlling the cell cycle and cell differentiation.