(N/A) $\Rightarrow$ Mitochondria are stained specifically to be observed under a microscope.
$\Rightarrow$ The number of mitochondria per cell is variable,depending on the physiological activity of the cells.
$\Rightarrow$ In terms of shape and size also,a considerable degree of variability is observed.
$\Rightarrow$ Typically,it is sausage-shaped or cylindrical,having a diameter of $0.2-1.0 \mu m$ (average $0.5 \mu m$) and a length of $1.0-4.1 \mu m$.
$\Rightarrow$ Each mitochondrion is a double membrane-bound structure,with the outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments,i.e.,the outer compartment and the inner compartment.
$\Rightarrow$ The inner compartment is called the matrix. The inner membrane forms a number of infoldings called cristae towards the matrix. The cristae increase the surface area.
$\Rightarrow$ The two membranes have their own specific enzymes associated with the mitochondrial function.
$\Rightarrow$ Mitochondria are the sites of aerobic respiration. They produce cellular energy in the form of $ATP$,hence they are called the 'powerhouses' of the cell.
$\Rightarrow$ The matrix also possesses a single circular $DNA$ molecule,a few $RNA$ molecules,ribosomes $(70S)$,and the components required for the synthesis of proteins.
$\Rightarrow$ The mitochondria divide by fission.