Since the functions of the mitochondria, chloroplast and peroxisomes are not coordinated with the organelles of endomembrane system, these are not considered as part of the endomembrane system.
Mitochondria (sing.: mitochondrion), unless specifically stained, are not easily visible under the microscope. The number of mitochondria per cell is variable depending on the physiological activity of the cells. In terms of shape and size also, considerable degree of variability is observed. The two membranes have their own specific enzymes associated with the mitochondrial function.