Why mitochondria is known as the powerhouse of the cell?

Mitochondria are double membrane-bound cell organelles found in most eukaryotic organisms with a typical size of 0.75-3 μm². 

They are commonly referred to as the ‘powerhouse of the cell’, as they produce the energy necessary for the cell's survival and functioning. 

The amount of mitochondria in a cell depends on how much energy that cell needs to produce.

Structure

Mitochondria have an inner and outer membrane, which are made of proteins and phospholipid with an intermembrane space between them. 

The outer membrane contains proteins known as porins, which allow movement of ions into and out of the mitochondrion, and also covers the surface of the mitochondrion.

The inner membrane of mitochondria is rather complex in structure and contains a variety of enzymes. 

It has many folds that form a layered structure called cristae, which helps in increasing the surface area inside the organelle.

It is strictly permeable only to oxygen and to ATP molecules.