What are the numerous small structures that function like organs in a cell?

They are called the organelles of the cell like the ribosomes, lysosomes, mitochondria, golgi body, endoplasmic reticulum; smooth and rough, nucleus, cytoskeleton, vacuoles and etc.

The different structures found within the cell is called the organelles and are assigned to perform specific function in the specific cell.  It is also said that organelles maintain the life of the cell. The organelles of a single cell like the organs of a multicellular organism is very essential for the survival of the body. The presence of organelles in both unicellular and multi-cellular organisms are very essential in orde to maintain life and carry on with its individual functions.