In cell biology, an organelle is a structure found within a cell that performs a particular specialized function. Organelles behave as the organs of the cell, and are found floating in a fluid called cytoplasm that is contained within a cell.
The nucleus of a cell, the mitochondria and the endoplasmic reticulum are all examples of organelles. Each of these internal structures of the cell plays an important role in the health and proper functioning of the cell. Like the outer layer of the cell itself, the cell's organelles are protected by lipid membranes, which can be seen through a microscope.