Cell is the basic structural and functional unit of life. It is the smallest unit of life and is often called the building unit of life. Robert Hooke discovered cells in 1665 when he examined thin slices of cork under a microscope and saw tiny compartments that he named cells. Cells have several organelles that allow them to carry out essential functions like the cell membrane that controls what enters and leaves the cell, the nucleus that controls the cell, and mitochondria that provide energy. Organisms can be unicellular, consisting of a single cell, or multicellular, made of many cells like humans. Plant and animal cells differ in structures like plant cells having cell walls and chloroplasts while animal cells lack these.