What is a cell?

The cell is the basic building block of all living organisms. Some life, like bacteria, consists of a single cell, while a human body contains an estimated 37 trillion of them. Each cell is enclosed by a membrane and carries the machinery to use energy, respond to its surroundings, and often reproduce. Complex cells store their genetic material in a nucleus, while simpler cells do not. The cell was first observed by Robert Hooke in 1665, who named it after monastery rooms.