There’s a range of complexity in life on earth. You can see an amoeba, a complete organism that consists of just one cell, under a microscope. Or you can look in a mirror and see a human being, made up of trillions of cells working together.
In both the amoeba and the human, the cell is a complex, functioning structure, with parts and chemical processes that define what the organism is and does. In molecular biology, you’ll study the cell and gain an understanding of how it works.