Cell biology

Cell structure, the cell membrane, the cell cycle, and endosymbiotic theory

What is cell biology?

Cell biology studies the cell — the basic structural and functional unit of every living thing. Every organism, from the simplest bacterium to a human being, is built from one or more cells, and understanding how cells work is fundamental to understanding life itself.

Two basic types of cells

Cells come in two broad categories. Prokaryotic cells (like bacteria) are relatively simple and lack a nucleus or other membrane-bound internal compartments — their genetic material floats freely within the cell. Eukaryotic cells (making up plants, animals, fungi, and more) are more complex, with a distinct nucleus enclosing their DNA and various specialized internal compartments, called organelles, each handling a different task.

Key organelles and their jobs

Inside a eukaryotic cell, different organelles perform distinct roles: the nucleus stores and protects the cell's DNA; mitochondria (covered further in the Anatomy & Physiology article) generate usable energy for the cell; ribosomes build proteins following the instructions covered in the Molecular Biology article; and the cell membrane, a thin, selectively permeable barrier, controls what enters and leaves the cell.

How cells divide

Cells reproduce by dividing into two new cells, a process called cell division. Before dividing, a cell first copies its entire DNA, ensuring each resulting new cell receives a complete, identical set of genetic instructions. This process allows organisms to grow, repair damaged tissue, and, in single-celled organisms, reproduce entirely.

Why cell biology matters

Cell biology provides the foundation for understanding development, aging, and disease — including how cancer arises from cells that divide uncontrollably — and it underlies medical fields ranging from cell-based therapies to understanding how infections and immune responses actually work at the cellular level.