Cell

A cell is the smallest unit of life. Every living thing is made of cells, from a tiny bacterium to a giant blue whale. Some living things are just one cell. Others, like humans, are made of trillions of cells working together. An adult human body holds around 37 trillion cells, more than the number of stars in the Milky Way galaxy.

Most cells are far too small to see without help. A typical human cell is about 20 micrometers wide. You could line up 50 of them across the width of a single human hair. People did not even know cells existed until the microscope was invented. In 1665, an English scientist named Robert Hooke looked at a piece of cork through a microscope. He saw tiny boxes that reminded him of the small rooms where monks lived, called cells. The name stuck.

Cells come in two main types. Simple cells, like bacteria, are called prokaryotes. They do not have a separate control center inside. Complex cells, called eukaryotes, do. Every cell in a plant, animal, fungus, or human is a eukaryote.

Each cell is like a tiny factory with many parts. The outside is wrapped in a thin skin called the cell membrane. It decides what gets in and what gets out. Inside, a jelly called cytoplasm fills the space. Floating in the jelly are small parts called organelles, each with its own job. The nucleus is the control center. It holds the cell's DNA, the instruction book for everything the cell does. Mitochondria turn food into energy the cell can use. Plant cells also have chloroplasts, which catch sunlight to make food through photosynthesis.

Cells are not all the same, even inside one body. A nerve cell in your leg can be three feet long and sends signals like a living wire. A red blood cell is shaped like a tiny disc and carries oxygen. A muscle cell stretches and pulls to move your body. Each type is shaped for its job.

Cells make more cells by splitting in two. One cell copies its DNA, then divides down the middle. This is how a single fertilized egg grows into a baby, and how a cut on your knee heals. Scientists are still learning how cells work. Questions like how cells age, and why some turn into cancer, are among the biggest mysteries in biology today.