What is a cell?
A cell is the smallest unit of life, the basic building block of all living things. Some organisms, like bacteria, are a single cell, while humans are made of trillions.
Every cell is surrounded by a membrane that controls what enters and leaves. Inside is a jelly-like fluid called cytoplasm, where many chemical reactions take place. Cells contain genetic material, DNA, which carries instructions for how they work and reproduce.
There are two main types. Prokaryotic cells, like bacteria, are simple and have no nucleus. Eukaryotic cells, found in plants, animals and fungi, are larger and have a nucleus and other structures called organelles, such as mitochondria, which produce energy. Plant cells also have a cell wall and chloroplasts for photosynthesis.
Discovering cells
English scientist Robert Hooke first described cells in 1665 after looking at cork through a microscope. He named them after small rooms, or cells, in a monastery. In the 1830s, scientists proposed cell theory, which states that all living things are made of cells and that all cells come from existing cells.
Did You Know?
The largest single cell is the ostrich egg yolk, while some nerve cells in the human body can be about a meter long.