What is a Cell?
A cell is the basic unit of all living things. Just as a house is made of bricks, every living organism--from blue whales to pine trees to Salmonella bacteria--is made up of one or more cells.
When we say cells are the basic unit of all living things, we mean that cells are the smallest biological unit that checks all the boxes to be considered alive. Cells can perform all the same basic functions of larger living organisms, such as taking in energy, growing and reproducing, replicating their genetic information, and responding to their environment.
Cells were first discovered and named by a man named Robert Hooke in 1665. Using an early version of the microscope, Hooke was able to see the outline of cell walls (a special structure found in plant cells) in a piece of cork. Hooke named the things he saw “cells” after the Greek word cella meaning “small room”.
In their most basic form, all cells consist of a cellular membrane enclosing a water-based fluid called cytoplasm. You can imagine this like a plastic bag full of water; the bag is the cellular membrane and the water is the cytoplasm.
All cells also contain genetic material like DNA, which gives the cell instructions on how to function, and ribosomes, which read and replicate the cell’s genetic information to make proteins.
The cells of bacteria and archaea, called prokaryotic cells, have their genetic material free-floating in the cytoplasm. The cells of animals, plants, and all other living things instead have their genetic material enclosed in a nucleus, and are called eukaryotic cells.
In addition to the nucleus, eukaryotic cells also have other membrane-bound structures called “organelles”. These organelles perform different jobs to help the cell function and stay alive, such as making proteins, generating energy, or transporting materials through the cell.
Cells from different types of organisms sometimes have different organelles and structures according to their different needs. For example, plant cells have special organelles called chloroplasts, which generate energy from light, and plant and fungi cells have rigid cells walls in addition to their cellular membranes.
We usually think of living things as having bodies made of many cells, but some consist of only a single cell. Single-celled organisms are called “unicellular” and organisms with multiple cells are called “multicellular”. All prokaryotes and some eukaryotes, like algae and protozoa, are unicellular.
Multicellular animals usually have many different types of cells to perform different functions within their bodies. Within the human body we have nerve cells (or neurons) with long, skinny branches for sending electrical messages, stretchy muscle cells for moving our body, donut-shaped red blood cells for carrying oxygen, and a huge variety of other cell types.
OpenClipart-Vectors. (artist). (2013). Cell Nucleus Cytoblast [digital art]. Retrieved from