Today, we'll look at the smallest types of living things, cells. The cell is the smallest unit of life as we know. Just as the atom is the smallest particle that retains the specific properties of an element, or a brick is the smallest unit of Legos that retains the specific properties of Legos, the cell is the smallest unit of life that retains the properties of a living thing. It can grow, metabolize food, adapt to environmental conditions, respond to stimuli, maintain homeostasis, and under certain conditions, it can reproduce. Some organisms like bacteria, amoeba, or plankton are unicellular, just a single cell all alone in the world. While others, like animals, plants, and fungi, are multicellular, a whole bunch of cells working together to get things done. Of course, this is biology, so there are always exceptions. There are plenty of single-celled organisms that live in colonies with millions of their closest friends, and there are others like Dictiostilium Discoedium that are sometimes single cells and sometimes part of a multicellular organism. Cells can be described as belonging to one of two categories, eukaryotic or pro-karyotic. The defining quality of the eukaryotic cell is the nucleus, basically a little pouch that they keep their DNA in. Eukaryotes have a skeleton, called a cytoskeleton, that gives them their shape and allows them to move. They have internal structures, called organelles, that help them to process food, excrete wastes, and respond to their environment. In fact, multicellular organisms, like animals and plants, are made up of trillions of eukaryotic cells, although there are still plenty of single-celled eukaryotes out there, just doing their thing. Pro-karyotes, on the other hand, like bacteria and archaea, tend to be much smaller than eukaryotes, and while they have cytoskeletons, they don't have a nucleus, or any other organelles. This means that they can replicate themselves a lot faster than eukaryotes, but it also means that they don't generally form multicellular organisms per se. Of course, this is biology, so there are always exceptions.