Seeing the word cell triggers so many memories. You may recall your last dropped call, solar panels, your current Fantasy Baseball spreadsheet, or your last school field trip to the local penitentiary. No? Anyway, onto real biology.
All living things (we like to call them organisms because no one wants to be called a "thing") have five characteristics in common. Cells are the basic units of life because they are the smallest entities on Earth to have all five of the attributes of Life.
(If you are totally confused right now, it probably means you need to go back to our handy dandy little intro unit. You might want to do that now, before reading on. Otherwise, on with regularly scheduled programming.)
Cells live and behave in a variety of ways. Some cells require the services of other cells to survive. The cells in your liver, and every other part of your body for that matter, require red blood cells to deliver oxygen. Without red blood cells, all the other cells in your body would die. Kaput. Lights out.
Other cells are able to grow and reproduce without the services of other cells. Many bacteria fall into this category. The entire bacterial "organism" is just a single cell. The human "organism," in contrast, is estimated to have between 10 and 100 trillion cells. You read that right: trillions of cells. Take that, US federal deficit. What’s more, some biologists estimate that there are 20 times more bacterial cells living inside the human body than there are actual human cells making up the body.1 We hope you aren't a germophobe, and if you are, this unit just might cure you.
There are two major types of cells:
While both types of cells have some attributes in common, we usually focus on the differences (glass-half-empty theory), like the fact that eukaryotic cells are generally larger and more complex than prokaryotic cells. The defining difference between the two cell types, however, is that eukaryotic cells have membrane-bound organelles (if you are thinking "mini organ," you are on the right track), including the nucleus. The name "eukaryote" means "true kernel," where "kernel" refers to the nucleus. "Prokaryote," on the other hand, means "before or without a kernel." In the end, it is the presence or absence of a membrane-bound nucleus that determines whether a cell is classified as prokaryotic or eukaryotic. Everything else is just details.
While you're here, we might as well give you the back story of the cell's discovery. A couple of geeks (a term of endearment around these parts), going by the names of Robert Hooke (in 1665) and Anton van Leeuwehoek (in 1673), were spending their days waiting around for the invention of DVR and YouTube. In an attempt to kill 300+ years of time, Bob used optical lenses to magnify the parts of a tree: the cork, the bark, the stems, the roots, and so on. He found these parts had little "boxes" in common that reminded him of the cells that the monks lived in. Hence, the name.
Also needing to occupy his hours, Anton ground the lenses further to produce more magnification (10x) so that he could observe the germs in pond water. Yep, you read that correctly. Someone wanted to study pond water. Biologists, what can you do…
These boxes were consistently observed in many living organisms over the next few years, and eventually, the cell theory was developed. With continued technological development and dedication to cell theory, scientists were able to decipher the inner workings of the cells. And so began a revolution that has fascinated scientists ever since. And…oh, you know…has saved hundreds of millions of lives every day through the practice of modern medicine.