Scientists — and people in general — love to divide things into categories. In some ways, life on Earth has done the same. Right now, scientists can divide cells into major categories — prokaryotes (or procaryotes; both spellings are okay) and eukaryotes.
Prokaryotes (PRO-kaer-ee-oats) are individualists. These organisms are small and single-celled. They might form into loose clumps of cells. But prokaryotes will never come together to take on different jobs within a single organism, such as a liver cell or a brain cell.
Eukaryotic cells are generally bigger — up to 10 times bigger, on average, than prokaryotes. Their cells also hold much more DNA than prokaryotic cells do. To hold up that big cell, eukaryotes have a cytoskeleton (Sy-toh-SKEL-eh-tun). Made from a network of protein threads, it forms a scaffold inside the cell to give it strength and help it move.
Keeping it simple
Prokaryotes make up two of the three big domains of life — those super kingdoms that scientists use to organize all living things. The domains of bacteria and archaea (Ar-KEY-uh) consist of prokaryotes only.
These single cells are small, and usually round or rod shaped. They might have one or more flagella (Fla-JEL-uh) — powered tails — hanging off the outside to move around. Prokaryotes often (but not always) have a cell wall for protection.
Inside, these cells throw together all they need to survive. But prokaryotes aren’t very organized. They let all their cell parts hang out together. Their DNA — the instruction manuals that tell these cells how to build everything they need — just floats around in the cells.
But don’t let the mess fool you. Prokaryotes are masterful survivors. Bacteria and archaea have learned to make meals of everything from sugars and sulfur, to gasoline and iron. They can get energy from sunlight or the chemicals spewed from deep-sea vents. Archaea in particular love extreme environments. They can be found in high-salt springs, rock crystals in caves or the acidic stomachs of other organisms. That means that prokaryotes are found on and in most places on Earth — including within our own bodies.
Eukaryotes keep it organized
Eukaryotes are the third domain of life. Animals, plants and fungi all fall under this umbrella, along with many other single-celled organisms, such as yeast. Prokaryotes might be able to eat almost anything, but these eukaryotes have other advantages.
These cells keep themselves tidy and organized. Eukaryotes tightly fold and pack their DNA into a nucleus — a pouch inside each cell. The cells have other pouches, too, called organelles. These neatly manage other cell functions. For example, one organelle is in charge of protein-making. Another disposes of trash.
Eukaryotic cells probably evolved from bacteria, and started out as hunters. They scooted around engulfing other, smaller cells. But some of those smaller cells didn’t get digested after they were eaten. Instead, they stuck around inside their larger host. These smaller cells now perform essential functions in eukaryotic cells.
Mitochondria (My-toh-KON-dree-uh) may have been one of these early victims. They now generate energy for eukaryotic cells. Chloroplasts (KLOR-oh-plasts) may have been another small prokaryote “eaten” by a eukaryote. These now hang out converting sunlight into energy inside plants and algae.
While some eukaryote are loners — like yeast cells or protists — others enjoy teamwork. They may band together into large conglomerations. These communities of cells often have the same DNA in each of their cells. Some of these cells, however, may use that DNA in different ways to perform special functions. One type of cell might control communication. Another might work on reproduction or digestion. The cell group then works as a team to pass on the organism’s DNA. These communities of cells evolved to become what are now known as plants, fungi and animals — including us.