The People's Share

GED Life Science, room two

Two Kinds of Cells

Prokaryotes and eukaryotes, the three domains of life, and a plant cell beside an animal cell.

Two kinds of cells

Every living thing is made of cells, but cells come in two kinds, and the difference comes down to one question: does the cell keep its DNA inside a room of its own?

A eukaryotic cell does. Its DNA sits inside a nucleus, a room with a double wall, and the cell has other walled rooms too: mitochondria, the ER, the Golgi body. Rooms with walls of membrane are called membrane-bound organelles. A prokaryotic cell has no nucleus. Its DNA floats in the cytoplasm, in a region called the nucleoid, and it has no walled rooms at all; the only working parts inside it are ribosomes. Bacteria are prokaryotes. So are archaea. Everything else, from yeast to you, is made of eukaryotic cells.

The names tell the story. Karyon is Greek for kernel: the nucleus. Pro means before, eu means true. A prokaryote is a cell from before the nucleus; a eukaryote has a true one. The earliest fossils of cells are prokaryotes, from about 3.5 billion years ago. Eukaryotes came about two billion years later.
A few parts are named here before room three teaches them one by one. For now: cytoplasm is the jelly that fills a cell; ribosomes are tiny machines that build proteins; the ER and the Golgi body are folded membranes that finish proteins and send them where they are needed.
prokaryote (a bacterium) eukaryote (an animal cell) flagellum (a tail) capsule (some have one) nucleoid: DNA, loose ribosomes: the dots cell wall, then membrane inside nucleus ER Golgi mito. lysosome
Tap a feature above to compare the two cells on it.

The three domains of life

Biologists sort every living thing into three domains, the largest groups there are. Two of the three are prokaryotes. For a long time all prokaryotes were simply called bacteria. Then, in the 1970s, a scientist named Carl Woese compared the genetic instructions of microbes and found that some of the "bacteria" were as different from the rest as they were from us. He gave them their own domain and named them archaea, from a Greek word for ancient. Under a microscope an archaean looks like a bacterium. Its chemistry and its genes say otherwise: it is closer to you.

all living things share one ancestor, at the root Bacteria Archaea Eukarya animals plants fungi protists prokaryotes, no nucleus: Bacteria and Archaea
Tap a branch, or a name, to read about that domain.

A plant cell beside an animal cell

Plant cells and animal cells are both eukaryotic. Both have a nucleus, mitochondria, ribosomes, an ER, a Golgi body, and a cell membrane. Plant cells have three things more: a stiff wall, green chloroplasts, and one big vacuole that fills most of the cell. Animal cells have a few small things that plant cells mostly lack. Tap any part.

animal cell plant cell nucleus lysosome small vacuoles central vacuole (water) cell wall, membrane just inside chloroplasts
Tap a part on either cell, or a name above.