Prokaryotes and eukaryotes

Prokaryotic cells are 0.1 to 5.0 µm across, while most eukaryotic cells are 10 to 100 µm.

Two big groups of cells

Wash your hands after touching a doorknob and you remove some of the most common living things on Earth: bacteria. Bacteria, together with a second group called Archaea, are prokaryotes. The name comes from Greek words meaning “before the nucleus”. Animals, plants, fungi and protists are eukaryotes, “true nucleus”. This is the deepest split in how cells are built.

All cells, of both kinds, share four parts. There is a plasma membrane that separates inside from outside. There is cytoplasm, a jelly-like cytosol holding everything else. There is DNA, the genetic material. And there are ribosomes that make proteins. Beyond those four, the two groups differ a lot.

Inside a bacterium

A prokaryote has no nucleus and no other organelles wrapped in membrane. Its DNA sits in a central region called the nucleoid, usually as a single circular chromosome, not closed off from the rest of the cell. Most bacteria have a cell wall made of peptidoglycan, a different material from the cellulose of plant walls. The wall protects the cell, keeps its shape and helps stop it drying out. Many bacteria also have a sticky capsule outside the wall that lets them cling to surfaces.

Some prokaryotes have extra parts. A flagellum is a long whip that turns to push the cell along. Pili are used to pass genetic material from one bacterium to another by direct contact, a process called conjugation. Fimbriae are short hairs that help a bacterium attach to a host cell. Not every bacterium has all of these.

Why being small helps a prokaryote

Prokaryotes are tiny, 0.1 to 5.0 µm. At that size, molecules that come in through the membrane spread to every part of the cell by diffusion alone, and wastes get out just as fast. A eukaryotic cell, ten or a hundred times wider, cannot rely on that. It divides the work among organelles and moves materials in vesicles along internal tracks. In the eukaryotic cell, a membrane wraps the nucleus, and the DNA is split into several linear chromosomes. Every species has its own number of chromosomes: 46 in humans and 8 in fruit flies.

Most microbes are not germs. A microbe is any living thing so small that you need a microscope to see it, and most bacteria do no harm at all. Some are useful: bacteria in your gut make vitamin K, which your body cannot make on its own, and in return they get shelter and a steady supply of food. Under a light microscope, bacteria such as Salmonella look like tiny stained dots. You can see that they are there and roughly what shape they have, but their insides stay hidden until an electron microscope is used.

Organelles that were once bacteria

Mitochondria and chloroplasts are odd organelles. Each has two membranes, its own DNA and its own ribosomes, and both are about the size of a bacterium. Strong evidence points to an explanation called endosymbiosis, “living inside”. Long ago, host cells took in bacteria that used oxygen and bacteria that photosynthesized, but did not digest them. Over many millions of years the guests became part of the host. The oxygen users became mitochondria and the photosynthesizers, related to today’s cyanobacteria, became chloroplasts.