History of the Universe

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Prokaryotes are very small cells that have no membrane around their nucleus nor any organelles. Prokaryotes include Bacteria and Archaea. They are the simplest organisms on Earth, and also the commonest. Every part of your skin is smothered in them. Their spores fill the air we breathe. Every glass of water we drink, even good clean drinking water, contains millions of them.

Prokaryotes have only one cell each. They can be round (coccus), rodlike (bacillus), or curved (vibrio, spirillum, or spirochete).

Prokaryotes live almost everywhere on Earth, including the soil, water, organic matter, and the bodies of multicellular animals (eukaryotes). Some prokaryotes give benefits to humans and other plants and animals, while some are harmful; bacteria are the chief cause of infectious diseases in humans.

The next diagram shows some of the main prokaryotic features.

Structure of a bacterium image by Mariana Ruiz Villarreal

On the outside, most prokaryotes have a strong cell wall. Beneath this is a membrane which will allow oily molecules to pass but prevents electrically charged molecules, and free protons and electrons, from passing through. Beneath this is a watery space full of molecules and ribosomes, messenger nucleic acids and all the other things needed to keep life going, all mixed up in a mostly chaotic way. There are some organelles within the cell, which function as storage compartments, but these do not have a membrane surrounding them as they do in eukaryotes. Instead they are surrounded by a protein coat.

At the center of the prokaryote is the nucleoid or nuclear region. This contains the DNA of the prokaryote, which is a long strand with its ends fixed together. Note that unlike the eukaryotes, the nucleoid of a prokaryote is not separated from the rest of the cell by a membrane.

The prokaryote may also contain shorter sequences of DNA called plasmids. These can move from one cell to another and carry information how to make proteins which inactivate antibiotics, for example.

At one end of the prokaryote there may be a wonderful little motor with a long stiff strand attached to it on the outside of the cell. As the motor rotates this strand is twirled around and pushes the cell along. So some prokaryotes can swim! The rod is called a flagellum or ‘whip'.

In times of hardship, prokaryotes can shut down most of their life processes and wrap themselves up in a very hard outer coating.

Bacteria on Soccearth

How large is a bacterium?

On Soccearth (the result of inflating a soccer ball to the size of the Earth) a typical bacterium is about 50 meters long and 25 meters across, the size of this hot air balloon for example.

A bacterium is this size on Soccearth

A bacterium's outer cell wall would be about 1 meter thick on Soccearth, and the oily membrane inside it would be around 15 centimeters thick.

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