Two Kinds of Cell — agentic bullshittery

Light microscopeno stain

Light cannot show anything finer than 0.2 µm. At this magnification there is only blur. Try the electron microscope.

Bench notes

The same comparison in numbers, and the places where the tidy two-way split stops being tidy.

Side by side

	Prokaryote	Eukaryote
Usual size	1 to 5 µm	10 to 100 µm
DNA	Usually one loop, lying loose in the cell	Several straight chromosomes, shut inside a nucleus
How much DNA	E. coli: 4.6 million letters, 1.6 mm	A human body cell: about 6.2 billion letters, 2 m
Rooms	One	Many: nucleus, endoplasmic reticulum, Golgi, lysosomes, mitochondria, and in plants chloroplasts and a vacuole
Ribosomes	About 20 nm across; tens of thousands	About 25 to 30 nm across; millions. The small kind again inside mitochondria and chloroplasts
Skeleton	A few simple fibres	Girders, rails and motors; the cell can crawl and swallow
Copying itself	Splits in two, as fast as every 20 minutes	Copies and sorts its chromosomes first; about a day for a human cell grown in a dish
Lives as	Single cells, sometimes chains or mats	Single cells, or bodies of trillions
Who	Bacteria and archaea	Animals, plants, fungi, algae, amoebas and their relatives
On Earth for	At least 3.5 billion years	At least 1.6 billion years by fossils; probably nearer 2 billion

Prokaryote

Eukaryote

Usual size

1 to 5 µm

10 to 100 µm

DNA

Usually one loop, lying loose in the cell

Several straight chromosomes, shut inside a nucleus

How much DNA

E. coli: 4.6 million letters, 1.6 mm

A human body cell: about 6.2 billion letters, 2 m

Rooms

One

Many: nucleus, endoplasmic reticulum, Golgi, lysosomes, mitochondria, and in plants chloroplasts and a vacuole

Ribosomes

About 20 nm across; tens of thousands

About 25 to 30 nm across; millions. The small kind again inside mitochondria and chloroplasts

Skeleton

A few simple fibres

Girders, rails and motors; the cell can crawl and swallow

Copying itself

Splits in two, as fast as every 20 minutes

Copies and sorts its chromosomes first; about a day for a human cell grown in a dish

Lives as

Single cells, sometimes chains or mats

Single cells, or bodies of trillions

Who

Bacteria and archaea

Animals, plants, fungi, algae, amoebas and their relatives

On Earth for

At least 3.5 billion years

At least 1.6 billion years by fossils; probably nearer 2 billion

How big

Each step along this ruler is ten times the one before. The two dashed lines are where the eye and the light microscope give up.

prokaryote, or a part of any celleukaryote

One tree, two mergers

Life has three great branches, and two of them are prokaryotes. Eukaryotes grew out of the archaea, and took in bacteria twice.

Not so tidy

Most of your cells have no nucleus. Red blood cells throw theirs out, and their mitochondria, as they mature, to make room for the protein that carries oxygen. By one careful count they are about 25 of the roughly 30 trillion cells in a body. They are still eukaryotic cells: they were built by the eukaryote plan and then stripped.

You are half bacteria, by count. The same study put the bacteria living in and on one person at about 38 trillion, nearly all of them in the gut. They are so small that together they weigh about 200 grams.

A bacterium you can pick up with tweezers. Thiomargarita magnifica, described in 2022 from sunken mangrove leaves in Guadeloupe, is a single cell about a centimetre long. It keeps its DNA in small membrane bags, which is not supposed to be a prokaryote habit.

“Prokaryote” is a floor plan, not a family. Archaea look like bacteria down a microscope. Their genes and chemistry are so different that in 1977 Carl Woese and George Fox split them off as a separate branch of life, and they are closer relatives of yours than bacteria are.

What is simplified

No one slide can be stained blue, dyed with three glowing dyes and sliced for an electron microscope. These are three preparations, drawn as if they were one scene. The electron views are drawings in the manner of micrographs, not photographs.

The bacterium on the tour is drawn to the measurements of E. coli, which lives in the gut. The bacteria really found on a cheek cell are other species, mostly round ones.

A cell from the surface of the cheek is flat and near the end of its life, with fewer working parts than this. The inside shown is a general animal cell.

A real slice 70 nm thick would rarely pass through flagella, a plasmid and the middle of the DNA at once. Bacteria lying on top of the cheek cell are left out of the slice, because a slice through the cell would miss them.

There are far more ribosomes than could be drawn in the big cell. Mitochondria are drawn as separate bodies; in a living cell they join into branching threads and split apart again all the time.

The glowing views are blurred to the same 0.2 µm limit as ordinary light. Newer “super-resolution” fluorescence microscopes beat that limit several times over.

The chloroplasts circle at a speed chosen to look right, not a measured one, and a real pondweed leaf is two layers of cells thick.

The magnification shown assumes a screen with 96 dots to the inch.

Sources

R. Milo and R. Phillips, Cell Biology by the Numbers (Garland Science, 2015), book.bionumbers.org: cell sizes and volumes, ribosome counts and sizes, DNA lengths, division times.

B. Alberts and others, Molecular Biology of the Cell, 6th edition (2015): cell structure; the limits of light and electron microscopes.

F. R. Blattner and others, “The complete genome sequence of Escherichia coli K-12”, Science 277 (1997). doi:10.1126/science.277.5331.1453

H. C. Berg, “The rotary motor of bacterial flagella”, Annual Review of Biochemistry 72 (2003). doi:10.1146/annurev.biochem.72.121801.161737

S. Anderson and others, “Sequence and organization of the human mitochondrial genome”, Nature 290 (1981). doi:10.1038/290457a0

L. Sagan (Lynn Margulis), “On the origin of mitosing cells”, Journal of Theoretical Biology 14 (1967). doi:10.1016/0022-5193(67)90079-3

C. R. Woese and G. E. Fox, “Phylogenetic structure of the prokaryotic domain: the primary kingdoms”, PNAS 74 (1977). doi:10.1073/pnas.74.11.5088

A. Spang and others, “Complex archaea that bridge the gap between prokaryotes and eukaryotes”, Nature 521 (2015). doi:10.1038/nature14447

H. Imachi and others, “Isolation of an archaeon at the prokaryote–eukaryote interface”, Nature 577 (2020). doi:10.1038/s41586-019-1916-6

H. C. Betts and others, “Integrated genomic and fossil evidence illuminates life’s early evolution and eukaryote origin”, Nature Ecology & Evolution 2 (2018). doi:10.1038/s41559-018-0644-x

R. Sender, S. Fuchs and R. Milo, “Revised estimates for the number of human and bacteria cells in the body”, PLoS Biology 14 (2016). doi:10.1371/journal.pbio.1002533

Y. M. Bar-On, R. Phillips and R. Milo, “The biomass distribution on Earth”, PNAS 115 (2018). doi:10.1073/pnas.1711842115

J.-M. Volland and others, “A centimeter-long bacterium with DNA contained in metabolically active, membrane-bound organelles”, Science 376 (2022). doi:10.1126/science.abb3634

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