How many ribosomes are in a cell?
Counts for bacterial, yeast, plant and human cells, and how big a ribosome is
Matic Broz, PhD8 min
A fast-growing E. coli cell carries about 72,000 ribosomes, a budding yeast cell about 200,000, and a typical mammalian cell roughly 1 million to 10 million. No single number fits every cell, because the count follows cell size and how much protein the cell has to make. The same bacterium holds ten times fewer ribosomes when it grows slowly, and a direct whole-cell count in a human cell line, published in 2025, found more than 10 million.
Plant cells hold two kinds: most of their ribosomes sit in the cytoplasm, and roughly a quarter work inside chloroplasts. All of them are tiny, about 20 to 30 nanometres across, small enough that seeing them at all takes an electron microscope.
How many ribosomes are in a cell?
A fast-growing E. coli cell has about 72,000 ribosomes, a budding yeast cell has about 200,000, and a typical mammalian cell has roughly 1 million to 10 million.[1][2][3] The table adds the smallest bacteria, a directly counted human cell line and the best available plant measurement.
| Cell | Ribosomes | Conditions or method |
|---|---|---|
| Spiroplasma melliferum (small wall-less bacterium) | About 1,000 per cell | Direct count in imaged cells |
| E. coli, slow growth | About 6,800 per cell | Doubling time about 100 minutes |
| E. coli, fast growth | About 72,000 per cell | Doubling time 24 minutes |
| Budding yeast (Saccharomyces cerevisiae) | About 200,000 per cell | Average growing cell |
| Typical mammalian cell | 1 million to 10 million per cell | Review range across cell types |
| Human hTERT-RPE-1 cell line | 9.8 million to 18.7 million per cell | Electron tomography (9.8 to 15.6 million) and rRNA quantification (18.7 million) |
| Plant leaf (Arabidopsis rosette) | About 60 trillion per gram of fresh leaf | rRNA quantification; about one-quarter in chloroplasts |
Prokaryotic cells show how much the count depends on growth. E. coli growing slowly, with a doubling time near 100 minutes, has about 6,800 ribosomes per cell. At a 24-minute doubling time the count rises to about 72,000, and ribosomes account for more than one-third of the cell's dry mass.[1]
Smaller bacteria carry far fewer: researchers counted about 1,000 ribosomes in a cell of Spiroplasma melliferum, which is much smaller than E. coli.[1] A single "ribosomes per bacterial cell" figure is therefore incomplete without the species and growth rate.
Yeast sits between bacteria and animal cells. Reviews of protein synthesis in Saccharomyces cerevisiae place the average near 200,000 ribosomes per cell, enough for a rapidly dividing cell to make nearly 13,000 protein molecules every second.[2]
Animal and human cells span the widest range. A Nature review gives 1 million to 10 million ribosomes for a typical mammalian cell and notes that the fraction translating at any moment is unknown.[3]
El Hankouri and colleagues then counted them directly in a human retinal cell line, hTERT-RPE-1. Electron tomography found 5,500 to 8,750 ribosomes per cubic micrometre of cytoplasm, depending on how strictly particles were detected, which gave 9.8 million to 15.6 million ribosomes across the cell's roughly 1,800 cubic micrometres of cytoplasm. Measuring ribosomal RNA in extracts of the same cells gave about 18.7 million.[4]
Those counts sit at and above the top of the review range, and volume is the simplest explanation. At a similar density, a cell with twice the cytoplasm holds roughly twice the ribosomes, so a large cell will carry more than a small one. For a human cell, "several million to more than 10 million" is a more defensible answer than any single number.
Plant cells add a second population. In Arabidopsis rosettes, Piques and colleagues measured about 0.10 nanomoles of ribosomes per gram of fresh leaf, roughly 60 trillion ribosomes. Cytoplasmic ribosomes outnumbered chloroplast ribosomes about three to one and mitochondrial ribosomes about thirty to one, consistent with earlier reports that 26 to 36 percent of leaf ribosomes sit in chloroplasts.[5] The study did not convert these values to a per-cell count, and leaf cells vary too much in size for one number to describe them all.
How big is a ribosome?
A ribosome is about 20 to 30 nanometres across.[4] The bacterial 70S ribosome of E. coli measures about 21 nanometres and is roughly two-thirds RNA and one-third protein by mass.[6] About 50 of them lined up side by side would span a single micrometre.
The "S" values name ribosomes by how fast they sediment in a centrifuge, measured in svedberg units. These do not add up arithmetically: a bacterial ribosome is built from a 30S and a 50S subunit yet sediments as 70S.[6] Eukaryotic ribosomes in the cytoplasm of yeast, plant and animal cells are larger 80S particles made of 40S and 60S subunits. Mass rises from about 2.3 megadaltons for a bacterial ribosome to 3.3 for yeast and 4.3 for human and other higher eukaryotes.[7] One dalton is roughly the mass of a hydrogen atom.
| Ribosome | Subunits | Mass | Found in |
|---|---|---|---|
| Bacterial 70S | 30S and 50S | About 2.3 MDa | Bacteria |
| Lower eukaryotic 80S | 40S and 60S | About 3.3 MDa | Cytoplasm of yeast |
| Higher eukaryotic 80S | 40S and 60S | About 4.3 MDa | Cytoplasm of human and other animal cells |
Size explains why ribosome counts matter to the cell's budget. Each extra ribosome is a large assembly of ribosomal RNA and dozens of proteins, so a fast-growing bacterium that devotes a third of its dry mass to them is making a real investment in speed.
How fast do ribosomes make proteins?
Ribosomes add roughly 10 to 20 amino acids per second in growing bacteria and about 5 to 10 amino acids per second in yeast and mammalian cells.[1][3][8][9]
In E. coli, translation runs near 20 amino acids per second during fast growth and closer to 10 during slow growth.[1] Ribosome-profiling work in yeast uses an average near 6 amino acids per second, while the U.S. National Institute of General Medical Sciences gives about 10 amino acids per second for a human ribosome.[8][9]
Protein length converts those rates into time. A typical bacterial protein of about 267 amino acids can be elongated in roughly 15 to 27 seconds. A typical human protein of about 375 amino acids takes roughly 40 seconds at 10 amino acids per second, or about one minute after allowing for initiation and termination.[8][10]
For other proteins, dividing the amino-acid length of a translated coding sequence by the elongation rate gives a baseline, though codon choice (which codon optimization adjusts), mRNA structure, nutrient supply, pauses and cellular stress all change the real translation time.
How many proteins does one ribosome make?
One active human ribosome usually completes an average-sized protein in about one minute, but there is no fixed lifetime total for one ribosome.[8]
The output depends on protein length and elongation speed. In one published mammalian-cell model, a ribosome translating a 500-amino-acid protein at 5 amino acids per second finishes one chain every 100 seconds, or 0.6 protein molecules per minute while active.[3]
That review also illustrates the scale at cell level. If a cell has 5 million ribosomes, half are active, and each makes a 500-amino-acid protein at 5 amino acids per second, the cell produces 1.5 million protein molecules per minute.[3] The result is a calculation built from explicit assumptions, not a measured constant for every human cell. It counts protein molecules, not the number of distinct proteins in the proteome.
How many ribosomes are in a polysome?
A polysome contains at least two ribosomes translating the same mRNA, but its count can range from a few ribosomes to several dozen.[11]
The number is constrained by the length of the coding sequence and how often new ribosomes initiate translation. In one highly translated bacterial mRNA, researchers measured a packing density of 1.3 ribosomes per 100 nucleotides.[12] Other mRNAs are less densely packed because initiation is often the limiting step.
Electron microscopy of eukaryotic double-row polysomes found an average of 8 to 10 ribosomes, while the heaviest observed structures contained up to 40.[11] Each ribosome makes its own protein chain, so ten ribosomes on one mRNA can produce ten copies in parallel without requiring ten separate mRNA molecules.
These numbers connect back to the count. A cell's protein output is its ribosome count, multiplied by the share that is loaded onto mRNA and by the speed of each one, which is why a bacterium preparing to divide every 24 minutes builds ten times the ribosomes of one dividing every 100.
