How much does life weigh?
It sounds like a strange question, but to biologists it makes all the sense in the world. Yeast cells tip the scales at about 100 picograms each. A single E. coli bacterium weighs only one picogram, about 60 million times lighter than a grain of sand.
That such a measurement is even possible seems absurd at first. Kitchen scales, after all, wobble at a tenth of a gram. An E. coli cell is 100 billion times lighter than that.
And yet, scientists have managed to pin down these numbers with remarkable precision. So, how is this even possible?
A Yeast Cell Sinks
In 1953, two biologists at Southern Illinois University set out to weigh yeast. They had no precision instruments, just a microscope, some sugar water, and a camera. Their funding partly came from the Anheuser-Busch brewery, a fitting sponsor for yeast research.
The researchers turned to a simple equation, written a century earlier by the Irish mathematician George Stokes. Stokes had shown how exactly a sphere sinks through a liquid, balancing gravity’s pull against the fluid’s resistance. If you know the size of the sphere, the viscosity of the fluid, and the speed of the fall, you can calculate its mass.
So, the scientists propped their microscope slides upright and filmed yeast cells drifting downwards in sugar water. Frame by frame, they measured how far each tiny sphere sank. They assumed the cells were perfectly round — close enough for yeast.
The math gave them an average: 79 picograms per cell. Astonishingly, that number has held up. Recent experiments with much more advanced tools put the figure around 100 picograms.
This feat of back-of-the-envelope physics is quite astonishing, but it’s not alone. Something similar was achieved way back in 1890, when Lord Rayleigh calculated the size of a single oil molecule by spreading a droplet on water and measuring how thin the film became. His estimate was off by less than 20 percent of today’s accepted value.
Vibrations of a Bacterium
But yeast cells are conveniently spherical. Other microbes aren’t so cooperative. E. coli are shaped more like rods. Dropping them in sugar water would only stir turbulence, ruining the math.
So, in 2010, scientists at MIT built something entirely new: a suspended microchannel resonator. Imagine a hollow beam, bent in a U-shape, that vibrates like a guitar string. Inside runs a fluid channel.
When a bacterium passes through, the beam’s vibration shifts ever so slightly. The heavier the cell, the larger the shift. By measuring this frequency change, researchers can calculate the cell’s buoyant mass with femtogram precision — a thousand times finer than a picogram.
Even better, the device can trap a single bacterium and weigh it repeatedly as it grows. At 37 °C, a small E. coli cell might grow by 0.06 picograms per hour; a large one grows faster, adding about 0.14 picograms in the same time.
When the researchers measured 48 cells, the average weight was 0.55 picograms.
These experiments, separated by half a century, share the same spirit. They show how simple principles — a falling sphere, a vibrating beam — can be stretched to capture the invisible.
Cells may seem like abstract units of life. But they are also physical things, with shapes, volumes, and weights. Once you think of them that way, measuring the basic unit of life doesn’t seem as daunting.
