The Weight of a Cell

Summary

Microbes are so small that tens of thousands could fit in the space of the period at the end of this sentence. And yet, for two of the most widely studied kinds — S. cerevisiae and E. coli — we know their weight with remarkable precision: A single yeast cell weighs about 100 picograms and a single E. coli bacterium weighs about one picogram, or 60 million times less than a grain of sand.At first blush, measuring the weight of a single cell seems an impossible task. How can we make such a precise measurement with reasonable certainty? A typical kitchen scale has a sensitivity of 0.1 grams, meaning it can detect the difference between a banana weighing 175 grams and another weighing 175.1 grams. By contrast, an E. coli cell weighs 100 billion times less than 0.1 grams.Today, such measurements are made using profoundly clever contraptions developed by scientists over the last couple of decades. These devices can measure individual cells with femtogram precision (a unit 1,000-times smaller than a picogram). But before such precision instruments, scientists had to make do with whatever was already in the lab.Back in 1953, for example, two biologists from Southern Illinois University (supported, in part, with funding from the Anheuser-Busch brewery) invented one of the first methods to determine the weight of an individual yeast cell. And they did it, incredibly, using little more than a microscope, sugar water, and a camera.Their method made use of Stokes’ law, which describes how a small sphere sinks through a liquid. If the sphere falls without causing turbulence, the liquid “pushes back” with a drag force. In 1845, Sir George Stokes — an Irish mathematician and the longest-serving Lucasian Professor of Mathematics at the University of Cambridge — showed that this drag force is proportional to the sphere’s size, the liquid’s viscosity, and the sphere’s velocity through the fluid. Written as a formula, where r is the sphere’s radius, η is the fluid’s viscosity, and v is...