Guessing the number of jellybeans in a jar is tricky, but try guessing the weight of a cell just by looking at it (left). Scientists have now accomplished this feat using nothing more than a standard laboratory microscope and a digital camera. By analyzing how an object bends or refracts light (middle), image-processing algorithms can deduce its composition. Knowing an object's makeup allows one to infer its density (right), and because the microscope can also be used to measure an object's dimensions and volume, that data altogether enables researchers to calculate the cell’s mass. This technique, reported online this week in Physical Review Letters, found that human red blood cells have an average dry mass of 27.2 trillionths of a gram. This is in line with past measurements, which have typically relied on custom-built instruments. Researchers say the new approach could help monitor how live cells grow and change over time, such as in response to disease or environmental changes.
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