The problem Calvin was investigating was how did the carbon in carbon dioxide get converted into the carbon in carbohydrates during phototsynthesis. This meant he had to find a way of tracing the path of the carbon as it was built up into carbohydrate: CO2 ---> substance A ---> substance B ---> etc ---> CH2O (carbohydrate) Calvin discovered the `Calvin cycle` by choosing a suitable organism to work with, devising a clever piece of apparatus and applying two carefully-chosen techniques. The organism was a single-celled alga called Chlorella. Each cell contains a single, large chloroplast. Because it is single-celled, it can absorb carbon dioxide rapidly and can be killed rapidly, making handling easier than it would be if a larger plant had been used. The apparatus was a flat glass flask called the `lollipop` apparatus after its shape. The flat shape meant that all of the Chlorella cells inside could receive the maximum amount of light, and so carry out...