Making Sugar from Carbon Dioxide: The Calvin Cycle

The process of photosynthesis is often described as turning sunlight into sugars, and while that's broadly true, there are two distinct biochemical reactions taking place. The first uses the sunlight to create energy inside the cell and the second takes carbon dioxide and uses it to make sugars. The second is the Calvin cycle although the name is a little unfashionable nowadays. It's politer to refer to it as the Calvin-Benson-Bassham cycle or the reductive pentose phosphate cycle, but with all due apologies to Misters Benson and Bassham, the Calvin Cycle is quicker to write.

Turning carbon dioxide into sugar may sound fairly magical, but it becomes a more conceivable when you consider that both carbon dioxide (CO2) and glucose (C6H12O6) contain roughly the same sort of elements. The Calvin cycle just adds on all the extra elements required. Having said that, the 'just' is still a fairly major task, requiring different enzymes all working in the correct order.

The carbon dioxide molecules diffuse into the cells through small holes in the underside of the leaf. The first enzyme that picks them up is called Rubisco. Despite sounding like a small corporate venture, Rubisco is actually one of the most important enzymes in the world. Without Rubisco, plants would not be able to make sugars, which means that animals would not be able to survive on plants.

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