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.

A simple outline of the process of photosynthesis, showing the light reactions and the Calvin cycle. By Daniel Mayer, credit link below.

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.

Rubisco is a relatively small molecule, containing 5 carbons as well as oxygen, hydrogen and phosphate. It bonds to the CO2 to create a 6 carbon molecule, which promptly splits into two small 3 carbon molecules as shown in the reaction scheme below:

Rubisco reacting with carbon dioxide. This reaction also requires water molecules. Image credit below.

These two 3 carbon molecules then go through a series of reduction stages, during which they react with the ATP (energy molecule) and NADPH (reducing molecule) that were produced during the light reactions of photosynthesis. Even though the Calvin cycle doesn?t require any light itself, it is completely reliant on molecules created by the light-reactions. This stage creates two molecules of the 3-carbon ?glyceraldehyde 3-phosphate? ? which can be turned into useful plant sugars by further reactions.

In order to continue running the Calvin cycle, and the reason that it is a cycle rather than just a process, the Rubisco must be recycled in order to go and pick up new carbon dioxide molecules. To do this also requires molecules of glyceraldehyde-3-phosphate ? which are modified and then joined together to re-form the Rubisco. The final result of all this is that for every 3 rounds of the cycle three molecules of Rubisco go in, 3 Rubiscos come out, and one new glyceraldehyde 3-phosphate is made.

When put like that it might seem like a lot of effort for very little, but in reality the constant cycling of Rubisco makes for a very efficient reaction. One molecule of hexose contains 2016 calories, while the number of Calvin cycles required to produce that uses up only 750 calories. When you factor in the light-reactions required to produce the energy, the entire process of photosynthesis works out as around 33% efficient. As biological processes go, it?s not bad!

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Credit link for image 1

Credit link for image 2

Featured Image by? Jon Sullivan. More of his awesome photos can be found here.

Source: http://rss.sciam.com/click.phdo?i=b531803298b4ee46a33b6dea7b1bb22b

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