Isotope labeling experiments revealed the steps of the Calvin Benson cycle

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To identify the sequence of reactions by which CO2 ends up in carbohydrates, it was necessary to label CO2 so that it could be followed after being taken up by a photosynthetic cell. In the 1950s, Melvin Calvin, Andrew Benson, and their colleagues used radioactively labeled CO2 in which some of the carbon atoms were not the normal 12C, but its radioisotope 14C. Although 14C is distinguished by its emission of radiation, chemically it behaves virtually identically to nonra-dioactive 12C. In general, enzymes do not distinguish between isotopes of an element in their substrates, so 14CO2 is treated the same way by photosynthesizing cells as 12CO2.

Calvin and his colleagues exposed cultures of the unicellular green alga Chlorella to 14CO2 for 30 seconds. They then rapidly killed the cells, extracted their carbohydrates, and separated the different compounds from one another by paper chromatography. Many compounds, including monosaccharides and amino acids, contained 14C (Figure 8.12).

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Good Carb Diet

Good Carb Diet

WHAT IT IS A three-phase plan that has been likened to the low-carbohydrate Atkins program because during the first two weeks, South Beach eliminates most carbs, including bread, pasta, potatoes, fruit and most dairy products. In PHASE 2, healthy carbs, including most fruits, whole grains and dairy products are gradually reintroduced, but processed carbs such as bagels, cookies, cornflakes, regular pasta and rice cakes remain on the list of foods to avoid or eat rarely.

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