Light-independent reaction

Light-independent reaction

In photosynthesis, the light-independent reactions, also somewhat misleadingly called the dark reactions (they don't require darkness, but they require the "products" of the light reactions), are chemical reactions that convert carbon dioxide and other compounds into glucose. It occurs in the stroma, the fluid filled area of a chloroplast outside of the thylakoid membranes. These reactions, unlike the light-dependent reactions, do not need light to occur; hence the term "dark reactions". These reactions take the products of the light-dependent reactions and perform further chemical processes on them. There are three phases to the light-independent reactions, collectively called the Calvin Cycle: Carbon Fixation, Reduction reactions, and ribulose 1,5-biphosphate (RuBP) regeneration.

However in CAM (Crassulacean acid metabolism) plants, carbon fixation actually "does" take place at night.

Carbon fixation

"Main article": "carbon fixation"

The carbon fixation reaction is the first step of the light-independent reactions. Carbon from carbon dioxide is "fixed" into a larger carbohydrate causing three pathways to occur: C3 carbon fixation (the most common), C4 carbon fixation, and CAM (Crassulacean Acid Metabolism). C3 fixation occurs as the first step of the Calvin-Benson cycle in all plants. C4 plants first fix carbon dioxide into oxaloacetate, a 4C acid, and then into malate, which is transported to the bundle sheath cells, where it is decarboxylated to supply carbon dioxide to the Calvin-Benson cycle. CAM plants perform a similar process, but instead of spatial separation of carbon dioxide fixation and the Calvin-Benson cycle, they separate the processes in time: at night they open their stomata and fix carbon dioxide into oxaloacetate which is converted to malate. During the day malate is decarboxylated and the thus released carbon dioxide enters the Calvin-Benson cycle.

Calvin cycle

The Calvin-Benson cycle takes carbon dioxide and shares it with glucose, which the plant uses for food.

External links

* [http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/dark.htm The Biochemistry of the Calvin Cycle at Rensselaer Polytechnic Institute]
* [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=stryer.chapter.2783 The Calvin Cycle and the Pentose Phosphate Pathway] from "Biochemistry", Fifth Edition by Jeremy M. Berg, John L. Tymoczko and Lubert Stryer. Published by W. H. Freeman and Company (2002).


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