Photosynthesis is a way of making organic molecules from carbon dioxide (CO2). These organic molecules contain many C—H bonds and are highly reduced compared with CO2. The Calvin Cycle is sometimes called the Dark Reactions, but do not be fooled by this name – the Calvin Cycle is the most active during the daylight hours when ATP and NADPH are plentiful.To build organic molecules, cells use raw materials provided by the light reactions:
1. Energy: ATP (provided by cyclic and noncyclic photophosphorylation) drives the endergonic reactions.
2. Reducing power: NADPH (provided by photosystem I) provides a source of hydrogen and the energetic electrons needed to bind them to carbon atoms. Much of the light energy captured in photosynthesis ends up invested in the energy-rich C—H bonds of sugars.
The key step in the Calvin cycle—the event that makes the reduction of CO2 possible—is the attachment of CO2 to a very special organic molecule. Photosynthetic cells produce this molecule by reassembling the bonds of two intermediates in glycolysis, fructose 6-phosphate, and glyceraldehyde 3-phosphate, to form the energy-rich five-carbon sugar, ribulose 1,5-bisphosphate (RuBP), and a four-carbon sugar.
CO2 binds to RuBP in the key process called carbon fixation, forming two three-carbon molecules of phosphoglycerate. The enzyme that carries out this reaction, ribulose bisphosphate carboxylase/oxygenase (usually abbreviated rubisco) is a very large four-subunit enzyme present in the chloroplast stroma. This enzyme works very sluggishly, processing only about three molecules of RuBP per second (a typical enzyme processes about 1000 substrate molecules per second). Because it works so slowly, many molecules of Rubisco are needed. In a typical leaf, over 50% of all the protein is rubisco. It is thought to be the most abundant protein on earth.
Three stages of the Calvin Cycle
Fixation, Reduction, and Regeneration
Plants store light energy in the form of carbohydrate, primarily starch and sucrose. The carbon and oxygen for this process come from CO2, and the energy for carbon fixation is derived from the ATP and NADPH made during photosynthesis. The conversion of CO2 to carbohydrate is called the Calvin Cycle and is named after Melvin Calvin who discovered it.
The Calvin Cycle requires the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase commonly called rubisco. The Calvin cycle generates the triose phosphates 3-phosphoglycerate (3-PGA), glyceraldehyde-3P (GAP) and dihydroxyacetone phosphate (DHAP), all of which are used to synthesize the hexose phosphates fructose-1,6-bisphosphate and fructose 6-phosphate.
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