Most Triose Phosphate Is Recycled to Regenerate RuBP, Not Turned Into ProductsThe Calvin cycle can only keep running if the plant remakes its carbon dioxide acceptor.That acceptor is ribulose bisphosphate (RuBP), a five-carbon molecule.Most of the triose phosphate (TP) made in the cycle is used to rebuild RuBP, using energy from ATP.NoteRegenerating RuBP keeps a supply of acceptor molecules ready for the next carbon dioxide.Without this step the cycle would run out of RuBP and stop.The 6-to-1 Split: Five Sixths of Triose Phosphate Goes Back Into the CycleTriose phosphate is the three-carbon sugar produced by the reduction stage of the Calvin cycle.For every six molecules of triose phosphate made, the plant divides them two ways.One molecule leaves the cycle to build carbohydrates and other products.Five molecules stay in the cycle to regenerate RuBP.Common MistakeIt is wrong to think all triose phosphate is used to make glucose.Only about one sixth leaves the cycle, while the rest is recycled.Counting the CarbonsFive molecules of triose phosphate carry fifteen carbons in total.These fifteen carbons are rearranged into three molecules of RuBP, each with five carbons.ExampleIf a plant makes 12 triose phosphate molecules, only 2 leave the cycle for products.The other 10 are converted back into 6 molecules of RuBP.ATP Powers the Rearrangement of Triose Phosphate Into RuBPTurning five three-carbon molecules into three five-carbon molecules takes a series of enzyme reactions.The final step adds a phosphate group using ATP, which is why the cycle needs an energy input here.Without ATP the cycle would stall, so RuBP could not be remade.Regeneration of RuBP in Calvin CycleExam techniqueYou do not need the names of the individual regeneration reactions.You do need that 5 triose phosphate regenerate 3 RuBP using ATP.Regeneration Is What Makes the Calvin Cycle a CycleRegenerating RuBP gives the cycle a fresh acceptor molecule to bind the next carbon dioxide.Because most triose phosphate is recycled, the cycle can keep fixing carbon over and over.This balance lets a plant fix carbon continuously while still sparing some triose phosphate for products.NoteIf all triose phosphate left the cycle, the plant would run out of RuBP.Recycling keeps a steady supply of acceptor for carbon fixation.Active recallFor every six triose phosphate made, how many are recycled to regenerate RuBP?How many triose phosphate are needed to regenerate three molecules of RuBP?What is ATP used for during the regeneration of RuBP?Why would the Calvin cycle stop if RuBP were not regenerated?
