Photosynthesis –The Details

09/04/2013

FIXING CARBON DIOXIDE: THE LIGHT-INDEPENDENT REACTIONS

SBI 4UI

Photosynthesis ? The Details

The Light Independent Reactions the Calvin cycle is the series of reactions that convert carbon dioxide into carbohydrates ? called CO2 assimilation occurs in the stroma of chloroplasts there are three phases:

Phase 1: Carbon fixation Phase 2: Reduction reactions Phase 3: Regenerating RuBP

The Light-Independent Reactions

Phase 1: Carbon Fixation

CO2 adds to ribulose 1,5-biphosphate (RuBP) to create an unstable 6 carbon intermediate this molecule instantly splits into two 3 carbon molecules called 3-phosphoglycerate (PGA) because these three-carbon compounds are the first stable products of the process, plants that use this method are called C3 plants (and the process is called C3 photosynthesis) catalyzed by enzyme called rubisco ? possibly the most abundant protein on Earth

The Light-Independent Reactions

Phase 2: Reduction Reactions each molecule of PGA is phosphorylated by ATP to form 1,3-biphosphoglycerate (1,3-BPG) A pair of electrons from NADPH reduces the 1,3-BPG to glyceraldehyde 3-phosphate (G3P), a sugar in their new reduced, high energy state some of the G3P will leave the cycle to be used to make glucose and other carbohydrates (the remaining G3P move to third phase of cycle)

The Light-Independent Reactions

Phase 3: RuBP regeneration most of the reduced G3P are used to make RuBP energy (from ATP) is required to make the 5carbon RuBP from G3P the Calvin cycle must be completed six times in order to make one glucose molecule

of the twelve G3P made from 6 cycles 10 used to regenerate RuBP, 2 used to make glucose

The Calvin Cycle

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The Light-Independent Reactions

the net equation for the Calvin cycle is:

6 CO2 + 18 ATP + 12 NADPH 2 G3P + 16 Pi + 18 ADP + 12 NADP+

the G3P that is produced can then be used for the synthesis of other molecules that plants require

G3P is used to produce sucrose, starch, cellulose and can combine with a source of nitrogen to make amino acids that are used to make proteins

Textbook Questions

Read pages 166 ? 167 and answer Learning Check questions 13 ? 18 on page 167

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