MP9: Alternative Mechanisms to Carbon Fixation

Calvin Cycle

Purpose to fix C → glucose

* enzyme: rubisco
* must first make 3-phosphoglycerate (3C)
* cycle known as C3 cycle

Problems w/ C3

Most plants use Calvin cycle - carbon fixation

* problem occurs when ^^**O₂**^^ binds w/ rubisco instead of CO₂
* photorespiration
* end up losing CO₂ to atmosphere - reduces # of carbon fixed
* alternatives to carbon fixation

C4 Plants

Dark reaction happens in ^^**bundle-sheath cell**^^

* another enzyme brings CO₂ into bundle-sheath for carbon fixation

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Happens in leaves of plants that live in hot & dry locations (e.g. corn, sugar cane)

* higher O₂ concentration near leaf
* mesophyll cell takes CO₂ + PEP → oxalacetate by PEP carboxylase
* oxalacetate reduced by NADPH → malate
* malate enter B-S cell through diffusion to make pyruvate
* REDOX reaction of malate to make CO₂ to enter calvin cycle
* pyruvate can go back → PEP
* energy intense

CAM Plants

^^**Night: stomata open**^^; release O₂ & CO₂ in

* C4 cycle happens
* CO₂ → malate

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^^**Day: stomata close**^^ to prevent ^^**water loss**^^

* malate → pyruvate; CO₂ in Calvin cycle

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Energy-intensive, but plants work harder at night when cooler

CAM Plants vs. C4 Plants

CAM: C4 & Calvin cycle happen in ^^**same place**^^**,** but ^^**different times**^^

C4: Happen in ^^**different places**^^, but ^^**same time**^^