photorespiration & alternative methods of carbon fixation
TRANSCRIPT
PhotorespirationPhotorespiration&&
Alternative Methods of Alternative Methods of Carbon FixationCarbon Fixation
PhotorespirationPhotorespiration
Stomata are pores in Stomata are pores in the leaf surface, the leaf surface, through which:through which: Carbon dioxide enters Carbon dioxide enters
the plantthe plant Water evaporates from Water evaporates from
the plantthe plant Oxygen leaves the Oxygen leaves the
plantplant
PhotorespirationPhotorespiration
In hot, dry climates, plants close their In hot, dry climates, plants close their stomatastomata Pro: Pro:
plant doesn’t lose as much waterplant doesn’t lose as much water Cons: Cons:
Plant doesn’t receive carbon dioxidePlant doesn’t receive carbon dioxide
Plant can’t get rid of oxygenPlant can’t get rid of oxygen
PhotorespirationPhotorespiration
Rubisco:Rubisco: Enzyme that unites COEnzyme that unites CO22 and RuBP in the first and RuBP in the first
step of the Calvin Cyclestep of the Calvin Cycle HOWEVER, it can also cause OHOWEVER, it can also cause O2 2 to join RuBPto join RuBP
Higher concentration (OHigher concentration (O22 or CO or CO22) “wins” ) “wins”
and is joined to RuBPand is joined to RuBP
When OWhen O22 is joined to RuBP, is joined to RuBP,
photorespiration occursphotorespiration occurs
Photorespiration as a ProblemPhotorespiration as a Problem
Photorespiration:Photorespiration: Does not produce any ATPDoes not produce any ATP Prevents the synthesis of glucosePrevents the synthesis of glucose
Therefore, some plants in hot, dry climates Therefore, some plants in hot, dry climates have developed ways to combat have developed ways to combat photorespirationphotorespiration
Alternative Methods of Alternative Methods of Carbon FixationCarbon Fixation
CC33 Pathway Pathway
Uses only the Calvin Uses only the Calvin CycleCycle
Called CCalled C33 because because
PGA is a 3-C PGA is a 3-C compoundcompound
Occurs in mesophyll Occurs in mesophyll cellscells
CC44 Pathway Pathway
Different Different locationlocation within the leafwithin the leaf
Mesophyll cells pump Mesophyll cells pump COCO22 into bundle- into bundle-
sheath cells, which sheath cells, which are impermeable to are impermeable to COCO22
This increases the This increases the concentration of COconcentration of CO22
CC44 Pathway, cont’d Pathway, cont’d
Since the Since the concentration of COconcentration of CO22
is greater than the is greater than the concentration of Oconcentration of O22, ,
the Calvin Cycle is the Calvin Cycle is favored over favored over photorespirationphotorespiration
Examples: Examples: CornCorn Sugar caneSugar cane
CC33 vs. C vs. C44
Different LOCATIONS within the leafDifferent LOCATIONS within the leaf
CAM pathwayCAM pathway
Occurs at a different Occurs at a different timetimeStomata open at night Stomata open at night and close during the and close during the dayday Pros:Pros:
Prevents water lossPrevents water loss
Prevents COPrevents CO22 from from leaving the leavesleaving the leaves
Con:Con:Plant grows slowlyPlant grows slowly
CAM Plants, cont’d…CAM Plants, cont’d…
COCO22 is stored is stored
overnight until the overnight until the light reactions can light reactions can supply ATP and supply ATP and NADPH to drive the NADPH to drive the Calvin CycleCalvin Cycle
Examples:Examples: CactiCacti PineapplesPineapples