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