Dark ReactionDark Reaction

The Calvin CycleThe Calvin Cycle

6CO6CO2 2 +6H+6H22O CO C66HH1212OO66 +6O+6O22

The dark reactions use the energy The dark reactions use the energy stored in ATP and NADPH the light stored in ATP and NADPH the light reactions.reactions. Carbon atoms from COCarbon atoms from CO22 are bonded, or are bonded, or

fixed, into organic compounds = carbon fixed, into organic compounds = carbon fixation. fixation.

THIS OCCURS IN THE STROMATHIS OCCURS IN THE STROMA

STEP 1STEP 1

An enzyme An enzyme (rubisco), combines (rubisco), combines COCO22 with a 5-carbon with a 5-carbon sugar RuBPsugar RuBP The product, 6-C The product, 6-C

sugar, immediately sugar, immediately splits into splits into

2, -3C molecules 2, -3C molecules (PGA)(PGA)

PGA– Phosphoglyceric PGA– Phosphoglyceric AcidAcid

Step 2Step 2 PGA is converted to PGA is converted to

another 3- Carbon another 3- Carbon molecule PGAL in a 2 molecule PGAL in a 2 part process:part process: Each PGA receives a P Each PGA receives a P

group from ATPgroup from ATP The resulting compound The resulting compound

receives a proton from receives a proton from NADPH and releases NADPH and releases the P, producing PGALthe P, producing PGAL

( ( ADP & NADP+ return to light ADP & NADP+ return to light rxn., to make ATP and rxn., to make ATP and NADPH)NADPH)

Step 3Step 3

Most of the PGAL Most of the PGAL is converted back is converted back to RuBPto RuBP Requires a P from Requires a P from

another ATPanother ATP

Some PGAL leave Some PGAL leave and used by plants and used by plants create organic create organic compoundscompounds

Balance Sheet for Balance Sheet for PhotosynthesisPhotosynthesis

How much ATP & NADH are How much ATP & NADH are required to make 1 molecule of PGA required to make 1 molecule of PGA from carbon dioxide?from carbon dioxide? Each turn fixes one COEach turn fixes one CO22

PGAL is a 3-C molecule (takes 3 turns PGAL is a 3-C molecule (takes 3 turns to make each molecule)to make each molecule)

Each turn of the cycle:Each turn of the cycle: 3 ATP ( 2 in step 2 & 1 in step 3)3 ATP ( 2 in step 2 & 1 in step 3) 2 NADPH (step 3)2 NADPH (step 3)

About 50% made to fuel cell About 50% made to fuel cell RespirationRespiration

Some of the PGAL is used to make Some of the PGAL is used to make amino acids, lipidsamino acids, lipids, carbohydrates , carbohydrates like like glucose and fructoseglucose and fructose, glycogen, , glycogen, starch, starch, and and cellulosecellulose..

Alternative PathwaysAlternative Pathways

Plants that fix carbon exclusively Plants that fix carbon exclusively through the Calvin Cycle- through the Calvin Cycle- C3 PlantsC3 Plants Because of the 3-C compound PGA, Because of the 3-C compound PGA,

that is initially formed.that is initially formed.

Example: Rice, wheat, oats, and soybeans

Alternative Pathways

Plants in Plants in hot, dryhot, dry climates use climates use alternative pathwaysalternative pathways Plant lose Plant lose HH22OO to the air-through small to the air-through small

pores called stomata (underside of leaf)pores called stomata (underside of leaf) Can be partially closed to prevent water Can be partially closed to prevent water

lossloss Stomata are the major passageways Stomata are the major passageways

through which through which COCO22 enters and enters and OO22 leavesleaves

(when stomata are closed CO(when stomata are closed CO22 levels levels decreasedecrease and O and O22 levels levels increaseincrease))

Transpiration-Transpiration-Evaporation of Evaporation of water from leaveswater from leaves

Rate of transpiration Rate of transpiration related to the related to the degree of stomata degree of stomata opening and opening and evaporation demand evaporation demand of environmentof environment

CC4 4 PlantsPlants

Fix COFix CO44 into 4-C into 4-C compoundscompounds

Partially close stomata Partially close stomata during hottest part of the during hottest part of the dayday

Enzymes fix COEnzymes fix CO22 into 4-C into 4-C compounds and transport compounds and transport them to cells where COthem to cells where CO22 is is released and enters calvin released and enters calvin cycle ( lose ½ as much cycle ( lose ½ as much H2O as C3)H2O as C3)

ExamplesExamples Corn, sugar cane, crabgrassCorn, sugar cane, crabgrass

CAM PathwaysCAM Pathways Open stomata at night; Open stomata at night;

close during the dayclose during the day

Take in COTake in CO22 at night at night and fix into compoundsand fix into compounds

Release ORelease O22 during the during the day and enter the day and enter the Calvin cycleCalvin cycle

Example: cacti, Example: cacti, pineapplespineapples

Rate of PhotosynthesisRate of Photosynthesis

Light Intensity:Light Intensity: Increase rate of photosynthesis, then Increase rate of photosynthesis, then

levels off ( max. rate of photosynthesis)levels off ( max. rate of photosynthesis) Higher intensity, excites more electrons Higher intensity, excites more electrons

in cholorphyllin cholorphyll @ same intensity, all available electrons @ same intensity, all available electrons

are excitedare excited

Amount COAmount CO22

Amount of COAmount of CO22:: Increases rate of photosynthesis to a Increases rate of photosynthesis to a

point, then levels offpoint, then levels off

TemperatureTemperature

Higher temperature accelerates the Higher temperature accelerates the chemical rxns. chemical rxns.

Peaks @ certain temp. because the Peaks @ certain temp. because the enzymes becomes ineffective and enzymes becomes ineffective and unstableunstable

Stomata closes-limiting HStomata closes-limiting H22O loss and O loss and COCO22 entry into the leaves entry into the leaves

Concentration of OConcentration of O22

Higher OHigher O22 will decrease the rate of will decrease the rate of photosynthesis and increase the rate photosynthesis and increase the rate of photorespirationof photorespiration Rubisco will bind with oxygenRubisco will bind with oxygen

Will send PGA into respiration, Will send PGA into respiration, instead of finishing photosynthesis instead of finishing photosynthesis Decreasing amount of organic Decreasing amount of organic

compound producedcompound produced