6-2: Calvin Cycle6-2: Calvin Cycle

What comes next?What comes next? In the second set of reactions of In the second set of reactions of

photosynthesis, plants use the energy that was photosynthesis, plants use the energy that was stored in stored in ATPATP and and NADPHNADPH during the Light during the Light Reactions to produce organic compounds in Reactions to produce organic compounds in the form of the form of SUGARSSUGARS

These organic compounds are then consumed These organic compounds are then consumed by autotrophs + heterotrophs alike for by autotrophs + heterotrophs alike for energyenergy

The most common way that plants produce The most common way that plants produce organic compounds is called the organic compounds is called the Calvin CycleCalvin Cycle

Carbon FixationCarbon Fixation Calvin cycle Calvin cycle is a series of enzyme-is a series of enzyme-

assisted chemical reactions that assisted chemical reactions that produces a 3-C sugarproduces a 3-C sugar COCO22 “fixed” into organic compounds “fixed” into organic compounds

Known as Known as Carbon FixationCarbon Fixation

3 CO3 CO22 molecules must enter to make 3-C molecules must enter to make 3-C

sugarsugar Occurs within the stroma of chloroplastOccurs within the stroma of chloroplast

Steps of Calvin CycleSteps of Calvin Cycle1.1. COCO22 diffuses into the stroma from diffuses into the stroma from

cytosol. An enzyme combines COcytosol. An enzyme combines CO22 + +

RuBPRuBP (5-C molecule). The 6-C (5-C molecule). The 6-C molecules immediately splits into 2 3-C molecules immediately splits into 2 3-C molecules called molecules called 3-PGA3-PGA

Let’s look @ STEP 1Let’s look @ STEP 1

Steps of Calvin CycleSteps of Calvin Cycle2.2. 3-PGA3-PGA is converted into another 3-C is converted into another 3-C

molecule called molecule called G3PG3P in two parts. in two parts. FirstFirst: : 3-PGA3-PGA receives a phosphate group receives a phosphate group

from from ATPATP SecondSecond: receives a : receives a protonproton ( (HH++) from ) from

NADPHNADPH, which releases the phosphate , which releases the phosphate groupgroup

ADP, NADPADP, NADP++, and phosphate can go back to , and phosphate can go back to the Light Reactions to make more the Light Reactions to make more ATPATP + + NADPHNADPH

Let’s look @ STEP 2Let’s look @ STEP 2

Steps of Calvin CycleSteps of Calvin Cycle3.3. One of the One of the G3PG3P molecules leaves molecules leaves Calvin Calvin

CycleCycle and is used to make organic and is used to make organic compounds (a carbohydrate) that is compounds (a carbohydrate) that is stored for later usestored for later use

4.4. The remaining The remaining G3PG3P molecules are molecules are converted back to converted back to RuBPRuBP through through addition of phosphate groups from ATP addition of phosphate groups from ATP molecules. molecules. RuBPRuBP then starts the then starts the Calvin Calvin CycleCycle over again. over again.

Let’s look @ STEP 3Let’s look @ STEP 3

Calvin Cycle Misc.Calvin Cycle Misc. Pathway named for American Pathway named for American

scientist Melvin Calvinscientist Melvin Calvin Calvin Cycle Calvin Cycle is most common is most common

pathway for carbon fixation to take pathway for carbon fixation to take placeplace

CC33 plants plants Plants that fix carbon exclusively Plants that fix carbon exclusively

through the through the Calvin CycleCalvin Cycle

CC33 = (C-C-C) = 3-C compound initially = (C-C-C) = 3-C compound initially

formed in processformed in process Examples: wheat, barley, potatoesExamples: wheat, barley, potatoes

Alternative PathwaysAlternative Pathways Under hot + dry Under hot + dry

climates, plants rapidly climates, plants rapidly lose lose waterwater to the air to the air

StomataStomata (or (or stomastoma) – a ) – a small pore usually small pore usually located on the located on the underside of a leafunderside of a leaf Used in Used in gas exchange gas exchange

Alternative Pathways Alternative Pathways (cont.)(cont.)

Plants can partially close their stomata to Plants can partially close their stomata to reduce the reduce the waterwater loss loss

Major passageways through which COMajor passageways through which CO22

enters and Oenters and O22 leaves a plant leaves a plant

Low COLow CO22 levels and high O levels and high O22 levels inhibit levels inhibit

the the Calvin Cycle Calvin Cycle from workingfrom working Plants must find a way to deal with those

conditions

CC44 Pathways Pathways CC44 plants plants

Plants that fix COPlants that fix CO22 into a 4-C compound into a 4-C compound

During hottest part of day, stomata are partially During hottest part of day, stomata are partially closed

Certain cells of CCertain cells of C44 plants have plants have enzymesenzymes that fix that fix

COCO22 when CO when CO22 levels are down + O levels are down + O22 levels are up levels are up

Compounds transported to other cells where they Compounds transported to other cells where they can enter the can enter the Calvin Cycle Calvin Cycle to produce to produce carbohydratescarbohydrates

Examples: Examples: corncorn, sugar cane, crabgrass, sugar cane, crabgrass CC44 plants do not lose as much plants do not lose as much waterwater as C as C33 plants plants

do – Why?do – Why? Stomata are partially closed during day

CAM PathwayCAM Pathway CAMCAM plantsplants include pineapples + cacti include pineapples + cacti

At night, the stomata open During the day, the stomata are

closed COCO22 that is taken in at night is fixed into a that is taken in at night is fixed into a

variety of organic compoundsvariety of organic compounds During the day, CODuring the day, CO2 2 is released by the is released by the

compounds and enters the compounds and enters the Calvin CycleCalvin Cycle CAM plants grow slowlyCAM plants grow slowly Lose less Lose less waterwater than either than either CC33 + + CC44 plants plants

Summary of PhotosynthesisSummary of Photosynthesis Two StagesTwo Stages::

Light ReactionsLight Reactions AKA AKA – – Light-Dependent Reactions, Light Light-Dependent Reactions, Light

Harvesting ReactionsHarvesting Reactions Calvin CycleCalvin Cycle

AKA – AKA – Light-Independent ReactionsLight-Independent Reactions, , Dark Dark ReactionsReactions

Ongoing cycle - Ongoing cycle - BiochemicalBiochemical pathwayspathways Simplest Equation for PhotosynthesisSimplest Equation for Photosynthesis

COCO22 + H + H22O + light energyO + light energy (CH(CH22O) + OO) + O22

GlucoseGlucose is not a direct product of is not a direct product of photosynthesis!!!photosynthesis!!!

Factors that Affect Factors that Affect PhotosynthesisPhotosynthesis

Light IntensityLight Intensity More light stimulates More light stimulates

more electrons in more electrons in Photosystems of Photosystems of ETCETC

Light reactions Light reactions happen more rapidlyhappen more rapidly

Peaks at a point Peaks at a point where there are where there are nono moremore electrons to electrons to exciteexcite

Factors that Affect Factors that Affect PhotosynthesisPhotosynthesis

Carbon DioxideCarbon Dioxide More COMore CO22 allows allows

“fixing” in Calvin “fixing” in Calvin Cycle to take Cycle to take place faster but place faster but reaches a reaches a saturation point saturation point as wellas well

Rate of PhotosynthesisRate of Photosynthesis TemperatureTemperature

Increases initially but Increases initially but declines at peakdeclines at peak

Why?Why? Because at some higher Because at some higher

temperatures, temperatures, enzymesenzymes stop working + become stop working + become ineffectiveineffective

Also, the Also, the stomatastomata close close limiting water loss + COlimiting water loss + CO22

entryentry

Calvin CycleCalvin Cycle

Video Recap

The Photosynthesis SongThe Photosynthesis Song

It’s Catchy!