PhotosynthesisPhotosynthesis

• An anabolic, endergonic, carbon dioxide anabolic, endergonic, carbon dioxide (CO(CO22)) requiring process that uses light light energy (photons)energy (photons) and water (Hwater (H22O)O) to produce organic macromolecules organic macromolecules (glucose).(glucose).

6CO2 + 6H2O C6H12O6 + 6O2 glucoseglucose

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photonsphotons

•Where does photosynthesis take place?Where does photosynthesis take place?

PlantsPlants

• Autotrophs:Autotrophs: self-producers.• Location:

1. Leavesa. stomab. mesophyll cells

StomaMesophyllCell

Chloroplast

Stomata (stoma)Stomata (stoma)

• PoresPores in a plant’s cuticle through which waterwater and gasesgases are exchanged between the plant and the atmosphere.

Guard Cell

Guard Cell

Carbon Dioxide (CO2)

Oxygen (O2)

Water (H2O)

Mesophyll Mesophyll CellCell

Cell WallNucleus

Chloroplast

Central Vacuole

ChloroplastChloroplast• OrganelleOrganelle where photosynthesisphotosynthesis takes

place.

GranumThylakoid

Stroma

Outer Membrane

Inner Membrane

ThylakoidThylakoid

Thylakoid Membrane

Thylakoid SpaceGranum

•Why are plants green?Why are plants green?

Chlorophyll MoleculesChlorophyll Molecules• Located in the thylakoid thylakoid

membranesmembranes.

• Chlorophyll have MgMg++ in the center.

• Chlorophyll pigmentsChlorophyll pigments harvest energy (photons) by absorbingabsorbing certain wavelengthswavelengths (blue-420 blue-420 nmnm and red-660 nm are most important).

• PlantsPlants are greengreen because the green wavelengthwavelength is

reflectedreflected, not absorbednot absorbed.

Wavelength of Light Wavelength of Light (nm)(nm)

400 500 600 700

Short wave Long wave(more energy) (less energy)

Absorption of Chlorophyll

wavelengthwavelength

Absorption

violet blue green yellow orange red

•During the fall, what causes the leaves to change colors?During the fall, what causes the leaves to change colors?

Fall ColorsFall Colors

• In addition to the chlorophyll pigments, there are other pigmentspigments present.

• During the fall, the green chlorophyllgreen chlorophyll pigments are greatly reducedgreatly reduced revealing the other pigmentspigments.

• CarotenoidsCarotenoids are pigments that are either redred or yellowyellow.

Other pigments of photosynthesis include

Other chlorophylls, xanthophyllsxanthophylls, carotenoids, fucoxanthins, anthocyanins, tannins

Redox ReactionRedox Reaction

• The transfertransfer of oneone or more electronsmore electrons from one reactantone reactant to anotheranother.

• Two types:Two types:

1.1. OxidationOxidation

2.2. ReductionReduction

Oxidation ReactionOxidation Reaction

• The lossloss of electronselectrons from a substance.• Or the gaingain of oxygenoxygen.

glucoseglucose

6CO2 + 6H2O C6H12O6 + 6O2

OxidationOxidation

Reduction ReactionReduction Reaction• The gaingain of electrons to a substance.• Or the lossloss of oxygenoxygen.

glucoseglucose

6CO2 + 6H2O C6H12O6 + 6O2

ReductionReduction

The Source of Oxygen Produced by Photosynthesis

Breakdown of Breakdown of PhotosynthesisPhotosynthesis

• Two main parts (reactions).Two main parts (reactions).

1. 1. Light Reaction oror Light Dependent ReactionLight Dependent Reaction

Produces energyenergy from solar powersolar power (photons)(photons) in the form of ATPATP and NADPHNADPH.

Breakdown of Breakdown of PhotosynthesisPhotosynthesis

2.2. Calvin Cycle orCalvin Cycle orLight Independent Reaction oror

Carbon Fixation orCarbon Fixation or

CC33 Fixation Fixation

Uses energyenergy (ATP and NADPH)(ATP and NADPH) from light rxnlight rxn to make sugar (glucose).sugar (glucose).

1. 1. Light Reaction (Electron Flow)(Electron Flow)

• Occurs in the Thylakoid membranesThylakoid membranes

• During the light reactionlight reaction, there are two possibletwo possible routes for electron flowelectron flow.

A.A. Cyclic Electron FlowCyclic Electron Flow

B.B. Noncyclic Electron FlowNoncyclic Electron Flow

A. Cyclic Electron Flow

• Occurs in the thylakoid membranethylakoid membrane.• Uses Photosystem I onlyPhotosystem I only• P700 reaction center- chlorophyll a • Uses Electron Transport Chain Electron Transport Chain

(ETC)(ETC)• Generates ATP only

ADP + ATPATPP

A. Cyclic Electron FlowA. Cyclic Electron Flow

P700

PrimaryElectronAcceptor

e-

e-

e-

e-

ATPATPproducedby ETC

Photosystem I

AccessoryPigments

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Photons

B. Noncyclic Electron B. Noncyclic Electron FlowFlow

• Occurs in the thylakoid membranethylakoid membrane

• Uses PS IIPS II and PS IPS I

• P680 rxn center (PSII) - chlorophyll a

• P700 rxn center (PS I) - chlorophyll a

• Uses Electron Transport Chain (ETC)Electron Transport Chain (ETC)

• Generates OGenerates O22, ATP and NADPH, ATP and NADPH

B. Noncyclic Electron B. Noncyclic Electron FlowFlow

P700

Photosystem IP680

Photosystem II

PrimaryElectronAcceptor

PrimaryElectronAcceptor

ETC

EnzymeReaction

H2O

1/2O1/2O22 + 2H+

ATPATP

NADPHNADPHPhoton

2e-

2e-

2e-

2e-

2e-

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Photon

B. Noncyclic Electron B. Noncyclic Electron FlowFlow

• ADP + ATPATP

• NADP+ + H NADPHNADPH

• Oxygen comes from the splitting Oxygen comes from the splitting of of HH22OO, not , not COCO22

HH22O O 1/2 O2 + 2H+

(Reduced)

P(Reduced)

(Oxidized)

ChemiosmosisChemiosmosis

• Powers ATP synthesisATP synthesis.

• Located in the thylakoid membranesthylakoid membranes.

• Uses ETC and ATP synthase (enzyme)(enzyme) to make ATP.

• Photophosphorylation:: addition of phosphatephosphate to ADPADP to make ATPATP.

ChemiosmosisChemiosmosisH+ H+

ATP Synthase

H+ H+ H+ H+

H+ H+ high Hhigh H++

concentrationconcentration

H+ADP + P ATP

PS II PS IE

TC

low Hlow H++

concentrationconcentration

H+ThylakoidThylakoidSpaceSpace

ThylakoidThylakoid

SUN (Proton Pumping)

Calvin Cycle• Carbon Fixation (light independent rxn).(light independent rxn).

• C3 plants (80% of plants on earth).

• Occurs in the stroma.

• Uses ATP and NADPH from light rxn.

• Uses CO2.

• To produce glucose: it takes 6 turns and uses 18 ATP and 12 NADPH.

ChloroplastChloroplast

GranumThylakoid

Stroma

Outer Membrane

Inner Membrane

Calvin Cycle (CCalvin Cycle (C33 fixation)fixation)

6CO2

6C-C-C-C-C-C

6C-C-C 6C-C-C

6C-C-C-C-C

12PGA

RuBP

12G3P

(unstable)

6NADPH 6NADPH

6ATP 6ATP

6ATP

C-C-C-C-C-CGlucose

(6C)(36C)

(36C)

(36C)

(30C)

(30C)

(6C)

6C-C-C 6C-C-C

C3

glucose

Calvin CycleCalvin Cycle

• Remember:Remember: C3 = Calvin CycleC3 = Calvin Cycle

C3

Glucose

Tracing the Pathway of CO2

Photorespiration/Calvin-Photorespiration/Calvin-Benson CycleBenson Cycle

• Occurs on hot, dry, bright dayshot, dry, bright days.

• Stomates close.

• Fixation of O2 instead of CO2.

• Produces 2-C molecules2-C molecules instead of 3-C 3-C sugar moleculessugar molecules.

• Produces no sugar molecules or no ATP.

PhotorespirationPhotorespiration

• Because of photorespirationBecause of photorespiration: PlantsPlants have special adaptationsspecial adaptations to limit the effect of photorespirationphotorespiration.

1.1. C4 plantsC4 plants

2.2. CAM plantsCAM plants

C4 PlantsC4 Plants

• Hot, moist environmentsHot, moist environments.

• 15% of plants (grasses, corn, 15% of plants (grasses, corn, sugarcane).sugarcane).

• Divides photosynthesis spatially.Divides photosynthesis spatially.

• Light rxn - mesophyll cells.

• Calvin cycle - bundle sheath cells.

C4 Pathway

PEP (phosphoenolpyruvate) + CO2 4C (oxaloacetic acid) using enzyme PEP carboxylase

PEP carboxylase has a higher affinity for CO2 than RuBP carboxylase

Ocaloacetic acid + NADPH malic acidMalic acid CO2 + pyruvic acid

CO2 enters C3 pathway

C4 PlantsC4 Plants

Mesophyll CellMesophyll Cell

CO2

C-C-C

PEP

C-C-C-CMalate

ATP

Bundle Sheath CellBundle Sheath Cell

C-C-C

Pyruvic Acid

C-C-C-C

CO2

C3

Malate

Transported

glucoseVascular Tissue

CAM PlantsCAM Plants• Hot, dry environmentsHot, dry environments.

• 5% of plants (cactus and ice plants).5% of plants (cactus and ice plants).

• Stomates Stomates closed during dayclosed during day..

• Stomates Stomates open during the nightopen during the night.

• Light rxn - occurs during the day.

• Calvin Cycle - occurs when CO2 is present.

CAM Pathway

• CO2 is captured at night and stored in vacuoles of mesophyll cells is then

• CO2 is then converted to crassulacean acid

• IN the morning, crassulacean acid is converted back to CO2 and can enter the C3 pathway

• Stomata close during the day• Found in cacti & succulents

CAM PlantsCAM PlantsNight (Stomates Open) Day (Stomates Closed)

Vacuole

C-C-C-CMalate

C-C-C-CMalate Malate

C-C-C-CCO2

CO2

C3

C-C-CPyruvic acid

ATPC-C-CPEP glucose

Question:Question:

• Why would CAM plants close their Why would CAM plants close their stomates during the day?stomates during the day?

• Okay, let’s try this one more time…Okay, let’s try this one more time…

Calvin Cycle/ C3

Pathway

C4 Pathway CAM (Crassulacean Acid Metabolism)

RibuloseRuBP 

   

  CO2+PEPoxaloacetic acid (4C)Enzyme=PEP carboxylaseOccurs in mesophyll

CO2 crassulacean acid CO2 stored in vacuoles of mesophyll cells

  Oxaloacetid acid + NADPHPMalic acid CO2 + pyruvic acidOccurs in bundle sheath cells

crassulacean acid CO2

CO2 + RuBPDPGA (3C)Enzyme=RuBP carboxylase 

CO2 + RuBPDPGA (3C)Enzyme=RuBP carboxylase

CO2 + RuBPDPGA (3C)Enzyme=RuBP carboxylase

DPGA + 2NADPH2PGAL 

DPGA + 2NADPH2PGAL DPGA + 2NADPH2PGAL

Stomata opened Stomata close partially to reduce H2O loss 

Stomata close during the day

Rice, wheat, soybeans, cotton, tomatoes,

Crabgrass, sugarcane corn, pineapple

Cacti & succulents

Resources

• Photosynthesis Tutorial• Plant Metabolism• Photosynthesis Problem Set• DNA Tube• How Do Proteins Help Chlorophyll Carry Out

Photosynthesis? Virtual Lab• Calvin Cycle Tutorial • Photosynthesis Animation• Transpiration Virtual Plant Lab