PHOTOSYNTHESISPHOTOSYNTHESIS

PhotosynthesisPhotosynthesis• An anabolicanabolic, , endergonicendergonic, carbon, carbon

dioxide (COdioxide (CO22)) requiring process that uses light energy (photons)light energy (photons) and water (Hwater (H22O)O) to produce organic organic macromolecules (glucose).macromolecules (glucose).

6CO2 + 6H2O C6H12O6 + 6O2

glucoseglucose

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photonsphotons

Question:Question:

• In what types of organisms In what types of organisms does photosynthesis take does photosynthesis take place?place?

•In plants, bacteria, and protists

PlantsPlants

• AutotrophsAutotrophs:: self-producers.

• Location:

1. Leaves

a. stoma

b. mesophyll cells

StomaMesophyllCell

Chloroplast

StomataStomata • 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)

Stomata Regulate Gas Exchange

Leaf Anatomy

ChloroplastChloroplast

• OrganelleOrganelle where photosynthesisphotosynthesis takes place.

GranumThylakoid

Stroma

Outer Membrane

Inner Membrane

• Chloroplasts

–Contain thylakoids and grana

Chloroplast

Mesophyll

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

Thylakoid

GranumStroma

1 µm

ThylakoidsThylakoids

Thylakoid Membrane

Thylakoid SpaceGranum

Mesophyll CellMesophyll Cell

Cell Wall

Nucleus

Chloroplast

Central Vacuole

QuestionQuestion::

• Why are plants green?Why are plants green?

Chlorophyll MoleculesChlorophyll Molecules• Located in the thylakoid membranesthylakoid membranes.• Chlorophyll have MgMg++ in the center.

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

• PlantsPlants are greengreen because the green wavelengthwavelength is reflectedreflected, not absorbednot absorbed.

Why leaves are green: interaction of light with chloroplasts

Location and Structure of Chlorophyll Molecules in Plants

Wavelength of LightWavelength of Light (nm)(nm)

400 500 600 700

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

The Electromagnetic Spectrum

Absorption of ChlorophyllAbsorption of Chlorophyll

wavelengthwavelength

Absorption

violet blue green yellow orange red

Evidence that chloroplast pigments participate in photosynthesis: absorption and action spectra for

photosynthesis in an alga

Question:Question:

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

Redox ReactionRedox Reaction• The transfertransfer of oneone or more more

electronselectrons from one reactantone reactant to anotheranother.

• Two typesTwo 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

Oxidation Oxidation (releases energy) (releases energy)• Combining with OxygenCombining with Oxygen•Loss of ElectronsLoss of Electrons•Loss of HydrogenLoss of Hydrogen

Reduction Reduction (absorbs energy)(absorbs energy)•Separation from OxygenSeparation from Oxygen•Gain of ElectronsGain of Electrons•Gain of HydrogenGain of Hydrogen

RememberRemember::““Leo says Ger”Leo says Ger”

Loss of electrons is oxidation;Loss of electrons is oxidation;Gain of electrons is reduction.Gain of electrons is reduction.

or or ““Oil Rig” Oil Rig”

Oxidation is loss; Reduction is Oxidation is loss; Reduction is gain.gain.

Breakdown of PhotosynthesisBreakdown of Photosynthesis

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

1. Light Reaction or1. Light Reaction or

Light Dependent ReactionLight Dependent Reaction

Produces energyenergy from solarsolar powerpower (photons)(photons) in the form of ATPATP and NADPHNADPH.

Breakdown of PhotosynthesisBreakdown of Photosynthesis

2.2. Calvin Cycle orCalvin Cycle or

Light Independent Reaction Light 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 sugar (glucose).(glucose).

An Overview of Photosynthesis: Cooperation of the Light Reactions and the Calvin Cycle

1. 1. Light ReactionLight Reaction (Electron Flow) (Electron Flow)• Occurs in the thylakoid thylakoid

membranesmembranes• 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. A. Cyclic Electron FlowCyclic 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

How a Photosystem Harvests Light

A. A. Cyclic Electron FlowCyclic Electron Flow

P700

PrimaryElectronAcceptor

e-

e-

e-

e-

ATPATPproducedby ETC

Photosystem I

AccessoryPigments

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Photons

Cyclic Electron Flow

B. B. Noncyclic Electron FlowNoncyclic Electron Flow• 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

• Produces NADPH, ATP, and oxygen

Figure 10.13Photosystem II

(PS II)

Photosystem-I(PS I)

ATP

NADPH

NADP+

ADP

CALVINCYCLE

CO2H2O

O2 [CH2O] (sugar)

LIGHTREACTIONS

Light

Primaryacceptor

Pq

Cytochromecomplex

PC

e

P680

e–

e–

O2

+

H2O

2 H+

Light

ATP

Primaryacceptor

Fd

ee–

NADP+

reductase

ElectronTransportchain

Electron transport chain

P700

Light

NADPH

NADP+

+ 2 H+

+ H+

1

5

7

2

3

4

6

8

B. B. Noncyclic Electron FlowNoncyclic Electron Flow

P700

Photosystem IP680

Photosystem II

PrimaryElectronAcceptor

PrimaryElectronAcceptor

ETC

EnzymeReaction

H2O

1/2O1/2O22 + 2H+

ATPATP

NADPHNADPH

Photon

2e-

2e-

2e-

2e-

2e-

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Photon

B. B. Noncyclic Electron FlowNoncyclic Electron Flow• 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)

How Noncyclic Electron Flow During the Light Reactions Generates ATP and NADPH

ChemiosmosisChemiosmosis• Powers ATP synthesisATP synthesis.

• Located in the thylakoid thylakoid membranesmembranes.

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

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

ChloroplastChloroplast

GranumThylakoid

Stroma

Outer Membrane

Inner Membrane

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)

The light reactions and chemiosmosis: the organization of the thylakoid membrane

Comparison of chemiosmosis in mitochondria and chloroplasts

Calvin Cycle• Carbon Fixation (light independent Carbon Fixation (light independent

rxn).rxn).

• C3 plants (80% of plants on earth).

• Occurs in the stroma.• Uses ATP and NADPH from light rxn

and also uses CO2 from air.

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

The Calvin Cycle

Calvin CycleCalvin Cycle (C (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

A Review of Photosynthesis

PhotorespirationPhotorespiration• 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 sugar molecules3-C sugar molecules.

• Produces no sugar molecules or no ATP.

PhotorespirationPhotorespiration• Because of photorespirationBecause of photorespiration:

PlantsPlants have special special adaptationsadaptations 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 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

C4 leaf anatomy and the C4 pathway

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 PlantsCAM Plants

Night (Stomata Open) Day (Stomata 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

C4 and CAM Photosynthesis Compared

QuestionQuestion::

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