PHOTOSYNTHESIS

Light Energy to Chemical Energy

Importance of Photosynthesis

• Source of atmospheric oxygen– Earth’s atmosphere

• 78% N2, 21% O2, 0.035% CO2

• Source of energy for food chain– Photoautotrophs vs. Heterotrophs

• Direct/indirect involvement in all products• Maintain stable ecosystem

Photosynthesis and TranspirationWhole Plant Perspective

• CO2 enters leaf through open stomata; water is lost

• CO2 is converted to carbohydrate in the leaf & transported as sucrose in phloem

• Water is taken up by roots & transported via xylem

• Loss of water from the leaf via stomata is evapotranspiration

Photosynthesis

6 H20 + 6 CO2 + Light Energy C6H12O6 + 6 O2

Chloroplast

GranumThylakoid

Stroma

Outer Membrane

Inner Membrane

Thylakoid

Thylakoid Membrane

Thylakoid SpaceGranum

Grana – Site of ‘Light Reactions’. Pigments Embedded in Thylakoid Membranes.

Stroma – Site of ‘Light-Independent Reactions’. CO2 Fixed into Sugar (Glucose). Membrane – Bounded by

2 smooth membranes

Visible light

Chlorophyll pigments harvest energy (photons) by absorbing certain wavelengths– Blue (420 nm)– Red (660 nm)

Plants are green because the green wavelength is reflected, not absorbed.

Photosynthetic Pigments

• Main photosynthetic pigment– Chlorophyll a

• Accessory Pigments– Absorb light & transfer energy to Chl a– Chlorophyll b– Carotenoids (Aids in photoprotection)

• Carotenes• Xanthophylls

Photosynthetic Unit

• Chlorophyll bound to proteins in PS

• Light harvesting molecules pass their energy at a reaction center

• Two photosystems– PS I (P700)– PS II (P680)

Processes in Photosynthesis

(1) light O2 CO2

sugar

starchH2O

(2) Light Reactions (3) Dark Reactions

NADPH

ATP

3 Processes in Photosynthesis

• Absorption of light energy– Pigments in thylakoids

• Light Reactions– Pigments & proteins in thylakoids– Generates ATP and NADPH

• “Dark” Reactions– Enzymes in stroma

– Reduce CO2 to carbohydrates using ATP & NADPH

The light-dependent reactions of photosynthesis produce chemical energy in the form of ATP and NADPH.

Lab 4B

• Expose samples to red wavelength– Why?

• Role of DPIP– Electron acceptor

• Works in place of NADP+– When DPIP is reduced, resultant increases

light transmittance

Light Reaction

• Light Dependent Reaction• Captures Light Energy

– Electrons exited to higher energy state • Produces energy from solar power (photons) in the

form of ATP and NADPH

• Splits H2O, Uses H+ & Electrons, Spits out O2 as waste

• Two possible routes for electron flow:– Noncyclic Electron Flow– Cyclic Electron Flow

Light ReactionNoncyclic Electron Flow

• Uses Photosystem II and Photosystem I• Uses Electron Transport Chain (ETC)

– Pq (Plastoquinone)– Cytochrome complex– Pc (Plastocyanin – Cu-containing proteins)

• Other proteins involved (P700)– Fd (Ferredoxin – Fe-containing protein)– NADP+ reductase

• Generates O2, ATP, and NADPH

Light ReactionCyclic Electron Flow

• Uses Photosystem I only• Uses Electron Transport Chain (ETC)

– Fd (Ferredoxin)– Cytochrome complex

• Generates ATP only– No production of NADPH

– No release of O2

Cyclic Electron Flow

Why Cyclic Electron Flow?

• Calvin cycle consumes more ATP than NADPH

• Makes up the difference needed for the Calvin cycle to function properly

Light Reactions

Inputs Outputs

Light

H2O

ADP

NADP+

O2 (Waste)

ATP

NADPH + H+

Energy Carrier Molecules Take Converted ‘Light’

Energy to Stroma

Chemiosmosis

• Powers ATP synthesis• Located in the thylakoid membranes• Uses ETC and ATP synthase (enzyme) to

make ATP• Photophosphorylation

– Addition of phosphate to ADP to make ATP

Dark Reaction

• Light Independent Reaction, Calvin Cycle, Carbon Fixation

• Occurs in the stroma • Uses energy (ATP and NADPH) from light reaction

to make sugar (glucose) from CO2

• Uses CO2 • Regenerates ‘Empty’ Carrier Molecules (NADP+ &

ADP) • To produce glucose

– It takes 6 turns and uses 18 ATP and 12 NADPH

3 Phases – Calvin Cycle

• Carbon fixation

• Reduction• G3P routes

PhotosynthesisPhotosynthesis

HH22OO COCO22

OO22 CC66HH1212OO66

Light Light ReactionReaction

Dark ReactionDark Reaction

Light is AdsorbedLight is AdsorbedBy By

ChlorophyllChlorophyll

Which splitsWhich splitswaterwater

ChloroplastChloroplast

ATP andATP andNADPHNADPH22

ADPADPNADPNADP

Calvin CycleCalvin Cycle

EnergyEnergy

Used Energy and is Used Energy and is recycled.recycled.

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Types of plantsProcesses of Photosynthesis

• C3 plants• C4 plants• CAM plants

C3 photosynthesis

• Called ‘C3’ - 3-carbon acid (PGA)• Catalyzed by enzyme Rubisco

– Most abundant enzyme on Earth! • C3 pathway used by 85% of plants• Most trees and many crops use C3

photosynthesis

Relatively high water loss

Photorespiration

• Occurs on hot, dry, bright days• Stomata close• Fixation of O2 instead of CO2

• Produces 2-C molecules instead of 3-C sugar molecules

• Produces no sugar molecules or no ATP• Plants have special adaptations to limit the

effect of photorespiration.– C4 plants & CAM plants

C4 Plants

• Hot, moist environments• 15% of plants (grasses, corn, sugarcane)• Divides photosynthesis spatially• Light rxn - mesophyll cells• Calvin cycle - bundle sheath cells

C4 photosynthesis

• Four carbon molecule as 1st intermediate• Catalyzed by phosphoenolpyruvate

carboxylase (PEP carboxylase)• PEP carboxylase reacts effectively with CO2

at low concentrations• C4 plants can achieve high photosynthesis

with small stomatal opening, thereby saving water

• Carbon Fixation & Calvin cycle occur in two types of cells

CAM Plants

• Crassulacean Acid Metabolism • Hot, dry environments• 5% of plants (cactus and ice plants)• Stomates closed during day• Stomates open during the night

– Lower temperatures at night reduce water loss• Light rxn - occurs during the day• Calvin Cycle - occurs when CO2 is present• Carbon Fixation and Calvin cycle occur in the

same cell but at different times