PHOTOSYNTHESISTHREE PHASE REACTION

• LIGHT ABSORBTION (CHLOROPHYLL)

• LIGHT REACTION (LIGHT DEPENDENT)

• DARK REACTION (LIGHT INDEPENDENT)

Green Plants and PhotosynthesisAll of our food ultimately results from the process of photosynthesisin green plants and algae. The pigment chlorophyll is responsible for the green color of plants as well as their ability to photosynthesize. In common terrestrial plants photosynthesis is usually carried out in the leaves, although it can also occur in the stem or other parts of the plant.

Dr. C.E. Jeffree, Oxford Scientific Films

PHOTOSYNTHESISNET REACTION

6 CO2 + 12 H20 SUNLIGHT

CHLOROPHYLL

C6H12O6 + 6 O2 + 6 H2O

6 CO2 + 6 H20 C6H12O6 + 6 O2

SUNLIGHT

CHLOROPHYLL

PHOTOSYNTHESISACTUAL REACTION

DISCOVERED THROUGH THE USE OF O-18 TAGGED WATER

LIGHT ABSORBTION BY CHLOROPHYLL

• CHLOROPHYLL A IS THE PRIMARY PHOTOSYNTHETIC PIGMENT

• CHLOROPHYLL B, CAROTENE, PHYCOBILINS, XANTHROPHYLL, AND OTHER PIGMENTS ARE ACCESSORY PIGMENTS

• LOCATED IN THYLAKOID MEMBRANE OF THE CHLOROPLAST

• ABSORBS RED AND BLUE LIGHT, REFLECTS GREEN LIGHT

• ACCESSORY PIGMENTS TRAP GREEN AND YELLOW LIGHT AND TRANSFER IT TO CHLOROPHYLL A (allow greater use of available energy)

Chlorophyll a absorption and efficiency of Photosynthesis

STRUCTURE OF CHLOROPLAST

• GRANA ARRANGED IN STACKS TO MAXIMIZE EXPOSURE TO LIGHT - LIGHT REACTIONS OCCUR HERE.

• SHAPE OF GRANA MAXIMIZES SURFACE AREA FOR PHOTOSYNTHESIS

• GRANA SURROUNDED BY STROMA - PERMITS EASY DIFFUSION OF PHOTOSYNTHETIC PRODUCTS

• PRODUCTS MOVE BACK AND FORTH BETWEEN THE LUMEN AND THE STROMA

• STROMA IS A PROTEIN RICH SOLUTION IN WHICH DARK REACTIONS OCCUR

Chloroplast Structure

CHLOROPLAST STRUCTURE

GRANA STROMA

CHLOROPLAST MEMBRANE

THYLAKOIDS

Chloroplasts in Onion Root CellsChloroplasts are minute, green, spherical structures that are essential to the process of photosynthesis. The chlorophyll molecule, a very complex organic molecule composed primarily of magnesium, carbon, and hydrogen, regulates the absorption of the red, violet, and blue portions of the visible light spectrum.

Richard Kirby, Oxford Scientific Films

LIGHT REACTIONS• TWO PARTS

– CONVERSION OF SUNLIGHT TO ELECTRICAL ENERGY (ELECTRON TRANSPORT)

– ELECTRICAL ENERGY IS THEN CONVERTED TO CHEMICAL ENERGY STORED IN BONDS

PHOTOSYSTEM II

– LIGHT STRIKES CHLOROPHYLL - ELECTRONS ABSORB PHOTON OF LIGHT ENERGY AND BECOME “EXCITED”

– “EXCITED” ELECTRONS MOVE TO ELECTRON ACCEPTORS WHERE THEY MOVE TO A LOWER ENERGY STATE (ELECTRON TRANSPORT CHAIN) TRANSFERED TO PHOTOSYSTEM I

PHOTOSYSTEM I

– LIGHT ALSO STRIKES CHLOROPHYLL IN PHOTOSYSTEM I WHICH ALSO MOVE ALONG AN ELECTRON TRANSPORT CHAIN

– ELECTRONS REPLACED BY THOSE FROM PHOTOSYSTEM II– ELECTRONS ARE PICKED UP BY NADP TO FORM NADP- -

WATER IS SPLIT INTO TWO H+ IONS, TWO ELECTRONS, AND AN OXYGEN ATOM» The H + ions are used to reduce NADP to form NADPH2

» The electrons go to photosystem II» The oxygen atom combines with another oxygen to form O2

» NADPH2 goes to stroma to be used in DARK REACTIONS

PHOTOSYSTEM I

e -

ee

e

e

-

-

-

-

ATP

ATP

CHLOROPHYLL

ELECTRONACCEPTORS

ADP + P

ADP + P ELECTRONTRANSPORT CHAIN

e-

cytochromes

SUN

PHOTOSYSTEM II

NADP --

H202 H+

2 e-

O O2

NADPH2

ADP+ PATP

ELECTRON TRANSPORT CHAIN

CHLOROPHYLL II

CHLOROPHYLL I

ELECTRON TRANSPORT CHAIN

Photosystem I and II

CHEMIOSMOSIS

• DIFFUSION OF CHEMICALS THROUGH THE MEMBRANE RESULTING IN PRODUCTION OF ATP– ELECTRONS MOVE ALONG ELECTRON TRANSPORT CHAIN

FROM HIGH TO LOW CONCENTRATION– CONCENTRATION OF HYDROGEN IONS IS HIGHER INSIDE THE

LUMEN THAN IN THE STROMA– ELECTROCHEMICAL ENERGY IS GENERATED BY THE

MOVEMENT OF IONS ACCROSS THE MEMBRANE– ENERGY IS USED TO PHOSPHORYLATE ADP TO FORM ATP

(using ATP synthetase)– ATP IS THEN USED IN CARBON FIXATION IN DARK

REACTIONS

DARK REACTIONSCALVIN

CYCLE (C3)

• DOES NOT REQIRE LIGHT ENERGY• USES ENERGY STORED IN LIGHT REACTIONS

IN THE FORM OF ATP AND NADPH2

• INVOLVES FIXATION OF CO2 INTO CARBON COMPOUNDS

• OCCURS IN MOST PLANT SPECIES IN MODRATE CLIMATES

• DIVIDED INTO FOUR BASIC STEPS

FOUR STAGES OF THE DARK REACTIONS

• CO2 DIFFUSES INTO THE STROMA AND AN ENZYME IN THE THYLAKOID BINDS IT TO A 5-CARBON RDP (RuBP) FORMING AN UNSTABLE 6-CARBON MOLECULE

• THIS 6-CARBON MOLECULE SPLITS TO FORM TWO 3-CARBON PGA MOLECULES

• PGA IS CHANGED TO PGAL BY PHOSPHORYLATON USING ATP AND THE ADDITION OF H FROM NADPH2

• TWO PGAL MOLECULES COMBINE TO FORM A 6-CARBON COMPOUND FDP (fructose diphosphate) WHICH EITHER IS CONVERTED INTO GLUCOSE (1 FOR EVERY 6 CO2 FIXED) OR USED TO REGENERATE RDP SO THE CYCLE CAN CONTINUE

CALVIN CYCLE (C3)

CO2

6-C

PGA 3-C

PGA 3-C

PGAL

RDP 5-C

PGAL

FDP 6-C

RP 5-CINTERMEDIATE COMPOUNDS

GLUCOSE

ATP ADP

ATP ADP

NADPH2 NADP

NADPH2 NADP

ADP

ATP