ENERGY-ACQUIRING PATHWAYS

PHOTOSYNTHESIS IN HIGHER PLANTS CONSISTS OF TWO STAGES

1. LIGHT-DEPENDENT REACTIONS
a. ENERGY FROM SUNLIGHT ABSORBED AND CONVERTED TO
ATP ENERGY
b. WATER MOLECULE IS SPLIT, OXYGEN, AND HYDROGEN
AND ELECTRONS ARE LIBERATED
c. NADP+ PICKS UP THE LIBERATED HYDROGEN AND
ELECTRONS TO FORM NADPH

2. LIGHT-INDEPENDENT REACTIONS
a. ATP ENERGY DONATED TO SITES WHERE GLUCOSE IS
MADE FROM CARBON, HYDROGEN AND OXYGEN
b. CARBON DIOXIDE PROVIDES CARBON AND OXYGEN
c. HYDROGEN IS BROUGHT IN AS NADPH

12H2O + CO2 -> 6O2 + C6H12O6 + 6H2O

sunlight

GLUCOSE MOLECULES COMBINE WITH EACH OTHER SIMPLE SUGARS TO FORM SUCROSE, STARCH AND
OTHER CARBOHYDRATES

WHERE THE REACTIONS TAKE PLACE

BOTH REACTIONS TAKE PLACE INSIDE THE CHLOROPLAST

THE CHLOROPLAST IS SURROUNDED BY A DOUBLE MEMBRANE AND CONTAINS A SERIES OF STACKS
OF FLATTENED SACS (THYLAKOID MEMBRANES) LOCATED IN THE STROMA.

THE LIGHT DEPENDENT REACTIONS OCCUR IN THE THYLAKOID MEMBRANES

THE LIGHT INDEPENDENT REACTIONS OCCUR IN THE STROMA

LIGHT TRAPPING PIGMENTS

PIGMENTS - MOLECULES THAT ABSORB LIGHT.

PHOTONS ARE PACKETS OF LIGHT ENERGY

1. CHLOROPHYLLS - ABSORB RED AND BLUE LIGHT AN
TRANSMIT GREEN LIGHT
a. CHLOROPHYLL a - MAIN PIGMENT
b. CHLOROPHYLL b

2. CAROTENOIDS - ABSORB VIOLET AND BLUE AND TRANSMIT
RED,ORANGE AND YELLOW

3. PHYCOBILINS - RED AND BLUE PIGMENTS

CHLOROPHYLL b, CAROTENOIDS AND PHYCOBILINS ARE ACCESSORY PIGMENTS THAT TRAP
WAVELENGTHS OF LIGHT THAT CHLOROPHYLL a CANNOT. THEY PASS THE TRAPPED ENERGY ON TO
CHLOROPHYLL a.

LIGHT-DEPENDENT REACTIONS

THREE EVENTS:

1. PIGMENTS ABSORB SUNLIGHT ENERGY AND GIVE UP
ELECTRONS
2. ATP AND NADPH FORMATION
3. PIGMENTS THAT GAVE UP ELECTRONS GAIN ELECTRONS

PHOTOSYSTEMS

THOUSANDS OF PHOTOSYSTEMS ARE EMBEDDED IN THE THYLLAKOID MEMBRANES

EACH PHOTOSYSTEM HAS SEVERAL HUNDRED PIGMENT MOLECULES

1. LIGHT ENERGY IS ABSORBED BY THE PIGMENT MOLECULES
2. THE SUNLIGHT ENERGY BOOSTS AN ELECTRON IN A
PIGMENT MOLECULE TO A HIGHER ENERGY STATE
3. THE ENERGIZED ELECTRON BOUNCES AROUND IN THE
LIGHT HARVESTING COMPLEX, LOSING ENERGY AT EACH
BOUNCE, UNTIL THE WAVELENGTH IS THE WAVELENGTH
NEEDED TO ACTIVATE THE CHLOROPHYLL MOLECULE THAT
GIVE OFF THE ELECTRONS USED FOR PS.

ATP AND NADPH FORMATION

TRAPPED ELECTRONS ARE TRANSPORTED DOWN AN ELECTRON TRANSPORT CHAIN TO PRODUCE ATP
AND NADPH

ELECTRON TRANSPORT CHAINS ARE ORGANIZED SEQUENCES OF ENZYMES AND OTHER PROTEINS
BOUND TO CELL MEMBRANES

1. ELECTRONS PASS FROM PROTEIN TO PROTEIN
2. EXTRA ENERGY IS RELEASED TO DRIVE SPECIFIC
REACTIONS

TWO KINDS OF PHOTOSYSTEMS GIVE UP ELECTRONS IN PLANTS TO MAKE ATP BY TWO DIFFERENT
PATHWAYS

THESE TWO PHOTOSYSTEMS ARE:

1. PHOTOSYSTEM I
2. PHOTOSYSTEM II

THE TWO PATHWAYS PRODUCING ATP ARE:

1. CYCLIC
2. NONCYCLIC

CYCLIC PATHWAY

1. ELECTRON EXITS CHLOROPHYLL (P700) IN PHOTOSYSTEM
I
2. EXCITED ELECTRON ACCEPTED BY AN ELECTRON ACCEPTOR
3. ELECTRON PASSED THROUGH A CHAIN OF ELECTRON
ACCEPTORS BACK TO P700

ATP FORMED FROM ADP AND PHOSPHATE AS THE ELECTRON PASSES DOWN THE CHAIN

THE CYCLIC PATHWAY IS THE OLDEST MEANS OF ATP PRODUCTION. NOT MUCH IS PRODUCED IN
THIS PROCESS

THE ELECTRONS THAT LEAVE PHOTOSYSTEM I RETURN

NONCYCLIC PATHWAY

THE ELECTRONS THAT EXIT PHOTOSYSTEM II DO NOT RETURN.

THEY ARE CARRIED AWAY AS PART OF NADPH

1. LIGHT HITS A CHLOROPHYLL MOLECULE(P680)IN
PHOTOSYSTEM II
2. CHLOROPHYLL GIVES UP AN ELECTRON AND A WATER
MOLECULE SPLITS INTO OXYGEN, HYDROGEN IONS AND
ELECTRONS
3. THE EXCITED ELECTRON IS ACCEPTED BY AN ELECTRON
ACCEPTOR AND THE ELECTRONS FROM THE WATER
MOLECULE REPLACE THE EXCITED ELECTRONS
4. THE ELECTRONS FROM P680 ARE PASSED ALONG AN
ELECTRON TRANSPORT CHAIN TO P700 WHERE THEY ARE
REACTIVATED
5. ACTIVATED ELECTRONS ACCEPTED BY ANOTHER ELECTRON
ACCEPTOR
6. ELECTRONS PASSED DOWN ANOTHER ELECTRON TRANSPORT
CHAIN
7. TWO ELECTRONS ALONG WITH A HYDROGEN ION COMBINE
WITH NADP+ TO FORM NADPH

ATP FORMATION IN CHLOROPLASTS

MADE ALONG THE ELECTRON TRANSPORT SYSTEM

1. ELECTRONS FLOW THROUGH THE MEMBRANE-BOUND
TRANSPORT SYSTEM
2. THE ELECTRONS PICK UP H+ IONS OUTSIDE THE
THYLAKOID MEMBRANE AND DUMP THEM OFF IN THE
INSIDE (LUMEN) SETTING UP A H+ CONCENTRATION AND
AN ELECTRIC GRADIENTS ACROSS THE MEMBRANE
3. H+ IONS FROM WATER BEING SPLIT IN THE LUMEN ADD
TO THE CONCENTRATION OF IONS
4. IONS FLOW OUT OF THE LUMEN ACROSS THE MEMBRANES
CAUSING AN ENERGY FLOW
5. ENERGY USED TO BIND PHOSPHATE TO ADP FORMING ATP

LECTURE 12

LIGHT-INDEPENDENT REACTIONS

SYNTHESIS PART OF PHOTOSYNTHESIS

USES ATP AND NADPH FROM THE LIGHT REACTION ALONG WITH CO2 TO FORM CARBOHYDRATES

CAPTURING CARBON

CARBON DIOXIDE FIXATION

1. CO2 DIFFUSES INTO THE AIR SPACES IN A LEAF
2. CO2 DIFFUSES ACROSS THE PLASMA MEMBRANE AND INTO
THE STROMA OF A CHLOROPLAST
3. CO2 ATTACHES TO RuBP (RIBULOSE BISPHOSPHATE)
BUILDING GLUCOSE SUBUNITS

CALVIN-BENSON OR CALVIN CYCLE - PRODUCES A SUGAR-PHOSPHATE MOLECULE AND REGENERATES
RuBP

NAMED IN HONOR OF MELVIN CALVIN WHO DISCOVERED THE CYCLE

CO2 + RuBP ---> UNSTABLE 6-C MOLECULE

6-C MOLECULE ---> 2PGA

(PGA=PHOSPHOGLYCERATE; EACH PGA HAS 3 CARBONS)

PGA + ATP + H+ + e- ---> PGAL

(PGAL = PHOSPHOGLYCERALDEHYDE)

THE ABOVE REACTION OCCURS SIX TIMES TO MAKE A 6-C MOLECULE AND 12 PGALís

TWO PGALís COMBINE TO FORM A 6-CARBON SUGAR PHOSPHATE WHICH GOES INTO MORE REACTIONS

THE OTHER 10 PGALís ARE REARRANGED TO FORM MORE RuBP TO REPLACE THE ONES USED (6)

THE ADP, NADP+, AND PHOSPHATES THAT ARE LEFT OVER DIFFUSE BACK TO THE SITES OF THE
LIGHT-DEPENDENT REACTIONS WHERE THEY ARE NEEDED AND CONVERTED BACK TO ATP AND NADPH

THE 6-C MOLECULE PRODUCED IS A BUILDING BLOCK FOR SUCROSE, STARCH OR CELLULOSE

HISTORY

THREE PATHWAYS OF CARBON FIXATION

1. C3 - FIRST PRODUCT OF CO2 FIXATION IS A 3 CARBON
COMPOUND
2. C4 - FIRST PRODUCT OF CO2 FIXATION IS A 4 CARBON
COMPOUND
3. CAM - CRASSULACEAN ACID METABOLISM

C3 PLANTS ARE CAPABLE OF PHOTORESPIRATION.

RuBisCO - ENZYME THAT ADDS CO2 TO RuBP TO FORM AN
UNSTABLE 6-C COMPOUND

CAN ALSO ADD O2 TO RuBP MAKING IT UNAVAILABLE FOR CARBON FIXATION

THIS MAKES FOR A LESS PRODUCTIVE PLANT

C4 METABOLISM

1. OCCURS IN MANY GRASSES AND WEEDS
2. HAS A STABLE 4-C COMPOUND AS ITíS FIRST PRODUCT
a. OAA (OXALOACETATE)
3. CLOSES STOMATES DURING TIMES OF WATER STRESS
a. C4 PHOTOSYNTHESIS OCCURS IN MESOPHYLL CELLS
AND OAA IS PRODUCED
b. OAA IS TRANSPORTED TO BUNDLE SHEATH CELLS
WHERE CO2 IS RELEASED AND FIXED AGAIN BY THE
C3 PATHWAY
4. 80% OF ALL PLANT SPECIES THAT EVOLVED IN FLORIDA
ARE C4 PLANTS.

CAM PLANTS

FIX CARBON AT NIGHT AND USE IT BY DAY

FOUND PREDOMINANTLY IN PLANTS GROWING IN SEVERELY DRY ENVIRONMENTS SUCH AS DESERTS

STOMATES ARE CLOSED DURING THE DAY AND OPEN AT NIGHT.

1. COLLECT CO2 AT NIGHT WHEN STOMATES ARE OPEN AND
STORE IT IN THE VACUOLE
2. USE STORED CO2 DURING THE DAY WHEN THE STOMATES
ARE CLOSED
3. CARBON IS FIXED IN THE SAME TISSUES AT DIFFERENT
TIMES
4. GROW SLOWLY

CHEMOSYNTHESIS

1. CONDUCTED BY SOME BACTERIA CALLED CHEMOAUTOTROPHS
a. ENERGY FROM AMMONIUM IONS, IRON OR SULFUR
COMPOUNDS

AUTOTROPHS - ORGANISMS CAPABLE OF PRODUCING THEIR OWN FOOD

HETEROTROPHS - ORGANISMS NOT CAPABLE OF PRODUCING THEIR OWN FOOD (ANIMALS)