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PHOTOSYNTHESIS
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YOU MUST KNOW…• HOW PHOTOSYSTEMS CONVERT
SOLAR ENERGY TO CHEMICAL ENERGY
• HOW LINEAR ELECTRON FLOW IN THE LIGHT REACTIONS RESULTS IN THE FORMATION OF ATP, NADPH, AND O2
• HOW CHEMIOSMOSIS GENERATES ATP IN THE LIGHT REACTIONS
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YOU MUST KNOW…• HOW THE CALVIN CYCLE USES THE
ENERGY MOLECULES OF THE LIGHT REACTIONS TO PRODUCE G3P
• THE METABOLIC ADAPTATIONS OF C4 AND CAM PLANTS TO ARID, DRY REGIONS
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CONCEPT 10.1• PHOTOSYNTHESIS CONVERTS
LIGHT ENERGY TO THE CHEMICAL ENERGY OF FOOD
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• AUTOTROPHS – PRODUCERS, SELF FEEDERSOURCE OF ORGANIC COMPOUNDS
• HETEROTROPHS – CONSUMER, LIVE ON COMPOUNDS PRODUCED BY OTHER ORGANISMS
• HETEROTROPHS INCLUDE THE DECOMPOSERS AND ARE DEPENDENT ON PHOTOSYNTHESIS FOR FOOD AND OXYGEN
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CHLOROPLASTS• SITES FOR PHOTOSYNTHESIS• MAKE UP THE MESOPHYLL TISSUE
FOUND IN THE INTERIOR OF THE LEAF
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• STROMA – ENCLOSED BY 2 MEMBRANES, A DENSE FLUID-FILLED AREA
• STROMA CONTAINS INTERCONNECTED MEMBRANOUS SACS CALLED THYLAKOIDS
• CHLOROPHYLL – LOCATED IN THE THYLAKOID MEMBRANES AND IS THE LIGHT-ABSORBING PIGMENT
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STOMATA• TINY PORES ON THE EXTERIOR OF
THE LOWER EPIDERMIS OF A LEAF• CARBON DIOXIDE ENTERS AND
OXYGEN AND WATER VAPOR EXIT THE LEAF
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PHOTOSYNTHESIS 6 CO2 + 6 H2O + LIGHT C6H12O6 + 6 O2
• OXYGEN WE BREATHE IS FROM SPLITTING OF A WATER MOLECULE
• WATER IS SPLIT FOR ITS ELECTRONS, WHICH ARE TRANSFERRED ALONG WITH HYDROGEN IONS FROM WATER TO CARBON DIOXIDE, REDUCING IT TO SUGAR
• PROCESS REQUIRES ENERGY (ENDERGONIC) WHICH IS PROVIDED FROM THE SUN
• PROCESS IS 2 STAGES
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LIGHT REACTIONS• OCCURS IN THYLAKOID MEMBRANES
WHERE SOLAR ENERGY IS CONVERTED TO CHEMICAL ENERGY
• LIGHT IS ABSORBED BY CHLOROPHYLL AND DRIVES THE TRANSFER OF ELECTRONS FROM WATER TO NADP+, FORMING NADPH
• WATER IS SPLIT & OXYGEN IS RELEASED• ATP IS GENERATED, USING CHEMIOSMOSIS
TO POWER THE ADDITION OF A PHOSPHATE GROUP TO ADP (PHOTOPHOSPHORYLATION).
• NET PRODUCTS ARE NADPH (STORES ELECTRONS), ATP, AND OXYGEN
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CHEMIOSMOSIS
•MOVEMENT OF IONS ACROSS A SELECTIVELY PERMEABLE MEMBRANE, DOWN THEIR ELECTROCHEMICAL GRADIENT•IT RELATES TO THE GENERATION OF ATP BY THE MOVEMENT OF HYDROGEN IONS ACROSS A MEMBRANE FROM AN AREA OF HIGH CONCENTRATION TO AN AREA OF LOW CONCENTRATION.
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CALVIN CYCLE• OCCURS IN THE STROMA• CO2 FROM THE AIR IS INCORPORATED
INTO ORGANIC MOLECULES IN CARBON FIXATION
• CALVIN CYCLE USED THE FIXED CARBON & NADPH & ATP FROM THE LIGHT REACTION IN THE FORMATION OF NEW SUGARS
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CONCEPT 10.2• THE LIGHT
REACTIONS CONVERT SOLAR ENERGY TO THE CHEMICAL ENERGY OF ATP AND NADPH
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• LIGHT IS MADE UP OF PARTICLES CALLED PHOTONS - IT HAS A FIXED QUANTITY OF ENERGY
• PIGMENTS – CHLOROPHYLL ABSORBS VIOLET-BLUE AND RED LIGHT WHILE REFLECTING GREEN LIGHT
• ABSORPTION SPECTRUM – A GRAPH PLOTTING A PIGMENT’S LIGHT ABSORPTION VS. WAVELENGTH
• ACTION SPECTRUM – GRAPHS THE EFFECTIVENESS OF DIFFERENT WAVELENGTHS OF LIGHT IN DRIVING PHOTOSYNTHESIS
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PHOTOSYSTEMS• PHOTONS OF LIGHT ARE ABSORBED
BY CERTAIN GROUPS OF PIGMENT MOLECULES IN THE THYLAKOID MEMBRANE OF CHLOROPLASTS
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LIGHT-HARVESTING COMPLEX• MADE UP OF MANY CHLOROPHYLL AND
CAROTENOID MOLECULES FOR GATHERING LIGHT EFFECTIVELY
• WHEN A PHOTON IS ABSORBED, ONE OF THE ELECTRONS IS RAISED TO AN ORBITAL OF HIGHER POTENTIAL ENERGY AND THE CHLOROPHYLL IS IN AN “EXCITED” STATE
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• THE ENERGY IS TRANSFERRED TO THE REACTION CENTER OF THE PHOTOSYSTEM
• THE CENTER CONSISTS OF 2 CHLOROPHYLL a MOLECULES, WHICH DONATE THE ELECTRONS TO THE SECOND MEMBER OF THE REACTION CENTER-PRIMARY ELECTRON ACCEPTOR
• THE ENERGY TRANSFER FROM THE REACTION CENTER TO THE PRIMARY ELECTRON ACCEPTOR IS THE FIRST STEP OF THE LIGHT REACTIONS
• CONVERSION OF LIGHT ENERGY TO CHEMICAL ENERGY OCCURS
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PHOTOSYSTEMS I AND II• PS I IS DESIGNATED P700 BECAUSE
THE CHLOROPHYLL a BEST ABSORBS RED LIGHT OF THIS WAVELENGTH THE BEST
• PS II IS DESIGNATED P680• THE KEY IS A FLOW OF ELECTRONS
THROUGH THE PHOTOSYSTEMS IN THE THYLAKOID MEMBRANE, A PROCESS CALLED LINEAR NONCYCLIC) ELECTRON FLOW
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CYCLIC ELECTRON FLOW
• USES A SHORT CIRCUIT OF LINEAR ELECTRON FLOW BY CYCLING THE EXCITED ELECTRONS BACK TO THEIR ORIGINAL STARTING POINT IN PS I
• CYCLIC ELECTRON FLOW PRODUCES ATP BY CHEMIOSMOSIS BUT NO NADPH IS PRODUCED AND NO OXYGEN IS RELEASED
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CONCEPT 10.3
• THE CALVIN CYCLE USES ATP AND NADPH TO CONVERT CO2 TO SUGAR
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CALVIN CYCLE• CARBON ENTERS IN THE FORM
OF CO2 AND LEAVES IN THE FORM OF A SUGAR
• SPENDS ATP AS AN ENERGY SOURCE AND CONSUMES NADPH AS REDUCING POWER FOR ADDING HIGH ENERGY ELECTRONS TO MAKE THE SUGAR
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• TO NET ONE MOLECULE
OF G3P, THE CYCLE
MUST GO THROUGH
3 ROTATIONS AND
FIX 3 MOLECULES
OF CO2
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MAJOR STEPS OF CYCLE• 3 CO2 MOLECULES ARE ATTACHED TO 3
MOLECULES OF THE 5-CARBON SUGAR RIBULOSE BISPHOSPHATE (RuBP)
• THESE REACTIONS ARE CATALYZED BY THE ENZYME RUBISCO AND PRODUCE AN UNSTABLE PRODUCT THAT IMMEDIATELY SPLITS INTO 2 THREE-CARBON COMPOUNDS CALLED 3-PHOSPHOGLYCERATE
• CO2 HAS NOW BEEN FIXED (INTO AN ORGANIC COMPOUND
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• THE 3-PHOSPHOGLYCERATE MOLECULES ARE PHOSPHORYLATED TO BECOME 1, 3-BISPHOSPHOGLYCERATE
• 6 NADPH THEN REDUCE THE 6 1,3-BISPHOSPHOGLYCERATE TO SIX GLYCERALDEHYDE-3-PHOSPHATE (G3P)
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• ONE G3P LEAVES THE CYCLE TO BE USED BY THE PLANT CELL
• TWO G3P MOLECULES CAN COMBINE TO FORM GLUCOSE
• RuBP IS REGENERATED AS THE 5 G3Ps ARE REWORKED INTO 3 OF THE STARTING MOLECULES WITH THE EXPENDITURE OF 3 ATP MOLECULES
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ENERGY NEEDED TO NET ONE G3P
• 9 MOLECULES OF ATP ARE CONSUMED ALONG WITH 6 MOLECULES OF NADPH (BOTH REPLENISHED BY THE LIGHT REACTIONS)
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CONCEPT 10.4• ALTERNATIVE MECHANISMS OF
CARBON FIXATION HAVE EVOLVED IN HOT, ARID CLIMATES
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• CO2 ENTERS THE LEAF THROUGH THE STOMATA, THE SAME PORE THAT WATER EXITS THE LEAF IN TRANSPIRATION
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C3 PLANTS• ON HOT, DRY DAYS C3 PLANTS PRODUCE
LESS SUGAR BECAUSE OF DECLINING LEVELS OF CO2 IN THE LEAF STARVING THE CALVIN CYCLE
• STOMATA CLOSES TO CONSERVE WATER• RUBISCO CAN BIND O2 IN PLACE OF CO2• CAUSES THE OXIDATION OF RuBP
RESULTING IN A LOSS OF ENERGY AND CARBON FOR THE PLANT (CALLED PHOTORESPIRATION)
• THIS CAN DRAIN AWAY AS MUCH AS 50% OF THE CARBON FIXED BY THE CALVIN CYCLE
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C4 PLANTS• HAVE 2 TYPES OF PHOTOSYNTHETIC
CELLS• BUNDLE-SHEATH CELL – GROUPED
AROUND THE LEAF’S VEINS• MESOPHYLL CELLS – DISPERSED
ELSEWHERE AROUND THE LEAF
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C4 PLANTS
• CO2 IS ADDED TO PHOSPHOENOLPYRUVATE (PEP) TO FORM THE FOUR-CARBON COMPOUND OXALOACETATE
• REACTION IS CATALYZED BY PEP CARBOXYLASE WHICH DOES NOT COMBINE WITH O2 AND DOES NOT PARTICIPATE IN PHOTORESPIRATION
• CARBON IS NOW FIXED
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• MESOPHYLL CELLS EXPORT THE OXALOACETATE TO THE BUNDLE-SHEATH CELLS, WHICH BREAK IT DOWN AND RELEASE CO2
• CO2 IN THE BUNDLE-SHEATH CELLS IS THEN CONVERTED INTO CARBOHYDRATES THROUGH THE NORMAL CALVIN CYCLE
• PHOTORESPIRATION IS REDUCED SINCE CO2 CONCENTRATION IS HIGH AND RUBISCO BINDS TO IT INSTEAD OF OXYGEN
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CAM PHOTOSYNTHESIS• ANOTHER ADAPTATION TO HOT, DRY
CLIMATES• PLANTS KEEP THEIR STOMATA
CLOSED DURING THE DAY • AT NIGHT, THE STOMATA OPEN AND
CO2 IS FIXED IN ORGANIC ACIDS AND STORED IN VACUOLES
• PHOTOSYNTHESIS CONTINUES DURING THE DAY
• LATER ENTERS THE CALVIN CYCLE