PHOTOSYNTHESIS

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

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

CONCEPT 10.1• PHOTOSYNTHESIS CONVERTS

LIGHT ENERGY TO THE CHEMICAL ENERGY OF FOOD

• 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

CHLOROPLASTS• SITES FOR PHOTOSYNTHESIS• MAKE UP THE MESOPHYLL TISSUE

FOUND IN THE INTERIOR OF THE LEAF

• 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

STOMATA• TINY PORES ON THE EXTERIOR OF

THE LOWER EPIDERMIS OF A LEAF• CARBON DIOXIDE ENTERS AND

OXYGEN AND WATER VAPOR EXIT THE LEAF

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

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

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.

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

CONCEPT 10.2• THE LIGHT

REACTIONS CONVERT SOLAR ENERGY TO THE CHEMICAL ENERGY OF ATP AND NADPH

• 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

PHOTOSYSTEMS• PHOTONS OF LIGHT ARE ABSORBED

BY CERTAIN GROUPS OF PIGMENT MOLECULES IN THE THYLAKOID MEMBRANE OF CHLOROPLASTS

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

• 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

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

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

CONCEPT 10.3

• THE CALVIN CYCLE USES ATP AND NADPH TO CONVERT CO2 TO SUGAR

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

• TO NET ONE MOLECULE

OF G3P, THE CYCLE

MUST GO THROUGH

3 ROTATIONS AND

FIX 3 MOLECULES

OF CO2

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

• 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)

• 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

ENERGY NEEDED TO NET ONE G3P

• 9 MOLECULES OF ATP ARE CONSUMED ALONG WITH 6 MOLECULES OF NADPH (BOTH REPLENISHED BY THE LIGHT REACTIONS)

CONCEPT 10.4• ALTERNATIVE MECHANISMS OF

CARBON FIXATION HAVE EVOLVED IN HOT, ARID CLIMATES

• CO2 ENTERS THE LEAF THROUGH THE STOMATA, THE SAME PORE THAT WATER EXITS THE LEAF IN TRANSPIRATION

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

C4 PLANTS• HAVE 2 TYPES OF PHOTOSYNTHETIC

CELLS• BUNDLE-SHEATH CELL – GROUPED

AROUND THE LEAF’S VEINS• MESOPHYLL CELLS – DISPERSED

ELSEWHERE AROUND THE LEAF

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

• 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

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