CHAPTER 10 PHOTOSYNTHESIS

I. PHOTOSYNTHESIS IN NATURE PLANTS AND OTHER AUTOTROPHS ARE THE PRODUCERS OF THE BIOSPHERE AUTOTROPHS NOURISH THEMSELVES WITHOUT INGESTING ORGANIC MOLECULES. PHOTOAUTOTROPHS USE THE ENERGY OF SUNLIGHT TO SYNTHESIZE ORGANIC MOLECULES FROM CO2 AND H2O. HETEROTROPHS INGEST ORGANIC MOLECULES FROM OTHER ORGANISMS TO GET ENERGY AND CARBON.

B. CHLOROPLASTS ARE THE SITES OF PHOTOSYNTHESIS IN PLANTS IN AUTOTROPHIC EUKARYOTES, PHOTOSYNTHESIS OCCURS IN CHLOROPLASTS (ORGANELLES CONTAINING THYLAKOID MEMBRANES THAT SEPARATE THE THYLAKOID SPACE FROM THE CHLOROPLAST’S STROMA.) *REMEMBER: STACKS OF THYLAKOIDS FORM GRANA

II. THE PATHWAYS OF PHOTOSYNTHESIS A. EVIDENCE THAT CHLOROPLASTS SPLIT WATER MOLECULES ENABLED RESEARCHERS TO TRACK ATOMS THROUGH PHOTOSYNTHESIS: SCIENCE AS A PROCESS PHOTOSYNTHESIS CAN BE SUMMARIZED AS: 6 CO2 + 6 H2O + LIGHT ENERGY  C6H12O6 + 6 O2 PHOTOSYNTHESIS IS A REDOX PROCESS: H2O IS OXIDIZED, CO2 IS REDUCED

C.     THE LIGHT REACTIONS AND THE CALVIN CYCLE COOPERATE IN CONVERTING LIGHT ENERGY TO THE CHEMICAL ENERGY OF FOOD THE LIGHT REACTIONS IN THE GRANA PRODUCE ATP AND SPLIT WATER, RELEASING O2 AND FORMING NADPH BY TRANSFERRING ELECTRONS FORM WATER TO NADP+. THE CALVIN CYCLE IN THE STROMA FORMS SUGAR FROM CO2, USING ATP FOR ENERGY AND NADPH FOR REDUCING POWER.

LIGHT IS A FORM OF ELECTROMAGNETIC ENERGY, WHICH TRAVELS IN WAVES. C. THE LIGHT REACTIONS TRANSFORM SOLAR ENERGY TO THE CHEMICAL ENERGY OF ATP AND NADPH LIGHT IS A FORM OF ELECTROMAGNETIC ENERGY, WHICH TRAVELS IN WAVES. SEE BOOK FOR DETAILS. (LIGHT SPECTRUM) A PIGMENT GOES FROM A GROUND STATE TO AN EXCITED STATE WHEN A PHOTON BOOSTS ONE OF ITS ELECTRONS TO A HIGHER-ENERGY ORBITAL. AN ANTENNA COMPLEX, CHLOROPHYLL, & PRIMARY ELECTRON ACCEPTOR MAKES UP A PHOTOSYSTEM THERE ARE TWO KINDS OF PHOTOSYSTEMS PHOTOSYSTEM II CONTAINS P680 MOLECULES PHOTOSYSTEM I CONTAINS P700 MOLECULES

NON-CYCLIC ELECTRON FLOW INVOLVES BOTH PHOTOSYSTEMS AND PRODUCES NADPH, ATP, & OXYGEN CYCLIC ELECTRON FOW EMPLOYS ONLY PHOTOSYSTEM I, PRODUCING ATP BUT NO NADPH OR OXYGEN ATP PRODUCTION IS THROUGH PHOTOPHOSPHORYLATION THE MECHANISM IS CHEMIOSMOSIS THE REDOX REACTIONS OF THE ELECTRON TRANSPORT CHAIN THAT CONNECTS THE TWO PHOTOSYSTEMS GENERATE AN H+ GRADIENT ACROSS THE THYLAKOID MEMBRANE AN ATP SYNTHASE USES THIS PROTON-MOTIVE FORCE TO MAKE ATP

D. THE CALVIN CYCLE USES ATP AND NADPH TO CONVERT CO2 TO SUGAR: A CLOSER LOOK THE CALVIN CYCLE IS A METABOLIC PATHWAY IN THE CHLOROPLAST STROMA. AN ENZYME (RUBISCO) COMBINES CO2 WITH RuBP, (A FIVE CARBON SUGAR). USING ELECTRONS FROM NADPH AND ENERGY FROM ATP, THE CYCLE SYNTHESIZES THE G3P. MOST OF THE G3P IS REUSED IN THE CYCLE TO RECONSTITUTE RuBP, BUT SOME EXITS THE CYCLE AND IS CONVERTED TO GLUCOSE AND OTHER ESSENTIAL ORGANIC MOLECULES.

ON DRY, HOT DAYS, PLANTS CLOSE THEIR STOMATA, CONSERVING WATER. E. ALTERNATIVE MECHANISMS OF CARBON FIXATION HAVE EVOLVED IN HOT, ARID CLIMATES ON DRY, HOT DAYS, PLANTS CLOSE THEIR STOMATA, CONSERVING WATER. OXYGEN FROM THE LIGHT REACTIONS BUILDS UP. WHEN O2 SUBSTITUTES FOR CO2 IN THE ACTIVE SITE OF RUBISCO, THE PRODUCT FORMED LEAVES THE CYCLE AND IS OXIDIZED TO CO2 AND H2O. THIS PROCESS IS KNOWN AS PHOTORESPIRATION BECAUSE IT OCCURS IN THE LIGHT, YET CONSUMES OXYGEN. C4 PLANTS/CAM PLANTS

F. PHOTOSYNTHESIS IS THE BIOSPHERE’S METABOLIC FOUNDATION: A REVIEW THE ORGANIC COMPOUNDS PRODUCED BY PHOTOSYNTHESIS PROVIDE THE ENERGY AND BUILDING MATERIAL FOR ECOSYSTEMS.