CHAPTER 10 PHOTOSYNTHESIS I PHOTOSYNTHESIS IN NATURE PLANTS

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 CO 2 AND H 2 O. • 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 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 CO 2 + 6 H 2 O + LIGHT ENERGY C 6 H 12 O 6 + 6 O 2 • PHOTOSYNTHESIS IS A REDOX PROCESS: H 2 O IS OXIDIZED, CO 2 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 O 2 AND FORMING NADPH BY TRANSFERRING ELECTRONS FORM WATER TO NADP+. • THE CALVIN CYCLE IN THE STROMA FORMS SUGAR FROM CO 2, USING ATP FOR ENERGY AND NADPH FOR REDUCING POWER.

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 P 680 MOLECULES – PHOTOSYSTEM I CONTAINS P 700 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 PROTONMOTIVE FORCE TO MAKE ATP

D. THE CALVIN CYCLE USES ATP AND NADPH TO CONVERT CO 2 TO SUGAR: A CLOSER LOOK • THE CALVIN CYCLE IS A METABOLIC PATHWAY IN THE CHLOROPLAST STROMA. • AN ENZYME (RUBISCO) COMBINES CO 2 WITH Ru. BP, (A FIVE CARBON SUGAR). • USING ELECTRONS FROM NADPH AND ENERGY FROM ATP, THE CYCLE SYNTHESIZES THE G 3 P. • MOST OF THE G 3 P IS REUSED IN THE CYCLE TO RECONSTITUTE Ru. BP, BUT SOME EXITS THE CYCLE AND IS CONVERTED TO GLUCOSE AND OTHER ESSENTIAL ORGANIC MOLECULES.

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 O 2 SUBSTITUTES FOR CO 2 IN THE ACTIVE SITE OF RUBISCO, THE PRODUCT FORMED LEAVES THE CYCLE AND IS OXIDIZED TO CO 2 AND H 2 O. • THIS PROCESS IS KNOWN AS PHOTORESPIRATION BECAUSE IT OCCURS IN THE LIGHT, YET CONSUMES OXYGEN. • C 4 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.