Photosynthesis   Done by Bacteria, protists (algae), plants

Photoautotrophs do psyn.  Photo = light  Auto = self  Troph = feeder  Photo…..energy provided by light  Autotroph get own carbon from inorganic source (feed themselves)  Chemoautotroph…

Light Dependent reactions... Be able to draw and explain  Photolysis  Photosystem  Electron transport  Chemiosmosis  Location in chloroplast

Light Independent Reaction – Be able to explain….  Carbon fixation - atmospheric CO 2, RUBP, Rubisco, unstable 6-C intermediate, PGA  Calvin Cycle – ATP used for…NADPH used for, what returns to light rxn, substrate level phosphorylation, 6 concurent cycles merge products to form 1 glucose  Know where in cell these rxns occur

Light Reactions  Begin at Photosystem II – chlorophyll a =p680  Light-harvesting complex – surrounding complex of proteins and pigments  chlorophyll a, chlorophyll b, carotenoids  pass energy along to reaction center  Reaction center: holds 2 special chlorophyll a molecules and primary e-acceptor protein  e- jumps out of chlorophyll a onto 1˚e- acceptor  chlorophyll a of photosystem II is now P680+

Step 2 light reaction -Photolysis  Photosystem II (p680)  More modern version PS w/ specialized enzyme  Enzyme splits water  e- from H replace those lost by p680+  H+ ions released into thylakoid space  O bonds to another O and diffuses out

Step 3 light reaction – electron transport  e - from p680+ chlorophyll a move down e- transport chain  Give energy to proton pumps  e- enter chlorophyll p700 in photosystem I  Re-energized e- from p700+ exit thylakoid  And are picked up by NADP+  (co-enzyme & e- carrier)  NADPH moves to Calvin cycle to provide  reducing power

Step 4 - light reaction = photophosphorylation  Proton pumps powered by e-transport chain  Pump H+ ions into center of thylakoid  H+ ions flow out of thylakoid space through  ATPase by facilitated diffusion  Energy of flowing H+ used by ATPase to make lots of ATP  ATP sent to Calvin cycle to provide energy for endergonic, anabolic reaction forming  glucose

Light Independent Reactions  Use ATP to make glucose  Take place in the stroma  Require ATP and NADPH from light rxn  Require CO 2 from atmosphere  Stroma is filled with thousands of enzymes  Hundreds of Calvin cycles are running simultaneously  For every 6 Calvin cycles 1 glucose is formed

1 C 6 H 12 O 6

Carbon Fixation  Atomospheric CO 2 is ‘fixed’ (attached) to an organic molecule called RuBP  The enzyme Rubisco mediates this rxn  RuBP + CO 2 forms unstable intermediate  Intermediate splits into 2 PGA (3 C each)

To make 1 glucose this step happens 6 times forming 12 PGA  RuBP PGA

Calvin Cycle – PGAL formation  ATP from light rxn gives a phosphate group to each PGA via substrate level phosphorylation  so…12 ATP phosphorylate 12 PGA priming them to react  NADPH reduces each PGA (adding H to them) making them into PGAL (P i is released)  (PGAL = G3P)

Calvin Cycle –reformation of RuBP  2 of the 12 PGAL are spit out of the cycle and made into glucose  The remaining PGAL are reformed into RuBP using energy from 6 more ATP  Most glucose is converted to sucrose for transport or into starch for storage

ADP, P i and NADP + Return to Light dependent reaction

Photosynthesis Chemical Equation  6 CO H 2 O  C 6 H 12 O O H 2 O  Or with net water shown:  6 CO H 2 O  C 6 H 12 O O 2

Evolution of Photosynthesis  1 st photosynthetic bacteria did cyclic psyn  Only makes ATP not sugar  Still used by many bacteria

Evolution of PS II  Non-cyclic psyn because e- obtained from H2O  (not recycled)  Allowed production of high energy carbs  A way to transport & store chemical energy  Allowed larger more complex organisms  All plants and some bacteria have PS II and do non-cyclic psyn

Chloroplast Location in plant  Any green part, most in leaves (p157)  A - cuticle (wax)  B - epithelial cell  C - palisade mesophyll  D - spongy mesophyll  E - stoma stomata

The Move to Land  Waxy cuticle to prevent dehydration  Stomata – open & close to let in CO 2  Water evaporates when stomata open  Closing stomata causes O 2 CO 2  Rubisco starts binding to O 2  Puts O 2 into Calvin cycle  Called photorespiration = bad no glucose  Most plants do this..called C3 plants

C4 plants avoid photorespiration  Close stomata but keep CO 2 levels near rubisco high by doing C4 cycle in mesophyll cells  In C4 cycle carbon fixation binds CO 2 to PEP instead of RuBP  The enzyme binds CO 2 to PEP ignores O 2  PEP + CO 2 = oxaloacetate ( a 4C molecule)  Oxaloacetate converts to malate which is sent  through plasmodesmata to bundle sheath cells

Malate drops off CO 2 to rubisco in bundle sheath cells that run C3 Calvin cycle 

CAM plants…cacti & desert plants  fix carbon by C4 cycle only at night  Store malate until day time  Close stomata and us malate as CO 2 source to run Calvin cycle in day time when ATP and NADPH become available

Photoprotective pigments  Carotenoids absorb violet and blue-green light  Also protect the leaf from excessive light  Very similar to the carotenoids that protect the human eye