Photosynthesis

Photosynthesis in Nature Autotrophs: biotic producers photoautotrophs/ chemoautotrophs obtains organic food without eating other organisms Heterotrophs: biotic consumers obtains organic food by eating other organisms or their by-products (includes decomposers)

Chloroplasts Sites of photosynthesis In the mesophyll of leaves Gas exchange: stomata (small pores on leaf surface) Structure: Double membrane Thylakoids- contain pigments (chlorophyll) Grana- stacks of thylakoids Stroma- surrounds thylakoid

6CO2 + 12H20 + light NRG  C6H12O6 + 6O2 + 6H2O The Reaction 6CO2 + 12H20 + light NRG  C6H12O6 + 6O2 + 6H2O Redox process H2O is split - e- (along w/ H+) are transferred to CO2, reducing it to sugar

Photosynthesis Overview 2 major steps: Light reactions (“photo”) NADP+ (e- acceptor) to NADPH Photophosphorylation: ADP  ATP Calvin cycle (“synthesis”) Carbon fixation: carbon into organic compounds

Light Light= electromagnetic NRG Light particles= photons Visible light broken up into colors The colors we see are the ones which are reflected. All others are absorbed Chlorophyll REFLECTS green light and absorbs others

Absorption of Light Absorption can be determined by a spectrophotometer Determines the amount of light transmitted at a certain wavelength Different wavelengths = different colors

Pigments in Photosynthesis Chlorophyll A Absorbs violet blue and red light Appear blue-green Chlorophyll B (Accessory pigment) Absorbs violet blue and orange light Appear yellow-green Carotenoids (Accessory pigment) Absorbs violet blue and blue green light Appear yellow or orange

Exciting Chlorophyll Chlorophyll absorbs photons of light Electrons move to a higher NRG level (outer orbital Molecule is unstable As electrons drop back down, they release heat and photons of light (fluorescence)

Photosystems Light harvesting units of the thylakoid membrane Pigment molecules make up an “antenna complex” which attracts photons Energy is passed to the reaction center (made up of 2 special chlorophyll a molecules) Excited e- from chlorophyll is trapped by a primary e- acceptor

Two Photosystems PS II and PS I… Named in order of discovery, but PS II happens first PS II Reaction center = P680 Most effective at 680 NM PS I Reaction center = P700 Most effective at 700 NM

Noncyclic electron flow PS II: photons excite chlorophyll e- to an acceptor e- replaced by splitting of H2O (release of O2) e-’s travel to PS I down an ETC as e- fall, ADP  ATP (noncyclic photophosphorylation) PS I: ‘fallen’ e- replace excited e- to primary e- acceptor 2nd ETC transfers e- to NADP+  NADPH (...to Calvin cycle…) These photosystems produce equal amounts of ATP and NADPH

Cyclic electron flow Alternative cycle when ATP is deficient Photosystem I used but not II Produces ATP but no NADPH or O2 Why? The Calvin cycle consumes more ATP than NADPH… Cyclic photophosphorylation of ATP keeps Calvin cycle going w/o extra NADPH

Chemiosmosis Movement of e- PS II  plastoquinone (Pq) cytochrome  plastocyanin (Pc)  PS I  ferredoxin (Fd)  NADP+ NRG trapped in thylakoid membrane used to pump H+ into membrane ATP synthase uses H+ flow to generate ATP

Haven’t we seen this before? ETC in photosynthesis is very similar to the one in cellular respiration H+ pumped into thylakoid H+ pumped into intermembrane space

The Calvin Cycle Carbon fixation Reduction Regeneration 3 molecules of CO2 are ‘fixed’ into glyceraldehyde 3-phosphate (G3P) Carbon fixation each CO2 is attached to RuBP (rubisco enzyme) Reduction e- from NADPH reduces G3P; ATP is used Regeneration G3P rearranged to RuBP; ATP used; cycle continues

Net Synthesis For each G3P (and for 3 CO2)… Consumption of 9 ATP’s & 6 NADPH (light reactions regenerate these molecules) G3P can then be used by the plant to make glucose and other organic compounds

C3 Plants Fix Carbon in to 3-phosphoglycerate Rice, wheat, soybeans Must have stomata open to take in CO2. Causes loss of water as well Photorespiration: on hot/dry days, stomata close, CO2 decrease, O2 increase in leaves O2 added to rubisco instead of CO2 no ATP or food generated

C4 Pathway C4 Plants 2 photosynthetic cells, bundle-sheath & mesophyll PEP carboxylase (instead of rubisco) fixes CO2 in mesophyll New 4-C molecule releases CO2 into the Calvin cycle corn, sugarcane, grasses

Alternative Pathway II CAM Crassulacean Acid Metabolism Open stomata during night, take up CO2 and store it as organic acids Close stomata during the day; CO2 released from acids and released into the Calvin cycle Cacti, pineapples, etc.

Review of Photosynthesis