Introduction to Photosynthesis Chapter 10 p. 181-188 Autotrophs: Producers of the Biosphere Autotroph: “self-feeding”; produce own organic molecules from CO2 & inorg. molec. in environment i.e.: plants, algae, some bacteria Heterotroph: “feeds on others”; must consume other orgs to obtain nutrients, O2, & energy i.e.: animals, fungi, most bacteria Chloroplasts: Sites of Photosynthesis Plants appear green due to pigment chlorophyll inside thylakoid space of chloroplasts Found in mesophyll tissue of leaves 30-40 per cell Stomata: pores through which CO2 enters & O2 leaves Stomata Chloroplast Structure Tracking Atoms through Photosynthesis O2 given off by plants comes from H2O; NOT CO2 Chloroplast splits H2O → 2H + O C.B. van Niel: proved 2H from split H2O goes to glucose; O released as atmospheric O2 Photosynthesis is a Redox Reaction Unlike cell respir., photosynthesis is endergonic Energy comes from sun Reverses direction of e- flow from H2O → CO2 (oxidized) CO2 is reduced to glucose CO2 + H2O + energy → C6H12O6 + O2 The 2 Stages of Photosynthesis 1) Light Reactions (“photo”) e- + H+ transferred to NADP+ (cousin of NAD+) O2 given off as byproduct Produces 1 ATP (“photophosphorylation”) Occurs in the thylakoid The 2 Stages of Photosynthesis 2) Calvin Cycle (“synthesis”) CO2 incorporated into organic molecules already present (“carbon fixation”) “Fixed” C is reduced to glucose (add e-) Powered by NADPH & ATP from light rxns Occurs during day in most plants; relies on light rxns Occurs in stroma Photosynthesis Overview Nature of Sunlight Light: electromagnetic energy (“radiation”) Travels in waves; distance between called wavelength Also acts as photons: particles of light energy Electromagnetic Spectrum: entire radiation spectrum Visible light = 380-750nm Amt energy inversely proportional to wavelength Purple photon > red photon Photosynthetic Pigments Pigment: substance that absorbs visible light at different wavelengths Reflected/transmitted wavelength is color we see Spectrophotometer: measures absorbed wavelengths Photosynthetic Pigments Chlorophyll a: main photosynthetic pigment Absorbs red & blue photons; reflects green Only pigment directly involved in light rxns Other pigments (Chlorophyll B & Carotenoids) transfer photons to chlorophyll a & provide photoprotection Excitation of Chlorophyll by Light When molecules absorb light energy (photons), e- “jump” to next orbital Ground state → excited state Specific to wavelength Unstable e- will “fall” back quickly, releasing energy (heat) Fluorescence: energy released as light Reactions of Photosynthesis Chapter 10 p. 189-198 Photosystems Consists of 3 sections: 1) Light-Harvesting Complex: 2) Reaction-Center: at center; receives energy from light-harvesting complex & becomes excited contain all 3 types pigments; ↑ surface area to absorb more light Contains special chlorophyll a molecules whose e-’s move to higher energy level 3) Primary Electron Acceptor: receives e-s from excited chlorophyll a molecules & “catches” them e-’s then enter into Noncyclic Electron Flow Types of Photosystems Photosystem II: absorbs 680nm best (“P680”) P700 & P680 identical, but surrounded by diff. proteins Work together to make ATP & NADPH for Calvin Cycle Photosystem I: Reaction center chlorophyll a absorbs 700nm best (“P700”) Noncyclic Electron Flow Predominant route for e-s Steps: 1) Photots. II absorbs light, P680 excited 2) e-s captured by Primary e- Acceptor P680 now has e- “hole” 3) e-s replaced in P680 by split H2O molecule; O2 released inside thylakoid 4) ETC takes e-s from Primary e- Acceptor to Photosystem I Composed of plastoquinone (Pq), 2 cytochromes, & plastocyanin (Pc) Noncyclic Electron Flow 5) As e-s “fall” down chain, energy is harvested to make ATP by chemiosmosis outside thylakoid 6) Final e- acceptor is P700 “Noncyclic Photophosphorylation” P700 e-’s excited by light energy are captured by Primary e- Acceptor Fills “hole” created by Primary e- Acceptor of Photo II 7) Primary e- Acceptor passes e-s to 2nd ETC → ferredoxin (Fd) 8) NADP+ reductase transfers e-’s from Fd to NADP+→ makes NADPH in stroma Summary of Noncyclic Electron Flow P680 → Primary eAcceptor → 1st ETC → P700 → Primary eAcceptor → Fd → NADP+ reductase → NADPH Cyclic Electron Flow Calvin cycle uses more ATP than NADPH If ATP runs low, chloroplast switches to cyclic Involves Photosystem I (P700) only Fd takes e-s to cytochrome complex of 1st ETC & returns them to P700 No NADPH produced; no O2 released Calvin Cycle C enters as CO2, leaves as sugar (G3P) Cycle must turn 3 x’s to make glucose Must “fix” 3 C’s into org. molecules Occurs in 3 phases: 1) C Fixation: 3C’s from 3CO2 are incorporated into RuBP, catalyzed by rubisco 2) Reduction: e-’s from NADPH reduce 6 1,3 biphosphate → 6 G3P (↑ energy) Spends 6 ATP 3) Regeneration of RuBP: G3P rearranged → RuBP (can pick up CO2 again) Spends 3 ATP Calvin Cycle - Summarized For each turn of Calvin: In: 9 ATP 6 NADPH 3 CO2 Out: 9 ADP 6 NADP+ 1 G3P (will become glucose) Alternate Methods of C Fixation In hot, dry climates, stomata remain closed to prevent H2O loss Also prevents CO2 in & O2 out Result is Photorespiration Photorespiration Most plants are called C3 because C fixation creates a 3-C compound Closed stomata ↓ [CO2] inside leaf, and ↑ [O2] O2 will be picked up by rubisco (instead of CO2) Photorespiration: uses light (“photo”) to consume O2 (“respiration”) No ATP produced; no sugar made May be ancient evolutionary adaptation C4 Plants C fixed by PEP carboxylase to form 4-C compound (oxaloacetate → malate) PEP carbox. has ↑↑ affinity for CO2; can “fix” CO2 when rubisco can’t 4-C cmpnd (malate) enters Bundle Sheath cells where CO2 breaks off & enters Calvin Keeps CO2 levels ↑ for rubisco Minimizes photorespiration & ↑ sugar production CAM Plants Water-storing plants (cacti, pineapple, etc.) close stomata during day, open at night Store org. molec. until day when light rxns can provide ATP & NADPH