Photosynthesis PDF

Photosynthesis (Ch. 10) Terrestrial plants Marine multicellular algae Unicellular protists Bacteria (prokaryotes) Purple sulfur bacteria (Photo) autotrophs (”self feeders”) use light energy to drive the synthesis of organic molecules from CO2...

Photosynthesis (Ch. 10) Terrestrial plants Marine multicellular algae Unicellular protists Bacteria (prokaryotes) Purple sulfur bacteria (Photo) autotrophs (”self feeders”) use light energy to drive the synthesis of organic molecules from CO2 and water (cf. heterotrophs)  Photosynthesis: key concepts Photosynthesis harnesses sunlight to make carbohydrate (sugar) Chloroplasts are the site of photosynthesis in plants Photosynthesis consists of an endergonic suite of REDOX reactions, i.e. it requires energy The light-capturing reactions convert solar energy to the chemical energy of ATP and NADPH (electron acceptor) in photosystems using chlorophyll (pigment) The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar = carbon fixation Fig. 10.2 Alternative mechanisms of carbon fixation have evolved in hot, arid climates (C4, CAM plants)  Photosynthesis as a REDOX process (Fig. 10.1) REACTANTS PRODUCTS Lower potential energy Higher potential energy Plants take in CO2 and H2O and with the aid of sunlight (electromechanical energy) synthesise carbohydrate (glucose) and release O2 Carbon is reduced; oxygen is oxidised Overall electrons are held more “loosely” in products than reactants i.e. their potential energy increases; requires energy (endergonic) Photosynthesis occurs in chloroplasts (Fig. 10.3) Have double membrane inside comprises fluid-filled stroma this contains hundreds of membrane- bound thylakoids (= 3rd membrane system) thylakoids are stacked “pancake-like” in grana (sing. granum) enzymes and pigments (chlorophyll) that run photosynthesis are embedded in thylakoid membrane or stroma Chloroplasts have their own circular chromosome and ribosomes!  Thylakoid membranes contain huge quantities of pigments, especially chlorophyll Pigments = substances that absorb visible light (a form of energy) Chlorophyll (and leaves) appear green because chlorophyll absorbs violet-blue and red wavelength light while reflecting green light Fig. 10.5 Ability of a pigment to absorb various wavelengths of light can be measured by a spectrophotometer and data plotted as an absorption spectrum (next weeks lab! )  How does chlorophyll capture light energy? When a photon of light strikes a See Fig. 10.9 chlorophyll molecule energy is transferred to an electron [in the chlorophyll] -in response the electron is “excited” or raised to a higher energy state -if the excited electron falls back to its “ground” or unexcited state it emits extra energy as heat and light (fluorescence) BUT in chloroplasts the energy from the electron can be passed among many chlorophyll molecules of an antenna complex then to a reaction center and then an electron can be transferred to an electron acceptor  Three fates for excited electrons in photosynthetic pigments (Fig. 10.10) Isolated molecules in the lab Chlorophyll molecules work in groups within chloroplasts 98% of energy! Antenna complex, reaction center and electron acceptor (multi-protein complexes) all occur in PHOTOSYSTEMS on the thylakoid membrane - makes up the light-capturing part of photosynthesis Thylakoid membranes in the chloroplast have 2 types of photosystem: PS II and PS I, which work together - electrons flow linearly through this pathway Electron Electron acceptor acceptor Chemiosmosis Antenna complex/ reaction centers  A mechanical analogy for linear electron flow during the light reaction (in case this helps!) Energy of electron LIGHT LIGHT Photosystem II feeds high-energy electrons to an electron transport chain (Fig. 10.12) Electron acceptor Energy reaching the reaction center excites an electron within a specialised chlorophyll molecules (P680) The excited electron binds to an electron acceptor = pheophytin “High energy” electrons that reach pheophytin are then passed to an electron transport chain (ETC) in thylakoid membrane P680 Reaction center The ETC here is very similar to that in the inner membrane of mitochondria - generates a proton-motive force and produces ATP by chemisosmosis  The ETC in photosystem II pumps protons creating a proton-motive force that drives ATP synthesis (Fig. 10.13) Within the ETC plastoquinone (PQ) carries electrons to a cytochrome complex Energy is used to transport protons (H+) from the stroma to the thylakoid lumen Creates large proton gradient across the thylakoid membrane Protons flow down their concentration gradient (exergonic) drives photophosphorylation of ADP to ATP by ATP synthase  Photosystem I produces NADPH via an electron transport chain not coupled to chemiosmosis (Fig. 10.14) Energy reaching the reaction center excites an electron within a specialised chlorophyll molecules (P700) High-energy electrons are passed through an electron transport chain (ETC) via a molecule called ferrodoxin and to NADP+ reductase NADP+ reductase transfers 2 P700 electrons + 1 H+ to NADP+ reducing it to NADPH This ETC does not involve chemiosmosis so NO ATP is produced - this photosystem stores energy (or reducing power) as NADPH  Organisation of photosystem II (cyanobacteria) PQ (ETC) electron acceptor (pheophytin) Reaction center special chlorophyll P680 The Z scheme model links photosystems II and I – follow the linear path of electron flow Fig. 10.15 Electrons lost from PS II are replaced by the “splitting” of water (the H + ions are released into thylakoid lumen) contributes to H+ gradient and proton-motive force Plastocyanin (PC) transfers electrons from cytochrome of PS II to PS I Outputs of light-capturing reaction = ATP and NADPH (and O2) Summary/interlude The light-capturing reactions (discussed so far) transform light energy to chemical energy in the form of ATP and NADPH -they are light-dependent The reactions that produce sugar from CO2 (Calvin cycle) are not directly light dependent -but they depend on ATP and NADPH produced by the light-capturing reactions The two sets of reactions are linked How is carbon dioxide fixed (reduced) to produce glucose in the Calvin cycle?  The Calvin cycle fixes carbon by addition of CO2 to an existing organic compound (RuBP) Calvin cycle is anabolic, i.e. it build complex CHO (initially a 3C sugar) from smaller molecules and consumes energy Fig. 10.19a CO2-fixing enzyme (rubisco) catalyses reaction between CO2 and ribulose 1,5-biphosphate (RuBP) Fixation = 1st of THREE phases of Calvin cycle - uses ATP and NADHP - regenerates RuBP (hence “cycle”) Glyceraldehyde-3-phosphate The Calvin cycle as a cycle (Fig. 10.19b) ONE turn of the cycle fixes 1 CO2 THREE turns of the cycle produces 1 x G3P (available to the plant) 3 RuBP + 3 CO2 → 3 RuBP + 1 G3P “sugar” 15C + 3C → 15C + 3C  A useful summary of photosynthesis? in thylakoid membrane in stroma converts light energy to uses ATP and NADPH to chemical energy of ATP and convert CO2 to sugar G3P NADPH returns ADP, inorganic P and splits H2O and releases O2 NADP+ to light reaction  Alternative mechanisms of carbon fixation have evolved in hot, arid climates Photosynthesis requires CO2 that enters via stomata on the leaf but stomata are the main route for evaporative loss of water via transpiration on hot, dry days plants close their stomata to conserve water this limits access to CO2 and reduces photosynthetic yield get build up of O2 in air spaces of leaf from light reactions leads to process called photorespiration  C3 plants and photorespiration Most plants are called C3 plants Rice because rubisco initially fixes carbon as a 3C molecule (3-phosphoglycerate) BUT rubisco is a slow and Wheat inefficient enzyme! It will bind O2 to RuBP equally as well as CO2 (when CO2 is scarce) Photorespiration consumes ATP and releases “fixed” CO2 so 2C it decreases the efficiency of photosynthesis (in C3 plants on hot, dry days) Fig. 10.21  Evolutionary solution to photorespiration I: C4 plants Trick: store CO2 when its available to maintain constant supply for Calvin cycle when stomata are closed In C4 photosynthesis CO2 is fixed Fig. 10.24 initially as a 4C-compound in outer Sugarcane mesophyll cells - PEP carboxylase has high affinity for CO2 and none for O2 so carbon fixation occurs even at low CO2 - 4C products are exported to bundle- sheath cells maintaining a supply of CO2 in these cells for Calvin cycle Spatial separation of 2 processes C4 pathway basically acts as a CO2 concentrator, maintaining CO2:O2 ratio in photosynthesising bundle-sheath cells limiting photorespiration Evolutionary solution to photorespiration II: CAM plants Trick: store CO2 when its available to maintain constant supply for Calvin cycle when stomata are closed Cacti NIGHT DAY Pineapple Crassulacean acid metabolism (CAM) Fig. 10.23 CAM plants take up CO2 at night when stomata can be open and incorporate it into organic acids which are stored in vacuole of mesophyll cells - during the day this stored CO2 is released Temporal separation of 2 processes