Photosynthesis Slides PDF

4/22/24 Photosynthesis Photosynthesis is a form of energy transformation Electrons are removed from an electron donor (water in green plant) and transferred...

4/22/24 Photosynthesis Photosynthesis is a form of energy transformation Electrons are removed from an electron donor (water in green plant) and transferred to NADP+ (giving NADPH) The NADPH passes electrons onto CO2 which becomes reduced and forms sugars These process occur in the light and the light independent reactions of photosynthesis respectively The light reactions involve two types of photosystem which are present in the thylakoid membrane An electron transport chain enables ATP production The light independent reactions are also called the Calvin cycle. Basically the cycle comprises CO2 fixation, reduction into sugar and then a recycling phase. The first reaction is catalysed by ribulose bisphisphosphate carboxylase/oxygenase (Rubisco) which is tightly regulated. Rubisco will also catalyse an oxygenase reaction which can lead to photorespiration. C3 plants can overcome this problem by keeping gas exchange pores open. Some plants there have specialised anatomies/biochemistry which enable the spatial (C4 plants) or temporal (CAM plants) separation of initial CO2 fixation from the initial fixation step in the Calvin cycle. 1 Photosynthesis Calvin cycle CO2 reduced to glucose reactions 6CO2 + 6H2O C6H12O6 + 6O2 Water oxidised to oxygen Light dependent reactions 2 1  4/22/24 Photosynthesis 1011 tons CO2 fixed annually Equivalent weight to 1014 copies of a typical undergraduate text book Sign up for FREE or Email or user Password ? Sign in Stack of these would be 40 billion Schoolboy screaming near the huge stack of books. Childhood, isolated. ROYA LT Y-FRE E S TOCK PHOTO kms high!! From here to the sun 268 times!!! FREE DOWNLOAD XS S M L XL MAX TIFF 3.3MB | jpg 4358x2752px 0 3 0 Extended licenses Schoolboy screaming near the huge stack of books isolated over white background 3 © Yuriy Zelenen'kyy ID 46239965 More similar stock images Photoautotrophs Confused schoolboy in funny glasses screaming near the huge st… Confused schoolboy in funny gl… Sulfur bacteria don’t use water as the electron source so note that the photosynthesis equation is different. 4 2  4/22/24 Photosynthesis ~70% of O2 produced in photosynthesis on the planet is produced by marine organisms ~>50% of O2 produced in photosynthesis on the planet is produced by marine prokaryotes (phytoplankton) More than 100 million tons of CO2 fixed per day 5 Photosynthesis - overview 6 3  4/22/24 Chloroplasts contain pigment molecules that absorbs light Absorb blue and red light Reflects/transmits green 7 In autumn…. Deciduous trees remove chlorophyll from their leaves before they fall Conserves N Why are leaves yellow/orange/red in autumn?? 8 4  4/22/24 Chlorophyll absorbs light energy Electron of one of C atoms Simplified + - chlorophyll molecule Nucleus of one of C atoms 9 Fates of light energy? + - hv Upon absorption of light energy 10 5  4/22/24 Fates of light energy? The electron jumps + - to a higher orbital giving what is called an excited state. There are four possible things that happen next. 11 Fates of light energy? The electron moves back to the original orbital. It loses energy which is released as heat. + - HEAT 12 6  4/22/24 Fates of light energy? + - 13 Fates of light energy? Alternatively the energy can be released as light. Fluorescence!! + - hv 14 7  4/22/24 Fates of light energy? + - - + A third alternative if another chlorophyll is close by is energy transfer to the neighbouring chlorophyll. Called resonance energy transfer. 15 Fates of light energy? + - - + This is the mechanism whereby energy from light falling anywhere within light harvesting complexes can be transferred to a reaction center chlorophyll. 16 8  4/22/24 Resonance energy transfer A third alternative if another chlorophyll is close by is energy transfer to the neighbouring chlorophyll. Called resonance energy transfer. This is the mechanism whereby energy from light falling anywhere within light harvesting complexes can be transferred to a reaction center chlorophyll. 17 Fates of light energy? + - The fourth alternative is energy transduction 18 9  4/22/24 Fates of light energy? Primary acceptor + - If a molecule (here termed primary acceptor) is close to the pigment molecule it can accept an electron. The electron is transferred to that molecule. 19 Fates of light energy? Primary acceptor + - Reduced primary acceptor 20 10  4/22/24 Fates of light energy? + Reduced e- primary acceptor Oxidised chlorophyll 21 Light reactions Reaction centre chlorophylls are positioned close to primary electron acceptors : in large protein complexes called photosystems Energy transduction or transformation Light energy into chemical energy (reducing power) Most life on this planet is dependent on this process!!!! 22 11  4/22/24 Light reactions: Key players 2 Photosystems: Photosystem I and Photosystem II – Reaction centre chlorophyll P700 in PSI P680 in PSII – Antennae chlorophyll, light harvesting complexes – PSII has a water splitting complex 23 Light reactions: Key players Cytochrome complex – Proton pump Plastoquinone Plastocyanin Ferredoxin NADP+ reductase Chloroplast ATP synthase 24 12  4/22/24 Light reactions – electron flow and energetics 25 PSII PSII – Light energy passed to reaction centre chlorophyll P680 by antennae chlorophylls. 26 13  4/22/24 PSII PSII – Electron in P680 excited and passed onto primary electron acceptor 27 PSII PSII – Lost electron in P680 is replaced by an electron which comes from water (water splitting complex) 28 14  4/22/24 PSII – electron transport chain PSII – Excited electron moves from primary electron acceptor onto electron transport chain (PQ, cyt complex, PC) in thylakoid membrane 29 Electron transport chain of chloroplast Plastoquinone, cytochrome complex, plastocyanin – Electron moves along chain – lose energy 30 15  4/22/24 Electron transport chain Plastoquinone, cytochrome complex, plastocyanin – Cytochrome complex uses this energy to pump protons from the stroma to thylakoid space 31 Electron transport chain Plastoquinone, cytochrome complex, plastocyanin – Creates a proton gradient – chloroplast ATP synthase uses this to produce ATP 32 16  4/22/24 PSI PSI – Light energy passed to reaction centre chlorophyll P700 by antennae chlorophylls. 33 PSI PSI – Electron in P700 excited and passed onto primary electron acceptor. 34 17  4/22/24 PSI PSI – Electron replaced by electron from plastocyanin (electron transport chain) 35 NADPH PSI – Excited electron moves from primary electron acceptor onto other electron transporters and then NADP+. – NADP+ reduced to NADPH – chemical energy!!! 36 18  4/22/24 NADPH and ATP fuel the Calvin cycle 37 ATP production - terminology Non cyclic photophosphorylation – Non cyclic – route of electrons (linear) – Photo – energised by light – Phosphorylation – production of ATP 38 19  4/22/24 ATP production - terminology Cyclic photophosphorylation – Cyclic – route of electrons (circular) – Photo – energised by light – Phosphorylation – production of ATP 39 Calvin cycle reactions Occur in stroma 3 Phases – Fixation – Reduction – Regeneration Produces sugar – sucrose 40 20  4/22/24 Rubisco Ribulose bisphosphate carboxylase oxygenase Adds CO2 to ribulose phosphate (carboxylase reaction) Slow turnover rate only catalyses 2-3 reactions s-1 Most abundant Garrett and Grisham: Biochemistry enzyme/protein in nature?? 41 Rubisco Very tightly regulated – Mg2+ – pH – reductants Only works when light reactions working (and producing ATP). So ATP used in Calvin cycle comes from light reactions 42 21  4/22/24 Rubisco Very tightly regulated – Mg2+ – pH – reductants Only works when light reactions working (and producing ATP). So ATP used in Calvin cycle comes from light reactions And not from respiration (thus avoids futile cycling) 43 Rubisco Ribulose bisphosphate carboxylase oxygenase Adds CO2 to ribulose phosphate BUT can also add O2 to ribulose phosphate – problem L When O2 added costs extra energy to make the product into useful sugar - photorespiration 44 22  4/22/24 Rubisco What determines activity is the CO2/O2 ratio High – carboxylase; Low - oxygenase So plants keep their gas exchange pores (stomata) open – O2 diffuses out of leaf – CO2 diffuses in Keeps the CO2 levels high in the leaf relative to O2 – Carboxylase reaction predominates 45 Rubisco However this will also lead to water loss through transpiration H2O This is a problem for plants in dryer environments – Need to keep their stomata closed to prevent water loss 46 23  4/22/24 C4 and CAM plants Certain plants (C4 and CAM) have evolved a special morphology and biochemistry – keep stomata closed AND maintain high CO2/O2 ratio where Calvin cycle occurs C4 - CO2 initially fixed in mesophyll cells and then delivered to bundle sheath cells where Calvin cycle occurs. – CO2 levels high where Rubisco is 47 C4 and CAM plants CAM – stomata open at night, CO2 enters leaf and is fixed into organic acid Released during day - used in Calvin cycle. Stomata closed but photorespiration not a problem as CO2 is high 48 24