Plant Photosynthesis.ppt

1、Lecture 6 Plant Photosynthesis,1. Chlorophyll,The conjugated 共轭的double bond of Porphyrin ring and central Mg (magnesium) atom are easily excitated by light and cause electron gains and losses, which involve in energy transfer.,Carotenoid is a 40C unsaturated alkenes(烯烃), composed by eight isoprene（异

2、戊二烯）units, relatively stable.,2. Carotenoid,5.2 Mechanism of Photosynthesis,According to whether need light or not， photosynthesis can be divided into two reactions: 1. light reaction 2. Carbon reaction (dark reaction ),Light reaction is a photochemistry reaction which works in light on thylakoid me

3、mbrane; Carbon reaction is an enzymatic reaction which work either in dark or light in chloroplast stroma.,- carotene,Lutein,The photosynthesis can be approximately divided into three steps: Primary reaction； Electron transport and photophosphorylation； Carbon assimilation process - Carbon reaction,

4、light reaction,5.2.1 Primary Reaction,Primary reaction: the process of absorption, transportation and conversion of light energy(Photons) to electric energy(electrons ) by photosynthetic pigments. Characteristics： (1) high speed (finished in 109sec)； (2) no relate to temperature (can work in liquid

5、nitrogen -196 or liquid helium -271).,Two kinds of photosynthetic pigments: (1) Reaction centre pigments: few Chla., having special state and photochemistry activity. (2) light-harvesting pigments (antenna pigments): most Chla , all Chlb and carotinoid. Absorb light, then transfer energy to reaction

6、 center only.,Photosynthetic unit: the number of photosynthetic pigments required for completing the photochemistry reaction of absorbing and transferring 1 photon to reaction center. (1) Photosynthetic unit is composed by light-harvesting pigment system and reaction centre.,(2) Photosynthetic unit

7、is the smallest structure function unit on thylakoid membrane which can complete photochemical reaction.,(photons),(250-300),light-harvesting pigment system,reaction center,Organization of light-absorbing antenna systems,Antenna funnels energy to the reaction center,Organization of light-absorbing a

8、ntenna systems,2-dimensional view of the structure of the LHCII (Light Harvesting Complex) antenna complex from higher plants Transmembrane pigment protein with three helical regions 14 Chl a and b, and 4 carotenoids molecules are associated with the complex,Basic components of reaction center： cont

9、ains at least a center pigment molecule or called primary electron donor (P), a primary electron acceptor (A), a secondary electron donor (D)，and necessary proteins that maintain homeostasis where light energy transforms to electric energy.,primary electron donor (P): few special Chla Primary electr

10、on acceptor (A): the substance accepted electrons directly which transferred from reaction centre pigment (P). Secondary electron donor (D): the substance donating electrons directly to reaction centre pigment (P).,h DPA DP*A D P+A-D+ PA- ground state excited state charge transfer reaction center re

11、action center reaction centre,In higher plant: The ultimate electron donor is H2O; The ultimate electron acceptor is NADP+,Primary reaction of photosynthesis：,5.2.2 Photosynthetic Electron Transfer and Photophosphorylation,5.2.2.1 Photosystem,Enhancement Effect,The characteristic of PS and PS PS : l

12、ight reaction belongs to short wave red light reaction (P680)，the main characteristic is water photolysis and release oxygen. PS: the light reaction is long wave red light reaction (P700), the main characteristic is NADP+ reduction.,5.2.2.2 Photosynthetic Chain,is a electron transfer orbit, composed

13、 with a series of electron transporters linking each other, located in photosynthetic membrane. “Z” scheme (R.Hill et al，1960). Transporters: 1. plastoquinone, PQ质体醌 (most amount, 1/7 ) ; 2. Cytochrome Cyt b6f complex; 3. ferredoxin, Fd铁氧还蛋白; 4. plastocyanin, PC质体蓝素;,PQ shuttle : PQ is a transporter

14、 of 2H+ with 2e-. 1. reduction of PQ occurs at the stroma side of the thylakoid and removes 2 protons from the stroma; 2. PQ shuttles electrons to the cyt b/f complex on the lumen side of the thylakoid. Only electrons are passed to the cyt b/f complex, the protons are expelled into the lumen. Thus,

15、PQ is oxidized on the lumen side of the membrane;,Oxygen-evolving organisms have two photosystems operating in series,P680 and P700 - Reaction Center pigments (Chl a),“Z” scheme,Electrons from chlorophyll travel through the carriers organized in the “Z scheme”,Cyclic ET,The transfer of electrons and

16、 protons is carried out by four protein complexes,Cyclic ET,Electron Transport,The photosystem II reaction center complex,The transfer of electrons and protons is carried out by four protein complexes,Cyclic ET,Mechanism of electron and proton transfer in the cytochrome b6f complex,Mechanism of elec

17、tron and proton transfer in the cytochrom b6f complex,The photosystem I reaction center complex,叶绿醌,The transfer of electrons and protons is carried out by four protein complexes,Cyclic ET,Electron Transport,How is ATP formed?,Electron and proton transport form a proton motive force (PMF) PMF is use

18、d to make ATP Where are protons produced? 1. Splitting of water 2. PQH2 oxidation,Light-dependent ATP synthesis: PHOTOPHOSPHORYLATION Peter Mitchell, 1960, photophosphorylation works via chemiosmotic mechanism,How is ATP formed?,Experiment showing energy in pH gradient is converted to ATP in the dar

19、k (Andre Jagendorf 1967).,Thylakoid Membrane,Stroma,Lumen,pH 8,pH 4,ATP synthase (ATPase),F0,5 polypeptides （3，3 1 1 1 ）,4 polypeptides （1 a，1 b, 1b 14 c）,Some herbicides block photosynthetic electron flow,+ O2 - O2,百草枯,敌草隆,Xanthophyll cycle,(e.g. ascorbate- glutathione cycle),Repair and Regulation

20、of the photosynthetic machinery,叶黄素循环,Summary Photosynthetic light reactions,Cyclic ET,5.2.2.3 H2O photolysis and O2 release,1. Hill reaction H2O was photolyzed in the isolated chloroplasts and release O2 under light. This reaction is Hill reaction.,R. Hill(1937)found that O2 was released from H2O w

21、hen isolated chloroplasts was added to the solution with hydrogen acceptors (A) under light. light 2H2O + 2A 2AH2 + O2 the isolated chloroplasts,2. Oxygen Release Mechanism,Oxygen-evolving complex(OEC), composed by Manganese stablizing protein (MSP) and Mn(2+ ,3+ ,4+), Ca2+, Cl-, participates oxygen

22、 releasing in PS. Mn is a component of OEC, Cl and Ca act as activators.,5.2.2.4 Pathways of Photosynthetic Electron Transport,Noncyclic Electron Transport The electrons, released by H2O photolysis, transport to NADP through two photosystems ( PS and PI ). The electron transport is a open pathway. H

23、2OPSPQCytb6/fPCPSFdFNRNADP According to noncyclic electron transport: 8 photons absorbed, 2 H2O photolysed, 4 electrons transported, 1 O2 release, 2 NADP+ reduced, and release 8 H into thylakoid lumen (4 H /photolysis, 4 H / PQ shuttle).,Characteristics： Electron transport pathway is opened, includi

24、ng O2 release, ATP and NADPH formation.,2 Cyclic Electron Transport： The electrons produced by PSis transported to Fd，then Cytb6/f complex and PC，finally back to PSI. So, the electron transport pathway is a closed circle. PSFd(NADPHPQ)Cytb6/fPCPS,Characteristics： Electron transport pathway is closed

25、 circle. neither O2 release, nor NADP+ reduction, only ATP synthesis.,3 Pseudocyclic Electron Transport： The electrons, released by H2O photolysis, transport to O2 through PS and PSI . Also named Mehler reaction. The only difference from noncyclic electron transport is that final electron acceptor i

26、s O2 , not NADP. It usually happens under strong light, and shortage of NADP+ . H2OPSPQCytb6/fPCPSFdO2,Characteristics： O2 release, ATP formation, but not NADPH formation. The final electron acceptor is O2 , form O2- (superoxide anion free radical).,Schema of electron transport in chloroplast,Cytb C

27、ytb Q FeSR Cytf,5.2.2.5 Photophosphorylation,Photophosphorylation, coupled with photosynthetic electron transport, has three types: ： noncyclic photophosphorylation； cyclic photophosphorylation； pseudocyclic photophosphorylation.,Pi and ADP synthesize ATP in chloroplast under light, this process is

28、called photophosphorylation.,Mechanism of Photophosphorylation -P.Mitchell “chemic osmosis theory”,（1） H2O was photolyzed in PS and H+ was released inside thylakoid membrane (lumen)； （2） In photosynthetic electron transporters, PQ shuttle transfer transport H+ from stroma into inside of thylakoid me

29、mbrane (lumen) in same time （3） H+ concentration in stroma decrease and H+ concentration in thylakoid lumen increase Proton motive force,The main points ：,（4）Proton motive force (pmf) is the primary force for Photophosphorylation. pmf = pH(质子浓度差) + E(电势差),Formation of Assimilation Force ：,Electrons

30、released by H2O photolysis transport through noncyclic electron chain. 2 mol H2O decomposed, 1mol O2 released, 4 mol electron transported, 8 mol H+ released to thylakoid lumen, coupling with the formation about 3 mol ATP and 2 mol NADPH. Assimilation Force : ATP and NADPH which will be used for assi

31、milation and reduction of CO2,Electron transport inhibitor: DCMU(敌草隆)inhibits electron transport from QA to QB in PS, hydroxylamine (羟胺) inhibit electron transport from H2O to PS; paraquat (百草枯) inhibit P700* NADP+ Photophosphorylation Uncoupler: DNP, NH4+ can increase membrane permeability to H+, o

32、ligomycin (寡霉素) can bind to CFo, both inhibit formation of ATP.,5.2.3 CO2 assimilation,CO2 assimilation: Assimilation force (ATP and NADPH) from light reaction convert CO2 into carbohydrates in Green plants,CO2 carbohydrates,ATP, NADPH,Light and Carbon reactions of Photosynthesis,Light Absorption: E

33、lectrons are “pulled” from water, and O2 is evolved (light reaction) 2. Electron Transport : NADPH is formed (light reaction) 3. Generation of ATP: (light reaction) 4. Conversion of CO2 into Carbohydrates (carbon reaction),There are three pathways of CO2 assimilation in higher plant ： C3 pathway - c

34、an synthesize starch and sucrose C4 pathway can not synthesize starch and sucrose CAM pathway can not synthesize starch and sucrose,5.2.3.1 C3 pathway In this pathway, CO2 acceptor is RuBP, it is also called reductive pentose phosphate pathway，RPPP. The first product of CO2 fixation is a 3-C compoun

35、d. So, it also called C3 pathway. Calvin reported this pathway in 1950s, so it is also called as the Calvin cycle. C3 pathway divided into three stages: 1.carboxylation羧化作用; 2. Reduction; 3. regeneration 再生,The Calvin Cycle C3 Carbon Fixation,Carboxylation,Reduction,Regeneration,CH2O P C=O Rubisco C

36、OOH HCOH + CO2 + H2O 2 HCOH HCOH Mg2+ CH2O P CH2O P 1,5-ribulose bisphosphate 3-phosphoglycerate carboxylase/oxygenase RuBP 3-PGA,1. Carboxylation phase,Rubisco二磷酸核酮糖羧化酶 is abbreviation of 1,5-ribulose bisphosphate carboxylase/oxygenase (RuBPC/RuBPO) Rubisco is the most abundant enzyme in the plant,

37、 accounting for approximately 40 % of the soluble proteins in plant leaves, and it consists of 8 big subunits and 8 small subunits, the active site is located on the big subunits. The big subunits are coded by chloroplast gene, and the small subunits are coded by nuclear gene.,RUBISCO Protein,RUBISC

38、O: 8 large subunits (green, blue) 13 kDa 8 small subunits (red) 55 kDa,540 kDa,2Reduction phase,Energy storage reaction is finished by 3-GAP formation,3. Regeneration phase The process of GAP transforms to RuBP by series of reactions This is accomplished by a series of reactions, involving in conver

39、tions of 3-、4-、 5-、6- and 7-C carbohydrates, finally catalyzed by Ru5PK consuming 1 ATP to regenerate RuBP and completing a cycle.,FBP,F6P,Fructose bisphosphate phosphatase FBPase,+,erythrose-4-phosphate E4P赤藓糖,xylulose-5-phosphate Xu5P,3-phosphoglycerate GAP,+,dihydroxyacetone phosphate DHAP,aldola

40、se,SBP,S7P,Ru5P,+,ATP,RuBP,Ru5PK,SBPase,酯酶,3CO2 + 3H2O + 9ATP + 6NADPH+6H+ GAP + 9ADP + 8Pi + 6NADP+ Assimilating 1 mol CO2 requires 3 mol ATP and 2 mol NADPH as energy, and exporting a GAP or DHAP needs 3 mol CO2 reduction. GAP or DHAP can synthesize starch in chloroplast, or transported from chlor

41、oplast to cytoplasm, and then to synthesize sucrose.,Total formula of C3 pathway:,Overview of the Calvin Cycle 3 Stages,Regulation of exporting rate of photosynthetic product :,Chloroplast,Cytoplasm,Triose phosphate,5.2.3.2 C4 pathway Some plants originate from tropic area, such as sugarcane，corn (m

42、aize) and Sorghum have another way to fix CO2 besides C3 pathway in most plants. The first product of CO2 fixation is a four-carbon dicarboxylic acid, so it is called C4dicarboxylic acid pathway, or Hatch Slack pathway,CO2 fixed by C4 pathway are called C4 plants Now it is known there are nearly 200

43、0 species of C4 plants, more than 20 different families in angiosperm. The acceptor of CO2 in C4 pathway: PEP The location: cytoplasm of mesophyll cells,C4 photosynthetic pathway,CO2 concentrates in bundle sheath cells,PPDK,MD,ME,PEPC,5.2.3.3 Crassulaceas acid metabolism (CAM) pathway: A special CO2

44、 assimilation pathway for plants such as Crassulacean, cactus, pineapple growing in dry area. Night (stoma open): CO2 + PEP OAA Mal Day (stoma close): Mal in vacuole CO2 These plants contain higher sugar content in the day , however, they accumulate organic acid at night. The photosynthesis carbon m

45、etabolism of organic acid synthesis with day-night change is called crassulaceas acid metabolism (CAM). PEP: phosphoenolpyruvate; OAA: oxaloacetic acid; Mal: malate,PEPC,NADP-MD,vacuole,cytoplasm,NAD/NADP-ME,C3 pathway,sugar，starch,Significance of the diversity of Carbon assimilation in plant Divers

46、ity of photosynthesis carbon assimilation in plants reflects their adaptation to different ecological environments. But C3 pathway is the most basic and universal one in plants. sucrose and starch are synthesized from C3 pathway only C4 and CAM pathways are the complement for C3 pathway.,Synthesis o

47、f Starch and Sucrose,Motivation,Sugarcane,Sucrose, a disaccharide, is synthesized in the cytosol.,Motivation,Rice,Starch, a poly-saccharide, is synthesized in the chloroplast.,Carbon mobilization in vascular plants,麦芽糖,Adenosine diphosphate-glucose,Synthesis, Export and Storage of Photosynthetic Pro

48、ducts,Synthesis: a. Starch synthesis in the chloroplast: G3P Hexose Starch b. Sucrose synthesis in the cytoplasm of mesophyll cells: G3P moves into the cytoplasm: G3P Hexose Sucrose Transport Sucrose is exported to sinks via the phloem. Fate of sucrose in sinks: a. Sucrose Hexose Respiration & Synth

49、esis b. Sucrose Hexose Starch for storage,Syntheses of starch and sucrose are competing processes in the chloroplast and cytosol, respectively,Adenosine diphosphate-glucose,Uridine diphosphate-glucose,Synthesis of starch,Adenosine diphosphate-glucose,Synthesis of sucrose,Uridine diphosphate-glucose,

50、Bundle sheath cell from maize,Chloroplast,Starch grain,5.3 Photorespiration,O2 CO2,5.3.1 Biochemical process of photorespiration Photorespiration is an oxidation process, oxidized substrate is glycolic acid.,Light,Green plant cells,1,5-ribulose bisphosphate,1. The whole process of photorespiration i

51、s completed by three organelles which are chloroplast, peroxisome and mitochondrion.,2. The substrate of photorespiration is glycolic acid，so it is called C2 Cycle.,3. O2 absorption occurs in chloroplast and peroxisome, and the CO2 release occurs in mitochondrion.,4. In C2 Circle，2 molecules of glyc

52、olic acid release 1 molecule CO2, and the lose fixed carbon more than 25%.,5.3.2 Physiological function of photorespiration,photorespiration has a major impact on cellular metabolism, particularly under high light, high temperatures, and CO2 or water deficits. Although the functions of photorespirat

53、ion remain controversial, it is widely accepted that this pathway influences a wide range of processes from bioenergetics, photosystem II function, and carbon metabolism to nitrogen assimilation and respiration. Crucially, the photorespiratory pathway is a major source of H2O2 in photosynthetic cell

54、s. Through H2O2 production and pyridine nucleotide interactions, photorespiration makes a key contribution to cellular redox homeostasis. In so doing, it influences multiple signaling pathways, particularly those that govern plant hormonal responses controlling growth, environmental and defense resp

55、onses, and programmed cell death. The potential influence of photorespiration on cell physiology and fate is thus complex and wide ranging.,5.4. Daily Variation of Photosynthesis,Reasons for Photosynthetic Midday Depression (1) drought of air and soil (leading to stomatal conductance decrease) (2) d

56、ecrease of CO2 uptake (3) Block photosynthetic product transportation, leading to feedback suppression of photosynthesis (4) photorespiration increase (5) Adjusted and controlled by physiological clock The loss caused by photosynthetic midday depression can reach over 30% of photosynthetic products.

57、,5.5 Loading and unloading of assimilate in phloem,5.5.1 Loading pathway,1. Symplast pathway: assimilate moves into companion cell through plasmodesma, and finally moves into sieve tube； 2. Alternate pathway，assimilate moves into apoplast through mesophyll cells，then moves into companion cell agains

58、t concentration gradient, and finally moves into sieve tube，called “symplast-apoplast-symplast” pathway.,(Source site),Symplastic loading,Polymer-trapping model P176,Raffinose棉子糖 Stachyose水苏糖,5.5.2 Loading mechanism,Loading is an active secretory process with high flow velocity and against concentra

59、tion gradient， which regulated by carrier.,Evidence： （1）need energy supply （2）selectivity for loading substances （3）saturation effect,ATP-dependent sucrose transport in sieve element loading,Regulation of sucrose loading?,sucrose - proton symport (cotransport),(SE-CC),Symplast,apoplast,5.5.3 Unloading pathway of assimila