Photosynthesis

How can chloroplasts induce photosynthesis in the dark?

-ATP synthesis is driven by a proton motive force -with light induced e- transfer, H+ released into the thylakoid lumen or taken up from the stroma--->acidic lumen -gradient maintained because thylakoid membrane is impermeable to protons -GRADIENT DETERMINED BY PH, NOT BY LIGHT -experiment: soaked chloroplasts in ph4 buffer, then rapidly mixed it with ph8 buffer w/ADP+Pi--->at first only stroma went to ph8, but then proton gradient disappeared with ATP synthesis

CAM PATHWAY

-Crassulacean acid metabolism permits growth in arid ecosystems -plants in hot, dry climates keep the stomata of their leaves closed in the heat of the day to prevent water loss- CO2 cannot be absorbed during daylight -CO2 enters the leaf at night when stomata is open -to store CO2, plants use CAM 1)CO2 fixed by C4 pathway into malate, which is stored in vacuoles 2)during the day, malate is decarboxylated and CO2 becomes available to the Calvin Cycle TEMPORAL SEPARATION- separates CO2 accumulation with CO2 utilization

Mode 1: much more R5P is needed than NADPH

-FOR EXAMPLE, RAPIDLY DIVIDING CELLS NEED R5P FOR SYNTHESIS OF NUCLEOTIDE PRECURSORS OF DNA -G6P IS USED FOR GLYCOLYTIC PATHWAYS---> F6P AND G3P -THEN TRANSALDOLASE AND TRANSKETOLASE CONVERT 2 F6P AND 1 G3P TO 3 R5P

Mode 2: need for R5P and NADPH balanced

-OXIDATIVE PHASE OF PPP USED TO GENERATE 2 NADPH AND 1R5P FROM G6P

Regulation of PPPathway

-RLS is dehydrogenation of G6P -NADP+ levels regulate dehydrogenase activity -low NADP+- inhibits the dehydrogenase (NADP+ is needed as e- acceptor) -high NADPH inhibits the dehydrogenase by competing with NADP+ for the active site

Photorespiration

-RUBISCO also catalyzes wasteful oxygenase reaction- instead of CO2 as substrate, O2 reacts with enediolate intermediate--->formation of phosphoglycolate and 3-phosphoglycerate -phosphoglycolate is not a versatile metabolite- salvage pathway recovers part of its carbon skeleton through photorespiration -phosphoglycolate undergoes a pathway in which it is eventually converted into serine- loses CO2 and ammonium ion -called photorespiration because O2 is consumed and CO2 is released -wasteful reaction because organic carbon is converted into CO2 without production of ATP, NADPH, or another energy-rich metabolite

SUMMARY

-absorption of 4 photons by PS II generates 1 molecule O2, releases 4 H+ into the lumen -2QH2---->2Q + 8H+ into the lumen -electrons from 4 Pcred--->4 Fdrec by absorption of 4 additional photons -4 Fdred-->NADPH -12H+ in lumen flow through ATP synthase- 1 rotation= 3 ATP, 8 photons= 3 ATP

RUBISCO

-catalyzes fixation of CO2- ribulose-1,5-BP------->3-phosphoglycerate -found on stromal surface of thylakoid membrane -catalyzes RLS of hexose synthesis -requires bound divalent metal ion for activity- activates bound substrate by stabilizing R-1,5-BP so that it will react with CO2 -CO2 (not substrate) adds to the uncharged Lys sidechain of Rubisco to form carbamate(-)- Mg2+stabilizes the negative charge -R-1,5-BP binds to Mg2+ ion linked to rubisco through the lysine carbamate and 2 other residues -R-1,5-BP gives up proton to form enediolate intermediate- intermediate reacts with CO2--->2 3-phosphoglycerates

MODE 3: MUCH MORE NADPH THAN R5P NEEDED

-for example, for FA synthesis -G6P completely oxidized to CO2 1)oxidative phase of PPP---> 2 NADPH and 1 R5P 2)R5P converted into F6P and G3P by transketolase and transaldolase 3)F6P and G3P are used to form G6P through gluconeogenesis IN ESSENCE, OXIDATIVE PHASE OF PPP IS USED TO GENERATE R5P WHICH CAN BE RECYCLED INTO G6P BY TRANSKETOLASE, TRANSALDOLASE, AND GLUCONEOGENIC PATHWAY

Pentose-Phosphate Pathway

-generates NADPH and synthesizes 5-C sugars IN CYTOPLASM -meets the need of all organisms for a source of NADPH to use in reductive biosynthesis- FA biosynthesis, neurotransmitter synthesis, nucleotide synthesis, cholesterol synthesis 1)oxidative phase- generation of NADPH -starts with dehydrogenation of G6P---->ribulose-5-phosphate which isomerizes to ribose-5-phosphate YIELD: 2 NADPH 2)nonoxidative phase -excess R-5-P can be converted to glycolytic intermediates 3 R-5-P---> 2F6P + G3P via transaldolase/transketolase

C4 PATHWAY

-in tropical climates- oxygenase activity of RUBISCO dominates over carboxylase activity- so tropical plants use c4 pathway to prevent wasteful photorespiration -solution:SPATIAL SEPARATION OF CARBON FIXATION AND USAGE -C4 compounds such as oxaloacetate and malate carry CO2 from mesophyll cells, which are in contact with air, to bundle-sheath cells, which are major sites of photosynthesis 1) condensation of CO2 and PEP--> oxaloacetate, which is sometimes converted into malate 2)malate enters bundle-sheath cell-->oxidative decarboxylation--->CO2 to calvin cycle by condensing with R-1,5-BP -pyruvate is formed--->returns to mesophyll cell for conversion to PEP -ENERGETIC EQUIVALENT OF 2 ATP MOLECULES ARE CONSUMED IN TRANSPORTING CO2 TO CHLOROPLASTS OF BUNDLE-SHEATH CELLS -CO2 CONCENTRATION IN BUNDLE-SHEATH CELLS CAN BE 20-FOLD AS GREAT IN COMPARISON TO MESOPHYLL CELLS -TROPICAL PLANTS WITH A C4 PATHWAY DO LITTLE PHOTORESPIRATION BECAUSE THE HIGH CONCENTRATION OF CO2 IN THEIR BUNDLE-SHEATH CELLS ACCELERATES THE CARBOXYLASE REACTION ELATIVE TO THE OXYGENASE REACITON

Starch and Sucrose

-major carbohydrate store in plants -starch- similar in structure to glycogen- less branching- synthesized in the chloroplast -sucrose- disaccharide- synthesized in the cytoplasm

PS I

-uses light energy to generate reduced ferredoxin, a powerful reductant -P700- initiates photoinduced charge separation Pc(red) + Fd (ox)---> Pc(ox) + Fd(red)- driven by absorption of photon -induces e- transfer from Pc-->P700 down an electron transfer pathway to a ferredoxin--->2 Fd(red) -each donates total 2e- and 2H+ to FAD prosthetic group of ferredoxin-NADP+ reductase--->donates H- to NADP+--->NADPH on stromal side of membrane NADPH is ultimate acceptor- used in reactions of Calvin cycle -P700+ captures e- from Pcred---> P700 so it can be excited again

cyclic photophosphorylation

-when there is not enough NADP+, Fdred gives its e- to Pcox---> Pcred -then Pcred gives its e- to P700+---> P700 -resulting proton gradient results from pumping of protons by cyt bf complex---ATP synthesis ATP GENERATED WITHOUT CONCOMITANT FORMATION OF NADPH

Three Stages of the Calvin Cycle

1)Fixation of CO2 2)Reduction 3)Regeneration -CALLED THE C3 CYCLE BECAUSE FIRST STEP, FIXATION OF CO2, RESULTS IN FORMATION OF 3 CARBON MOLECULE-->2 3-PHOSPHOGLYCERATE

How is the activity of the Calvin Cycle dependent on Environmental Conditions?

1)Influence of Light on Rubisco- -RLS of Calvin cycle is R-1,5-BP--->3-phosphoglycerate -activity of rubisco increases markedly on illumination- light facilitates carbamate formation necessary to enzyme activity a)ph in stroma increases--->carbamate formation is favored at alkaline ph b)Mg2+ ions from thylakoid space are released into the stroma to compensate for influx of protons 2)Thioredoxin -cycles betwen reduced/oxidized form -reduced form activates many biosynthetic enzymes- reduces disulfide bridges to activate biosynthetic enzymes/inhibit degradative enzymes -presence of reduced ferredoxin and NADPH are good signals that conditions are right for biosynthesis -in chlorpolasts, ferredoxin reduces thioredoxin via f-thioreductase- couples 2 1e- oxidations of Fred to 2 e- red of thioredoxin THUS THE ACTIVITIES OF THE LIGHT AND DARK REACTIONS OF PHOTOSYNTHESIS ARE COORDINATED THROUGH E- TRANSFER FROM REDUCED FERRDOXIN TO THIOREDOXIN, AND THEN TO COMPONENT ENZYMES CONTAINING REGULATORY DISULFIDE BONDS

TWO PARTS OF PHOTOSYNTHESIS

1)Light Reactions -occur at the thylakoid membrane -transforms energy of light into ATP and biosynthetic reducing power; NADPH 2)Dark Reactions -occur in the stroma -use ATP and NADPH to reduce carbon atoms from CO2 to hexose -CALVIN CYCLE

How does Photosystem II generate a proton gradient?

1)P680 (special pair in photosystem II) is excited by light--->transfers excited electron to nearby pheophytin-->Qa--> mobile plastoquinone QB -with the arrival of the second e- and the uptake of 2 H+ from the stroma- Qb--QH2 2)when photon kicks out e- from P680 (P680-->P680+), nearby tyrosine residue donates an e-,-->formation of tyrosine radical -tyrosine radical then removes e- from Mn2+ -after extraction of 4 e- (with the absorption of 4 photons), then O2 is generated 2H20---> O2, 2H+, 2Q--->2QH2 -4H+ taken up in reduction of Q on stromal side of thylakoid membrane -4H+ released in lumen upon oxidation of H2O--->O2 THUS FORMATION OF PROTON GRADIENT--->HIGHPH IN STROMA, LOW PH IN LUMEN

How does cyt bf generate a proton gradient?

1)QH2 is oxidized to Q, gives 2 electrons to Plastocyanin, 1 e- at a time QH2-->Q + 2e- + 2H+ 2 Pc(Cu2+) + 2e- --> 2 Pc(Cu+), 2H+ to lumen 2)cyt bf reduces molecule of Q from the Q pool Q--->QH2 (takes up 2 proton from the stroma), then cyt bf reoxidizes it in the lumen- 2 H+ released in lumen total: 2H+ taken up from stroma, 4H+ released in lumen

Mode 4: Both NADPH and ATP are required

1)oxidative phase of PPP---> 1 R5P and 2 NADPH 2)R5P--->F6P and G3P---> pyruvate through glycolysis ATP AND NADPH ARE CONCOMITANTLY GENERATED, AND 5 OF 6 CARBONS OF G6P EMERGE IN PYRUVATE -pyruvate can be oxidized to generate more ATP or it can be used as a building block in a variety of biosyntheses

Reduction and Regeneration of Calvin Cycle

2)reduction- 2 3-phosphoglycerates are converted into Fructose-6-phosphate at the expense of ATP and NADPH from the light reactions 3)Regeneration of R-1,5-BP -formation of 5-c sugars using transketolase and aldolase--->ribose-5-phosphate and xylulose-5-phosphate--->ribulose-1,5-bisphosphate -3 molecules of ATP and 2 molecules of NADPH are consumed in incorporating a single CO2 into a hexose

How are the Calvin Cycle and PPP Mirror Images?

CC begins with fixation of CO2 and proceeds to use NADPH in synthesis of glucose. The PPP begins with oxidation of glucose-derived carbon atom to CO2 and concomitantly generates NADPH -regeneration phase of CC---> Ribulose-1,5-BP -PPP converts R5P--->C6 and C3 intermediates of glycolytic pathway

What is the difference between photoinduced charge separation and resonance energy transfer?

PHOTOINDUCED CHARGE SEPARATION -molecule absorbs light energy, which excites an electron from a lower energy state to a higher energy state -this high-energy electron moves from donor (D) to acceptor (A)---> D+ and A- -occurs at reaction center -electron now has reducing power- can reduce other molecules to store the energy originally obtained from light in chemical forms RESONANCE ENERGY TRANSFER -done by accessory pigments such as chlorophyll b and carotenoids because 1)chlorophyll a is specific for only certain range of wavelengths- chlorophyll b can capture photons in wavelengths outside this range 2)even ones that do not fall in this gap sometimes pass through because density of chlorophyll a is not very high -ENERGY TRANSFERRED, NOT ELECTRON TRANSFER -excitation energy transferred from one molecule to another through electromagnetic interactions in space -energy transfer must be from donor in excited state to an acceptor of equal or lower energy -accessory pigments have higher energy in excited state than do special pair in reaction center- thus they are able to transfer energy to reaction center -RESONANCE ENERGY TRANSFER ALLOWS ENERGY TO MOVE FROM THE SITE OF INITIAL ABSORBANCE TO THE REACTION CENTER ACCESSORY PIGMENTS ABSORB LIGHT AND FUNNEL THE ENERGY TO THE REACTION CENTER FOR CONVERSION INTO CHEMICAL FORMS.