Chapter 19 photosynthesis

Electron Chain in the Photosynthetic Bacterial Reaction Center

1.P960 absorbs a photon, entering the excited state. 2.P960 passes an electron to BPh, forming BPh− and creating a photoinduced charge separation. 3.BPh− transfers the electron to QA, forming Q 4.P960 absorbs another electron from cytochrome c2. 5.Q − donates an electron to Q , which binds a proton. A B 6.Steps 1-5 occur again, generating fully reduced QB (QH2)B. 7.Reduced QB moves into the Q pool of the membrane.

A light harvesting system that relied only on chlorophyll a molecules of a special pair would be inneficient for 2 reasons

1: they only absorb at a specific wavelenght, theres a large gap present in the middel of the visible region between 450 and 650 that falls at the peak of the solar spectrum, so failing to collect this light would be alot of lost opportunity. 2nd: even when theres alot of sun, mayn photons will pass through the chloroplast without being absorbed because the density of chlorophylla molecules in a reaction center is not very great so we use accessory pigments, both additional chlorophylls and other classes of moelcules, that are closely associated with the reaction centers. these pigments absorb light and funnel energy to the reaction center for conversion into chemical forms

Structure of a Bacterial Light- harvesting Complex

Chlorophyll arranged with carotenoid Can transfer energy directly to reaction center so next up in terms of funnel stage, carefully arranged to funnel energy into every chlorophyll in reaction center

Cytocrome bf links PS2 and PS1

Electrons flow from PS2 to PS1 through cytochrome bf complex. it catalyzes the transfer of electrons from plastoquinnol (Qh2) to plastocyanin(Pc) a small soluble copper protein in thylakoid lumen, 2 protons from QH2 are relased into thylakoid lumen Cytochrome bf contains 2 b type hemes, a rieske type Fe-S protein, a cytochrome f with a c type cytochrome and a chain goes through q cycle, first half turns QH2 into Q one electron at a time, electrons from QH2 go to FE-S protein to convert Pc into reduced form. in second half, cytochrome bf reduces a molecule of Q from Q pool into QH2, takes 2 protons from one side of the memrbane, reoxidizes the QH2 to release protons onto other side. released into lumen taken from stroma, more proton gradient.

Two Photosystems

Electrons go from water to PPS2, to cytochrome bf mobile, to plastocynanin also mobile then give to PS1, receiver is NADP+ to produce reducing power NADPH If catabolism is oxidative, anabolis is inverse building reduction need reducing reagents and NADPH is our reducing power First light comes in at PS2, starts photoinduced charge separation and begins electron transfer, eventually during the process generates ph gradient forms ATP and when it reaches PS1 you have to excite again Slightly differed by absorption peak, first is p680, PS1 is called P700,

Light Absorption

Empty excited state orbital, once light energy is absorbed, and one electron drops to excited state, depends on the photoreceptor Depend on the photoreceptor, they like to du to chem structure conjugated systems groups state and excited state they absorb at characteristic spectroscopy peaks Chlorophyll is major color of earth, green color because of this spectroscopic light absorption Once electrons are excited they can transfer

Primary events of photosynthesis take place in thylakoid membrane

It contains energy transforming machinery which include the light harvesting proteins, reaction centers, ETC and ATP synthase memrbane has equal amounts of lipids and proteins 75% galactoliids, 10% sulfolipids, 10% phospholipids THylakoid memrbane and inner membrane are impermeable to molecules and ions Outer membrane of the chloroplast is highly permeable stroma contains soluble enzymes that use the NADPH and the ATP made by the thylakoids to convert CO2 into sugar.

Bacterial Photosynthetic Reaction Center

One carrier to another once its been generated, because you have ETC, you have a defined organixation of components and carriers, in the oxidative phosphorlyaiton you have complex 1,2,3,ato synthase Large protein complex, special pair reaction center is in the middle, another bchl-b nearby and bph next to that, electron will be moved at special apir end up at a quinone Quinone can be released is mobile can go to different protein complexes while others are stiff wont move. Highly organized because the series of events in sequence that will happen.

How is charge recombination prevented?

P960+ and BPh- can go back to natural states, this would waste the high energy electron and convert it into heat its prevented by Qa being so close to BPh- to rapidly transfer the electron, 2nd one of the hemes on the C subunit is very close form the special pair, so its able to neutralize the P960 positive charge by transferring electrons from the reduced cytochrome.

PS1 uses light enery to generate reduced ferredoxin

PS1 is transmembrane complex of 15 chains and other proteins and cofactors. Made of psaA and psaB, which binds 80 chlorophyll moelcules special pair of chlorophyll a molecules lies at center of structure and absorbs light at 700nm (p700) p700 initiates photoinduced charge separation, electron travels from p700 down through chlorophyll at site Ao and quinione at site A1, toa 4Fe-4S cluster. electron gets sent to ferrodoxin, soluble protein with 2fe-2s cluster coordinated to 4 cysteine residues, this goes on to transfer electrons to NADP+ P700+ capturs electron from reduced Pc provided by PS2 to return to p700

Electron Flow Through Photosystem I to Ferredoxin

Pc takes from bf complex, excitation happens at reaction center at p700, Ao and A1, no pheophytin carrier identifies are different as well, 4fe-4s are carriers as well, ferredoxin Ferredoxin is small protin that has iron sulfur cluser, 2fe-2S with 4 cysteine residues

Electron Flow Through Photosystem II

Special pair in center, D1 D2 subunits special pair of chlorophyl A, magnesium in center Thylakoid membrane, electron carriers nearby. Special pair generates charge separation and one electron moved to pheophytin, Qa then Qb, go about until QB becomes QH2, releasing mobile quinone unit Refill after separation is not from cytochrome c There is a manganese center that splits water to generate O2 and strips electrons to go into it Keep consuming water

Pathway of Electron Flow from H2O to NADP+ in Photosynthesis "Z"Scheme

Summary of entire thing, electron transport depends on redox potential, high energy carrier goes to lower excited state Y axis needed, relative highes of each component must be scientifically accurate Function: split water strip the electrons and goes all the way

What is the functional significant of this laterial differentiation of the thylakoid membrane?

THe postioning of PS1 in unstacked memrbanes gives it direct access to the stroma for the reduction of NADP+. ATP synthase is also in the unstacked region to provide space for its large CF1 globule and give access to ADP. tight space is not a problem for PS2, which interacts with small polar electron donors H2O and highly lipid soluble electron carrier (plastoquinone)

Proton-gradient Direction

To generate oxygen is to supply continuous flw of electrons Split water you get oxygen gas and protons Quinone needs to take up protons from the outside of the thylakoid lumen, consuming protons outside producing protons inside generates a gradient, get a lower pH outside higher pH inside Once the proton gradient is generated, that's energy

Protective Function of Accessory Pigments

•Accessory pigments help transfer energy to reaction centers, but also serve a protective role. •They are able to assist in suppressing harmful photochemical reactions. In bright sunlight, they use nonphotochemical quenching (NPQ) to direct photons away from the light-harvesting complex and release energy as heat, preventing formation of damaging reactive oxygen species (ROS). •Plants lacking carotenoids are quickly killed on exposure to light and oxygen.

Electron Transfer

•Another fate for the excited electron is to move to a nearby molecule that has a lower excited state, a process called electron transfer. •Electron transfer results in photoinduced charge separation because the initial molecule is now positively charged and the molecule that accepted the electron is negatively charged. •Separation of charge occurs at a site called the reaction center. Starts from photoinduced charge separation, because electrons at ground state can be excited to higher state, if there's another carrier nearby that can provide a more stable ground state and its close enough then its released to it because its more stable and becomes a lower excited state in different carrier Once it jumps, removed from other and generated a positive and negative charge Separation of charge occurs at site called reaction center,

Accessory Pigments Funnel Energy into Reaction Centers

•Chlorophyll b and carotenoids, such as lycopene and β- carotene, are light-harvesting pigments that funnel energy to the reaction center. Narrow use of light, what about the solar light coming in at wide spectrum covers a lot, wateful if you cant get it, if you don't absorb it, you cant usd it and instead it causes harm Photosyntehtic organisms have light harvesting system

The Activity of Chloroplast ATP Synthase is Regulated

•Chloroplast ATP synthase is activated when a specific disulfide bond in the γ subunit is reduced to two cysteines. •The reductant is reduced thioredoxin, which is formed from ferredoxin generated in photosystem I, by ferredoxin- thioredoxin reductase. •An increase in the proton-motive force causes a change in the ε subunit of ATP synthase, which also enhances synthase activity. •These control mechanisms ensure that synthase activity is maximal when biosynthetic reducing power and a proton gradient are available and coordinate the light reactions with ATP synthesis. When do you want to use this, when p gradient is high, when light reaction is going well and we can generate more ATP ferredoxin-thioredoxin reductase Thioredoxin primarily regulatory protein that can conduct redox sensing, instead of iron sulfide cluster its disulfide bond, if its reduces to 2 free thiols its reduced, using a disulfide bond to sense redox potential ATP synthase will have higher activity if light reaction is going a lot of ferredoxin going to produce thiorodoxin, which will then control enzyme activity enhance synthase activity

Chloroplasts Arose from an Endosymbiotic Event

•Chloroplasts contain their own DNA and the machinery for replicating and expressing it, but they are not autonomous since they also require nuclear-encoded proteins. • •Chloroplasts in higher plants and green algae are believed to be the descendants of an ancestor of a cyanobacterium that was engulfed by a eukaryotic host; chloroplasts of red and brown algae are derived from at least one additional event. Small bacteria engulfed in bigger one and eukaryotic cells retained them Those bacteria in the chloroplasts in higher plants are called cyanobacteria photosynthetic bacteria

Cytochrome bf Links Photosystem II to Photosystem I

•Cytochrome bf transfers electrons from plastoquinol (QH2) to plastocyanin (Pc). •Protons from plastoquinol are released into the thylakoid lumen, and cytochrome bf pumps two more protons from the stroma into the lumen, generating a proton-motive force. •The mechanism is similar to the Q cycle of Complex III in the electron-transport chain of cellular respiration.

Many Herbicides Inhibit the Light Reactions of Photosynthesis

•Diuron and atrazine are herbicides that inhibit photosystem II, blocking electron flow. •Paraquat inhibits photosystem I and generates reactive oxygen species. It can then react with many cellular molecules (e.g., damaging membrane lipids).

The Absorption of 8 Photons Yields 1 O2, 2 NADPH, and 3 ATP Molecules

•Eight photons are required to yield two molecules of NADPH and three molecules of ATP. •In cyclic photophosphorylation, two photons yield one molecule of ATP but no NADPH.

Cyclic Electron Flow Reduces the Cytochrome of the Reaction Center

•Electrons from QH2 are returned to cytochrome c2 by the cytochrome bc1 complex. •Cytochrome bc1 complex is a proton pump that generates a proton gradient as it passes electrons to cytochrome c2.

Ferredoxin-NADP+ Reductase Converts NADP+ into NADPH

•Ferredoxin-NADP+ reductase transfers electrons from ferredoxin to form NADPH, biosynthetic reducing power. •The formation of NADPH occurs on the stromal side of the thylakoid membrane, where it is used for carbohydrate synthesis.THis takes place on the stromal side of the memrbane, so it contributes to the proton gradient. •The electron flow from H2O to NADP+ is called the Z scheme of photosynthesis. Structure of the reductase Yellow part is the reductase, binds NADP+ and ferredoxin reduced, cluster located very close to NADP+, electrons can transfer from one cluster to another, becomes NADPH, not used in energy generation instead biosynthesis. Flavin FAD coenzyme Electron jumps on FAD then NADPH

Cyclic Photophosphorylation

•If the NADPH needs are met, cyclic electron flow generates ATP without forming NADPH. •The electrons of photosystem I flow from ferredoxin through cytochrome bf to plastocyanin and then return to P700. •The protons pumped by cytochrome bf are used to synthesize ATP. •This process is called cyclic photophosphorylation. Photosystem II does not participate, and no O2 is generated.

P680+ is a strong oxidant that removes electrons from water

•P680+ is a strong oxidant that removes electrons from water. This reaction, the photolysis of water, occurs at the water-oxidizing complex (also called the manganese center) of photosystem II. •Four photons are required to generate one oxygen molecule. •The four electrons extracted from water are used to reduce two Q to two QH2. •The four protons used to reduce the Q molecules come from the stroma, and the four protons liberated from water are released into the lumen. •The photolysis of water is the source of O2 for all life.

Light Absorption by Chlorophyll Induces Electron Transfer

•Photosynthesis begins with the absorption of light by a photoreceptor molecule, also called a pigment. The principal photoreceptor in the chloroplasts of green plants is chlorophyll a. • •When a photon of the appropriate energy is absorbed by a pigment, an electron in the pigment molecule jumps to a higher energy state. • •The excited electron may fall to its original state, releasing the energy as light or heat. Substituted tetrapyrrole 4 N atoms of the pyrolls are corridnated to Mg unlike heme, it has reduced pyrrole ring and an additional 5-carbon ring fused to one of the pyrrole rings also has phytol, hydrophobic 20 carbon alcohol esterfied to side chain

Two Photosystems Generate a Proton Gradient and NADPH in Oxygenic Photosynthesis (Green Plants)

•Photosynthesis in green plants consists of two photosystems. •Photosystem I generates biosynthetic reducing power in the form of NADPH. •Photosystem II replenishes the electrons of photosystem I while generating a proton gradient that is used to synthesize ATP. •The missing electrons in photosystem II are replaced by the photolysis of water.

Photosynthesis Converts Light Energy into Chemical Energy

•Photosynthesis is the opposite of cellular respiration in terms of chemical reactants and products, although in both cases, the generation of high-energy electrons is an essential feature.

Photosynthesis

•Photosynthesis uses light energy to convert electromagnetic radiation into chemical energy. •High-energy electrons are used to create a proton-motive force that powers the synthesis of ATP. •The high-energy electrons are also used to form NADPH (biosynthetic reducing power). •The reactions that are directly powered by sunlight are called the light reactions. •Photosynthetic organisms are called autotrophs, whereas organisms that obtain energy from chemical fuels only are heterotrophs.

The Ability to Convert Light into Chemical Energy Is Ancient

•Photosystems from different organisms have structural features in common, suggesting a common evolutionary origin. •Geological evidence suggests that oxygenic photosynthesis became important approximately 2 billion years ago. •No archaean photosynthesizers have been discovered, suggesting that photosynthesis evolved in bacteria after archaea and bacteria diverged from a common ancestor. •Some organisms use electron donors other than water.

The ATP Synthase of Chloroplasts Closely Resembles Those of Mitochondria and Prokaryotes

•The ATP synthase of the chloroplast is also called the CF1-CF0 complex, where C stands for chloroplast. •The chloroplast CF1-CF0 complex is very similar to the mitochondrial F1-F0 complex. •Newly synthesized ATP is released into the stroma, where it is used in carbohydrate synthesis. •The membrane orientation of the CF1-CF0 complex is reversed compared to the mitochondrial ATP synthase.

Photosynthesis Takes Place in Chloroplasts

•The chloroplast is a double-membrane organelle. •The inner membrane surrounds a space called the stroma, which is the site of the dark reaction: the synthesis of glucose from CO2 and H2O using ATP and NADPH formed in the light reactions. •In the stroma are membranous sacs called thylakoid membranes. Thylakoid membranes are the location of the light reactions of photosynthesis. outer membrane and inner membrane, with an intervening intermembrane space innner membrane that surrounds this space called the stroma, site of dark rxns Stroma contain membranous structures called thylakoids thylakoids are flattened sacs Granum: stacks of thylakoid sacs Stroma lamellae:How different grana are linked by regious of thylakoid membrane Thylakoid membranes: separate the thylakoid space from the stroma space, so chloroplasts have 3 different membranes (outer, inner, thylakoid membrane) and 3 spaces (intermembrane, stroma, thylakoid space. Thylakoids arise from budding of the inner memrbane, analogous to cristae, site of redox rxns of light reactions that generate proton motive force

A Proton Gradient across the Thylakoid Membrane Drives ATP Synthesis

•The flow of electrons from H2O to NADP+ generates a proton-motive force that is used to power the synthesis of ATP. •Thylakoid membranes exposed to an artificial pH gradient synthesize ATP. •In chloroplasts, most of the energy of the proton-motive force consists of the proton gradient, with the membrane potential contributing little energy. •In chloroplasts, electroneutrality is maintained because Mg2+ moves into the stroma when two H+ are pumped from the stroma into the thylakoid lumen. •In mitochondria, the membrane potential also contributes to the proton-motive force.

A Special Pair of Chlorophylls Initiate Charge Separation (*Bacterial Model)

•The photosynthetic reaction center of Rhodopseudomonas viridis (a bacterium) is well characterized. •Light absorption depends on two photosynthetic pigments—bacteriochlorophyll b (BChl-b) and bacteriopheophytin (BPh). •The reaction center, P960, consists of a pair of BChl-b molecules.

Photosystem I Uses Light Energy to Generate Reduced Ferredoxin, a Powerful Reductant

•The reaction center in photosystem I is called P700. •The electrons from excited P700 (P700*) flow down an electron-transport chain to the iron-sulfur protein ferredoxin. •Plastocyanin (Pc) replaces the electrons of P700. Special pair at the bottom, enxited and then happens again, how do you refill electron hole? Plastocyanin comes in to give electron, 4fe-4s clusters, has ability to carry electrons

Photosystem II Transfers Electrons from Water to Plastoquinone and Generates a Proton Gradient

•The reaction center in photosystem II is called P680. •Excited P680 (P680*) transfers electrons from water to photosystem I in an electron- transport chain that includes pheophytin, a chlorophyll with protons replacing magnesium; plastoquinone (Q), which is similar to ubiquinone; and the cytochrome bf complex. •P680, the reaction center for photosystem II, is bound by the D1 and D2 subunits of the photosystem. •Upon excitation of P680, electrons flow to pheophytin and then to plastoquinone. Finally, they flow to reduce plastoquinone (Q). •P680+ extracts electrons from water bound at the manganese center to maintain redox balance.

The Primary Events of Photosynthesis Take Place in Thylakoid Membranes

•The thylakoid membranes contain the components required for the light reactions of photosynthesis: 1.Light-harvesting proteins 2.Reaction centers 3.Electron-transport chains 4.ATP synthase •Like mitochondria, chloroplasts have a highly impermeable inner membrane and a more permeable outer membrane.

The Components of Photosynthesis Are Highly Organized

•Thylakoid membranes are organized into stacked (appressed) and unstacked (nonappressed) regions. •Photosystem II is located in the stacked regions. •Photosystem I and ATP synthase are in unstacked regions, allowing their products—NADPH and ATP, respectively—ready access to the carbohydrate- synthesizing enzymes of the stroma. Stacking incrreases the amount of thylakoid membrane in a given chloroplast volume. Both regions surround a common internal thylakoid space, but only unstacked regions make direct contact with the chloroplast stroma.

Resonance Energy Transfer Allows Energy to Move From the Site of Initial Absorbance to the Reaction Center

•When a photon of the appropriate energy is absorbed by a pigment, an electron in the pigment molecule jumps to a higher energy state. •When the excited electron falls to its original state, the released energy may be absorbed by a neighboring electron, which jumps to a higher energy state. •Transfer of energy by this means is called resonance energy transfer, and its rate depends strongly on the distance between the energy donor and the energy acceptor molecules. •Electrons are ultimately trapped by the special pair at the reaction center of the light-harvesting complex. Through a biophysical process called resonance energy transfer Each pigment will have absorption spectrum and emission spectrum it can emit at higher wavelength, if emission of first one falls into wrentch of absorption of 2nd one it can transfer and be absorbed again When things become properly aligned spacially, and the absorption and emission range are properly overlapped then this energy transfer happens continuously Because all those accessory pigments not absorbing at reactioncenter they will aborb other ones and give it