Bot 120 Notes Photosynthesis PDF

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Bot 120 Notes Photosynthesis...

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PHOTOSYNTHESIS Reference: Page 111 (Taiz) I. Photosynthesis definition II. Leaf as a photosynthetic system: mesophyll III. Nature of light A. Wave B. Particle (Photon) IV. Phases of Photosynthesis A. Light Reaction (Light Phase) 1. Light Harvesting: Photosystem (PS) a. Reaction Center Complex: Reaction Center, Primary Electron Acceptor b. Light Harvesting Complex (LHC) 2. Photochemical Reaction 3. Electron Transport: Z-scheme of photosynthetic electron transport a. 4 Supramolecular Membrane Complexes i. PS I ii. PS II iii. Cytochrome b/f iv. ATP synthase b. Steps in Photosynthetic Electron Transport *Electron Evolving Complex *Plastoquinone Reaction 4. Photophosphorylation a. Cyclic Electron Flow b. Noncyclic Electron Flow *Energy not absorbed in LR: heat, fluorescence, energy transport, photochemistry B. Carbon Assimilation Phase (Photosynthetic Carbon Reduction Cycle or PCR Cycle) 1. C3 or Calvin Cycle a. Carboxylation b. Reduction c. Regeneration *Photorespiration 2. C4 or Hatch and Slack Pathway a. Steps i. Carboxylation ii. Transfer iii. Decarboxylation iv. Transfer v. Regeneration b. Chemical Variants i. NADP: malic enzyme type ii. NAD: malic enzyme type iii. Phosphoenolpyruvate carboxykinase type NADP: malic enzyme type NAD: malic enzyme type Phosphoenolpyruvate carboxykinase type

Chloroplast of mesophyll, Cytosol of mesophyll, Cytosol of mesophyll, cytosol of bundle sheath mitochondria of bundle chloroplast of bundle sheath cell sheath cell cell C4 acid: malate Aspartate (transamination) Aspartate (transamination) 3. Crassulacean Acid Metabolism (CAM) Pathway a. Dark b. Day PRC Summary Attributes C3 C4 CAM Leaf Anatomy Mesophyll with no Mesophyll with Mesophyll with distinct bundle distinct bundle large vacuole sheath sheath Carboxylating enzyme RUBISCO PEPcase and PEPcase and RUBISCO (spatial RUBSCO (temporal separation) separation) Primary products of CO2 C3 acid: C4 acids: C4 acids: fixation phosphoglycerate oxaloacetate, oxaloacetate, malate, aspartate malate Photorespiration High Very low or absent Very low Leaf chlorophyll a to b ratio 3: 1 4: 1 3: 1 Occurrence Temperate (cool Tropics (hot and Desert (arid and moist) dry) conditions) Energy requirements (CO2: 1:3:2 1:5:2 1:6:5:2 ATP: NADPH) *Regulation of Carbon Assimilation Reactions V. Product Synthesis A. Starch Synthesis B. Sucrose Synthesis 1. Principal pathway provided by enzymes 2. Sucrose synthase (SS Pathway) Starch Synthesis Sucrose Synthesis Polysaccharide of glucose Disaccharide of glucose and fructose Chloroplast stroma Cytosol stored translocatable ATP activated by glucose UTP activated by glucose Primary P. Sucrose Synthase P. Use activated form of glucose and fructose Non-spontaneous (requires energy) Operates in reverse direction to break down sucrose (enzyme as an invertase) Translocated sucrose stored as starch

PHOTOSYNTHESIS - “synthesis using light” o Acquire energy but does not produce food - Use of solar energy to synthesize organic compounds - Cannot be done without the input of energy - Energy stored used later to: 1. Power cellular processes in the plant (only form of energy that can power a cell (ATP)) 2. Serves as the energy source for all forms of life *Energy stored: energy that can be transformed *Type of stored form of energy: organic compounds (after redox reaction where bonds are broken, energy will be released) SUMMARY OF PHOTOSYNTHESIS PROCESS • The most active photosynthetic tissue in higher plants is the mesophyll of leaves o Mesophyll cells have many chloroplasts, which contain the specialized lightabsorbing green pigments, the chlorophylls • Plant uses the solar energy to oxidize water (releasing oxygen), and to reduce carbon dioxide into organic compounds (primary sugars) o everytime you oxidize, there is something reduced: to release energy; electrons with high energy are removed o carbon dioxide is reduced forming an organic compound storing energy Thylakoid Reactions of Photosynthesis - Thylakoid: internal membranes of the chloroplast - End products: ATP and NADPH o High-energy compounds o Used for synthesis of sugars in the carbon fixation reactions (stroma: aqueous region surrounding thylakoid) A. THE LEAF, CHLOROPLAST, AND LIGHT LEAF AS PHOTOSYNTHETIC SYSTEM • Leaves are primary sites of photosynthesis in higher plants • Leaf arrangement (phyllotaxy), morphology (e.g. broad or flat), and anatomy (parts) makes the leaves ideal organs for photosynthesis • Most leaves are efficient interceptor of light because of their flat shape • Chloroplasts are usually near the surface of leaves • Mesophyll o Palisade layer: more compact ▪ Cells near the upper surface of the leaf are numerous and compact to absorb light o Spongy layer

THE NATURE OF LIGHT - Light has the characteristic of both particle and wave •

Wave: Characterized by: o Wavelength (λ): distance between two crests ▪ the longer the wavelength, the slower, the lower the energy o Frequency (v): number of crests that passes through a given distance at the speed of light (c) → 3 x 108 ms−1

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Photon (Particle) o Contains an amount of energy called quantum o Sunlight is a rain of photons of different frequencies o Energy of photon (E) depends on the frequency of light known as Planck’s law: E = hv Where: E = energy in joules (J) h = Planck’s constant (6.625x10−34 𝐽𝑠) v = frequency = c/ λ c = speed of light (3 x 108 ms−1 ) o o o o

Photons of light travels with the same speed but different frequency Energy contents vary along the electromagnetic spectrum of light Blue (450 nm) and Red (660 nm) photon of light has the same effect on photosynthesis although their energy contents are different Energy in a mole (contains 6.02 × 1023 𝑝ℎ𝑜𝑡𝑜𝑛 𝑚𝑜𝑙 −1 = 𝐴𝑣𝑜𝑔𝑎𝑑𝑟𝑜′ 𝑠𝑛𝑢𝑚𝑏𝑒𝑟 [𝑁]) of photons light can be computed using E = hv Computation Example: 450 nm and 660 nm 450 nm 𝐸 = ℎ𝑣 𝑐 𝐸 = ℎλ 𝐸 = 6.625 × 10−34 𝐽𝑠

3×10 𝑚/𝑠 −1

= 4.41 × 10−19 𝐽 *450 × 10−9 because nm was converted m *1000000000 nm = 1 m * 1 nm = 10−9 450 × 10−9𝑚

𝐸𝑛𝑒𝑟𝑔𝑦 𝑖𝑛 𝑚𝑜𝑙𝑒𝑠 = 4.41 × 10−19 𝐽 × 𝐴𝑣𝑜𝑔𝑎𝑑𝑟𝑜′𝑠 𝑁𝑢𝑚𝑏𝑒𝑟 𝐸𝑛𝑒𝑟𝑔𝑦 𝑖𝑛 𝑚𝑜𝑙𝑒𝑠 = 4.41 × 10−19 𝐽 × 6.02 × 1023 𝑝ℎ𝑜𝑡𝑜𝑛 𝑚𝑜𝑙 −1 660 nm 𝐸 = 2.8 × 10−19 𝐽 •

Absorption Spectrum - Displays the amount of light energy taken up or absorbed by a molecule or substance as a function of the wavelength of the light

B. PHASES OF PHOTOSYNTHESIS The light driven synthesis of carbohydrates using carbon dioxide and water from the environment with the subsequent release of oxygen (carbon dioxide is reduced) 1. Light Reaction (Light Phase): when harvesting of light is happening - Involves the capture of light by photosynthetic pigments - Conversion of light energy of ATP and NADPH 2. Carbon Assimilation Phase (Photosynthetic Carbon Reduction Cycle or PCR Cycle) - Entails the use of ATP and NADPH to reduce CO2 - Yields triose phosphate which is eventually converted to glucose and other carbohydrates

LIGHT REACTIONS - photolysis: decomposition or separation of molecules by the action of light o photosynthetic transfer of electron from H2O (ultimate electron donor) to NADP+ (ultimate electron acceptor) - R. Hill's hypothesis 2H2O → O2 + 4H+ + 4e(Hill's Reaction)

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2 photochemical reactions 2 photosystems: demonstrated by the Emerson Enhancement Effect o If light of shorter wavelength is provided at the same time as the longer wavelength, photosynthesis is faster than the sum of the two rates for each wavelength Enhancement effect by Emerson o When red and far-red light were given together, the rate of photosynthesis was greater than the sum of the individual rates o Ex. Blue light in plant A + plant B < red and blue light in plant C

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Led to the discovery that 2 photochemical complex (PSI and PSII) operate in a series to carry out the early energy storage reactions of photosynthesis

Photosystem (PS) - Functional units that convert light energy to chemical energy o Absorbed light energy is used to power the transfer of electrons through a series of compounds that act as electron donors and electron acceptors - light-harvesting unit in the thylakoid membrane - harvest light and independently evolve O2 (PS II) and reduced NADP+ (PS I) o Majority of the electrons reduce NADP+ to NADPH and oxidize H2O to O2 o Light energy generates proton motive force across the thylakoid membrane to synthesize ATP - PS I and PS II PSI o Preferentially absorbs far-red light of wavelengths greater than 680 nm o Stroma lamella and edges of the grana lamellae PSII o

Preferentially absorbs red light of 680 nm and is driven very poorly by far-red light o Produce reductant that re-reduces the oxidant produced by photosystem I o Grana lamellae PSI and PSII are spatially separated in the thylakoid membrane

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consist of the following: a. Light Harvesting Complex (LHC) / Antenna Complex o receives and gives off light energy o majority of pigments serve as antenna complex o collect light and transfer the energy to the reaction center complex b. Reaction center (RC) o minimum unit where photochemical reaction takes place o where chemical oxidation and reduction reactions leading to long-term energy storage takes place o chl of the 2 photosystems absorb at different wavelengths c. Primary Electron Acceptor

1. Reaction Center Complex Reaction center Primary electron acceptor 2. LHC/Antenna Complex

Photosystem I

Photosystem II

P700 A0 100 chl a+b (4:1)

P680 Pheophytin 250 chl a+b (1:1) xanthophylls

Benefit from division of labor between antenna and reaction center pigments - If many pigments can send energy into a common reaction center, the system is kept active a large fraction of the time. Z-scheme of photosynthetic electron transport - Basis for understanding O2-evoving (oxygenic) photosynthetic organisms - Accounts for the operation of 2 physically and chemically distinct photosystems (I and II), each with its own antenna pigments and photochemical reaction center - light energy cannot be destroyed so it is transformed into another form - light energy is all about the transfer of electrons and conversion to ATP - electrons ejected from chlorophyll travel through a series of electron carriers organized in the “Z Scheme” - Photochemical reaction: triggered by light followed by reduction-oxidation reaction o PS I and PS II linked by an electron transport chain o PS I and PS II cooperate (coupled reactions) o Photosynthetic electron transport occurs in the thylakoid membrane (stroma) o there are 60 polypeptide species arranged in Four Supramolecular Membrane Complexes: 1. PS II o Oxidation of 2 water molecules produces 4 electrons, 4 protons, and a single oxygen ▪ Protons produced by the oxidation of water diffuse to the stroma region where ATP is synthesized (lumen) 2. Cytochrome b/f o Connects the photosystems o Evenly distributed between stroma and grana o Receives electrons from PSII and delivers them to PSI o Transports additional protons into the lumen from the stroma 3. PS I

Reduces NADP+ to NADPH in the stroma by the action of ferredoxin (Fd) and the flavoprotein ferredoxin-NADP reductase (FNR) 4. ATP synthase o Produces ATP as protons diffuse back through it from the lumen into the stroma o

*Cytochrome b/f, plastoquinone, and NADPH is in the ETC PHOTOSYNTHETIC ELECTRON TRANSPORT - first stage of photosynthesis that produces chemically stored energy and uses solar photons to drive electron transport against a thermodynamic gradient. Steps Involved in Photosynthetic Electron Transport (A series of oxidation-reduction reactions) 1. Light absorption by LHC II excites P680 • Light Harvesting Complex (LHC): antenna complexes (e.g. carotenoids) • Reaction center: always contains chlorophyll a and contains smallest maximum energy LHC energy < Reaction Center • Resonance transfer o Physical mechanism by which excitation energy is conveyed from the chlorophyll that absorbs the light to the reaction center o Energy from LHC to another LHC to the Reaction Center o Excitation energy is transferred from one molecule to another by a nonradiative process o Efficient because 95-99% photons absorbed by antenna pigments have their energy transferred to the reaction center • Reaction Center can only absorb 700 nm in PSI and 680 nm in PSII 2. P680 = strongly oxidized → drives photolysis (because replacement electron is needed; oxygen revolving complex) 3. e- from P680 transferred to Pheophytin to PQ (plastoquinone) → PQ becomes PQH2 (Plastohydroquinone) 4. e- from PQH2 is donated to cyt b/f complex (reduction) 5. e- from cyt b/f are transferred to Plastocyanin (PC) 6. Once P700 has been energized by light e- from P700, it will be transferred to Ferrodoxin (Fd) 7. Fd reduces NADP+ via NADP- Reductase 8. P700 will accept the e- from PC •

ATPase (ATP synthase): protein to produce ATP; works through chemiosmotic pressure by the difference in H+ gradients

Transfer of electrons • Light energy bounces from one antenna complex protein to another until it reaches P680 • Light harvesting complex can absorb blue to yellow light • Resonance transfer: transfer from one harvesting complex to another; similar to transfer of sound (vibrate when receive light energy) • Photolysis: guided by the oxygen evolving complex • Electron reaches high excitation state at P680 o Status: Unstable ▪ There can be sudden release of energy so there must be a step by step transfer of energy so that energy will go back to ground state ▪ Spontaneous combustion: when all energy is released suddenly (causes an anomaly) • Pheophytin: suddenly reduced • ETC • Cytochrome b/f complex • Electron reduces highly oxidized P700 (highly oxidized like in the P680 is: resonance transfer) • THE WHOLE PROCESS IS A TRANSFER OF ELECTRONS

Electron Evolving Complex • Oxygen-evolving complex is a protein complex in the periphery of P680 • Oxygen-evolving complex: helps in splitting of water into H, O, and e• Pheophytin is reduced by excited e- from the P680 • Platoquinone (Q) is responsible for reducing electrons in cytochrome b/f complex • Plastocyanin reduced • Plastocyanin reduces P700

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These are protein complexes, a pool of proteins, functioning as one body. Pheophytin will be oxidized by Plastoquinone o Pheophytin acts as an early acceptor in PSII followed by a complex of 2 plastoquinones in close proximity to an iron atom

Pheophytin is a chlorophyll in which the central magnesium atom has been replaced by 2 hydrogen atoms Reduce cytochrome b/f complex (cytochrome c made of cytochrome b, f, superprotein) in lumen area where proton will be deposited Proton transfer is facilitated by the oxidation-reduction in the Plastoquinone o

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Plastoquinone Reaction

*Diagram: harvesting, transfer of electrons • PQ → PQH2: 2 electrons are required for PQ to be reduced • First PQH2 reduced, Second PQH2 reduced • Proteins in Cytochrome b/f complex: have 5 sites o Cytochrome f: Near plastocyanin because is oxidized by PC o Cytochrome b: 2 cytochrome b o Rieske Fe-S (FeSR): First to be reduced by plastoquinone ▪ 2 iron atoms are bridged by 2 sulfur atoms o Quinone: 2 areas in cyt b/f complex because reduction of quinone will happen twice

* PQ is reduced to PQH2 How did it become H2 when only 1 electron is received? • Q and PQ is the same but o Q: ready to accept electron; ground state o PQ: once receives electron, becomes ion; plastosemiquinone is produced o Q that is ready to accept e becomes semiquinone • Steps o Q ▪ Electron binds to double bond in Plastosemiquinone (O-) making it an anion ▪ Oxygen on top does not have a charge (not normal) o 2 H+ is required o Plastohydroquinone produced ▪ Hydroquinone: need one electron to bind to uncharged particle • First QH2 oxidation: linear process o PSII → QH2 • Electron is transferred to cytochrome b and FeSR • FeSR reduces Cytochrome f • Cyt f reduce plastocyanin • Plastocyanin reduces P700 • Electron is Cyt b goes to Q producing Q- (semiquinone) • Q outside the cyt b/f complex can go inside • Second QH2 is oxidized: same process • 2 e- generates movement of 4 H+ towards the lumen Plastoquinone and Plastocyanin Carry Electrons between PSII and PSI

Photophosphorylation - process of utilizing light energy from photosynthesis to convert ADP to ATP. - process of synthesizing energy-rich ATP molecules by transferring the phosphate group into ADP molecule in the presence of light. • •

Light absorbed by complex PSII Light Reaction must produce ATP and NADPH

Types of Photophosphorylation 1. CYCLIC e- FLOW: Recycling of e- from Fd to P700 via PQ and cyt b/f • No PSII • No plastoquinone • Only in lower vascular plants 2. NONCYCLIC e- FLOW: e- transport from P680 to NADPH (shown in diagram above) • For higher plants Summary of Light Reaction: 1. Light Harvesting 2. Photochemical Reaction 3. Electron transport 4. Photophosphorylation NOTE: Some of the energy absorbed is not used in light reactions and given to the surroundings as: 1. HEAT 2. FLUORESCENCE: emission of a photon of light of longer wavelength by an excited pigment molecule as it returns to ground state What happens when a molecule absorbs light? • Chlorophyll mainly absorbs red and blue light • Absorption of light is represented by the equation:

Chl + hv = Chl* (Excited chlorophyll) Chl in its lowest-energy (ground state) absorbs a photon (hv) and makes a transition to a higher-energy (excited state) in higher excited state, chlorophyll is extremely unstable - very rapidly gives up some of its energy to the surroundings as heat - enters the lowest excited state o

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blue light has higher excitation state than red light because it has more energy (shorter wavelength)

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In the lowest excited state, the excited chlorophyll has four alternative pathways for disposing of its available energy: 1. Fluorescence o Excited chl can re-emit a photon and return to its ground state o Wavelength of fluorescence is slightly longer and of lower energy than the wavelength of absorption because a portion of the excitation energy is converted into heat before the fluorescent photon is emitted o Chl fluoresce in red region 2. Heat (emit excess) o No photon released o When light was absorbed, heat is released, electron goes to lower state, oxidation-reaction occurs 3. Energy transfer (going to a nearby protein) 4. Photochemistry (oxidation-reduction) o Excited state causes chemical reactions to occur These 4 can occur but it is most probable that photochemistry reaction will take place because it is among the fastest known chemical reactions

Photosynthetic Pigments • All pigments active in photosynthesis are found in the chloroplast • Carotenoids o characteristic orange color is given by the absorption bands in the 400-500 nm region o found in all p/s organisms o integral constituents of thylakoid membrane o associated with antenna and reaction center pigment proteins o light absorbed transferred to chl for p/s: function as accessory pigments • light excite specialized chlorophyll in the reaction center, either by direct absorption or energy transfer from an antenna pigment

Diagram: Funneling effect of the light harvesting complex - going from higher excited state to lower excited state • • • • • • • •

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P680: can absorb or contain maximum of 680 nm P700: can absorb or contain maximum of 700 nm P680 and P700 cannot contain blue or yellow light (red light only) P680 and P700 have chlorophyll a If 350 nm, only carotenoid can absorb it but it can still reach the reaction center through resonance transfer. The molecule nearest the reaction center can absorb lower light (red light) Not all energy is transferred. Others are emitted as heat How to ensure that it will go to P680? o 350 is the minimum and it cannot return to carotenoid because it is not enough (backflow does not happen) Goes to chlorophyll a that has lesser absorption capacity than chlorophyll b Action Spectra - Relate light absorption to photosynt...