Photosynthesis 2 PDF

Photosynthesis 2 - Pigments Photons are Absorbed by Photosynthetic Pigments = The First Step in Photosynthesis • The photosynthetic pigments absorb the energy of the photons. excited state Chl ground state ↑ Energy  • Photon absorption process: Chl* excited state This is the energy that power...

Photosynthesis 2 - Pigments Photons are Absorbed by Photosynthetic Pigments = The First Step in Photosynthesis • The photosynthetic pigments absorb the energy of the photons. excited state Chl ground state ↑ Energy  • Photon absorption process: Chl* excited state This is the energy that powers photosynthesis. Chl* I goes Chl grounda Chl Time  back to down ground ChI • In Chl* an electron has received the energy of a photon, and transiently moves to a higher energy electron orbital (Chl* has a very short lifetime). Absorption of a photon leads to the movement of an electron ( ) to a higher energy orbital (further away from the nucleus). Nucleus Higher energy orbital Lower energy orbital State Photosynthesis 2 - Pigments • The lifetime of the excited state is very brief. • The electron quickly drops back to the lower energy orbital (= the ground state). • In photosynthesis, the energy of the excited state is passed in to an adjacent Chl (more about that very soon). Higher energy orbital Nucleus Lower energy orbital • In Chl* an electron has received the energy of a photon, and transiently moves to a higher energy electron orbital (Chl* has a very short lifetime). Car ground state Car* excited state Energy  • Same deal with carotenoids: Car* Car Time  • All of this is happening in the thylakoids. • All of the photosynthetic pigments (Chl & Car) are in the thylakoids. Car Photosynthesis 2 - Pigments • Photosynthesis uses the energy of Chl* and Car* to perform work. Energy  Photons are Absorbed by Photosynthetic Pigments = The First Step in Photosynthesis • Chl* and Car* represent potential energy; excited state -Chl* and Car* represent the energy of a photon • Chl and Car do not represent potential energy. Chl* Chl Time  • The ultimate goal of photosynthesis is to produce carbohydrates (sucrose, starch); -many sub-processes ground state Energy  • The energy represented by carbohydrates comes from photons. • (The electrons in carbohydrates come from water; the “water-splitting reaction”) Chl Car* Car Car Time  Photosynthesis 2 - Pigments • Photosynthetic pigments are in the thylakoid membranes; nowhere else. • Thylakoid membranes are green; nothing else in a leaf is green. • The thylakoid lumen does not have pigments. Modified from Chow (2003) Journal of Biological Physics 29: 447-459 Photosynthesis 2 - Pigments Energy is Transferred Between Pigments (in the thylakoids) • This is not electron transfer (that’s later). • Energy transfer: ChlChlChl CarChl Chl*ChlChl Car*Chl ChlChl*Chl ChlChlChl* CarChl* • There are a LOT of pigments, packed tightly together in the thylakoids. • Energy always ends up at Chl (Chla to be precise); -if Chlb absorbs photon  energy transferred to Chla • The energy being transferred originally came from a photon. • The energy transfer is the transfer of the excited state from one pigment to an adjacent pigment. Photosynthesis 2 – Light Reactions Concept of a Photosystem • The photosynthetic pigments are organized into photosystems. • Lots of chlorophylls and also some carotenoids. • One “job” of a photosystem is “light harvesting” (= photon absorption). • The absorbed photons are funnelled to the reaction centre chlorophyll. energy transfer e- transfer PS ETC Regular Chlorophyll Molecules Reaction Centre Chlorophyll PS ETC = photosynthetic electron transport chain Photosynthesis 2 – Light Reactions - Photosystems • In a photosystem, a lot of regular Chl molecules funnel energy to the reaction centre Chl. • At the reaction centre Chl, the energy is used to convert a low energy electron to a high energy electron. • The high energy electron enters the PS ETC. e- transfer Reaction Centre Chlorophyll • There are two types of photosystems (PSII and PSI). • The general outline of how they work is the same, but the details are different. Photosynthesis 2 – Concept of a Photosystem = energy of a photon moving from Chl to Chl to Chl until the energy reaches the RChl A, B, C = electron carriers (for high energy e-s); RChl = reaction centre chlorophyll stroma – site of the Calvin cycle photon (particle of light) B A transfer of high energy electrons C thylakoid RChl light-harvesting complex reaction centre complex lumen of thylakoids There are two sub-types of photosystems: PSI and PSII, which differ in their proteins and electron carriers. Photosystems are in the thylakoid membrane. Photosynthesis 2 – Concept of a Photosystem = energy of a photon moving from Chl to Chl to Chl until the energy reaches the RChl A, B, C = electron carriers (for high energy e-s); RChl = reaction centre chlorophyll stroma – site of the Calvin cycle photon (particle of light) B A transfer of high energy electrons C thylakoid RChl light-harvesting reaction centre complex complex • Lots of Chl + less Car. • Pigments tightly packed in, side by side. • Pigment-binding proteins. • Energy transferred from pigment to pigment. lumen of thylakoids Photosynthesis – Concept of a Photosystem = energy of a photon moving from Chl to Chl to Chl until the energy reaches the RChl A, B, C = electron carriers (for high energy e-s); RChl = reaction centre chlorophyll stroma – site of the Calvin cycle photon (particle of light) B A transfer of high energy electrons C thylakoid RChl light-harvesting complex lumen of thylakoids reaction centre complex • Some Chl + Car (less than LHC). • Reaction centre Chl (RChl). • Electron carriers. • Many different proteins. Photosynthesis 2 – Reaction Centre Chlorophylls • Most Chl molecules have two possible states: 1) ground state 2) excited state Energy  excited state Chl* Chl Chl Time  ground state • Reaction centre chlorophylls (RChl) have three possible states: RChl RChl* RChl+ ground state  excited state  oxidized state (= loss of an e-) Photosynthesis 2 – Reaction Centre Chlorophylls • RChl has three possible states: RChl RChl* RChl+ • For RChl+, the e- that was lost was the high energy e- (HEE) that was in the higher energy orbital. • That HEE is transferred through various e- carriers in the photosystem and in the rest of the photosynthetic electron transport chain; it is replaced by a low energy e- (LEE). Chl* Chl RChl RChl* A reaction centre Chl (RChl) cycles through three states. A e(low E) A = e- acceptor A- = acceptor with a high energy eRChl+ A- Photosynthesis 2– Photosynthetic Electron Transport Chain & ATP Synthase • There are two types of photosystems: PSII and PSI. • The photosystems are embedded in the thylakoid. • The photosystems contain e- carriers, and are also linked by additional e- carriers. ATP synthase stroma – site of the Calvin cycle low [H+] light PSII e- nH+ ATP NADPH nH+ light high E e-s e- ADP + Pi high E e-s e- thylakoid e- 4elow E e-s 2H2O OEC 4H+ + O2 water-splitting reaction low E e-s nH+ ePSI nH+ thylakoid lumen high [H+] OEC = oxygen-evolving complex. Both PSII and PSI are composed of a reaction centre complex and light-harvesting complexes. Photosynthesis 2 – Photosystems Photosystem II (PSII) • HEE = high energy e-s • LEE = low energy e-s • P680 is the RChl for PSII stroma – site of the Calvin cycle photon (particle of light) PSII reaction centre complex C A transfer of high energy electrons B HEE thylakoid P680 LHCII LEE 2H2O OEC 4H+ lumen of thylakoids + O2 water-splitting reaction • OEC = oxygen-evolving complex • OEC is a multi-subunit protein attached to the PS reaction centre complex. • OEC catalyzes the water-splitting reaction (2H2O  4e- + 4H+ + O2), and supplies low energy e-s to P680. Photosynthesis 2 – Photosystems Photosystem II (PSII) has the Oxygen-Evolving Complex (OEC) • Multi-protein complex that also contains metal ions. • Attached to PSII, but protrudes into the thylakoid lumen. • Catalyzes the water-splitting reaction. • The water-splitting reaction produces: 1) LEEs – which are supplied to P680 and converted into HEEs and enter the PS ETC 2) H+s – released into the thylakoid lumen, contributing to the Δ[H+] 3) O2 – by-product (plant doesn’t care about this) Water-splitting reaction: 2H2O  4e- + 4H+ + O2 Photosynthesis 2 – Photosystem II (PSII) • The RChl in PSII is P680. • Cycling of P680 through three states (ground state, excited state, oxidized state): Chl* Chl P680 P680* A water-splitting reaction e(low E) OEC water A = e- acceptor A- = acceptor with a high energy eP680+ A- Water-splitting reaction: 2H2O  4e- + 4H+ + O2 important not important/ a by-product Photosynthesis 2 – Electron Travel Between Photosystems • The HEEs leave PSII and enter PSI as LEEs. • The energy was used to pump H+s from the stroma to the thylakoid lumen;  contributes to the Δ[H+] Photosynthesis 2 – Photosystem I (PSI) Photosystem I (PSI) • LEE = low energy e-s • HEE = high energy e-s • P700 is the RChl for PSI photon (particle of light) stroma – site of the Calvin cycle HEE B A LEE transfer of high energy electrons C thylakoid P700 lumen of thylakoids PSI reaction centre complex LHCI Photosystem I (PSI) RChl = P700 • The RChl in PSI is P700. • PSI does not have an OEC; low energy electrons are from the photosynthetic electron transport chain (ultimately from PSII, via the water-splitting reaction). • Cycling of P700 through three states (ground state, excited state, oxidized state): Chl* Chl P700 P700* A e(low E) PSII PS ETC P700+ A- A = e- acceptor A- = acceptor with a high energy e- Photosynthesis 2 – Photosynthetic Electron Transport Chain & ATP Synthase • HEEs leave PSI. • Stroma also has a pool of NADP, a two e- carrier similar to NAD. • NADP+ + 2HEE (from PSI) + H+ NADPH (= e- source for the Calvin cycle) ATP synthase stroma – site of the Calvin cycle low [H+] light PSII e- nH+ ATP NADPH nH+ light high E e-s e- ADP + Pi high E e-s e- thylakoid e- 4elow E e-s 2H2O OEC 4H+ + O2 low E e-s nH+ ePSI nH+ thylakoid lumen high [H+] OEC = oxygen-evolving complex. Both PSII and PSI are composed of a reaction centre complex and light-harvesting complexes. Photosynthesis 2 Water-Soluble Electron Carriers in the Stroma • NADP is a two e- carrier, structurally very similar to NAD (which we saw in ACR). • NADP+ + 2e- + H+ NADPH • NADPH = a pair of high energy electrons = chemical potential energy. • NADPH is the molecule used to reduce C in photosynthesis. Photosynthesis 2 – Photosynthetic Electron Transport Chain & ATP Synthase • • • • ATP synthase regenerates ATP: ADP + Pi ATP ΔG = +7.3 kcal/mole Need a coupling reaction  H+s moving down ΔC (Δ[H+]) Δ[H+] = CPE. Two mechanisms that lead to the Δ[H+]. mechanism #1 for the Δ[H+] mechanism #2 for the Δ[H+] OEC = oxygen-evolving complex. Both PSII and PSI are composed of a reaction centre complex and light-harvesting complexes. Photosynthesis 2 – Photosynthetic Electron Transport Chain & ATP Synthase • ATP synthase is located in the thylakoids. • Same location as the PS ETC, but not part pf the PS ETC. • Very similar in structure and function to the mitochondrial ATP synthase that is part of oxidative phosphorylation. • ATP synthase regenerates ATP for use in the Calvin cycle. Chloroplast ATP Synthase Reaction: ADP + Pi + nH+lumen ATP + nH+stroma (+ H2O) There are two mechanisms that lead to the Δ[H+]. Relative Electron Energy Levels in the Photosynthetic Electron Transport Chain (PS ETC) – The “Z Scheme” P700* Relative Energy of Electrons  P680* high energy electrons NADPH photon photon H2O P680 P700 PSI PSII Path of Electrons Through the PS ETC  low energy electrons Photosynthesis 2 – Photosynthetic Electron Transport - HEEs Photosynthesis 2 – Photosynthetic Electron Transport & ATP Synthase • There are a lot of electron carriers in the photosynthetic electron transport chain. • ATP synthase is a very complex multi-protein enzyme. ATP synthase. By Somepics - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=38088695 These diagrams are far more detailed than you need to know. By The original uploader was Asw-hamburg at German Wikipedia. Transferred from de.wikipedia to Commons by Leyo using CommonsHelper., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8993938 Photosynthesis 2 Chloroplast ATP Synthase Reaction (Complete Reaction, and Energy Transformations) ADP + Pi nH+lumen ADP + Pi + nH+lumen ATP nH+stroma ΔG > 0 ΔG < 0 ATP + nH+stroma ΔG < 0 The ATP synthase reaction. • The chloroplast and mitochondrial ATP synthases are very similar. • Both are located in a membrane. • Both the chloroplast and mitochondrial ATP synthase convert the CPE of a Δ[H+] into the CPE of ATP. (coupling reaction) n ≈ 4 H+/ATP Photosynthesis 2 – Photosynthetic Electron Transport - HEEs Quick Review – Why/How is There a Δ[H+]? • Two mechanisms for generating the Δ[H+]: 1) water-splitting reaction by the OEC at PSII 2) H+ pumping by the PS ETC • Reason for producing a Δ[H+]: -ATP synthase needs a Δ[H+] in order to regenerate ATP Mechanism #1 Mechanism #2 Photosynthesis 2 Energy Transformations So Far photons @ PSII water-splitting H+ pumping photons @ PSI NADPH Δ[H+] ATP The Light Reactions Convert the Energy of Photons into the Chemical Potential Energy of ATP and NADPH • The light reactions are all of the reactions/processes that are directly light-dependent: -photon absorption by pigments, and transfer of energy to the RChls -photosynthetic electron transport chain -water-splitting reaction -H+ pumping -NADPH production -ATP synthase activity • But we have not fixed/reduced any inorganic carbon (CO2) yet. Photosynthesis 2 – Enzymatic Reactions • Photosynthesis takes place in two stages: light-dependent reactions (= the light reactions) and the Calvin cycle (= the photosynthetic carbon reduction cycle = several other names as well) and associated reactions. • Both stages take place in chloroplasts, although in different parts of that organelle. • The light reactions, which take place in the thylakoid membrane, use light energy to regenerate ATP and NADPH. The Calvin cycle, which takes place in the stroma, uses energy derived from these compounds to make 3C sugars from CO2. + other enzymatic reactions 3C sugars Starch Modified from OpenStax Biology 2e Sucrose Photosynthesis 2 – Enzymatic Reactions • The light reactions produce ATP and NADPH for the enzymatic reactions, and it’s the enzymatic reactions (we will focus on the Calvin cycle) that fix CO2 and produce carbohydrate. • The enzymatic reactions occur in the stroma. • The enzyme Rubisco catalyzes the first reaction of the Calvin cycle: RuBP + CO2 5C sugar 2 3-PGA 3C molecule • Rubisco is considered to be the most abundant protein on earth by mass. • Rubisco is a large enzyme, with 16 subunits of two types (has 4° structure). • Rubisco does not actually use either of the products of the photosynthetic light reactions (NADPH, ATP), but subsequent steps in the Calvin cycle use NADPH and ATP to produce a 3C sugar. • Calvin cycle only operates in the light because it requires the products of the light reactions. Photosynthesis 2 – Enzymatic Reactions CO2 RuBP Rubisco (high energy electrons) nNADPH 2 3-PGA nATP nADP + nPi nNADP+ Calvin cycle 3-Carbon Sugars Not part of the Calvin cycle (but still very important) Starch (polymer of glucose) Sucrose (12C Sugar) Export to rest of plant Photosynthesis 2 – Enzymatic Reactions • Starch is insoluble, and serves as a temporary energy storage molecule. Modified from Zhou et al. (2019) PeerJ 7(1):e7938 • The stroma contains starch grains. Electron micrograph of a chloroplast with a starch grain. • Starch is also found in the roots and in underground storage organs (e.g. potato tubers). • Sucrose is water-soluble, and is used by plants as an energy transport molecule;  can be moved from leaves to other parts of the plant  sucrose is continually exported from leaves in the daytime • Chloroplast starch is converted to sucrose at night;  exported from the leaves starch grain (glucosen) Sucrose By NEUROtiker - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2951918 Photosynthesis 2 – Enzymatic Reactions What does the Calvin cycle actually do? Why is it important? • The Calvin cycle converts CO2 (inorganic C) into carbohydrate (organic C). • To do this, it uses the products of the light reactions of photosynthesis (NADPH, ATP). • The operation of the Calvin cycle depends on the operation of the light reactions. • The light reactions capture the energy of light (photons) as the energy of NADPH and ATP. • The Calvin cycle converts the energy of NADPH and ATP into the form of carbohydrates. Photosynthesis 2 • Overall: photosynthesis converts energy of photons into chemical potential energy (C-C and C-H bonds of carbohydrate) -takes 2 NADPH to reduce one CO2 (i.e. 4 e-s) -takes 12 NADPH to produce the equivalent of one glucose  split 12 water molecules Energy Transformations During Photosynthesis 1) Energy of photons CPE of the Δ[H+] across the thylakoids CPE of NADPH + Heat ` 2) Chem pot E of the Δ[H+] 3) Chem pot E of NADPH Chem pot E of ATP Overall: Energy of photons ` CPE of ATP + Heat CPE of carbohydrate + Heat ` light reactions CPE of carbohydrate + Heat Calvin cycle & associated reactions Summary of Light Reactions and Enzymatic Reactions Modified from OpenStax Biology 2e. Chloroplast envelope 3C Sugars Starch A summary of photosynthesis, as it takes place in the chloroplast. Photosynthesis may be conceptually divided into the light reactions and the enzymatic reactions (carbon fixation via the Calvin cycle & associated reactions). Light reactions harness energy from the sun to produce ATP and NADPH. These energy-carrying molecules are made in the stroma where carbon fixation (Calvin cycle) takes place. Sucrose A Comparison of Electron Transport Chains • The photosynthetic and respiratory electron transport chains (PS ETC and RS ETC respectively) have a lot in common. • Both create a Δ[H+] across a membrane, and both pump H+s against a Δ[H+], which requires energy. • In both cases, the Δ[H+] is used to power ATP synthase (catalyzes ATP regeneration from ADP and Pi). • In both cases, ATP synthase is in the same membrane as the ETC (it must be, in order to use the Δ[H+]. • Differences between the PS ETC and the RS ETC: 1) Source of energy is photons for the PS ETC, and NADH/FADH2 for the RS ETC. 2) The Δ[H+] is really the only “product” of the RS ETC, while the PS ETC produces both a Δ[H+] and NADPH. 3) High energy electrons (NADH/FADH2) enter the RS ETC, and leave as low energy electrons (O2 as the garbage can). Low energy electrons (from H2O) enter the PS ETC, and leave as high energy electrons (NADPH). 4) The ETC and the Δ[H+] is thylakoid-based in the PS ETC, and based on the mitochondrial inner membrane for the RS ETC. Chloroplast Trivia: Kleptoplasty (= theft of plastids) • There are several groups of animals that feed on marine algae and retain the algal chloroplasts for a period of time; these chloroplasts maintain their capacity for photosynthesis and provide reduced carbon to the organism (i.e. photosynthetic animals). • The sea slug Elysia chlorotica, which acquires chloroplasts by eating the marine filamentous alga Vaucheria litorea, can live 10-12 months without ingesting food (in the presence of light). This will not be on a test. https://www.nature.com/articles/sdata201922 https://alchetron.com/Vaucheria-litorea Atmospheric Carbon Dioxide – Photosynthesis and Fossil Fuels southern summer northern summer https://gml.noaa.gov/ccgg/trends/ • There is more vegetation north of the equator. The info on this slide will not be on a test. Modified from https://earthobservatory.nasa. gov/images/1863/globalenhanced-vegetation-index