Photosystems Photosystem (fig 10.12) = rxn center surrounded by several light-harvesting complexes Light-harvesting complex = pigment molecules bound to proteins (act as antenna for rxn center) Rxn center = protein complex that includes 2 special chlorophyll a molecules + primary e- acceptor molecule First step of light rxns: special chlorophyll a molecule transfers its excited e- to the primary e- acceptor

(INTERIOR OF THYLAKOID) A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes A photosystem Is composed of a reaction center surrounded by a number of light-harvesting complexes Thylakoid Photon Photosystem STROMA Light-harvesting complexes Reaction center Primary election acceptor Thylakoid membrane e– Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) Figure 10.12

Light-harvesting Complexes and Reaction Centers The light-harvesting complexes consist of pigment molecules bound to particular protein They funnel the energy from photons of light to the reaction center When a reaction-center chlorophyll a molecule absorbs energy, one of its electrons gets bumped up to a primary electron acceptor

Photosystems Two types of photosystems embedded in the thylakoid membranes of land plants (fig 10.13) 1. Photosystem I (PS I) Rxn center chlorophyll a = P700 Cyclic and noncyclic e- flow 2. Photosystem II (PS II) Rxn center chlorophyll a = P680 Noncyclic e- flow Noncyclic e- flow (fig 10.13) Uses PS II & PS I Excited e- from PS II  goes through ETC  produces ATP Excited e- from PS I  ETC  used to reduce NADP+ Electrons ultimately supplied from splitting water  releases O2 and H+ Cyclic e- flow (fig 10.15) Uses only PS I Only generates ATP Excited e- from PS I cycle back from 1st ETC No O2 release & no NADPH made

Two Photosystems The thylakoid membrane Is populated by two types of photosystems, I and II

Noncyclic Electron Flow – Involves both Photosystems Produces NADPH, ATP, and oxygen, and is the primary pathway of energy transformation in the light rxns. Figure 10.13 Photosystem II (PS II) Photosystem-I (PS I) ATP NADPH NADP+ ADP CALVIN CYCLE CO2 H2O O2 [CH2O] (sugar) LIGHT REACTIONS Light Primary acceptor Pq Cytochrome complex PC e P680 e– + 2 H+ Fd reductase Electron Transport chain Electron transport chain P700 + 2 H+ + H+ 7 4 2 8 3 5 1 6

A mechanical analogy for the light reactions Mill makes ATP e– Photon Photosystem II Photosystem I NADPH Figure 10.14 

Cyclic Electron Flow Under certain conditions Photoexcited electrons take an alternative path Uses Photosystem I only

In cyclic electron flow Electrons cycle back to the first ETC Only ATP is produced Primary acceptor Pq Fd Cytochrome complex Pc NADP+ reductase NADPH ATP Figure 10.15 Photosystem II Photosystem I

Light Reactions and Chemiosmosis The light reactions and chemiosmosis: (fig 10.17) As e- are brought down their E gradient through the ETCs embedded in the thylakoid membranes  that E is being used to pump H+ into the thylakoid space ATP synthase is the only place H+ can flow along its concentration gradient  E from this flow is used to join ADP + Pi  ATP NADPH is made by NADP+ reductase ATP and NADPH  used to power Calvin cycle

Light reactions and chemiosmosis: organization of the thylakoid membrane As e- are brought down their E gradient through the ETCs embedded in the thylakoid membranes  that E is being used to pump H+ into the thylakoid space ATP synthase is the only place H+ can flow along its concentration gradient  E from this flow is used to join ADP + Pi  ATP LIGHT REACTOR NADP+ ADP ATP NADPH CALVIN CYCLE [CH2O] (sugar) STROMA (Low H+ concentration) Photosystem II H2O CO2 Cytochrome complex O2 1 1⁄2 2 Photosystem I Light THYLAKOID SPACE (High H+ concentration) Thylakoid membrane synthase Pq Pc Fd reductase + H+ NADP+ + 2H+ To Calvin cycle P 3 H+ 2 H+ +2 H+ Figure 10.17

Light Reactions and Chemiosmosis Comparison of chemiosmosis in mitochondria & chloroplasts (fig 10.16): ATP generation basically the same (ETC + ATP synthase) Mitochondria use high-E e- extracted from organic molecules (glucose) Chloroplasts use light E to drive e- to top of the ETC

Comparison of Chemiosmosis in Chloroplasts and Mitochondria Generate ATP by the same basic mechanism: chemiosmosis But use different sources of energy to accomplish this. Chloroplasts use light energy and mitochondria use the chemical energy in organic molecules.

The spatial organization of chemiosmosis Differs in chloroplasts and mitochondria Key Higher [H+] Lower [H+] Mitochondrion Chloroplast MITOCHONDRION STRUCTURE Intermembrance space Membrance Matrix Electron transport chain H+ Diffusion Thylakoid Stroma ATP P ADP+ Synthase CHLOROPLAST Figure 10.16

Chemiosmosis: Chloroplasts vs. Mitochondria In both organelles Redox reactions of electron transport chains generate a H+ gradient across a membrane ATP synthase Uses this proton-motive force to make ATP

The Calvin Cycle The Calvin cycle uses ATP + NADPH to convert CO2  sugar Calvin cycle (fig 10.18) = anabolic process that builds carbohydrates from smaller molecules Consumes E (ATP) Uses NADPH as reducing agent for adding high E e- to make sugar Phase 1: Carbon fixation CO2 incorporated by attaching to 5-C sugar (ribulose bisphosphate = RuBP) Catalyzed by Rubisco (ribulose bisphosphate carboxylase/oxygenase) = most abundant protein on Earth! Unstable 6-C intermediate  2 molecules of 3-phosphoglycerate (3-C sugar) Phase 2: Reduction 3-phosphoglycerate has phosphate added  NADPH reduces intermediate  G3P (glyceraldehyde-3-phosphate) One G3P exits the cycle to be used by the plant cell Other 5 recycled to regenerate RuBP Phase 3: Regeneration of CO2 acceptor (RuBP) Requires ATP Five G3P (3-C)  Three 5-C molecules of RuBP

Calvin cycle uses ATP & NADPH to convert CO2 to sugar The Calvin cycle Is similar to the citric acid cycle Occurs in the stroma

The Calvin Cycle Calvin cycle (fig 10.18) = anabolic process that builds carbohydrates from smaller molecules (CO2 and H2O Consumes E (ATP) from the light reactions Uses NADPH (from the light reactions) as the reducing agent for adding high energy e- to make sugar

The Calvin cycle The Calvin cycle Figure 10.18 Input Light 3 CO2 (G3P) Input (Entering one at a time) CO2 3 Rubisco Short-lived intermediate 3 P P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 P 6 1,3-Bisphoglycerate 6 NADPH 6 NADPH+ Glyceraldehyde-3-phosphate 6 ATP 3 ATP 3 ADP CALVIN CYCLE 5 1 G3P (a sugar) Output Light H2O LIGHT REACTION ATP NADPH NADP+ ADP [CH2O] (sugar) CALVIN CYCLE Figure 10.18 O2 6 ADP Glucose and other organic compounds Phase 1: Carbon fixation Phase 3: Regeneration of the CO2 acceptor (RuBP) Phase 2: Reduction

The Calvin cycle has three phases Carbon fixation Reduction Regeneration of the CO2 acceptor

The Calvin cycle Phase 1: Carbon fixation CO2 incorporated by attaching to 5-C sugar (ribulose bisphosphate = RuBP) Catalyzed by Rubisco (ribulose bisphosphate carboxylase/oxygenase) = most abundant protein on Earth! Unstable 6-C intermediate  2 molecules of 3-phosphoglycerate (3-C sugar) (G3P) Input (Entering one at a time) CO2 3 Rubisco Short-lived intermediate 3 P P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 P 6 1,3-Bisphoglycerate 6 NADPH 6 NADPH+ Glyceraldehyde-3-phosphate 6 ATP 3 ATP 3 ADP CALVIN CYCLE 5 1 G3P (a sugar) Output Light H2O LIGHT REACTION ATP NADPH NADP+ ADP [CH2O] (sugar) CALVIN CYCLE O2 6 ADP Glucose and other organic compounds Phase 1: Carbon fixation Phase 3: Regeneration of the CO2 acceptor (RuBP) Phase 2: Reduction

The Calvin cycle Phase 2: Reduction (G3P) Input (Entering one at a time) CO2 3 Rubisco Short-lived intermediate 3 P P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 P 6 1,3-Bisphosphoglycerate 6 NADPH 6 NADPH+ Glyceraldehyde-3-phosphate 6 ATP 3 ATP 3 ADP CALVIN CYCLE 5 1 G3P (a sugar) Output Light H2O LIGHT REACTION ATP NADPH NADP+ ADP [CH2O] (sugar) CALVIN CYCLE O2 6 ADP Glucose and other organic compounds Phase 2: Reduction 3-phosphoglycerate has phosphate added  NADPH reduces intermediate  G3P (glyceraldehyde-3-phosphate) One G3P exits the cycle to be used by the plant cell Other 5 recycled to regenerate RuBP Phase 1: Carbon fixation Phase 3: Regeneration of the CO2 acceptor (RuBP) Phase 2: Reduction

The Calvin cycle Phase 3: Regeneration of CO2 acceptor (RuBP) Requires ATP Five G3P (3-C)  Three 5-C molecules of RuBP (G3P) Input (Entering one at a time) CO2 3 Rubisco Short-lived intermediate 3 P P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 P 6 1,3-Bisphoglycerate 6 NADPH 6 NADPH+ Glyceraldehyde-3-phosphate 6 ATP 3 ATP 3 ADP CALVIN CYCLE 5 1 G3P (a sugar) Output Light H2O LIGHT REACTION ATP NADPH NADP+ ADP [CH2O] (sugar) CALVIN CYCLE O2 6 ADP Glucose and other organic compounds Phase 1: Carbon fixation Phase 3: Regeneration of the CO2 acceptor (RuBP) Phase 2: Reduction

Balancing gas exchange against water loss Terrestrial plants have to balance gas exchange (CO2 + O2) w/ H2O loss Happens at the stomata on leaves If a plant closes its stomata during hottest part of day  accumulation of O2 & no CO2 uptake Photorespiration = process that uses O2 instead of CO2  only generates 1/2 the amount of 3-phosphoglycerate  decreases Ps output by siphoning organic material from Calvin cycle (up to 50%) Rubisco can act on both CO2 & O2 (not discriminate) C3 plants Most plants Use only Calvin cycle to fix CO2 in mesophyll Limited by photorespiration

On hot, dry days, plants close their stomata Conserving water but limiting access to CO2 Causing oxygen to build up

Photorespiration: An Evolutionary Relic? In photorespiration O2 substitutes for CO2 in the active site of the enzyme rubisco The process consumes oxygen and releases CO2 The photosynthetic rate is reduced

Adaptations to Hot, Arid Climates Alternative mechanisms of carbon fixation have evolved in hot, arid climates C4 plants separate initial carbon fixation from the Calvin cycle in space CAM plants separate initial carbon fixation from the Calvin cycle in time

Adaptations to Hot, Arid Climates C4 plants (fig 10.21a) - exhibit a spatial separation of C-fixation Preface Calvin cycle with alternate mode of C-fixation that forms a 4-C compound (occurs in mesophyll cells) – Ex. Corn and sugarcane Associated w/ unique leaf anatomy (Kranz anatomy, fig 10.19) PEP carboxylase adds CO2 to PEP (phosphoenolpyruvate)  4-C oxaloacetate  converted to malate  transported to bundle sheath cells  broken down to CO2 (Calvin cycle) + pyruvate (3-C goes back to mesophyll to regenerate PEP) PEP carboxylase has a much higher affinity for CO2 than rubisco does, and no affinity for O2 CAM plants (fig 10.21b) Ex. Succulents, cacti, pineapples, etc. Crassulacean Acid Metabolism plants adapted to arid environments Temporal separation of C-fixation: open stomata at night and close during day Take up CO2 at night  oxaloacetate  malate  malic acid stored in vacuole during the day ATP + NADPH synthesized during day  malic acid transported out of vacuole  malate  CO2 + pyruvate at night

Adaptations to Hot, Arid Climates C4 plants (fig 10.21a) CAM plants (fig 10.21b) These types of plants separate initial carbon fixation from the Calvin Cycle either in space or time. This allows them to keep their stomata partially closed during the day to minimize water loss.

C4 Plants C4 plants minimize the cost of photorespiration By incorporating CO2 into four carbon compounds in mesophyll cells These four carbon compounds Are exported to bundle sheath cells, where they release CO2 used in the Calvin cycle

C4 leaf anatomy and the C4 pathway Separates initial carbon fixation from the Calvin cycle in space CO2 Mesophyll cell Bundle- sheath cell Vein (vascular tissue) Photosynthetic cells of C4 plant leaf Stoma Mesophyll C4 leaf anatomy PEP carboxylase Oxaloacetate (4 C) PEP (3 C) Malate (4 C) ADP ATP Sheath Pyruate (3 C) CALVIN CYCLE Sugar Vascular tissue Figure 10.19

CAM Plants CAM plants Open their stomata at night, incorporating CO2 into organic acids During the day, the stomata close And the CO2 is released from the organic acids for use in the Calvin cycle So they separate initial carbon fixation from the Calvin cycle in time.

CAM pathway is similar to the C4 pathway Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in different types of cells. (a) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times. (b) Pineapple Sugarcane Bundle- sheath cell Mesophyll Cell Organic acid CALVIN CYCLE Sugar CO2 C4 CAM CO2 incorporated into four-carbon organic acids (carbon fixation) Night Day 1 2 Organic acids release CO2 to Calvin cycle Figure 10.20

Importance of Photosynthesis About 50% of the organic material made by Ps is consumed as fuel for respiration in the plant body Not all plant cells make their own food  have to be supplied by Ps cells Two most important products we derive from plants come directly from Ps (what are they?) Fig 10.21 = nice overview of Ps

The Importance of Photosynthesis: A Review A review of photosynthesis Light reactions: • Are carried out by molecules in the thylakoid membranes • Convert light energy to the chemical energy of ATP and NADPH • Split H2O and release O2 to the atmosphere Calvin cycle reactions: • Take place in the stroma • Use ATP and NADPH to convert CO2 to the sugar G3P • Return ADP, inorganic phosphate, and NADP+ to the light reactions O2 CO2 H2O Light Light reaction Calvin cycle NADP+ ADP ATP NADPH + P 1 RuBP 3-Phosphoglycerate Amino acids Fatty acids Starch (storage) Sucrose (export) G3P Photosystem II Electron transport chain Photosystem I Chloroplast Figure 10.21

Two most important products of photosynthesis Organic compounds produced by photosynthesis Provide the energy and building material for ecosystems Oxygen produced by photosynthesis provides an aerobic environment that allows for cellular respiration