PS Lecture Outline I. Introduction A. Structures B. Net Reaction II. Overview of PS A. Rxns in the chloroplast B. pigments III. Closer looks A. LD Rxns B. LI Rxns 1. non-cyclic e- flow 2. cyclic e- flow 3. chemiosmosis C. Calvin Cycle D. Alternative C-fixations IV. Variables in PS effectiveness
A. Structures of PS Site of PS in plants: mostly in leaves in chloroplasts Leaf cross section Vein Mesophyll Stomata CO 2 O 2
Inside the chloroplast Identify the structures in the diagram
Mesophyll Chloroplasts Chloroplast thylakoids 5 µm grana stroma Outer membrane Stroma Granum Thylakoid Thylakoid space Inner membrane Intermembrane space 1 µm
B. The Net Reaction of Photosynthesis 6H2O + 6CO2 ------> C6H12O6+ 6O2 6 waters (plus light E, makes...) 6 carbon dioxides 1 glucose 6 oxygen
hydrolysis e- of H --> sugar O --> O2 Net Reaction Reactants: 6 CO 2 12 H 2 O Products: C 6 H 12 O 6 6 H 2 O 6 O 2 Figure 10.4 Photosynthesis is a redox process Water is oxidized, carbon dioxide is reduced
II. PS Overview we will stay (mostly) conceptual follow the energy
A. Light Dependant Rxns Occur in the grana 1. capture photons 2. split water 3. release oxygen 4. produce ATP a. photophosphoryllation driven by chemiosmosis b. E transfer 5. form NADPH E transfer
B. Light Independent Rxns aka The Calvin cycle in the stroma 1. forms sugar from carbon dioxide 2. uses ATP for energy 3. uses NADPH for reducing power
PS Overview
PS Overview
PS Overview
Photosynthetic Pigments: The Light Receptors How does the chloroplast absorb light? Pigments chlorophyll a,b accessory pigments
III. Closer look at LD Rxns
Excitation of Chlorophyll by Light When a pigment absorbs light ground state --> excited state (unstable) e Excited state Energy of election Heat Photon (fluorescence) Photon Chlorophyll molecule Ground state Figure 10.11 A
Photosystems Light harvesting complexes aka antenna pigment mols pigment mols bound by proteins funnel energy to reaction center e- gets bumped p680 (PSII) p700 (PSI) Figure 10.12 Thylakoid Thylakoid membrane Photon Transfer of energy Photosystem Light-harvesting complexes e Special chlorophyll a molecules Reaction center STROMA Primary election acceptor Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)
Analogy for the Lt Rxns ATP e e e NADPH e e e Mill makes ATP Photon e Figure 10.14 Photon Photosystem II Photosystem I
Noncyclic electron flow Produces NADPH, ATP, and oxygen
The biomass (dry weight) of a tree comes primarily from A. soil. B. water. C. air. D. organic fertilizer (manure, detritus). E. light. 2014 Pearson Education, Inc.
The light reactions, which involve the very hydrophobic chlorophyll, are located here in the chloroplast. B. A. 2014 Pearson Education, Inc. E. C. (lumen) D.
What colors of light will drive photosynthesis by green plants most efficiently? A. red only B. yellow only C. green only D. blue only E. red and blue 2014 Pearson Education, Inc.
How are the light reactions and the Calvin cycle connected? A. The light reactions provide ATP to the Calvin cycle, and the Calvin cycle provides NADPH for the light reactions. B. The light reactions provide ATP and NADPH to the Calvin cycle, and the Calvin cycle returns ADP and NADP + to the light reactions. C. The light reactions provide ATP and NADPH to the Calvin cycle, and the Calvin cycle returns reduced sugars to the light reactions. D. The light reactions provide NADPH to the Calvin cycle, and the Calvin cycle provides RuBP to the light reactions. E. The light reactions provide RuBP to the Calvin cycle, and the Calvin cycle returns G3P to the light reactions. 2014 Pearson Education, Inc.
Calvin Cycle Occurs: Sim to Citric Acid Cycle (aka Krebs) in CR opposite of anaerobic glycolysis 3 phases C-fixation Reduction Regeneration of the CO 2 acceptor
The Calvin Cycle Light H 2 O CO 2 LIGHT REACTION ATP O 2 NADP + ADP NADPH 3 ATP CALVIN CYCLE [CH 2 O] (sugar) 3 P P Ribulose bisphosphate (RuBP) (G3P) Input 3 (Entering one CO at a time) 2 Rubisco 3 ADP CALVIN CYCLE Phase 3: Regeneration of the CO 2 acceptor (RuBP) 5 P 3 P P Short-lived intermediate 6 6 Phase 1: Carbon fixation 3-Phosphoglycerate 6 P 1,3-Bisphoglycerate Glyceraldehyde-3-phosphate (G3P) P P P 6 NADPH 6 NADPH + 6 P 6 ATP 6 ADP Phase 2: Reduction Figure 10.18 1 G3P (a sugar) Output P Glucose and other organic compounds
The Calvin Cycle Starts w/ CO 2 + enzyme Rubisco +RuBP 3 turns of Calvin make... 3-C sugar 6 turns = 1 glucose
The Calvin Cycle Three phases... Fixation CO 2 + RuBP + Rubisco Reduction requires ATP (for E) requires H (from NADPH) produces 3-C sugars: G3P 1/6 of which form Glu Regeneration requires ATP 5/6 of 3-C sugars regenerate RuBP
Cyclic Electron Flow aka Cyclic Phosphoryllation Under certain conditions ATP deficit excess NADPH, deficit of NADP+ bumped e- s take an alternative path generates ATP
Cyclic Electron Flow Only photosystem I is used Only ATP is produced (no O2 or NADPH) Primary acceptor Fd Primary acceptor Fd Pq Cytochrome complex NADP + reductase NADP + NADPH Pc Figure 10.15 Photosystem II ATP Photosystem I
Alternative C-Fixations What do plants do on hot, arid days? close their stomata What happens to PS reactants/prods? water conserved less CO 2 O 2 build up Leads to Photorespiration O 2 substitutes for CO 2 in the active site of the enzyme rubisco Rate PS decreases
Minimizing Photorespiration C 4 plants avoid photorespiration spatially CO 2 stored as 4-C compounds stored in mesophyll cells exported to bundle sheath cells release CO 2 to the Calvin cycle
C4 Path Photosynthetic cells of C 4 plant leaf Mesophyll cell Bundlesheath cell Mesophyll cell PEP carboxylase CO CO 2 2 Vein (vascular tissue) Oxaloacetate (4 C) PEP (3 C) ADP Malate (4 C) ATP C 4 leaf anatomy Bundle- Sheath cell CO 2 Pyruvate (3 C) Stoma CALVIN CYCLE Sugar Vascular tissue Figure 10.19
Minimizing Photorespiration CAM plants avoid photorespiration temporally Open their stomata at night incorporating CO 2 into organic acids CO 2 released during the day to CC
C4 vs CAM Sugarcane Pineapple Mesophyll Cell Bundlesheath cell (a) Spatial separation of steps. In C 4 plants, carbon fixation and the Calvin cycle occur in different igure 10.20 types of cells. C 4 Organic acid CALVIN CYCLE Sugar CAM CO 2 CO 2 1 CO 2 incorporated into four-carbon organic acids (carbon fixation) 2 Organic acids release CO 2 to Calvin cycle Organic acid CALVIN CYCLE Sugar Night Day (b) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times.
Effect of Light Intensity low light => not enough ATP, NADPH what would be rate limiting step in Calvin Cycle? unusual unless the plant is heavily shaded
Effect of CO2 Concentration low CO 2 => what would be rate-limiting step? this is a common limiting factor
Effect of Temperature low temps => rate of rxn = high temps => CO 2 fixation doesn t work