Chapter 10 Photosynthesis Crosby High School AP Biology

Pathways of Photosynthesis Chemical equation: 6CO2 + 6H2O + Light Energy  C6H1206 + 6O2 Splitting of Water Originally thought that oxygen released came from carbon dioxide C.B. van Niel challenged in 1930’s Used bacteria that used H2S Radioactive O in carbon dioxide and water

Light Reactions Takes place in Thylakoids Drives e- from water to NADP+ H2O splits NADP+ reduced by electrons to NADPH Adds P to ADP generating ATP

Calvin Cycle Named for Melvin Calvin Begins with Carbon Fixation Reduces C to Carbohydrates by adding electrons Reducing power from NADPH ATP required to convert CO2  Carb. Dark Reactions (Light Independent Reactions) Takes place in Stroma

Absorbing Sunlight Photons act as particles of Light Photosynthetic Pigments Spectrophotometer: measures absorbtion Chlorophyll a Blue-green and participates directly in L.R. Cholorphyll b Yellow-green and transfers E to Chlorophyll a Carotenoids Red and Yellow Hydrocarbons Photoprotection: Absord and dissipate excessive light energy Excitation of Chlorophyll

Absorption and Transmittance

Excitation of Chlorophyll Photon absorbed and e- excited Must be same E as E dif. btwn. Ground and Excited state Fluorescence: When an e- drops from excited to ground state it emits a photon

Noncyclic Electron Flow Photosystem II (P680) H2O donates e- ETC Noncyclic Photophosphorylation Photosystem I (P700) Fd to NADP+ reductase

Noncyclic Electron Flow

Cyclic Electron Flow Ferrodoxin to Cytochrome, back to P700 No O or NADPH Cyclic Photophosphorylation Usefullness There is more ATP required in the Calvin Cycle than NADPH

Chemiosmosis: Mitochondria vs. Chloroplast Uses chemical energy Ion pump creates proton-motive force Hydrogen ion gradient formed in intermembrane space Chloroplast Uses light energy Ion pump creates proton-motive force Hydrogen ion gradient formed in Thylakoid space (Lumen)

Spatial organization

Calvin Cycle (Dark Reactions) Makes Glyceraldehyde-3-phosphate Requires three cycles to make one G3P Net synthesis Uses 9 ATP Uses 6 NADPH Makes 1 G3P G3P is used to make other compounds for the plant

Phases of Calvin Cycle Phase 1: Carbon Fixation Phase 2: Reduction Each CO2 attaches to Ribulose biphosphate RuBP carboxylase (Rubisco) Results in 6-Carbon molecule Unstable Splits to form 3-phosphoglycerate Phase 2: Reduction Each 3-phosphoglycerate receives Pi 1,3-biphosphate formed and reduced to G3P by NADPH

Phases (cont.) Phase 3: Regeneration of CO2 acceptor (RuBP) One molecule of G3P exits cycle for use Five molecules of G3P are used to form 3 molecules of RuBP Requires the use of 3 ATP

Calvin Cycle

Photorespiration Carbon is normally fixed by Rubisco (C3 plants) On hot days Stomata close and prevent CO2 uptake Rubisco will take up O2 instead producing a 2-C compound Mitochondria and Peroxisomes convert to CO2 No food or E produced

C4 Plants Anatomy Bundle-Sheath cells: Mesophyll Cells: Tightly packed around the veins Location of Calvin Cycle Mesophyll Cells: Loosely packed between B-S and leaf surface Location of Carbon fixation

C4 Plant (cont.) Fixation CO2 forms 4-C Oxaloacetate in the Mesophyll Phosphoenolpyruvate Carboxylase combines PEP with CO2 4-C product enters B-S through Plasmodesmata 4-C releases CO2 for Calvin Cycle

CAM Plants Crassulacean Acid Metabolism Open Stomata at night Take in CO2 Convert CO2 to organic acids Organic acids stored in vacuoles Organic acids released when L.R. begin Separated by time of day

Products of Photosynthesis 50% of products used for fuel Carbs. Transported as Sucrose Large amounts of products used to form cellulose Extra stored as Starch