6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O Photosynthesis Overview: The Process That Feeds the Biosphere Solar Energy Chemical Energy

Plants are Photoautotrophs (a) Plants (c) Unicellular protist 10 m 40 m (d) Cyanobacteria 1.5 m (e) Pruple sulfur bacteria Figure 10.1 Figure 10.2 plants, algae, certain other protists, and some prokaryotes

Chloroplasts: The Sites of Photosynthesis in Plants Vein Leaf cross section Figure 10.3 Mesophyll CO2 O2 Stomata Note: spongy parenchyma (irregular shaped cell)  allow for diffusion of gases within the leaf Stomata  allow for gas exchange and transpiration Guard cells  controls opening and closing of stomata Vein  vesticular bundle (xylem and pholem) for transport material in tplant

Chloroplasts Organelle where photosynthesis occurs THREE MEMBRANES!! Mesophyll 5 µm Outer membrane Intermembrane space Inner Thylakoid Granum Stroma 1 µm Organelle where photosynthesis occurs THREE MEMBRANES!!

The Splitting of Water Reactants: 6 CO2 12 H2O 6 O2 6 H2O Products: C6H12O6 6 H2O 6 O2 Figure 10.4 Photosynthesis is a redox process Water is oxidized, carbon dioxide is reduced

An overview of photosynthesis CO2 Light LIGHT REACTIONS CALVIN CYCLE Chloroplast [CH2O] (sugar) NADPH NADP  ADP + P O2 Figure 10.5 ATP The light reactions Occur in the grana (in our analogy, the “pancakes”) Split water, release oxygen, produce ATP, and form NADPH The dark reaction: The Calvin cycle Occurs in the stroma (in our analogy, the “syrup”) Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power

The “Light” Reactions Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH Gamma rays X-rays UV Infrared Micro- waves Radio 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1 m 103 m 380 450 500 550 600 650 700 750 nm Visible light Shorter wavelength Higher energy Longer wavelength Lower energy Light Reflected Chloroplast Absorbed light Granum Transmitted Electromagnetic Spectrum (Think about why pigments are important  used as light receptors) ch

Wavelength of light (nm) The Absorption Spectra Absorption of light by chloroplast pigments Chlorophyll a (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. Wavelength of light (nm) Chlorophyll b Carotenoids Figure 10.9 The absorpion spectra of three types of pigments in chloroplasts provide clues to the relative effectiveness of different wavelengths for driving photosynthesis (a)  main pigment, (b) accessory pigment, carotenoid  for other part of the spectrum and pass energy to chlorophyll

Excitation of Chlorophyll by Light Excited state Energy of election Heat Photon (fluorescence) Chlorophyll molecule Ground e– Figure 10.11 A When a pigment absorbs light It goes from a ground state to an excited state, which is unstable

(INTERIOR OF THYLAKOID) Photosystem Primary election acceptor Photon Thylakoid Light-harvesting complexes Reaction center Photosystem STROMA Thylakoid membrane Transfer of energy Special chlorophyll a molecules Pigment THYLAKOID SPACE (INTERIOR OF THYLAKOID) Figure 10.12 e– Is composed of a reaction center surrounded by a number of light-harvesting complexes The light-harvesting complexes Consist of pigment molecules bound to particular proteins Funnel the energy of photons of light to the reaction center

Electron transport chain What Comes Out? Produces NADPH, ATP, and oxygen (as a byproduct) 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 Photophosphorylation (producing ATP by using light!) Noncyclic electron flow Is the primary pathway of energy transformation in the light reactions 5 1 6

The “Light” Reactions Figure 10.14 NADPH e– Mill makes ATP Photon Photosystem II Photosystem I NADPH Figure 10.14 

Cyclic Electron Flow Primary Fd acceptor Pq NADP+ reductase Cytochrome complex Pc NADP+ reductase NADPH ATP Figure 10.15 Photosystem II Photosystem I Under certain conditions Photoexcited electrons take an alternative path In cyclic electron flow Only photosystem I is used Only ATP is produced

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 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 light reactions and chemiosmosis: the organization of the thylakoid membrane 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

The “Dark” Reaction Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO2 to 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 Figure 10.18 O2 6 ADP Glucose and other organic compounds The Calvin cycle has three phases Carbon fixation Reduction Regeneration of the CO2 acceptor The Calvin cycle Is similar to the citric acid cycle Occurs in the stroma

Summary Stages of Photosynthesis Location Input Output Light Dependent Reactions (noncyclic flow) Thylakoid membrane Photosystem I (P680) Photosystem II (P700 Photons H2O NADPH ATP O2 Light Dependent Reactions (cyclic flow) Photosystem I (P700) Light Independent Reactions (Calvin Cycle) Stroma 3 CO 3 RuBP 9 ATP 6 NADPH 1 glyceraldhyde-3-phosphates (G3P)

Photorespiration: An Evolutionary Relic? In photorespiration O2 substitutes for CO2 in the active site of the enzyme rubisco The photosynthetic rate is reduced Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates Conserving water but limiting access to CO2 Causing oxygen to build up

C4 Plants C4 plants minimize the cost of photorespiration By incorporating CO2 into four carbon compounds in mesophyll cells

C4 leaf anatomy and the C4 Pathway 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 These four carbon compounds Are exported to bundle sheath cells, where they release CO2 used in the Calvin cycle

CAM Plants CAM plants During the day, the stomata close 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 Carassulacean acid metabolism photosynthesis

The Importance of Photosynthesis: A Review 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