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PHOTOSYNTHESIS Photosynthesis • Anabolic (small molecules combined) • Endergonic (stores energy) • Carbon dioxide (CO2) requiring process that uses light energy (photons) and water (H2O) to produce organic macromolecules (glucose). SUN photons 6CO2 + 6H2O C6H12O6 + 6O2 glucose 2 Question: Where does photosynthesis take place? 3 Plants • Autotrophs – produce their own food (glucose) • Process called photosynthesis • Mainly occurs in the leaves: a. stoma - pores b.mesophyll cells Mesophyll Cell Chloroplast Stoma 4 Stomata (stoma) Pores in a plant’s cuticle through which water and gases are exchanged between the plant and the atmosphere. Oxygen (O2) Carbon Dioxide (CO2) Guard Cell Guard Cell Found on the underside of leaves 5 Mesophyll Cell of Leaf Nucleus Cell Wall Chloroplast Central Vacuole Photosynthesis occurs in these cells! 6 Chloroplast Organelle where photosynthesis takes place. Stroma Outer Membrane Inner Membrane Thylakoid Granum Thylakoid stacks are connected together 7 Thylakoid Thylakoid Membrane Granum Thylakoid Space Grana make up the inner membrane 8 Question: Why are plants green? 9 Chlorophyll Molecules • Located in the thylakoid membranes • Chlorophyll have Mg+ in the center • Chlorophyll pigments harvest energy (photons) by absorbing certain wavelengths (blue-420 nm and red660 nm are most important) • Plants are green because the green wavelength is reflected, not absorbed. 10 Wavelength of Light (nm) 400 500 600 700 Short wave Long wave (more energy) (less energy) 11 Absorption of Light by Chlorophyll Absorption violet blue green yellow wavelength orange red 12 Question: During the fall, what causes the leaves to change colors? 13 Fall Colors • In addition to the chlorophyll pigments, there are other pigments present • During the fall, the green chlorophyll pigments are greatly reduced revealing the other pigments • Carotenoids are pigments that are either red, orange, or yellow 14 Redox Reaction The transfer of electrons from another Two types: 1. Oxidation is 2. Reduction is one or more one reactant to the loss of ethe gain of e- 15 Oxidation Reaction The loss of electrons from a substance or the gain of oxygen. Oxidation 6CO2 + 6H2O C6H12O6 + 6O2 glucose 16 Reduction Reaction The gain of electrons to a substance or the loss of oxygen. Reduction 6CO2 + 6H2O C6H12O6 + 6O2 glucose 17 Two Parts of Photosynthesis Two reactions make up photosynthesis: 1.Light Reaction or Light Dependent Reaction Produces energy from solar power (photons) in the form of ATP and NADPH. SUN 18 Two Parts of Photosynthesis 2. Calvin Cycle or Light Independent Reaction • Also called Carbon Fixation or C3 Fixation • Uses energy (ATP and NADPH) from light reaction to make sugar (glucose). 19 Light Reaction (Electron Flow) • Occurs in the Thylakoid membranes • During the light reaction, there are two possible routes for electron flow: A.Cyclic Electron Flow B. Noncyclic Electron Flow 20 Cyclic Electron Flow • • • • Occurs in the thylakoid membrane. Uses Photosystem I only P700 reaction center- chlorophyll a Uses Electron Transport Chain (ETC) • Generates ATP only P ADP + ATP 21 Cyclic Electron Flow Primary Electron Acceptor SUN e- e- ePhotons P700 e- ATP produced by ETC Accessory Pigments Photosystem I Pigments absorb light energy & excite e- of Chlorophyll a to produce ATP 22 Noncyclic Electron Flow • Occurs in the thylakoid membrane • Uses Photosystem II and Photosystem I • P680 reaction center (PSII) chlorophyll a • P700 reaction center (PS I) chlorophyll a • Uses Electron Transport Chain (ETC) • Generates O2, ATP and NADPH 23 Noncyclic Electron Flow Primary Electron Acceptor Primary Electron Acceptor Enzyme Reaction 2e- 2e- ETC SUN 2e- 2e- Photon H2O 1/2O2 + 2H+ 2e- P700 NADPH ATP P680 Photon Photosystem I Photosystem II H2O is split in PSII & ATP is made, while the energy carrier NADPH is made in PSI 24 Noncyclic Electron Flow • ADP + P ATP • NADP+ + H NADPH • Oxygen comes from the splitting of H2O, not CO2 H2O 1/2 O2 + 2H+ 25 Chemiosmosis • Powers ATP synthesis • Located in the thylakoid membranes • Uses ETC and ATP synthase (enzyme) to make ATP • Photophosphorylation: addition of phosphate to ADP to make ATP 26 Chemiosmosis SUN H+ H+ Thylakoid (Proton Pumping) E T PS II PS I C H+ H+ H+ H+ H+ H+ ADP + P H+ H+ high H+ concentration ATP Synthase ATP Thylakoid Space low H+ concentration 27 Calvin Cycle • Carbon Fixation (light independent reaction) • C3 plants (80% of plants on earth) • Occurs in the stroma • Uses ATP and NADPH from light reaction as energy • Uses CO2 • To produce glucose: it takes 6 turns and uses 18 ATP and 12 NADPH. 28 Chloroplast Outer Membrane Inner Membrane STROMA– where Calvin Cycle occurs Thylakoid Granum 29 Calvin Cycle (C3 fixation) (36C) 6C-C-C-C-C-C (6C) 6CO2 (unstable) (30C) 6C-C-C-C-C RuBP (30C) glucose 6C-C-C 12PGA (36C) 6ATP 6ATP 6NADPH 6NADPH 6C-C-C 6ATP C3 6C-C-C (36C) 6C-C-C 12G3P (6C) C-C-C-C-C-C 30 Glucose Calvin Cycle Remember: C3 = Calvin Cycle C3 Glucose 31 Photorespiration • • • • Occurs on hot, dry, bright days Stomates close Fixation of O2 instead of CO2 Produces 2-C molecules instead of 3-C sugar molecules • Produces no sugar molecules or no ATP 32 Photorespiration Because of photorespiration, plants have special adaptations to limit the effect of photorespiration: 1. C4 plants 2. CAM plants 33 C4 Plants • Hot, moist environments • 15% of plants (grasses, corn, sugarcane) • Photosynthesis occurs in 2 places • Light reaction mesophyll cells • Calvin cycle - bundle sheath cells 34 C4 Plants Malate-4C sugar C-C-C-C Malate C-C-C-C Transported CO2 CO2 C3 glucose C-C-C PEP ATP Mesophyll Cell Vascul Tissu C-C-C Pyruvic Acid Bundle Sheath Cell 35 CAM Plants • Hot, dry environments • 5% of plants (cactus and ice plants) • Stomates closed during day • Stomates open during the night • Light reaction - occurs during the day • Calvin Cycle - occurs when CO2 is present 36 CAM Plants Night (Stomates Open) Day (Stomates Closed) Vacuole CO2 C-C-C-C Malate C-C-C-C Malate C-C-C-C Malate CO2 C3 C-C-C PEP ATP C-C-C Pyruvic acid glucose 37 Question: Why do CAM plants close their stomata during the day? 38 Cam plants close their stomata in the hottest part of the day to conserve water