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AP Bio Photosynthesis 2 Photosynthesis • Uses light energy as energy source • Uses CO2 as a carbon source • Synthesizes energy rich organic molecules (sugar and others) • Supplies energy, stored in sugars, to most living organisms • Occurs in the chloroplasts of autotrophs (green plants) 3 Photosynthesis Phacts • On a global scale, photosynthesis makes about 160 billion metric tons of carbohydrates a year. • Sucrose is the transport form of carbohydrates in most plants. • Plants consume about 50% of the photosynthate as fuel for cellular respiration. • Extra sugars are synthesized into starch and stored in cells of roots, tubers, seeds, and fruits. 4 Summary Equation One Overall Equation: 6 CO2 + 12 H2O + light energy C6H12O6 + 6 O2 + 6 H2O In Two Stages: a) Light Dependent Reactions b) Light Independent Reactions 5 Chloroplast Structure Thylakoids Stroma Granum Inner & Outer Membranes 6 Organization of Photosynthesis 7 Leaf Structure Vein Upper Epidermis Mesophyll Cells Stoma Lower Epidermis 8 The Light-Dependent Reactions 1. Occur in thylakoid membranes 2. O2 released by splitting water 3. Light energy boosts P.E. of electrons 9 Photosynthetic Pigments •Chlorophyll a - main photosynthetic pigment in all organisms except bacteria •Accessory pigments absorb slightly different wavelengths of light • Chlorophyll b • Carotenoids 10 Absorption Spectra The combination of all of the pigments increases the range of colors that plants can use in photosynthe sis. 11 The Photosystems 12 Photosystem II 13 14 The Light-Dependent Reactions 1. Occur in thylakoid membranes • Photosystems 2. O2 released by splitting water • Photolysis 3. Light energy boosts P.E. of electrons • Photoexcitation 4. ATP is generated • Photophosphorylation Chemiosmosis 1 15 16 Chemiosmosis 2 17 http://www.cst.cmich.edu/users/baile1re/bio 101fall/enzphoto/photoanima.htm# 18 http://www.stolaf.edu/people/giannini/flash animat/metabolism/photosynthesis.swf 19 Light-Dependent Reactions 20 Overview of Photosynthesis Light-Dependent Reactions H2O (in thylakoids) Depleted Carriers Energized Carriers (ADP, NADP+) Glucose O2 (ATP, NADPH) Light-Independent Reactions (in stroma) CO2+H2O 21 Rubisco (RuBP carboxylase) 22 The Light-Independent Reactions 1. Occur in the stroma 2. Does not require light, but does require the products of the light reactions (ATP and NADPH) 3. Calvin Cycle (Melvin Calvin) • 3 Phases Assimilation of atmospheric carbon dioxide into carbohydrates. 23 The C3 Cycle of Carbon Fixation 24 A Summary of Photosynthesis Light-dependent reactions occur in thylakoids Lightindependent reactions (c3 cycle) occur in stroma 25 C3 Plants & the Pathway C3 plants use the C3 pathway In a C3 plant, most chloroplasts are in mesophyll cells. 26 C3 Plants Much photorespiration occurs under hot, dry conditions 27 C4 Plants & the C4 Pathway C4 plants use the C4 pathway In a C4 plant, both mesophyll & bundlesheath cells contain chloroplasts. Mesophyll cell in a C4 plant 28 CO2 is captured with a highly specific enzyme. C4 Plants & the C4 Pathway Much glucose synthesis occurs. Almost no photorespiration occurs under hot, dry conditions. Bundle-sheath cell in a C4 plant Relationship of Photosynthesis & Respiration 29 AP Bio The end