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Unit 3 - Photosynthesis

Unit 3 - Photosynthesis. The Basis of Life. Overall Process. 6CO 2 + 12H 2 O + Light Energy  C 6 H 12 O 6 + 6O 2 + 6H 2. Structure of a Leaf. Stomata Mesophyll. Chloroplast. Site of photosynthesis Light Reaction – thylakoids Calvin Cycle – stroma (fluid). Redox in Biology.

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Unit 3 - Photosynthesis

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  1. Unit 3 - Photosynthesis The Basis of Life

  2. Overall Process 6CO2 + 12H2O + Light Energy  C6H12O6 + 6O2 + 6H2

  3. Structure of a Leaf • Stomata • Mesophyll

  4. Chloroplast • Site of photosynthesis • Light Reaction – thylakoids • Calvin Cycle – stroma (fluid)

  5. Redox in Biology • OIL – Oxidation Is Loss • RIG – Reduction Is Gain • In chemistry, “loss” or “gain” refers to e- • In biology, “loss” or “gain” refers to H atom • Ex: • NADPH is reduced (because it gained an H) • NADP+ is oxidized (because it lost the H)

  6. Phosphorylation • Accomplished by enzymes called kinases • Attach a phosphate group (PO4) to a molecule • Makes molecule less stable due to increase in free energy (more ordered)

  7. ADP Phosphorylation

  8. Two stages: • Light Rxn: • Reactants – Light (photons) and H2O • Products – O2, ATP, and NADPH • Calvin Cycle: • Reactants – ATP, NADPH, and CO2 • Products – CH2O (sugar!), ADP + Pi, NADP+

  9. Light Rxn is divided into 2 parts • Photosystem II • Photosystem I • Photosystem – reaction center located on thylakoid membrane • Contain many light-harvesting complexes (contain chlorophyll)

  10. Photosystem II (PSII) • Light, in the form of photons, hits leaf and is absorbed by chlorophyll (usually chlorophyll a) in PSII rxn center • e- with chlorophyll a gets excited and moves to higher energy state (gains PE) • @ same time, enzymatic rxn splits H2O  2e-, 2H+, and ½ O2 • e-’s from splitting of water (photolysis) replace e- that was excited in chlorophyll a

  11. 4. Electron Transport Chain (ETC) – e- is passed from PSII to PSI via a series of molecules • Each “pass” results in e- falling to a lower energy state • Exergonic processprovides energy to pump H+ (protons) into the thylakoid space.

  12. Chemiosmosis • Light rxn creates a proton gradient due to ETC • Results in a pH, charge, and [] gradient • ATP Synthase– couples the diffusion of H+ ions back down their concentration gradient to stroma with phosphorylation of ADP to make ATP

  13. pH gradient Concentration gradient Charge gradient

  14. Photosystem I (PSI) • Light energy transfers from PSII  PSI which excites e- • e- travels through PSI and enters another ETC • As e- travels down ETC, the exergonic process produces energy to convert NADP+ to NADPH (reduction!). • NADPH is the final electron acceptor!

  15. Thus…. • Light rxns convert light energy to chemical energy stored in NADPH (@PSI) and ATP (via chemiosmosis). • Light rxns also produce O2 (@PSII) as a by-product.

  16. Calvin Cycle (aka dark rxns aka light-independent rxns) • ATP and NADPH from light reactions provide energy to drive carbohydrate synthesis. • CO2 (from air) is reduced and “fixed” into carbohydrate • NADPH and ATP provide reducing power • Anabolic due to building up of sugar from smaller molecules and consumption of energy

  17. Fates/uses of glucose • Provides plant with chemical energy and carbon (for synthesis of all organic macromolecules) • Proteins, lipids, nucleic acids • STARCH and CELLULOSE • Heterotrophs (us) consume autotrophs (plants)

  18. Evolutionary background • Photosynthesis first evolved in prokaryotes • Evidence supports that prokaryotic photosynthesis was responsible for production of oxygenated atmosphere • Big point – prokaryotic photosynthetic pathways were foundation of eukaryotic photosynthesis

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