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Chapter 8: Photosynthesis Chapter 9: Respiration Photosynthesis: 2 RXNS Light-Dependent Reactions Needs light to occur (produces ATP and NADPH) Needs water (source of electrons and hydrogen ions) Takes place in the thylakoid membranes Oxygen is released as a by-product Light-Independent Reactions Does NOT need light Takes place in stroma Needs carbon dioxide ATP and NADPH (produced in light rxn) are used to produce high-energy sugars Photosynthesis: 2 RXNS Light-Dependent Reactions Light-Independent Reactions The Light-Dependent Reactions Requires light Uses energy from the sun to: Produce oxygen Convert ADP to ATP Convert NADP+ to NADPH Occurs in the thylakoid membrane which contain: Clusters of chlorophyll Photosystems – proteins surrounded by accessory pigments Absorb sunlight Generate high-energy electrons that are passed to a series of electron carriers embedded in the thylakoid membrane Photosystem II Electron Transport Chain Photosystem I Short ETC NADPH Chemiosmosis The Light-Dependent Reactions Photosystem II Light energy absorbed by pigments Electrons’ energy levels increase e- passed to ETC Enzymes break apart water – releasing O2, H+, and 2 e2e- replace electrons lost The Light-Dependent Reactions Electron Transport Chain Series of electron carrier proteins that shuttle high-energy electrons during ATPgenerating reactions High energy e- move down the ETC Energy from the e- is used by the proteins in the chain to pump H+ ions from the stroma into the thylakoid e- then passed to… The Light-Dependent Reactions Photosystem I e- contain less energy because some energy has been used to pump H+ ions Use pigments to reenergize eShort ETC transfers eto NADP+ NADP+ picks up e- and H+ to make NADPH The Light-Dependent Reactions Hydrogen Ion Movement & ATP Inside of thylakoid is positively charged (lots of H+) relative to the outside (=gradient) H+ pass over thylakoid membrane through ATP synthase ATP synthase rotates and the energy produced is used to convert ADP to ATP (chemiosmosis) The Light-Independent Reactions AKA Calvin Cycle Does NOT require light Uses the energy that ATP and NADPH contain to build stable high-energy compounds that can be stored for a long time Uses 6CO2 to produce 1 6carbon sugar The Light-Independent Reactions Carbon dioxide enters the cycle from the atmosphere Energy from ATP and NADPH: Combine 6 carbon dioxide molecules with six 5-carbon compounds and then… Produce 12 3-carbon compounds Midcycle 2 3-carbon molecules are removed These become the building blocks of sugars, lipids, amino acids, etc. Finally 10 3-carbon molecules are converted back into 6 5-carbon molecules – go on to repeat the cycle Factors Affecting Photosynthesis Temperature Photosynthesis enzymes function best between 0°C and 35°C Light Intensity High light intensity increases the rate of photosynthesis After a certain point the maximum rate of photosynthesis is reached Availability of water Questions for Review What is the first process in the lightdependent reaction? What is the source of the e- in the lightdependent reaction? What are 2 products of the lightdependent reaction that are used as a source of energy in the Calvin cycle? What are three factors that affect the rate at which photosynthesis occurs?