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Photosynthesis Chapter 6 Carbon and Energy Sources Photoautotrophs Carbon source is carbon dioxide Energy source is sunlight Heterotrophs Get carbon and energy by eating autotrophs or one another Linked Processes Photosynthesis Aerobic Respiration Energy-storing pathway Energy-releasing pathway Releases oxygen Requires oxygen Requires carbon dioxide Releases carbon dioxide Autotrophs are the producers of the biosphere Photoautotrophs capture sunlight energy and use it to carry out photosynthesis. Plants, algae, some protists, and bacteria are photosynthetic autotrophs They are the ultimate producers of food consumed by virtually all organisms On land, plants such as oak trees and cacti are the predominant producers In aquatic environments, algae and photosynthetic bacteria are the main food producers Photosynthesis occurs in chloroplasts In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts A chloroplast contains: Stroma (fluid) grana (stacks of thylakoids) The thylakoids contain chlorophyll Chlorophyll is the green pigment that captures light for photosynthesis Location and Structure of Chloroplasts Photosynthesis Equation LIGHT ENERGY 12H2O + 6CO2 6O2 + C2H12O6 + 6H2O Photosynthesis is a redox process, as is cellular respiration Reduction Oxidation Water molecules are split apart and electrons and H+ ions are removed, leaving O2 gas These electrons and H+ ions are transferred to CO2, producing sugar Two Stages of Photosynthesis Visible Light Humans perceive different wavelengths as different colors Violet (380 nm) to red (750 nm) Longer wavelengths, lower energy Shorter wavelengths, higher energy Pigments Light-absorbing molecules Absorb some wavelengths and transmit others Color you see are the wavelengths NOT absorbed chlorophyll a chlorophyll b Wavelength (nanometers) Excitation of Electrons Excitation occurs only when the quantity of energy in an incoming photon matches the amount of energy necessary to boost the electrons of that specific pigment Amount of energy needed varies among pigment molecules Pigments in Photosynthesis Bacteria Plants Pigments found in plasma membranes Pigments embedded in thylakoid membrane system Pigments and proteins organized into photosystems Photosystems located next to electron transport systems Pigments include chlorophyll a, chlorophyll b and carotenoids Light-Dependent Reactions Pigments absorb light energy, give up ewhich enter electron transport systems Water molecules are split, ATP and NADPH are formed, and oxygen is released Pigments that give up electrons get replacements from photosystem Photosystem Function: Pigments When excited by light energy, these pigments transfer energy to adjacent pigment molecules Each transfer involves energy loss Photosystem Function: Reaction Center Energy is reduced to level that can be captured by molecule of chlorophyll a This molecule (P700 or P680) is the reaction center of a photosystem Reaction center accepts energy and donates electron to acceptor molecule Light Dependent Reactions Making Sugar from CO2: The Calvin Cycle The Calvin–Benson cycle makes sugar from CO2. Overall reactants Carbon dioxide ATP NADPH Overall products Glucose ADP NADP+ Making Sugar from CO2: The Calvin Cycle The Calvin cycle has four phases: fixation of CO2 Energy consumption and reduction carbohydrate production (release of G3P) regeneration of RuBP. Calvin Cycle Using the Products of Photosynthesis Phosphorylated glucose is the building block for: sucrose • The most easily transported plant carbohydrate starch • The most common storage form in plants Photosynthesis Summary Photorespiration in C3 Plants On hot, dry days stomata close Inside leaf O2 levels rise CO2 levels drop Rubisco attaches RuBP to oxygen instead of carbon dioxide Only one PGA forms instead of two Photorespiration in C4 Plants Carbon dioxide is fixed twice In mesophyll cells, carbon dioxide is fixed to form four-carbon compound (oxaloacetate) Carbon dioxide is released and fixed again in Calvin cycle Photorespiration in CAM Plants Carbon is fixed twice (in same cells) Night Carbon dioxide is fixed to form organic acids Day Carbon dioxide is released and fixed in Calvin cycle
