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Photosynthesis

Photosynthesis. Photosynthesis is the process of converting light energy to chemical energy. Plants, algae, cyanobacteria, and some protists produce organic compounds from inorganic materials and light energy and are classified as photoautotrophs. Chloroplasts are the site of photosynthesis.

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Photosynthesis

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  1. Photosynthesis • Photosynthesis is the process of converting light energy to chemical energy. Plants, algae, cyanobacteria, and some protists produce organic compounds from inorganic materials and light energy and are classified as photoautotrophs. Chloroplasts are the site of photosynthesis. • Photosynthesis: The production of carbon compounds with the use of light energy • Photosynthesis like cellular respiration is performed through a series of redox reactions with glucose being reduced (gaining charge) through the oxidation of water. • 6CO2 + 12H2O +light energy gC6H12O6 + 6 O2 + 6H2O • 6CO2 + 6H2O +light energy g C6H12O6 + 6 O2 (simplified formula) • CO2 + H2O g [CH2O]n + O2 (reduced formula) • the reduced formula is necessary to visualize that the sugar molecule is built one carbon at a time • Tracking the atoms through photosynthesis • Production of O2 is from the splitting of water (not CO2) • confirmed by C.B. van Neil using the isotope O-18 • Carbon in the glucose comes from CO2 • Hydrogen in the new water and in the water come from reactant water • Oxygen in the glucose and the new water come from the CO2 in the reactants

  2. Structures necessary in photosynthesis • Chloroplasts - site of photosynthesis • primary pigment is chlorophyll (green) • found in the mesophyll - tissue of the leaf's interior • contains 30-40 chloroplasts • stroma - fluid filled compartment surrounded be 2 membranes • thalakoids - stack of membraneous sacs that separate the stroma from other compartments (place where chlorophyll resides) • grana - stack of thalakoids • CO2 enters through holes in the leaf's surface called stomata • veins carry water to the leaves and export sugar to various parts

  3. The Light Reaction • The light reaction of photosynthesis takes advantage of the photoelectric effect (ability of photons to exert inertia on electrons) and the photosynthetic pigment chlorophyll. There are 3 main pigments (2 types of chlorophyll) used in plants. The action spectrum describes which wavelength of light works best for each (studied by looking at CO2 uptake and O2 production). • Chlorophyll a • blue-green in color • absorbs violet and red best • reflects blue & green • Chlorophyll b • yellow-green in color • absorbs blue and red best • reflects yellow & green • Carotenoids • various shades of red, orange, and yellow • called accessory pigments • absorbs violet & blue-green

  4. Photosystems • Photosystems - complexes containing the pigments where light energy is harvested • When a photon is absorbed by a pigment it causes an e- to move from its ground state to an excited state. The electron (because it is unstable in its new configuration) immediately returns to its ground state releasing the stored energy. The energy is then harvested in a reaction center where it is transferred to an electron acceptor. • photon energy is transferred from pigment to pigment (light harvesting complex) until it reaches the reaction center • the reaction center (composed of a chlorophyll a molecule & a primary electron acceptor) • chlorophyll a boosts its e- to an excited state where it is oxidized as the primary electron acceptor removes it • this is the 1st step in changing light energy to chemical energy (glucose)

  5. The thylakoid membranes are populated by two types of photosystems (PS I & PS II). The two work together together to produce ATP & NADPH (8 steps in the non-cyclic flow of electrons) • A photon strikes a pigment in PS II & is transferred until in reaches P680 in the reaction center. • The excited electron is captured by an electron acceptor. • Water is split • 2 H+ are produced (reduced) • 2 e- are given back to the P680 • O2 is produced from the oxidized O • The excited e- is passed through an electron transport chain (ETC) to PS I • plastoquinine (Pq) g cytochrome complex gplastocyanin (Pc) • The exergonic "fall" of electrons provides energy for the synthesis of ATP (via proton-motive force & ATP synthase) • PS I absorbs a photon and donates an e- from P700 to its electron acceptor • 2 electrons from PS II fill in the space (electron hole) • Electrons captured by P700 are transfered down a 2nd ETC through the protein ferredoxin (Fd) • Enzyme NADP+ reductase transfers the 2 e- to NADP+ to NADPH (requires 2 e-) • NADPH is then transferred to the Calvin Cycle • Occasionally the system may skip PSII and operate under cyclic flow. In cyclic flow, ATP is generated but no NADPH is produced for transfer into the Calvin Cycle

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