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Chapter 7

BIOL 1000 CHAPTER 7 textbook notes

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BIOL 1000
Nicole Nivillac

Chapter 7 – Photosynthesis - Photosynthetic organisms are photoautotroughs and are primary producers of Earth The Two Parts of Photosynthesis - 2 stages: light-dependent reactions (sometimes called the light reactions) and the light- independent reactions (called the Calvin Cycle) - Light reactions involve the capture of light energy by pigment molecules and the utilization of that energy to synthesize both NADPH and ATP - ^ This involves an electron transport chain that in plants, algae, and cyanobacteria utilizes electrons donated from water - In the Calvin cycle, the electrons carried by NADPH and the energy of ATP are used to convert CO2 from inorganic to organic form called CO2 fixation - Reduction is a process in which electrons are added to CO2 - CO2 is converted to a carb in the ratio: 1C:2H:1O - Equation for photosynthesis: 6CO2 + 12 H2O  C6H12O6 + 6O2 + 6H2O Eukaryotes: Photosynthesis Takes Place in Chloroplasts - Light reactions and the Calvin cycle take place within the chloroplast - Chloroplast consists of outer membrane, inner membrane, and an intermembrane (in between the other 2 membranes) - The aqueous environment within the inner membrane is the stroma - Within the stroma is the third membrane system, the thylakoid membranes (or thylakoids) - ^ flattened sacs and the space enclosed by a thylakoid is called the thylakoid lumen - The enzymes that catalyze the reactions of the Calvin Cycle are found in the stroma of the chloroplast - Organisms without mitochondria may still undergo cellular respiration, cells lacking chloroplasts may still be photosynthetic 7.2 – The Photosynthetic Apparatus Pigment Molecules Absorb Light Energy 1. The absorption of a photon by a pigment molecule excites a single electron, moving it from the ground state to an excited state 2. The difference in energy level between the ground state and the excited state is equivalent to the energy of the photon of light that was absorbed. If the energies do not match, the photon is not absorbed by the pigment - In the first mechanism, the excited electron may simply return to its ground state, releasing its energy either as heat or as florescence - The energy of the excited electron can be transferred to a neighbouring pigment molecule, a process called inductive resonance - This requires 2 molecules to be closely aligned with one another - The excited-state electron may itself be transferred to a nearby electron-accepting molecule - In Photosynthesis, this molecule is called the primary acceptor Chlorophylls and Carotenoids - Chlorophylls are the major photosynthetic pigments in plants, green algae, and cyanobacteria - Dominant types are chlorophyll a and b which are just structurally different - The second major group of pigments involved in photosynthesis is the carotenoids - During photosynthesis it is only chlorophyll a that becomes oxidized and donates an electron to the primary electron acceptor - Carotenoids and chlorophyll b are referred to as accessory pigments because after light absorption they donate excitation energy by indiuctae resonance to molecules of chlorophyll a - Photosynthesis is dependent on the absorption of light by chlorophylls and carotenoids - Action spectrum: is a plot of the effectiveness of light of particular wavelengths in driving photosynthesis Photosynthetic Pigments into Photosystems - Pigment molecules are bound to difference proteins - These pigment-proteins are organized into photosystems - Each photosystem is composed of a large antenna complex of pigment-proteins that surrounds a central reaction centre - The reaction centre of a photosystem comprises a small number of proteins that bind a pair of specialized chlorophyll a molecules as well as the primary electron acceptor - 2 kinds of photosystems: photosystem I and photosystem II (PSI and PSII) - Chlorophyll a of photosystem I is called P700 because it absorbs light optimally at a wavelength of 700 nm and photosystem II of chlorophyll a is called P680 - The function of a photosystem is to trap photons of light and use the energy to oxidize a reaction centre chlorophyll, with the electron being transferred to the primary electron acceptor 7.3 – Photosynthetic Electron Transport - Photosystem I and photosystem II are the 2 major light-trapping components involved in photosynthetic electron transport in most photoautotrophs The Structure and Function of Photosystem II 1. The absorption of photons by the antenna complex and funnelling of energy to the reaction centre results in an electron within P680 being raised from the ground state to an excited state. (becomes P680*) 2. Once in the excited state, P680* can be easily oxidized to P680+ by the primary electron acceptor of photosystem II, a molecule called pheophytin (pheo), which initiates electron transport by donation to plastoquinone (PQ) which is analogous to ubiquinone of respiratory electron transport 3. P680 is re-formed by P680+ gaining an electron by oxidizing water which is the strongest oxidant Linear Electron Transport 1. Oxidation of P680 – Absorption of light energy by photosystem II results in the formation of excited state P680 (P680*). This molecule is rapidly oxidized by the primary electron acceptor pheophytin 2. Oxidation-Reduction of the Plastoquinone pool – From the primary acceptor, the electrons transfer to plastoquinone (PQ), which migrates through the lipid bilayer and acts as an electron transfer link between PSII and the cytochrome complex. - Plastoquinone is analogous in structure and fu
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