UNIT 2: Metabolic Processes Chapter 5: Photosynthesis: The Energy of Life pg. 210-240 5.2: Pathways of Photosynthesis pg. 220-228 Light Dependent Reactions Photosystem II and I are the two light capturing complexes used by photoautotrophs to trap sunlight energy, transform it into ATP and NADPH. Photosystem II In photosystem II, water is split into electrons and protons and oxygen gas is released. Sunlight energy (photons) strikes the antenna complex and excites an electron in chlorophyll a molecule P680 becoming P680 +. The electron goes from ground state to excited state and then is passed onto the primary electron acceptor. The P680 + is now electronegative and requires an electron to become neutral again. The P680 + molecule will remove an electron from a water molecule, P680 + becomes P680, and water is split into a hydrogen proton and oxygen molecule. This process is enzyme facilitated in the water splitting complex inside the thylakoid membrane. The electron released by P680 is accepted by plastoquinone (PQ). Photosystem II can now start up again, accepting photons. This process occurs twice to completely split a water molecule.
Figure 2: a) The actions of photosystem II begins when a photon energizes an electron in P680, forming P680*. b) The energized chlorophyll (P680*) then transfers the high energy electron to acceptor A in the reaction centre. P680 is now positively charged. c) The positive P680* ion then oxidizes water, and the high energy electron is transferred from the reaction centre to the carrier molecule plastoquinone (PQ). This process releases both oxygen gas and protons into the lumen. Linear Electron Transport and ATP Synthesis The major steps of the Electron Transport System; 1. Oxidation P680: The absorption of light energy by photosystem II results in the formation of an excited-state P680 (P680+) molecule. This molecule is rapidly oxidized, transferring a high-energy electron to the primary acceptor. 2. Oxidation-reduction of Plastoquinone: From the primary acceptor, the electrons transfer to plastoquinone (PQ), which moves trough the lipid bilayer and acts as an electron shuttle between photosystem II and the Cytochrome complex. As plastoquinone accepts electrons from photosystem II, it also gains protons (H + ) from the stroma. When PQ donates electron to the Cytochrome complex, it also releases protons into the lumen, increasing the proton concentration there. 3. Electron transfer from Cytochrome complex and shuttling by plastocyanin: From the Cytochrome complex, electrons pass to the mobile carrier plastocyanin, which shuttles electrons from the cytochrome complex to photosystem I.
4. Oxidation-reduction of P700: When a photon of light is absorbed by photosystem I, an electron is excited and P700 + forms. The P700 + chlorophyll transfers its electron to the primary electron acceptor of photosystem I, forming P700 +. P700 + can now act as an electron acceptor and is reduced back to P700 by the oxidation of plastocyanin. 5. Electron transfer to NADP + by ferredoxin: The first electron from P700+ is transported by a short sequence of carriers within photosystem I. It is then transferred to ferredoxin, an iron-sulfur protein. The oxidation of ferredoxin results in the transfer of the electron to NAHP +, reducing it to NADPH. 6. Formation of NADPH: A second electron is transferred to NADP by another molecule of ferredoxin. This second electron and a proton (H + ) from the stroma are added to NADP from NADP + reductase to form NADPH. NADPH is now carrying two high-energy electrons. The concentration of protons in the stroma decreases as a result of this NADPH formation. Along with the movement of protons from stroma to lumen by plastoquinone and the splitting of water into protons, these three processes create a much higher proton concentration inside the lumen than outside in the stroma. ** This pathway is known as the non-cyclic pathway or linear. H2O 4 H + + 4 e - + O2 For each electron passed from through the electron transport chain 2 photons are required. One photon for each photosystem II and I. The oxidation of water to release oxygen, therefore to get 4 electrons, it requires 8 photons.
Figure 3: Electron Transport in the Thylakoid, This model of the eukaryotic thylakoid membrane illustrates the major protein and redox cofactors required for non-cyclic electron transport and ATP synthesis by Chemiosmosis. The four major protein complexes are photosystem II, the cytochrome complex, photosystem I, and ATP synthase. The blue arrow illustrates the pathway of electron transport. Chemiosmotic Synthesis of ATP In the thylakoid membrane there are three mechanisms to synthesize ATP; 1. Protons are taken into the lumen by the reduction and oxidation of plastoquinone as it moves from photosystem II to the Cytochrome complex and back again. 2. The concentration of protons inside the lumen is increased by the addition of two protons for each water molecule that split in the lumen. 3. The removal of one proton from the stroma for each NADPH molecule formed decreases the concentration of protons in the stroma outside the thylakoid. There is a higher concentration of hydrogen protons in the thylakoid lumen then in the stroma, creating a substantial proton-motive force. Hydrogen protons are unable to pass through the thylakoid membrane but are forced to leave the lumen and enter the stroma, passing through protein complexes of ATP synthase, embedded in the membrane, by a process known as Chemiosmosis. The free energy released by the protons is used to synthesize ATP.
Figure 4: 1) a proton gradient is established by the carrying of protons across the membrane by PQ, 2) by the releasing of protons into the lumen during the oxidation of water, and 3-4) by the removal of protons from the stroma during the reduction of NADP +. 5) ATP is then synthesized as protons move through the ATP synthase complex. In photosynthetic electron transport is the opposite of electron transport in cellular respiration. Photosynthesis starts with a low energy molecule of water, and ends in a high energy molecule of NADPH, created from the movement of protons because of the proton gradient. Each step of the represents an energy level change. Light energy is the input of energy that drives the production of NADPH and ATP. Figure 5: Energy Levels in the Electron Transport Chain, In the electron transport chain of the light-dependent reactions, the energy level of the electrons is increased dramatically by the light-absorbing actions of both photosystem II and I. The high energy electrons at the end of the electron transport chain ultimately produce NADPH. pg. 224
Light Independent Reactions Carbon dioxide is a low energy molecule, it is a waste molecule from cellular respiration because is has no usable free energy. Sugar molecules, such as; glucose (C 6 H 12 O 6 ) store higher levels of free energy that can be utilized by the cell for energy. Photosynthesis is an eleven step process that uses NADPH and CO 2 to produce a high energy molecule in the form of a sugar molecule. This endergonic reaction is facilitated by an enzyme during each step. The Calvin cycle is responsible for the production of G3P and eventually glucose. How the Calvin cycle Produces Carbohydrates The Calvin cycle is divided into three phases: carbon fixation, reduction and regeneration. 1. Carbon Fixation: During carbon fixation, carbon dioxide (inorganic) is transformed into glucose (organic). Carbon dioxide enters the Calvin cycle as an inorganic compound and reacts with ribulose 1-5 bisphosphate (RuBP), a 5 carbon sugar, to produce three carbon molecules of 3-phosphoglycerate. (C3 metabolism) 2. Reduction: The molecules of 3-phosphoglycerate, are phosphorylated by the hydrolysis of ATP, and oxidation of NADPH, creating glyceraldehydes-3-phosphate (G3P) molecules. One molecule of G3P is removed from the Calvin Cycle. 3. Regeneration: In this multi-step process, G3P molecules are combined and rearranged to regenerate molecules of RuBP. RuBP is reused over and over again in the Calvin Cycle.
Figure 7: The Calvin cycle reactions consist of three phases: carbon fixation (each CO2 is incorporated into a 6-C compound), reduction (G3P is produced), and regeneration (RuBP is re-formed). The sum of three turns of the cycle (three CO2 molecules) ultimately produces one molecule of a 3-carbon sugar, G3P, which can then return to participate in the light-dependent reactions. Rubisco: The Most Abundant Protein on Earth Rubisco is ribulose 1.5-bisphosphate carboxylase oxygenase; a critical enzyme that acts as a catalyst for the reduction of carbon dioxide in the Calvin cycle of photosynthesis. Ribulose-1,5-bisphosphate carboxylase oxygenase is the enzyme that catalyzes the first reaction of the Calvin cycle. It catalyzes the CO 2 fixation in all photoautotrophs, which provides the organic carbon molecule (glucose) for most of the world s organisms. A Diversity of Organic Products