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    PHOTOSYNTHESIS

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    1 : 1 PHOTOSYNTHESIS Light-dependent Reactions (Light Reactions) Use light energy to split water and use electrons to reduce NADP+ and make ATP- produce ATP, NADPH & O2 Light-independent reactions (Dark Reactions) (Calvin Cycle) Use ATP & NADPH to convert CO2 to sugars The light reactions take place within the membranes of the Thylakoids, whilst the reactions of the Calvin Cycle are catalysed by enzymes in the Stroma
    2 : 2 PHOTOSYNTHETIC ELECTRON TRANSPORT CHAIN Water PSII Plastoquinone Cytochrome b-f complex Plastocyanin PSI Ferredoxin NADP+ (ATP SYNTHASE)
    3 : 3 PHOTOSYSTEM II (PSII) Concentrated in the grana thylakoids Consists of a complex of about ten polypeptides embedded in the thylakoid membranes Consists of: a reaction centre a water-splitting unit light-harvesting complexes (LHCII) The reaction centre is called P680
    4 : 4 Reaction Centre &Water Splitting The 'core' of PSII consists of about six polypeptides. These include: D1 and D2 (each ca 32 kDa) to which quinones, Mn ions and P680 reaction centre 33 kDa, 17 kDa & 23 kDa proteins involved in splitting of water 43 and 47 kDa proteins which contain chlorophyll which also serve as antenna molecules. Each P680 molecule receives the energy absorbed by about 250 chlorophyll molecules The primary acceptor of PS II is pheophytin This is a chlorophyll molecule without an Mg atom
    5 : 5 Water Splitting Complex Responsible for the water splitting reaction 2H2O ---> O2 + 4H+ + 4e- Poorly understood - both Mn II and Cl- are required Synthetic electron donors can substitute for water Contains 4 Mn atoms, which collect electrons from water and pass them on to P680 The 33 kDa, 17 kDa & 23 kDa proteins and a tyrosine residue on the D1 protein are involved in this process. The primary acceptor for PS II, to which an electron is transferred from P680 is pheophytin.
    6 : 6 Water Splitting Complex Electron is transferred to a molecule of plastoquinone (PQ) permanently bound to D2 The electron from this molecule is passed to another plastoquinone molecule bound to D1. This molecule remains bound until it has accepted a second electron to fully reduce it - then released. By this means the two electrons needed for the reduction of PQ can be supplied, one at a time, by pheophytin.
    7 : 7 Light Harvesting Complex II (LHCII) This consists of: a polypeptide (ca 26 kDa) approx equal numbers of chlorophyll a chlorophyll b molecules a few carotenoids.
    8 : 8 CYTOCHROME b-f COMPLEX Consists of a complex of four subunits: cytochrome f (33 kDa) cytochrome b552 (b6) - 23 kDa, containing 2 haems Iron-sulphur protein (20 kDa) another 17 kDa chain. Pumps protons across the membrane as electrons flow from plastoquinone (PQ) to plastocyanin (PC). Oxidation of PQ releases 2 electrons, whilst PC can accept only one electron at a time. Cytochrome b-f complex accepts two electrons from PQ and hands them on one at a time to PC.
    9 : 9 PLASTOCYANIN (PC) This is a blue 11 kDa copper-containing protein. The copper atom co-ordinates with the S atoms of a methionine and cysteine residue and with 2 histidine residues and can be oxidised and reduced. Passes electrons to PSI
    10 : 10 PHOTOSYSTEM I (PSI) The units of PSI are found only in the stroma thylakoids and non-appressed regions of the grana that face the stroma PSI is a trans-membrane complex consisting of at least 13 polypeptides. Consists of: a core complex a light-harvesting complex (LHCI), Reaction centre of PSI is a chlorophyll a molecule (or possibly a pair of chlorophyll a molecules) in a special environment and is called P700
    11 : 11 The core complex Main components of the core complex are two nearly identical proteins psaA (83 kDa); psaB (82 kDa) Also contains: P700 reaction centre a quinone an Iron-sulphur complex chlorophyll a ?-carotene Electron is transferred from P700 to A0 - v strong reducing agent - chl a in special environment From A0 transferred to a quinone, to an Fe-S centre & then to ferredoxin on the stroma side of the membrane.
    12 : 12 Light Harvesting Complex I (LHCI) light-harvesting antenna complex which surrounds the core contains 80 molecules of chlorophyll a and 20 of chlorophyll b. absorbs light and transfers energy to the reaction centre (P700)
    13 : 13 REDUCTION OF NADP+ Ferredoxin is a red protein of low molecular weight (approx. 10.7 kDa). It is a non-haem iron protein, in which the Fe is associated with sulphur. Spinach ferredoxin has 2 Fe atoms bound to 2 S atoms. One of the Fe atoms undergoes Fe III----> Fe II changes Reduces NADP+ to NADPH under the influence of ferredoxin-NADP+ reductase. During this last reaction protons are released on the stroma side of the membrane.
    14 : 14 ATP SYNTHESIS As electrons pass through the Electron Transport Chain, protons are pumped from the Stroma to the Thylakoid Space. Generates a Proton Gradient. This is used to make ATP as electrons pass back through ATP synthase complex
    15 : 15 THE DARK REACTIONS (CALVIN CYCLE) Light reactions release NADPH and ATP into the chloroplast stroma. These compounds are used to convert CO2 into sugars in Calvin Cycle. Calvin cycle enzymes are in the stroma or on the stromal side of the thylakoid membranes. The first step is catalysed by the enzyme RUBP carboxylase/oxygenase (Rubisco) located on the stromal surface of the thylakoid membranes.
    16 : 16 THE DARK REACTIONS (CALVIN CYCLE) Rubisco catalyses the reaction of ribulose-1,5-bisphosphate (5C) with CO2 to form an unstable, six carbon compound, which rapidly breaks down to form two molecules of 3-phosphoglycerate. The enzyme also catalyses reaction with O2 instead of CO2. This leads to PHOTORESPIRATION which reduces photosynthetic efficiency. The next step uses ATP to convert 3-phosphoglycerate to 1,3-diphosphoglycerate.
    17 : 17 THE DARK REACTIONS (CALVIN CYCLE) 1,3-diphosphoglycerate is then reduced to glyceraldehyde-3-phosphate using NADPH. This reaction is catalysed by NADP+-specific glyceraldehyde-3-phosphate dehydrogenase Practical Class Most of the glyceraldehyde-3-phosphate is used to regenerate ribulose-1,5-bisphosphate These reactions are catalysed by the enzymes transaldolase and transketolase. Steps are virtually identical to parts of the pentose phosphate pathway.
    18 : 18 THE DARK REACTIONS (CALVIN CYCLE) Six molecules of ribulose-1,5-bisphosphate (6 x 5C) and 6 molecules of carbon dioxide (6 x 1C) give rise to 12 molecules of glyceraldehyde-3-phosphate (12 x 3C). 10 molecules of glyceraldehyde-3-phosphate (10 x 3) are used to re-form 6 molecules of ribulose-1,5-bisphosphate (6 x 5). Two molecules of glyceraldehyde-3-phosphate are left over. These represent the net output of the cycle.
    19 : 19 THE DARK REACTIONS (CALVIN CYCLE) The sugars produced are either converted into starch within the chloroplasts or exported as triose phosphate (glyceraldehyde-3-phosphate), into the cytoplasm. Starch accumulates in the light - temporary store of carbon during times of rapid carbon fixation. The inner membrane of the chloroplast is practically impermeable to polar molecules such as free sugars, Pi, and triose phosphates. Specific carrier facilitates the counter-transport of triose phosphates and Pi.
    20 : 20 THE DARK REACTIONS (CALVIN CYCLE) Pi is required for the Calvin cycle enters the chloroplasts as triose phosphates move into the cytoplasm. Some glyceraldehyde-3-phosphate is converted to dihydroxyacetone phosphate. These react together to form fructose-1,6,-bisphosphate, from which glucose, starch and other sugars are formed. Overall equation for operation of the Calvin cycle is: 6CO2 + 12NADPH + 12H+ + 18ATP + 11H2O ----> fructose-6-phosphate + 12NADP+ + 18ADP + 17Pi
    21 : 21 CONTROL OF PHOTOSYNTHESIS Light increases rate of photosynthesis. Increases availability of ATP, NADPH from light reactions. Stimulates development of chloroplasts Changes activity of Calvin Cycle enzymes. Five Calvin cycle enzymes are activated by light. Rubisco glyceraldehyde 3-phosphate dehydrogenase* fructose bisphosphatase* sedoheptulose bisphosphatase* phosphoribulokinase*
    22 : 22 CONTROL OF PHOTOSYNTHESIS Due to changes in pH resulting from proton movement from stroma into thylakoid lumen Also due to action of 'light effect mediators'. proteins containing cysteine residues that can exist either in the -SH (reduced) form or in the oxidised -S-S- form. Best known of these is called THIOREDOXIN. In the light, reduced ferredoxin converts thioredoxin into its reduced (-SH) form. This reduces -S-S- bonds in enzymes marked * on previous slide, activating them. May also inhibit enzymes involved in sugar breakdown.
    23 : 23 PHOTOSYNTHESIS IN C4 PLANTS Tropical grasses use the Hatch and Slack or C4 pathway for photosynthesis. Initial products of CO2 fixation are four-carbon compounds, hence the name. Plants using the Calvin cycle are called C3 plants as 3 carbon compounds are formed first. The C4 pathway involves the co-operation of two types of cell - mesophyll cells and bundle sheath cells.
    24 : 24 PHOTOSYNTHESIS IN C4 PLANTS In mesophyll cells CO2 is initially fixed into 4C compounds (oxaloacetate & malate). CO2 fixation is catalysed by PEP carboxylase Practical Class. Malate is transported into the bundle sheath cells. CO2 is released and enters the Calvin cycle. This system acts as a 'pump' for CO2. PEP carboxylase has a high affinity for CO2 and none for O2. O2 is excluded from Rubisco, eliminating photorespiration.
    25 : 25 PHOTOSYNTHESIS IN C4 PLANTS The Calvin Cycle uses 2 NADPH and 3 ATP molecules for each molecule of CO2 fixed. C4 mechanism uses a further 2 high energy phosphate bonds (ATP is converted to AMP). In bright light and at high temperatures, the extra ATP requirement of the C4 pathway is less important than losses resulting from photorespiration. In many tropical crops C4 photosynthesis is favoured. In temperate conditions light is more likely to be a limiting factor so the C3 pathway is preferred.

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