12) Photosynthesis summary 9744 2018

Prepared by: Mrs Selvamani Nair (2018) Raffles Institution (Yr 5-6) 1 Photosynthesis ( 6CO2 + 6H2O  C6H12O6 + 6O2) Light Dependent Reactions (on thylakoid membrane) * Primary pigment: special chl a mlc, P680 & P700. P680 is found in the reaction centre (RC) of Photosystem II (PSII) & P700 in Photosystem I (PS I) * Accessory pigments: other chl a, chl b mlcs & carotenoids (fd in the light harvesting complex (LHC) Non-cyclic photophosphorylation * When a photon of light is absorbed by an accessory pigment molecule in the light harvesting com plex ( LHC) of PS II, one of its electrons is excited to a higher energy level. As the excited electron drops to its ground state, the energy released is passed on to the next pigment molecule. This resonance transfer of energy continues until P680, the special chlorophyll a molecule in the reaction centre (RC) is reached. * When P680 absorbs the energy from the accessory pigments of the light harvesting apparatus , it loses an electron, leaving an electron hole in PSII. The displaced electron is accepted by a primary electron acceptor (X) in the reaction centre. * The electron hole in PSII is filled by an electron released from the splitting of water in an enzyme-catalysed reaction in the thylakoid space. During the splitting of water, the H + released contributes to a high concentration of H + in the thylakoid space while the O atom combines with another O atom, forming molecular oxygen (O2) as a by-product * The electron from the primary e - acceptor (X) is then passed down a series of increasingly electr onegative electron carriers (of the 1st ETC) losing energy during the transfer. The energy lost during this electron flow is used to actively pump H + from the stroma to the thylakoid space , generating a proton gradient across the membrane. Chemiosmosis occurs when H+ diffuse down the proton gradient back into the stroma via ATP synthase, & ADP is phosphorylated to ATP. * Meanwhile, PSI loses an electron in a manner similar to PSII . When P700 absorbs the energy from the accessory pigments in the light har vesting apparatus, it loses an electron, leaving an electron hole in PSI. The displaced electron is accepted by a primary electron acceptor (Y) in the reaction centre. The electron hole in PSI is filled by the displaced electron from PSII when it reaches the end of the first electron transport chain. * The electron from the primary electron acceptor (Y) is then is passed down a series of electron carriers of a 2nd ETC. (Energy is not released during electron transfer down this 2 nd ETC.). The electron is finally accepted by NADP (the final electron acceptor) which is reduced to NADPH (NADP+e- +H+NADPH) by NADP reductase which is found on the thylakoid membrane. * The ATP & NADPH produced during non-cyclic photophosphorylation will be used in the Calvin cycle. Cyclic photophosphorylation * In cyclic photophosphorylation, electrons displaced from P700 of PSI & accepted by the primary electron acceptor Y are transferred to the middle of the 1st ETC. The electron is transported down the ETC & is finally recycled back to PSI. * Energy lost during electron transfer is coupled to the formation of ATP in a manner similar to non-cyclic photophosphorylation. * Only PSI is involved & only ATP is produced during cyclic photophosphorylation. NADPH is not produced. The ATP produced is used in the Calvin cycle. Non-cyclic photophosphorylation: Cyclic photophosphorylation: X Y

Prepared by: Mrs Selvamani Nair (2018) Raffles Institution (Yr 5-6) 2 Light Independent Reactions / Calvin Cycle (in stroma) Some terms: Phosphorylation = addition of a phosphate group to a molecule [eg: ADP + Pi (inorganic phosphate) ATP] Photophosphorylation = formation of ATP from ADP + Pi using light energy in photosynthesis Non-cyclic photophosphorylation = Electrons obtained from PS II  Primary electron acceptor (X) electron transport chain  PSI  Primary electron acceptor (Y)  electron transport chain NADP. Electron from the photolysis of water replaces the electron lost form PSII. Cyclic photophosphorylation = Electrons that are raised to a higher energy level are lost from PSI, but are recycled back to PSI through the 1st electron transport chain. Together cyclic & non-cyclic photophosphorylation produce sufficient ATP & NADPH to drive the Calvin cycle. Chemiosmosis: an energy coupling mechanism that uses energy stored in a proton gradient across a membrane to synthesise ATP. Photoactivation: When a chlorophyll molecule absorbs light, the energy from this light raises one of its electrons to a higher energy level. That chlorophyll molecule is said to be photoactivated. Resonance transfer: When a chlorophyll molecule absorbs light, the energy from light raises one of its electrons to a higher energy level. When the excited electron returns to its ground state, the energy released is transferred to another pigment molecule. This is called resonance transfer. * Substances required from light reaction: NADPH & ATP * Carbon fixation: CO2 combines with RuBP (5C) in the presence of the enzyme ribulose bisphosphate carboxylase (Rubisco) to form an unstable 6C compound which breaks down into 2 molecules of GP/PGA (3C) * Reduction and sugar formation: GP is reduced to G3P/TP/PGAL(3C). ATP and NADPH are needed for the reaction. NADPH provides the reducing power for the reaction * Regeneration of RUBP: G3P molecules can either be converted to sugars and then polymerized to starch or enter a series of reactions driven by ATP to regenerate RuBP to allow CO2 fixation to continue. * C & O atoms of sugar (C6H12O6) come from CO2 & H atoms come from NADPH * Products of light independent reaction : 1) G3P (a triose sugar) 2) NADP & ADP (which are recycled to the light reactions) (Note: GP: Glycerate-3-phosphate / Glycerate phosphate; G3P: Glyceraldehyde-3-phosphate; TP: Triose phosphate) S: Light saturation point: Light intensity beyond which an increase in light intensity will not increase the rate of photosynthesis Limiting factor: Any environmental factor that - by its decrease or increase, absence or presence - alters the growth, metabolic processes or distribution of organisms and populations most significantly. If you increase a particular variable and there continues to be a proportional relationship between the values on the x & y axes, it is referred to as the only limiting factor. At P: light intensity is a limiting factor (note linear relationship between x and y values) At Q: light intensity is not the only limiting factor. Some other factor is also limiting. (eg: CO2 concentration)