PowerPoint Presentation: (Plants are cool!) Photosynthesis : How Plants Feed the World PowerPoint Presentation: Photo+ synthesis= making food with the help of light Light: A Review?: Light: A Review? Visible light is a part of the electromagnetic spectrum Humans can see light between 380 and 750 nm Shorter wavelengths have more energy Light is also particles of energy: Light is also particles of energy A metal exposed to a particular wavelength becomes positively charged , because the light hitting it dislodges electrons This is the photoeletric effect The effect is independent of the light intensity ( brightness = number of photons ) More intense light will cause more electrons to be dislodged , but not at a higher velocity For some metals , the critcal wavelength is within the visible spectrum Pigments: Pigments A pigment is a substance that absorbs light Many substances absorbs light of certain wavelengths and transmit others . This is the pigment’s absorption spectrum What is the absorption spectrum of chlorophyll ? Pigments: Pigments Pigment molecules absorb photons of light Electrons in the pigment are boosted to a higher energy level ( excited state ) When they return to ground state, the energy released has 4 possible fates: Fates of excited electrons: Fates of excited electrons HEAT The energy is all dissipated as heat Fates of excited electrons: Fates of excited electrons FLUORESCENCE Energy is converted partly to heat and released mostly as a photon at a lower energy level than the excited one. Fates of excited electrons: Fates of excited electrons RESONANCE ENERGY TRANSFER The energy is transfered from the excited pigment molecule to a nearby molecule , and the excited pigment returns to ground state. Fates of excited electrons: Fates of excited electrons ELECTRON TRANSFER The high-energy electron itself is transferred to the neighbouring molecule, starting an electron transport chain. Plant pigments: Plant pigments Chlorophyll a is the main photosynthetic pigment in green plants and cyanobacteria It is the only one able to split water to generate oxygen Chlorophyll b is an accessory pigment, that transfers its energy to chlorophyll a Plant pigments: Plant pigments Other accessory pigments are: carotenoids (in green algae and plants) carotenes and xanthophylls red, yellow and orange colours present in chloroplasts where they prevent oxidative damage to chlorophyll by light phycobilins (in cyanobacteria and red algae) Significance of photosynthesis: Significance of photosynthesis Maintain O 2 equilibrium. Provides food; directly through vegetables; indirectly meat or milk Fuel reserves such as coal, oil, peat, wood History: History In 1648 Van Helmont planted willow shoots in 200 pounds of soil. After 5 years of watering, he noticed that soil wt remained same while tree got large mass. He suggested mass came from water alone. Aristotle; Plant feeds oh humus. History: History Stephan Hales (1727); plants get nutrition from air and sunlight plays a role in it. Priestley (1772); plants might restore CO 2 produced by burning of candle. Ingenhousz (1779); green parts purify air in the presence of sunlight History: History Jean Senebir (1782); CO 2 is fixed and O 2 is liberated in the presence of sunlight. Nicolas theodore de Saussure (1804) showed wt of organic matter generated is more than CO 2 and water is also a part of photosynthesis. Mayer (1845) recognized the role of light in overall process of photosynthesis as conversion of water and CO 2 into O 2 and organic matter The balanced equation for photosynthesis is:: The balanced equation for photosynthesis is: light 6CO 2 + 12H 2 O C 6 H 12 O 6 + 6O 2 + 6H 2 O PowerPoint Presentation: Julius Sachs (1864); Photosynthesis occurs in chloroplasts and results in the synthesis of starch. Reactions of Photosynthesis: Reactions of Photosynthesis The light reactions light energy is used to form ATP and to reduce NADP + to NADPH water molecules are split and O 2 is liberated The carbon-fixation reactions (Calvin Cycle) carbon is fixed into sugars ATP and NADPH are used Photosynthetic Apparatus: Photosynthetic Apparatus CHLOROPLAST Discoid or ellipsoidal 4-6 μ in length and 1-2 μ thick. Two membranes; app. 50Å thick, made up of lipid bilayer and proteins. Thickness of two membranes including IMS is 300 Å. Photosynthetic Apparatus: Photosynthetic Apparatus Chloroplast is filled with hydrophilic matrix ; Stroma . Stroma are embedded in Grana . Each granum has a diameter of 0.25-0.8 μ and have 5-25 disk shaped grana lamella placed one above the other like the stack of coins. Lamellae are paired to for sac like structure called thylakoids Photosynthetic Apparatus: Photosynthetic Apparatus Grana lamella or thylakoid encloses a space, the loculus . The ends of disk shaped thylakoids are called margins. Thylakoids are interconnected with somewhat thinner stroma -lamellae or fret membranes. Chloroplast structure: AEBI 210 2012 Lec. 7 Chloroplast structure Photosynthetic Apparatus: Photosynthetic Apparatus Chlorohylls and other pigments are found in form of protein complexes in thylakoid memebranes of grana . These are the sites for primary phytochemical reaction. Some protein pigment complexes are also found in Stroma . These are site for dark reactions. Photosynthesis: Overview: AEBI 210 2012 Lec. 7 Photosynthesis: Overview 1. Light reactions: AEBI 210 2012 Lec. 7 1. Light reactions Chlorophylls and other pigment molecules are embedded in the thylakoid membranes in groups of 250-400 These are photosystems There are two of them: PS I and PS II Each photosystem consists of an antenna complex and a reaction center PowerPoint Presentation: AEBI 210 2012 Lec. 7 Two types of photosystems cooperate in the light reactions PowerPoint Presentation: AEBI 210 2012 Lec. 7 Accessory pigments transfer their energy through resonance energy transfer Only the 2 reaction center molecules actually transfer an electron In both Photosystems I and II: There are two photosystems: AEBI 210 2012 Lec. 7 There are two photosystems Photosystem II Reaction center molecules are P 680 Located in grana thylakoids Splits the water to yield oxygen and electrons and generates ATP Cannot generate ATP independently without Photosystem I There are two photosystems: AEBI 210 2012 Lec. 7 There are two photosystems Photosystem I Reaction center molecules are P 700 Located in stroma thylakoids and at margins of grana thylakoids Can operate independently to generate ATP Reduces NADP + to NADPH Light reactions: flow of energy: AEBI 210 2012 Lec. 7 Light reactions: flow of energy light PS II PS I C-fixation (Calvin Cycle) Photosystem II: AEBI 210 2012 Lec. 7 Photosystem II Photosystem II: AEBI 210 2012 Lec. 7 Photosystem II Light energy is absorbed and passed on to the P 680 molecules in the reacton center, which lose electrons The electrons are replaced by the splitting of water (water is oxidized) 2H 2 O 4 e - + 4H + + O 2 The protons are released into the thylakoid lumen, the oxygen into the air Photosystem II: AEBI 210 2012 Lec. 7 Photosystem II The electrons generated at P 680 pass down the electron transport chain through the cytochrome b 6 /f complex to PS I At each step, protons accumulate in the thylakoid lumen The electrochemical proton gradient drives the formation of ATP by ATP synthase in the thylakoid membrane ATP accumulates in the chloroplast stroma PowerPoint Presentation: AEBI 210 2012 Lec. 7 Photosystem I: AEBI 210 2012 Lec. 7 Photosystem I Photosystem I: AEBI 210 2012 Lec. 7 Photosystem I Light energy photoexcites the antenna molecules, which pass their energy to the P 700 reaction centre The electrons are passed down an electron transport chain to the enzyme NADP reductase NADP + is reduced to NADPH NADPH accumulates in the stroma The electrons are replaced by incoming electrons from PS II PowerPoint Presentation: AEBI 210 2012 Lec. 7 ATP Synthesis: AEBI 210 2012 Lec. 7 ATP Synthesis ATP is made from ADP and P i by the ATP synthase complex, embedded in the thylakoid membrane near PS I The synthesis is driven by the H+ protons which accumulate in the thylakoid lumen As they move down an electrochemical gradient and through the membrane, they release energy which is used to make the ATP The ATP accumulates in the stroma Non-cyclic electron flow: AEBI 210 2012 Lec. 7 Non-cyclic electron flow Distribution of the components of photosynthesis within the chloroplast membranes: AEBI 210 2012 Lec. 7 Distribution of the components of photosynthesis within the chloroplast membranes Cyclic photophosphorylation: AEBI 210 2012 Lec. 7 Cyclic photophosphorylation PS I is capable of producing ATP on its own by cyclic photophosphorylation The electron released by P 700 passes down the electron transport chain but is shunted back to the cytochrome b 6 /f complex This causes accumulation of protons in the lumen and drives the formation of ATP, but without splitting water or liberating oxygen . Cyclic photophosphorylation: AEBI 210 2012 Lec. 7 Cyclic photophosphorylation Cyclic photophosphorylation: AEBI 210 2012 Lec. 7 Cyclic photophosphorylation Carbon fixation (Calvin cycle) requires ATP and NADPH in a ratio of 3:2 Cyclic phosphorylation allows the accumulation of the extra ATP Some bacteria carry out photosynthesis this way, without splitting water Which statement is NOT true about Photosystem II?: Which statement is NOT true about Photosystem II? Water is split to replace the missing electron The reaction centre is P 700 It is concentrated in the inner membranes of the grana The reaction centre molecule is chlorophyll a ATP is generated AEBI 210 2012 Lec. 7 Which statement is NOT true about Photosystem I?: Which statement is NOT true about Photosystem I? Water is split to replace the missing electron The reaction centre is P 700 It is concentrated in the outer membranes of the grana The reaction centre molecule is chlorophyll a NADPH is generated AEBI 210 2012 Lec. 7 The order in which the reactions of photosynthesis occur is:: The order in which the reactions of photosynthesis occur is: PS I; PS II; Calvin cycle carbon fixation; PS I; PS II PS II; PS I; carbon fixation Calvin cycle; PS II; PS I AEBI 210 2012 Lec. 7 The order of the locations for the reactions of photosynthesis are:: The order of the locations for the reactions of photosynthesis are: stroma thylakoids; grana thylakoids; stroma; grana thylakoids; stroma thylakoids; stroma stroma; grana thylakoids; stroma thylakoids AEBI 210 2012 Lec. 7 It's not that easy bein ' green… but it is essential for life on earth!