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AN OVERVIEW OF PHOTOSYNTHESIS
Autotrophs are the producers of the biosphere Plants are autotrophs, producing their own food and sustaining themselves without eating other organisms
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Plants, algae, and some bacteria are photoautotrophs Producers of food consumed by virtually all organisms
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Plants produce O2 gas by splitting water
The O2 liberated by photosynthesis Is made from the oxygen in water
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Photosynthesis uses solar energy
Photosynthesis uses solar energy To produce glucose and O2 from CO2 and H2O CO2 H2O Glucose O2 ATP ECOSYSTEM Sunlight energy Photosynthesis in chloroplasts Cellular respiration in mitochondria (for cellular work) Heat energy +
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To make sugar and oxygen gas from carbon dioxide and water
Photosynthesis is the process by which certain organisms use light energy To make sugar and oxygen gas from carbon dioxide and water Light energy PHOTOSYNTHESIS 6 CO2 + H2O Carbon dioxide Water C6H12O6 O2 Glucose Oxygen gas
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Electrons can capture light energy
Move into a higher-energy orbit in the atom Fluorescence and phosphorescence occur when excited electrons release their energy as light energy, to return to their original energy state Can also release energy as heat {Remember that energy transformations increase entropy}
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Plants capture sunlight using pigments = molecules having electrons that absorb light energy
Chloroplast membranes hold photosystems containing pigments Chlorophyll a molecules fluoresce when their electrons absorb UV light
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Wave Model of Electromagnetic Energy
Light energy is Electromagnetic Energy Two models to describe behavior: Photon and Wave Wave Model of Electromagnetic Energy Shorter wavelengths carry higher energy light Our eyes sense part of this energy as visible light Our brains interpret the different energies as colors
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Pigments filter light waves
Wavelengths that the pigment does not absorb are passed through (reflected) The color of the pigment (what we see) are the colors of light the pigment does not absorb Chlorophyll is green because it absorbs all visible wavelengths except green
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Photosynthesis – Overall Reaction make sugar and oxygen gas
Photosynthesis – Overall Reaction make sugar and oxygen gas from carbon dioxide and water Light energy PHOTOSYNTHESIS 6 CO2 + H2O Carbon dioxide Water C6H12O6 O2 Glucose Oxygen gas
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Photosynthesis occurs in chloroplasts
In plants, photosynthesis Occurs primarily in the leaves, in the chloroplasts, which contain stroma, and stacks of thylakoids called grana Leaf Cross Section Leaf Mesophyll Cell Mesophyll Vein Stoma CO2 O2 Chloroplast Grana Stroma TEM 9,750 Granum Thylakoid space Outer membrane Inner Intermembrane LM 2,600
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Photosynthesis is a redox process, as is cellular respiration
Photosynthesis is a redox process, as is cellular respiration In photosynthesis H2O is oxidized and CO2 is reduced Reduction Oxidation 6 O2 6 H2O 6 CO2 C6H12O6
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Overview: Photosynthesis occurs in two stages linked by ATP and NADPH
Overview: Photosynthesis occurs in two stages linked by ATP and NADPH The complete process of photosynthesis consists of two linked sets of reactions The light reactions and the Calvin cycle
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The Calvin cycle assembles sugar molecules from CO2
The light reactions Convert light energy to chemical energy and produce O2 The Calvin cycle assembles sugar molecules from CO2 Using ATP and NADPH from the light reactions Light CO2 H2O Chloroplast LIGHT REACTIONS (in thylakoids) CALVIN CYCLE (in stroma) NADP+ ADP + P ATP NADPH O2 Sugar Electrons
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THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY
Visible radiation drives the light reactions Certain wavelengths of visible light, absorbed by pigments Drive the light reactions of photosynthesis Increasing energy 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1 m 103 m Gamma rays X-rays UV Infrared Micro- waves Radio Visible light 400 500 600 700 750 650 nm Wavelength (nm) Transmitted light Absorbed Reflected Light Chloroplast 380
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Chlorophyll a molecule
Each photosystem consists of Light-harvesting complexes of pigments A reaction center with a primary electron acceptor that receives excited electrons from a reaction-center chlorophyll Energy of electron Photon Excited state Heat (fluorescence) Ground state Chlorophyll molecule e– Photosystem Light-harvesting complexes Reaction center Primary electron acceptor To electron transport chain Pigment molecules Chlorophyll a molecule Transfer of energy Thylakoid membrane
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In the light reactions, electron transport chains generate ATP and NADPH Two connected photosystems absorb photons of light in pigments that transfer the energy to chlorophyll molecules in each of the photosystems {P680 and P700} Photosystem regains electrons by removing electrons from water molecules, releasing O2 gas
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Electron transport chain
The excited electrons Are passed from the primary electron acceptor to electron transport chains Thylakoid space Photon Stroma Thylakoid membrane 1 Photosystem I Photon P700 e– 5 Photosystem II e– P680 2 + NADP+ H+ NADPH 6 ATP Electron transport chain Provides energy for synthesis of by chemiosmosis 4 H2O 1 2 + 2 O2 H+ 3
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Chemiosmosis powers ATP synthesis in the light reactions
The electron transport chain Pumps H+ into the thylakoid space
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The diffusion of H+ back across the membrane through ATP synthase
Powers the phosphorylation of ADP to produce ATP (photophosphorylation) Chloroplast Stroma (low H+ concentration) Light NADP+ + H+ NADPH ATP P ADP + Thylakoid membrane H2O 1 2 O2 Photosystem II Electron transport chain Photosystem I ATP synthase Thylakoid space (high H+ concentration)
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THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS
ATP and NADPH power sugar synthesis in the Calvin cycle The Calvin cycle Occurs in the chloroplast’s stroma Consists of carbon fixation, reduction, release of G3P, and regeneration of RuBP Input CO2 ATP NADPH CALVIN CYCLE G3P Output:
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Using carbon from CO2, electrons from NADPH, and energy from ATP
The cycle constructs G3P, which is used to build glucose and other organic molecules 3 P CO2 Step Carbon fixation. An enzyme called rubisco combines CO2 with a five-carbon sugar called ribulose bisphosphate (abbreviated RuBP). The unstable product splits into two molecules of the three-carbon organic acid, 3-phosphoglyceric acid (3-PGA). For three CO2 entering, six 3-PGA result. 1 Input: In a reaction catalyzed by rubisco, CO2 is added to RuBP. 6 RuBP 3-PGA 3 G3P 6 P 2 Step Reduction. Two che- mical reactions (indicated by the two blue arrows) consume energy from six molecules of ATP and oxidize six molecules of NADPH. Six molecules of 3-PGA are reduced, producing six molecules of the energy-rich three-carbon sugar, G3P ATP ADP + NADPH NADP+ 3 3 ADP ATP 4 Step Regeneration of RuBP. A series of chemical reactions uses energy from ATP to rearrange the atoms in the five G3P molecules (15 carbons total), forming three RuBP molecules (15 carbons).These can start another turn of the cycle. 5 P G3P CALVIN CYCLE 3 Step Release of one molecule of G3P. Five of the G3Ps from step 2 remain in the cycle. The single molecule of G3P you see leaving the cycle is the net product of photosynthesis. A plant cell uses two G3P molecules to make one molecule of glucose. Output: 1 P G3P Glucose and other compounds
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PHOTOSYNTHESIS REVIEWED AND EXTENDED
Review: Photosynthesis uses light energy to make food molecules Light H2O CO2 NADP+ Photosystem II Photosystem I Electron transport chains ADP P + RUBP CALVIN CYCLE (in stroma) 3-PGA Stroma G3P NADPH ATP O2 LIGHT REACTIONS CALVIN CYCLE Sugars Cellular respiration Cellulose Starch Other organic compounds Thylakoid membranes Chloroplast
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PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH’S ATMOSPHERE CONNECTION
Photosynthesis moderates global warming Greenhouses used to grow plants Trap solar radiation, raising the temperature inside
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Average composition of the atmosphere up to an altitude of 25 km.
Gas Name Chemical Formula Percent Volume Nitrogen N2 78.08% Oxygen O2 20.95% *Water H2O 0 to 4% Argon Ar 0.93% *Carbon Dioxide CO2 0.0360% Neon Ne 0.0018% Helium He 0.0005% *Methane CH4 % Hydrogen H2 % *Nitrous Oxide N2O % *Ozone O3 % * variable gases
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Excess CO2 in the atmosphere Is contributing to global warming
Sunlight ATMOSPHERE Some heat energy escapes into space Radiant heat trapped by CO2 and other gases
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