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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 • Plants, algae, and some bacteria are photoautotrophs – Producers of food consumed by virtually all organisms Plants produce O2 gas by splitting water • The O2 liberated by photosynthesis – Is made from the oxygen in water Photosynthesis uses solar energy – To produce glucose and O2 from CO2 and H2O Sunlight energy ECOSYSTEM Photosynthesis in chloroplasts CO2 Glucose + + H2O O2 Cellular respiration in mitochondria ATP (for cellular work) Heat energy Photosynthesis is the process by which certain organisms use light energy – To make sugar and oxygen gas from carbon dioxide and water Light energy 6 CO2 + Carbon dioxide 6 H2O Water C6H12O6 PHOTOSYNTHESIS Glucose + 6 O2 Oxygen gas 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} 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 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 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 Photosynthesis – Overall Reaction – make sugar and oxygen gas – from carbon dioxide and water Light energy 6 CO2 + Carbon dioxide 6 H2O Water C6H12O6 PHOTOSYNTHESIS Glucose + 6 O2 Oxygen gas Photosynthesis occurs in chloroplasts In plants, photosynthesis – Occurs primarily in the leaves, in the chloroplasts, which contain stroma, and stacks of thylakoids called grana Mesophyll Cell Leaf Cross Section LM 2,600 Leaf Mesophyll Chloroplast Vein Stoma CO2 O2 TEM 9,750 Chloroplast Grana Stroma Stroma Granum Thylakoid Thylakoid space Outer membrane Inner membrane Intermembrane space Photosynthesis is a redox process, as is cellular respiration • In photosynthesis – H2O is oxidized and CO2 is reduced Reduction 6 CO2 + 6 H2O C6H12O6 + 6 O2 + 6 H2O Oxidation Oxidation C6H12O6 + 6 O2 6 CO2 Reduction 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 • 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 H2O CO2 Chloroplast Light NADP+ ADP +P CALVIN CYCLE (in stroma) LIGHT REACTIONS (in thylakoids) ATP NADPH O2 Sugar 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 Gamma rays X-rays 1 nm 103 nm UV 1m 106 nm Microwaves Infrared 103 m Radio waves Light Reflected light Visible light 380 400 500 600 700 750 Wavelength (nm) 650 nm Chloroplast Absorbed light Transmitted light • 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 Photosystem e– Heat Photon Photon (fluorescence) Ground state Chlorophyll molecule Photon Excited state Thylakoid membrane Energy of electron Light-harvesting Reaction center complexes Primary electron acceptor To electron transport chain e– Pigment molecules Transfer of energy Chlorophyll a molecule 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 • The excited electrons – Are passed from the primary electron acceptor to electron transport chains Photon Photon Photosystem II 1 6 Thylakoid membrane Stroma Photosystem I NADP+ + H+ e– 2 e– 4 5 P700 P680 Thylakoid space 3 H2O 1 O + 2 + 2 H 2 Electron transport chain Provides energy for synthesis of by chemiosmosis ATP NADPH Chemiosmosis powers ATP synthesis in the light reactions • The electron transport chain – Pumps H+ into the thylakoid space • 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 H+ Light ADP + P H+ NADP+ + H+ H+ NADPH H+ Thylakoid membrane H2 O 1 O +2 H+ 2 2 Photosystem II Thylakoid space (high H+ concentration) H+ H+ Electron transport chain H+ H+ H+ H+ Photosystem I H+ H+ H+ H+ ATP synthase ATP 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 Output: G3P • 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 Step 1 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. Input: 3 CO2 1 3 Step 2 Reduction. Two chemical reactions (indicated by the two blue arrows) consume energy from six molecules of ATP and oxidize six molecules of NADPH. Six molecules of 3PGA are reduced, producing six molecules of the energyrich three-carbon sugar, G3P Step 3 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. In a reaction catalyzed by rubisco, CO2 is added to RuBP. P P P 6 RuBP 3-PGA 6 ATP 3 ADP 3 6 CALVIN CYCLE ATP 4 5 ADP + 2 P 6 G3P 6 NADPH 6 NADP+ P G3P 3 Step 4 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. Output: P 1 G3P Glucose and other compounds P PHOTOSYNTHESIS REVIEWED AND EXTENDED Review: Photosynthesis uses light energy to make food molecules H2O CO2 Chloroplast Light NADP+ ADP +P RUBP Photosystem II Thylakoid membranes Electron transport chains Photosystem I CALVIN CYCLE 3-PGA (in stroma) ATP NADPH Stroma G3P Cellular respiration Cellulose O2 LIGHT REACTIONS Sugars CALVIN CYCLE Starch Other organic compounds PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH’S ATMOSPHERE CONNECTION Photosynthesis moderates global warming • Greenhouses used to grow plants – Trap solar radiation, raising the temperature inside 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 0.00017% Hydrogen H2 0.00005% N2O 0.00003% O3 0.000004% *Nitrous Oxide *Ozone * variable gases http://www.physicalgeography.net/fundamentals/7a.html • Excess CO2 in the atmosphere – Is contributing to global warming Some heat energy escapes into space Sunlight ATMOSPHERE Radiant heat trapped by CO2 and other gases