9.Luz y reacciones luz.pdf

Light and the Light Reactions Chapter 7 1 THE BASICS OF PHOTOSYNTHESIS • Almost all plants are photosynthetic autotrophs, as are some bacteria and protists – Autotrophs generate their own organic matter through photosynthesis – Sunlight energy is transformed to energy stored in the form of chemic...

Light and the Light Reactions Chapter 7 1 THE BASICS OF PHOTOSYNTHESIS • Almost all plants are photosynthetic autotrophs, as are some bacteria and protists – Autotrophs generate their own organic matter through photosynthesis – Sunlight energy is transformed to energy stored in the form of chemical bonds (c) Euglena (b) Kelp (a) Mosses, ferns, and flowering plants (d) Cyanobacteria Photosynthesis: 6 CO2 + 6 H2O carbon dioxide + water C6 H12 O6 + 6 O2 = sugar + oxygen photosynthetic products often stored as starch •Starch = glucose polymer Tracking atoms STARCH THE SUN: MAIN SOURCE OF ENERGY FOR LIFE ON EARTH LIGHT LIGHT • Both!! – Waves with wavelength (λ) & frequency (ν) c = λv – Particles: photons Quantum -> amount of energy. Discrete packets E = hv (h: Planck´s const) Wavelength: the distance between trough and trough, or peak and peak. Wavelength is thus measured in units of length, (e.g. m, nm, km) Frequency: the number of cycles per second (units = Herz). As wavelength decreases, frequency increases. a Why is just visible light used in photosynthesis? • Infra-red isn’t high enough in energy to build bonds between atoms (e.g. NADP+H = NADPH; ADP + Pi = ATP) • High-energy radiation (e.g. UV) is too dangerous; can mutate DNA and form free radicals. Thus, these wavelengths are largely reflected and/or attenuated by the waxy cuticle and phenolics in the upper epidermis to protect subjacent cells. Cuticle Epidermis Why is just visible light used in photosynthesis? • Visible light has enough energy to build bonds, but is also safe to absorb. More energy Less energy Visible light is electromagnetic energy that falls between 400-700nm in wavelength WHY ARE PLANTS GREEN? Different wavelengths of visible light are seen by the human eye as different colors. The feathers of male cardinals are loaded with carotenoid pigments. These pigments absorb some wavelengths of light and reflect others. R ligh d e t eflec t Sunlight minus absorbed wavelengths or colors equals the apparent color of an object. THE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED Why are plants green? ted c fle e R h lig t Transmitted light WHY ARE PLANTS GREEN? Light Reflected light The thylakoid membrane of the chloroplast is impregnated with photosynthetic pigments (i.e., chlorophylls, carotenoids). Absorbed light Plant Cells have Green Chloroplasts Transmitted light Chloroplast Chloroplasts: Sites of Photosynthesis • Photosynthesis Chloroplast LEAF CROSS SECTION MESOPHYLL CELL – Occurs in chloroplasts – All green plant parts have chloroplasts and carry out photosynthesis • The leaves have the most chloroplasts • The green color comes from chlorophyll in the chloroplasts • The pigments absorb light energy LEAF Mesophyll CHLOROPLAST Intermembrane space Outer membrane Granum Grana Stroma Stroma Inner membrane Thylakoid Thylakoid compartment Chloroplast Pigments • Chloroplasts contain several pigments – Chlorophyll a – Chlorophyll b – Carotenoids: xanthophylls and beta-carotene chromophore Chlorophyll Structure Porphyrin ring: contains N and a Mg Hydrophobic tail: attaches to hydrophobic proteins in thylakoid membrane Hemoglobine! Leaf pigments } Chlorophylls Xanthophylls Photopigments B-carotene Anthocyanin Tannins Chloroplast Cyanobacteria & red algae (red) Leaf pigments Chlorophylls Xanthophylls Other pigments B-carotene Vacuole Chloroplasts Anthocyanin Tannins The wavelengths absorbed by a molecule correspond with the excitation energy taken in to boost e- to a higher energy state; wavelengths not absorbed are reflected or transmitted. Ionizing radiation has enough energy to excite e- so much that they are ejected (“ionized”). This is bad because it results in production of free radicals (molecules with unpaired e-, which are highly reactive). Absorption of energy can excite e- to multiple different higher energy states (higher energy orbitals), depending on the energy contained in those wavelengths Absorption of blue and red light by chlorophyll causes excitation of an e- in the S0 orbital to either the S2 (blue light) or S1 (red light) orbital. Chl alternative pathways 1. Chl can re-emit photon and return to its ground state => fluorescence. Longer wavelength: has heat therefore in red spectrum 2. Excited chl returns to ground state by excitation energy -> heat No photon emitted 3. Chl transfers energy to another molecule 4. Photochemistry: energy of excited state causes chemical reactions to occur Among the fastest known chemical reactions Energy of electron e– Excited state Heat Photon (fluorescence) Photon Chlorophyll molecule Ground state (a) Excitation of isolated chlorophyll molecule (b) Fluorescence Different pigments absorb light differently 1. 2. 3. 4. 5. Bacteriochl Chl a Chl b Phycoerythrobilin ß-carotene Still drives photosynthesis! How? MOST ACTIVE TISSUE Photopigments are embedded in the thylakoid membranes stroma (fluid) & grana (stacks of thylakoids) Photopigments are held in place in the thylakoid membrane by light harvesting complex proteins (beadlike structure shown here). 27 Organelles (including mitochondria and chloroplasts) move around to maximize absorption of light, nutrients, and gases. Motor proteins, incl. kinesin and myosin, allow movement of organelles or vesicles along substrates (e.g. microtubules in the cell) Energy transfer is very efficient 95-99% of photons absorbed à transferred to reaction center What is the advantage of the Antenna complex? 1. Single chl molecules can’t absorb all the photons. There are too many coming at the same time! 2. Photosynthetic system becomes saturated with light 3. There’s a problem with excited e- TWO PHOTOCHEMICAL COMPLEXES Photosystem I Absorbs red light (680 nm) Very strong oxidant à can oxidize H2O Produces weak reductant Absorbs far-redPlight (700 nm) Strong reductant àreduces NADP+ Produces weak oxidant There’s an excess of PSII in chloroplasts Ratio 1.5:1 (PSII to PSI): PSII is more prone to photodamage ~10 photons to produce 1 molecule O2. Photochemical quantum yield ~100% but energy efficiency is much less (~27%) How to explain the discrepancy? Almost all absorbed photons engage in photochemistry but only ~1/4 of the energy in each photon is stored. Remainder: heat About ½ of the solar spectrum is absorbed by the plant Overall energy conversion efficiency into biomass à 4.3% C3, 6% C4 Light absorbed by carotenoids FAR-RED > 680 nm Light is less efficient AN OVERVIEW OF PHOTOSYNTHESIS • The light reactions convert solar energy to chemical energy Light Chloroplast NADP+ ADP +P – Produce ATP & NADPH • The Calvin cycle makes sugar from CO2 – ATP generated by the light reactions provides the energy for sugar synthesis – The NADPH produced by the light reactions provides the electrons for the reduction of CO2 to glucose Light reactions Calvin cycle Introduction to the main players e- derived from H2O in the Oxygen Evolving Complex; Used to reduce P680 Stroma e- Lumen Photosystem Photon Thylakoid membrane Light-harvesting Reaction-center complex complexes STROMA Primary electron acceptor e– Transfer of energy Special pair of chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) The electrons that run through the electron transport chain are originally derived from H2O Each H2O donates 2 e- to P680. After two H2O have been split, the remaining oxygen atoms are combined to create one molecule of molecular oxygen (O2) Stroma e- Lumen Stroma e- Lumen Stroma e- Lumen Stroma H+ H+ e- Lumen Stroma H+ H+ e- Lumen Two pathways 1. Transfers to PC 2. Cyclic: picks up more protons from the stroma (4 H+) Stroma eH+ H+ Lumen Stroma e- H+ H+ Lumen Stroma e- H+ H+ Lumen Stroma e- H+ H+ Lumen Stroma e- H+ H+ Lumen Ferredoxin: reduces NADP+ to NADPH Also: reduces nitrate, regulates Cfixation enzymes Iron-sulfur proteins Stroma NADPH H+ H+ Lumen Stroma H+ H+ H+ H+ H+ Lumen H+ Stroma H+ H+ H+ H+ H+ Lumen H+ Stroma H+ H+ H+ H+ H+ Lumen H+ PHOTOPHOSPHORYLATION ATP Synthesis dependent of light e– ATP e– e– NADPH e– Mill makes ATP e– n Photo e– Photon e– Photosystem II Photosystem I Again! Stroma e- Lumen Stroma e- Lumen Stroma e- Lumen Stroma e- Lumen Stroma H+ H+ e- Lumen Stroma H+ H+ e- Lumen PC (plastocyanin) can move around! Transport products between PS Stroma eH+ H+ Lumen Stroma e- H+ H+ Lumen Stroma e- H+ H+ Lumen Stroma e- H+ H+ Lumen Stroma e- H+ H+ Lumen Stroma e- H+ H+ Lumen Stroma NADPH H+ H+ Lumen Stroma H+ H+ H+ H+ H+ Lumen H+ Stroma H+ H+ H+ H+ H+ Lumen H+ Stroma H+ H+ H+ H+ H+ Lumen H+ Stroma H+ H+ H+ H+ H+ Lumen H+ STROMA (low H+ concentration) Cytochrome Photosystem I complex Light Photosystem II 4 H+ Light Fd NADP+ reductase H2 O THYLAKOID SPACE (high H+ concentration) 1 e– Pc 2 1/ 2 NADP+ + H+ NADPH Pq e– 3 O2 +2 H+ 4 H+ To Calvin Cycle Thylakoid membrane STROMA (low H+ concentration) ATP synthase ADP + Pi ATP H+ Because the Calvin cycle occur in the stroma, the ATP synthases and Photosystem I are positioned on the OUTER surfaces of the grana, while Photosystem II and cytochromes are on the INTERIOR surfaces. Summary of Light Reactions • Their goal: take energy from the sun to convert ADP and NADP to ATP and NADPH through: – Oxidation/reduction reactions (NADPH) – Chemiosmosis (ATP) • Those molecules then go into the stroma for the Calvin Cycle (where glucose is made) REPAIR & REGULATION • Light energy can produce toxic reactants: superoxide, singlet oxygen, peroxide à VERY reactive, damages lipids/DNA especially carotenoids – Essential role in photoprotection. Vent excessive energy – Quench excited state of chl if PS are saturated Non-photochemical quenching: quenching of chl fluorescence by processes different from photochemistry à HEAT! Reversible in early stages, but if prolonged à PSII is disassembled and repaired. Main target: D1 Unbalanced photosynthesis High energy compounds Photo Bright light produces lots of energy Real time Synthesis Low-capacity biochemical pathways use it slowly Potential for destruction