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Transcript
Photosynthesis Chapter.7 Introduction • Plants use water and atmospheric carbon dioxide to produce a simple sugar and liberate oxygen – Earth s plants produce 160 billion metric tons of sugar each year through photosynthesis, a process that converts solar energy to chemical energy – Sugar is food for humans and for animals that we consume • Scientists have suggested that plants can be used in energy plantations to create a fuel source to replace fossil fuels – This would be an excellent energy solution, because air pollution, acid precipitation, and greenhouse gases could be significantly reduced Overview of photosynthesis 6 CO2 +6 H 2O Carbon dioxide Water Light energy C6H12O6 Photosynthesis + 6 O2 Glucose Oxygen gas Autotrophs • Autotrophs are living things that are able to make their own food without using organic molecules derived from any other living thing – Autotrophs that use the energy of light to produce organic molecules are called photoautotrophs – Most plants, algae and other protists, and some prokaryotes are photoautotrophs – Producers in a food chain – cf) Heterotrophs : the consumers • The ability to photosynthesize is directly related to the structure of chloroplasts – Chloroplasts are organelles consisting of photosynthetic pigments, enzymes, and other molecules grouped together in membranes – Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria (b) Multicellular alga (a) Plants (d) Cyanobacteria (c) Unicellular protists 40 µm 10 µm (e) Purple sulfur 1 µm bacteria 5 Photosynthesis occurs in chloroplasts • Chloroplasts are the major sites of photosynthesis in green plants – Chlorophyll, an important light absorbing pigment in chloroplasts, is responsible for the green color of plants – Chlorophyll plays a central role in converting solar energy to chemical energy. • Chloroplasts are concentrated in the cells of the mesophyll, the green tissue in the interior of the leaf • Stomata are tiny pores in the leaf that allow carbon dioxide to enter and oxygen to exit • Veins in the leaf deliver water absorbed by roots • An envelope of two membranes encloses the stroma, the dense fluid within the chloroplast • A system of interconnected membranous sacs called thylakoids segregates the stroma from another compartment, the thylakoid space – Thylakoids are concentrated in stacks called grana Anatomical structure of leaf Mesophyll: Middle leaf in Greek The assimilation tissue is the primary location of photosynthesis in Plant Stomata (stoma) : gas exchange 7 Leaf cross section Chloroplasts Vein Mesophyll Stomata Chloroplast Thylakoid Stroma Granum Thylakoid space 1 µm CO2 O2 Mesophyll cell Outer membrane Intermembrane space Inner membrane 20 µm 9 1. Outer membrane 2. Intermembrane space 3. Inner membrane 4. Stroma (aqueous fluid) 7 Granum (stack of thylakoids) 8 Thylakoid 9 Starch 10 Ribosome 11 Plastidial DNA 10 Plants produce O2 gas • Scientists have known for a long time that plants produce O2, but early on they assumed it was extracted from CO2 taken into the plant – Using a heavy isotope of oxygen, 18O, they showed with tracer experiments that O2 actually comes from H2O Experiment 1 6 CO2 + 12 H2O C6H12O6 + 6 H2O + 6 O2 Not labeled Experiment 2 6 CO2 + 12 H2O C6H12O6 + 6 H2O + 6 O2 Labeled Copyright © 2009 Pearson Education, Inc. Reactants: Products: 6 CO2 C6H12O6 12 H2O 6 H 2O 6 O2 Photosynthesis is a redox process • Photosynthesis, like respiration, is a redox (oxidationreduction) process – Water molecules are split apart by oxidation, which means that they lose electrons along with hydrogen ions (H+) – Then CO2 is reduced to sugar as electrons and hydrogen ions are added to it • Recall that cellular respiration uses redox reactions to harvest the chemical energy stored in a glucose molecule – This is accomplished by oxidizing the sugar and reducing O2 to H2O – The electrons lose potential as they travel down an energy hill, the electron transport system – In contrast, the food-producing redox reactions of photosynthesis reverse the flow and involve an uphill climb Photosynthesis (Use light energy) Reduction 6 CO2 + 6 H2O C6H12O6 + 6 O2 Oxidation Cellular respiration (release chemical energy) Oxidation C6H12O6 + 6 O2 6 CO2 + 6 H2O Reduction Photosynthesis Cont d • In photosynthesis, electrons gain energy by being boosted up an energy hill – Light energy captured by chlorophyll molecules provides the boost for the electrons – As a result, light energy is converted to chemical energy, which is stored in the chemical bonds of sugar molecules 16 Light reactions • Occur in thylakoids • light energy is converted in the thylakoid membranes to chemical energy and O2 • Water is split to provide the O2 as well as electrons and proton (H+) – O2 generation as a byproduct – Transfer of H+ and electron to NADP+ reducing it to NADPH – NADPH : electron carrier, provides reducing power to the Calvin cycle • ATP generation by photo-phosphorylation Calvin cycle • occurs in the stroma of the chloroplast – It is a cyclic series of reactions that builds sugar molecules from CO2 and the products of the light reactions – CO2 is incorporated into organic compounds, a process called carbon fixation • NADPH produced by the light reactions provides the electrons for reducing carbon in the Calvin cycle – ATP from the light reactions provides chemical energy for the Calvin cycle – The Calvin cycle is often called the dark (or light-independent) reactions • No light is required during reaction • Occur during daytime due to activation of light reaction during daytime supplying it with NADPH and ATP CO2 H 2O Chloroplast Light NADP+ ADP ! P LIGHT REACTIONS CALVIN CYCLE (in stroma) (in thylakoids) ATP NADPH O2 Sugar Visible radiation • Sunlight contains energy called electromagnetic energy or radiation – Visible light is only a small part of the electromagnetic spectrum, the full range of electromagnetic wavelengths – Electromagnetic energy travels in waves, and the wavelength is the distance between the crests of two adjacent waves • Light behaves as discrete packets of energy called photons – A photon is a fixed quantity of light energy, and the shorter the wavelength, the greater the energy Electromagnetic spectrum Increasing energy 10–5 nm 10–3 nm Gamma rays X-rays 1 nm 103 nm UV 1m 106 nm Infrared Microwaves 103 m Radio waves Visible light 380 400 600 500 Wavelength (nm) 700 650 nm 750 Visible radiation for light reaction • Pigments, molecules that absorb light, are built into the thylakoid membrane – Plant pigments absorb some wavelengths of light and transmit others – We see the color of the wavelengths that are transmitted; for example, chlorophyll transmits green – Wavelengths that are not absorbed are reflected or transmitted – Leaves appear green because chlorophyll reflects and transmits green light Light Reflected light Chloroplast Thylakoid Absorbed light Transmitted light Visible radiation Cont d • Chloroplasts contain several different pigments and all absorb light of different wavelengths – The main photosynthetic pigment : Chlorophyll a absorbs blue violet and red light and reflects green – The accessory pigments : Chlorophyll b absorbs blue and orange and reflects yellow-green – The accessory pigments : The carotenoids absorb mainly blue-green light and reflect yellow and orange • Photoprotection: absorbs and dissipate excessive light energy that may damage chlorophyll or produce ROS • Phytochemicals from the plant : Antioxidants 25 Photosystems capture solar power • Pigments in chloroplasts are responsible for absorbing photons (capturing solar power), causing release of electrons – The electrons jump to a higher energy level—the excited state— where electrons are unstable – The electrons drop back down to their ground state, and, as they do, release their excess energy • The energy released could be lost as heat or light, but rather it is conserved as it is passed from one molecule to another molecule – All of the components to accomplish this are organized in thylakoid membranes in clusters called photosystems – Photosystems are light-harvesting complexes surrounding a reaction center complex – Protein with pigment molecules (chlorophyll a, b and carotenoids) Excitation of isolated chlorophyll by light. Photosystems capture solar power • The energy is passed from molecule to molecule within the photosystem – Finally it reaches the reaction center where a primary electron acceptor accepts these electrons and consequently becomes reduced – This solar-powered transfer of an electron from the reaction center pigment to the primary electron acceptor is the first step of the light reactions The structure and function of a photosystem 29 Photosystems I and II • Two types of photosystems (photosystem I and photosystem II) – Photosystem II, which functions first, – The reaction-center chlorophyll a of PS II is called P680 because its pigment absorbs light with a wavelength of 680 nm – Photosystem I, which functions next, – The reaction-center chlorophyll a of PS I is called P700 because it absorbs light with a wavelength of 700 nm • During the light reactions, light energy is transformed into the chemical energy of ATP and NADPH – To accomplish this, electrons removed from water pass from photosystem II to photosystem I and are accepted by NADP+ – The bridge between photosystems II and I is an electron transport chain that provides energy for the synthesis of ATP • NADPH, ATP, and O2 are the products of the light reactions e– ATP e– e– e– NADPH e– Mill makes ATP n Photo e– Photon e– Photosystem II Photosystem I Electron transport chain Provides energy for synthesis of by chemiosmosis Photon ATP Photosystem II Stroma NADP+ + H+ Photon Photosystem I 1 Primary acceptor Primary acceptor 2 e– e– Thylakoid membrane 4 P700 P680 Thylakoid space 3 H 2O 1 2 5 O2 + 2 H+ 6 NADPH Chemiosmosis for ATP synthesis • Interestingly, chemiosmosis is the mechanism that not only is involved in oxidative phosphorylation in mitochondria but also generates ATP in chloroplasts – ATP is generated because the electron transport chain produces a concentration gradient of hydrogen ions across a membrane • ATP synthase couples the flow of H+ to the phosphorylation of ADP – The chemiosmosis production of ATP in photosynthesis is called photo-phosphorylation Chemiosmosis for ATP synthesis Chloroplast Stroma (low H+ concentration) Light H+ Light H+ NADP+ + H+ ADP + P NADPH H+ ATP H+ Thylakoid membrane H 2O 1 2 O2 + 2 H+ H+ Photosystem II Electron transport chain Thylakoid space (high H+ concentration) H+ H+ H+ H+ H+ Photosystem I H+ H+ H+ H+ ATP synthase Chloroplast Mitochondrion CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE H+ Intermembrane space Inner membrane Matrix Diffusion Electron transport chain Thylakoid membrane ATP synthase Stroma ADP + P i Key [H+ ] Higher Lower [H+ ] Thylakoid space H+ ATP 35 STROMA (low H+ concentration) Photosystem II Light 4 H+ Cytochrome complex Photosystem I Light NADP+ reductase 3 Fd Pq H 2O NADPH Pc 2 1 THYLAKOID SPACE (high H+ concentration) 1/ 2 NADP+ + H+ O2 +2 H+ 4 H+ To Calvin Cycle Thylakoid membrane STROMA (low H+ concentration) ATP synthase ADP + Pi ATP H+ 36 Calvin cycle • The Calvin cycle makes sugar within a chloroplast – To produce sugar, the necessary ingredients are atmospheric CO2, ATP, and NADPH, which were generated in the light reactions – Using these three ingredients, an energy-rich, three-carbon sugar called glyceraldehyde-3-phosphate (G3P) is produced • A plant cell may then use G3P to make glucose and other organic molecules • The starting material for the Calvin cycle is a five-carbon sugar named ribulose bisphosphate (RuBP) – The next step is a carbon (CO2) fixation step aided by an enzyme called rubisco (ribulose bisphosphate carboxylase/oxygenase) – This is repeated over and over, one carbon at a time – For One G3P, nine ATP and six NADPH are consumed Overview of Calvin cycle Input CO2 ATP NADPH CALVIN CYCLE Output: G3P 39 40 41 Photosynthesis uses light energy, CO2, and H2O • The chloroplast, which integrates the two stages of photosynthesis, makes sugar from CO2 – All but a few microscopic organisms depend on the food-making machinery of photosynthesis – Plants make more food than they actually need and stockpile it as starch in roots, tubers, and fruits • Some plants have evolved a means of carbon fixation that saves water during photosynthesis – One group can shut its stomata when the weather is hot and dry to conserve water but is able to make sugar by photosynthesis – These are called the C4 plants because they first fix carbon dioxide into a four-carbon compound H 2O CO2 Chloroplast Light NADP+ ADP + P Photosystem II Thylakoid membranes RuBP CALVIN CYCLE 3-PGA (in stroma) Electron transport chains Photosystem I ATP NADPH Stroma G3P O2 Sugars LIGHT REACTIONS CALVIN CYCLE Cellular respiration Cellulose Starch Other organic compounds Photorespiration • In hot climates, most plants (C3 plants) stomata close to reduce water loss so oxygen builds up – Rubisco adds oxygen instead of carbon dioxide to RuBP and produces a two-carbon compound, a process called photorespiration – Unlike photosynthesis, photorespiration produces no sugar, and unlike respiration, it produces no ATP • Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle • In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle Photorespiration 45 C4 Plants • C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells – This step requires the enzyme PEP (phosphoenolpyruvate) carboxylase – PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low – These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle 46 C4 pathway The C4 pathway C4 leaf anatomy Mesophyll cell PEP carboxylase Mesophyll cell Photosynthetic cells of C4 Bundleplant leaf sheath cell Oxaloacetate (4C) Vein (vascular tissue) PEP (3C) ADP Malate (4C) Stoma Bundlesheath cell CO2 ATP Pyruvate (3C) CO2 Calvin Cycle Sugar Vascular tissue 47 48 CAM (crassulacean acid metabolism) • Another adaptation to hot and dry environments has evolved in the CAM (crassulacean acid metabolism), such as pineapples and cacti – CAM plants conserve water by opening their stomata and admitting CO2 only at night – Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle – When CO2 enters, it is fixed into a four-carbon compound, like in C4 plants, and in this way CO2 is banked – It is released into the Calvin cycle during the day 49 Sugarcane Pineapple C4 Mesophyll Organic acid cell CAM CO2 1 CO2 incorporated (carbon fixation) Organic acid Calvin Cycle Night CO2 CO2 Bundlesheath cell CO2 2 CO2 released to the Calvin cycle Sugar (a) Spatial separation of steps Calvin Cycle Day Sugar (b) Temporal separation of steps 51 Global warming • The greenhouse effect results from solar energy warming our planet – Gases in the atmosphere (often called greenhouse gases), including CO2, reflect heat back to Earth, keeping the planet warm and supporting life – However, as we increase the level of greenhouse gases, Earth s temperature rises above normal, initiating problems • Increasing concentrations of greenhouse gases lead to global warming, a slow but steady rise in Earth s surface temperature – The extraordinary rise in CO2 is mostly due to the combustion of carbon-based fossil fuels – The consequences of continued rise will be melting of polar ice, changing weather patterns, and spread of tropical disease • Perhaps photosynthesis can mitigate the increase in atmospheric CO2 – However, there is increasing widespread deforestation, which aggravates the global warming problem Some heat energy escapes into space Sunlight Atmosphere Radiant heat trapped by CO2 and other gases Southern tip of South America Antarctica Notice for Mid-term exam • • • • Type I (50%) : From Item pool (5점짜리 10문제) – 5 items (with right answer) from each student • Due on 16th April (11:59 PM) – Upload to cyber campus – 10 % of mid-term grade – File name should be “ID number-name.pdf” (no other format) » Pdf format only!!!!!! • Creative and comprehensive items will be accepted and presented. – With extra point. – Short answered question (Free format but no multiple choice) – 문제당 5점.. • Wrong question and answer will not be accepted – Item pool will be found in cyber-campus on no later than 18th of April Type II (30%) : Simple short answered question (5점짜리 6문제) – For minimum grade Type III (20%) : Creative and comprehensive write-out answered question (10점짜리 2문제) – For assessment of the students – Prepare the answers for Why or How in the classes – May take 10 – 20 min each question – May be important for ‘A grade – Consists of multiple sub-questions (ex. III-1: 5점, III-2, 5점) Mid-term Exam on 22th (Fri) April – Where: K302 55 56