Module 5: Photosynthesis PDF

Module 5: Photosynthesis FC BIO 111: General Botany by: Jayson F. Enciso, M.Sc. Autotroph Organisms that can manufacture its own foods from simple inorganic substances. Autotroph Organisms that can manufacture its own foods from simple inorganic substances. Photosynthesis Plants undergo photosy...

Module 5: Photosynthesis FC BIO 111: General Botany by: Jayson F. Enciso, M.Sc. Autotroph Organisms that can manufacture its own foods from simple inorganic substances. Autotroph Organisms that can manufacture its own foods from simple inorganic substances. Photosynthesis Plants undergo photosynthesis, a process by which the energy captured from the sun is converted and stored in the chemical bonds of glucose. Algae, unicellular eukaryotes, and some prokaryotes undergo photosynthesis. Photosynthesis Plants undergo photosynthesis, a process by which the energy captured from the sun is converted and stored in the chemical bonds of glucose. Algae, unicellular eukaryotes, and some prokaryotes undergo photosynthesis. Heterotrophs Heterotrophs obtain organic material by the second major mode of nutrition. Unable to make their own food, they live on compounds produced by other organisms. Decomposer Most fungi and many types of prokaryotes are decomposers. They get their nourishment by consuming remains of other organisms such as dead organisms, feces, and fallen leaves. Decomposer Almost all heterotrophs, including humans, are completely dependent, either directly or indirectly, on photoautotrophs for food—and also for oxygen, a by-product of photosynthesis. Chloroplast Chloroplasts is where photosynthesis really happens. All green parts of a plant, including green stems, have chloroplasts but photosynthesis mainly occurs in leaves. Chloroplast A chloroplast has an outer and inner membranes. The dense fluid inside the chloroplast is the stroma that contains DNA and ribosomes. Suspended within the stroma is the thylakoids that contains photosynthetic green pigments called chlorophyll. Chlorophyll gives leaves their color. The stack of thylakoids are called grana.  chlorophyll c Chlorophyll Structure of Chlorophyll chlorophyll d Carotenoid Carotenes -carotene lycopene Xanthophylls lutein zeaxanthin Chemical Reaction of Photosynthesis Any factor whose presence is required before a reaction proceeds is termed as essential factor. If one factor is absent, the reaction cannot take place. In photosynthesis, the essential factors are CO2, water, energy (light), pigments, enzymes, carrier molecules and suitable temperature. Chemical Reaction of Photosynthesis Carbon dioxide that we continually add to the air diffuses through the stomata into the intercellular spaces of the leaf. Once in the intercellular spaces, the molecules of CO2 dissolve in the water that saturates the walls of the cells and diffuse into the cytoplasm and eventually to the chloroplasts of these cells where photosynthesis takes place. Photosynthesis The energy source in photosynthesis is light energy. Photosynthesis The pigments absorb energy from the sun and use this energy in the production of sugars. There are actually several chlorophylls labeled as a, b, c and so forth. Chlorophyll a is the most abundant pigment and the key light-absorbing pigment involved in photosynthesis. Chlorophyll a and chlorophyll b are green in color that absorbs blue and red light and reflects green. Photosynthesis The pigments absorb energy from the sun and use this energy in the production of sugars. There are actually several chlorophylls labeled as a, b, c and so forth. Chlorophyll a is the most abundant pigment and the key light-absorbing pigment involved in photosynthesis. Chlorophyll a and chlorophyll b are green in color that absorbs blue and red light and reflects green.  Chlorophyll a Chlorophyll b Chlorophyll c Chlorophyll d Bacteriochlorophylls Photosynthesis Temperature is also an essential factor. Photosynthesis takes place over a range of temperature from 5 to 35°C. If the temperature exceeds 35°C, the photosynthetic rate will decrease. Photosynthesis The final products of photosynthesis are carbohydrates, in the form of glucose, and oxygen. In reality, photosynthesis is not a single process. There are two reactions involved in it. The light reaction (the photo part of photosynthesis) and the Calvin cycle (the synthesis part of photosynthesis). Light Reaction Light Reaction Light reaction occurs in the thylakoid membranes and it converts light energy into chemical energy of ATP and NADPH. Before we proceed to the process of light reaction, let us first understand what light really is. Light Light is a form of electromagnetic energy, also called as electromagnetic radiation. Light is composed of tiny particles called photons that travels in rhythmic waves just like the waves you create when you drop a stone into the water. The Nature of Light 1. Wave C – light speed (3 x 108 m s-1) Λ – wavelength Ν - frequenncy 1. Wave c = λν c – light speed (3 x 108 m s-1) λ– wavelength ν - frequenncy 2. Particle or Photon E = hν E – light speed (3 x 108 m s-1) H – wavelength ν - frequency E = h c/λ E=hc/ Blue Light −1 −1 (6.62 𝑥 10 − 34 𝐽 𝑠 𝑝ℎ𝑜𝑡𝑜𝑛 )(3 𝑥 108 𝑚 𝑠 ) 𝐸= 4.35 𝑥 10 − 7 𝑚 −1 −1 𝐸 = 4.56 𝑥 10 − 19 𝐽 𝑝ℎ𝑜𝑡𝑜𝑛 (6.023 𝑥 1023 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑚𝑜𝑙 ) −1 𝐸 = 274 𝑘𝐽 𝑚𝑜𝑙 E=hc/ Red Light −1 −1 (6.62 𝑥 10 − 34 𝐽 𝑠 𝑝ℎ𝑜𝑡𝑜𝑛 )(3 𝑥 108 𝑚 𝑠 ) 𝐸= 6.60 𝑥 10 − 7 𝑚 −1 −1 𝐸 = 3.01 𝑥 10 − 19 𝐽 𝑝ℎ𝑜𝑡𝑜𝑛 (6.023 𝑥 1023 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑚𝑜𝑙 ) −1 𝐸 = 181 𝑘𝐽 𝑚𝑜𝑙 Radiation of Principal Interest to Biologists Color Wavelength Average Energy Range (nm) (kJ mol-1 photons) Ultraviolet 100-400 UV-C 100-280 471 UV-B 280-320 399 UV-A 320-400 332 Visible 400-740 Violet 400-425 290 Blue 425-490 274 Green 490-550 230 Yellow 550-585 212 Orange 585-640 196 Red 640-700 181 Far-red 700-740 166 Infrared Longer than 740 85 Light Sources A. Thermal radiation sources - emit light due to thermal excitation 1. Natural = Sun H + H → He + gamma rays + energy Gamma rays → electrons + photons + energy  Light Sources A. Thermal radiation sources - emit light due to thermal excitation 2. Artificial = Incandescent lamp - light emission from electrically heated tungsten filament placed in an inert gas Light Sources B. Electric discharge lamp - current flows between two electrodes through metal vapors or ionized gas; - spectral energy determined by metal vapor or gas and pressure inside the lamp 1. Low pressure discharge lamps a. Sodium arc lamp – emits 589 nm b. Mercury arc lamp – emits 185 and 254 nm c. Fluorescent lamp – emits sunlight without UV & IR d. Hydrogen and Deuterium lamp – emits broad UV Fluorescent vs Incandescent Lamp Incandescent Lamp Fluorescent Lamp Advantages Advantages Low initial and replacement cost Inexpensive Fluence rates rather constant over time Low power sources High yield of visible light Useful for long term irradiation of large areas Disadvantages Low yield of visible light compared to electric energy used High fraction of infrared radiation High sensitivity to fast changes in temperature Alteration of the spectral distribution due to voltage alteration Relatively short lifetime  Light Sources B. Electric discharge lamps - current flows between two electrodes through metal vapors or ionized gas; - spectral energy determined by metal vapor or gas and pressure inside the lamp 2. High pressure discharge lamps a. Sodium vapor lamp – emits broad 590 nm b. Mercury vapor lamp – emits 365 nm c. Metal halogen lamp – emits visible range d. Xenon lamp – emits all wavelengths transmitted by quartz e. LASER – emits monochromatic radiation Radiation Measurements Photometric measurement of visible radiation with a sensor having a spectral responsivity curve equal to the average human eye, that is from 390 to 780 nm expressed as illuminance (density of luminous flux incident at a point in surface) with unit as lux (lm/m2) or foot-candle (lm/ft2) Radiometric measurement of the properties of radiant energy (SI unit: joule, J) emitted by a source or incident on a receiver expressed as irradiance (radiant flux incident on a per unit area of surface) with unit as W m-2 (J s-1 m-2) Quantum measures the photosynthetically active radiation (PAR) in the 400-700 nm band expressed as quantum or photosynthetic photon flux density or fluence rate (number of photons in the 400-700 nm waveband incident per unit time on a unit surface) with unit as 1 mol m-2 s-1 = 6.022 x 1017 photons s-1m-2 Measurements of sunlight in different units Conditions Photometric Radiometric Photon flux units units density kilolux W m-2 (400-700 nm) µmol m-2 s-1 Full sun 100-130 750-1000 (total radiation) 400-520 1840-2400 (400-700 nm) Overcast 14-16 55-65 250-300 (noon, cloudy) (400-700 nm) Deeply shaded 0.8 3 15 (forest floor) (400-700 nm) Fundamental Principles in Photobiology Gotthaus-Draper Law Only light that is absorbed can be active in a photochemical process Stark-Einstein Law A molecule can absorb or be activated by only one photon at a time Excitation of Molecule by Photon Excited States Singlet states Triplet states Basic scheme of photobiological events Deactivation of Excited States Thermal dissipation - emitting the energy difference as heat Photoluminescence (radiation emission) Fluorescence - light emission by a molecule in the singlet state returning to the ground state Phosphorescence - light emission by a molecule in the triplet state returning to the ground state Chemiluminescence - light emission when molecules are excited exclusively by chemical reaction Bioluminescence - an enzymatically catalyzed chemiluminescence in biological systems Inductive resonance - energy transfer of excited states to another molecule Photochemistry - induction of a chemical reaction Fluorescence Fluorescence microscopy Phosphorescence Glow sticks Chemiluminescence Bioluminescence Why Use Bioluminescence? Firefly: to attract mates of opposite sex Squid: for predation (there are some squids having bioluminescent spots that confuse the squid’s prey since he can be mistaken for the sparkle of water around him making him harder to see) Why Use Bioluminescence? Polynoid worm: prevent predation (when the worm is attacked, it leaves behind a piece of its glowing body to attract the attention of the hunter, then the rest of the worm goes dark and escapes to later regenerate the piece left behind) Flashlight fish: to see better in the dark sea environment Why Use Bioluminescence? Angler fish: as a lure to attract prey Lantern fish: protection from predators (lights on the side and belly break up silhouette of the fish’s body, making the fish harder to see from below) Why Use Bioluminescence? Bacteria: as a defense mechanism? Dinoflagellates: as a response to agitation Why Use Bioluminescence? Mushroom: to attract insects that can spread the fungus’ spores for more widespread reproduction Bioluminescent Plants? Luciferin-Luciferase Reaction Resonance energy transfer Inductive resonance Photochemistry  P700 P680  Principal Plant Photoreceptors/Pigments ◼ Chlorophylls ◼ Phytochromes ◼ Carotenoids ◼ Cryptochromes ◼ Phycobilins* ◼ Phototropins ◼ Flavonoids * Blue-green and Red Algae  Chlorophylls ◼ Chlorophyll a ◼ Chlorophyll b ◼ Chlorophyll c ◼ Chlorophyll d ◼ Bacteriochlorophylls Chlorophyll Biosynthesis Chlorophylls chlorophyll c chlorophyll d  Carotenoids ◼ Carotenes -carotene lycopene ◼ Xanthophylls lutein zeaxanthin Carotenoid Biosynthesis Carotenoids Phycobilins Phycobilin-containing Organisms Blue-green algae / Cyanobacteria Red algae Photosynthetic Pigment Distribution Among Organisms  Pigment abs (1) Bacteriochlorophyll UV & IR (2) Chlorophyll a 420 & 660 nm (3) Chlorophyll b 470 & 650 nm (4) Phycoerythrobilin 567 nm (5) Carotenoid 425, 445 & 475 nm  Chromatophores (purple bacteria) Chlorosomes (green bacteria) Phycobilisome Flavonoids Flavonoid Biosynthesis Major Flavonoids Anthocyanin in Vacuoles Different Functions of Plant Pigments ◼ Mass pigments Light-collecting chlorophyll, carotenoid, phycobilin Light-signalling anthocyanin Light-filtering xanthophylls, flavones/flavonols ◼ Photosensor pigments phytochrome, cyrptochrome, phototropin Light Reaction Photosystem Now, chlorophylls molecules are organized along with other small organic molecules and proteins into complexes called photosystems. A photosystem is composed of a reaction-center complex surrounded by several light-harvesting complexes. Photosystem A reaction-center complex is an organized association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor, a molecule capable of accepting electrons and becoming reduced. Light-harvesting complexes consists of various pigment molecules (which may include chlorophyll a, chlorophyll b, and multiple carotenoids) bound to proteins. Photosystem A reaction-center complex is an organized association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor, a molecule capable of accepting electrons and becoming reduced. Light-harvesting complexes consists of various pigment molecules (which may include chlorophyll a, chlorophyll b, and multiple carotenoids) bound to proteins. Photosystem II There are two groups of photosystem that drive photosynthesis. Photosystem II functions first. The reaction-center chlorophyll a in photosystem II absorbs best in light with a wavelength of 680nm. That is why photosystem II is also called as P680. Photosystem I Photosystem I comes after photosystem II. It is best at absorbing light having a wavelength of 700nm and is called P700. Light Reaction The chlorophyll is non polar and contains magnesium. This makes it easy for a chlorophyll molecule to be excited with a photon of light. Excited chlorophyll may release the electron if the energy of light is high enough. Now, there are two possible routes for electron flow. Linear Electron Flow The linear electron flow and the cyclic electron flow. Let us first discuss the linear electron flow. Linear electron flow is the primary pathway of electrons. It involves both photosystems and produces ATP and NADPH using light energy. Linear Electron Flow The linear electron flow and the cyclic electron flow. Let us first discuss the linear electron flow. Linear electron flow is the primary pathway of electrons. It involves both photosystems and produces ATP and NADPH using light energy. Cyclic Electron Flow The second route of electron is the cyclic electron flow. This route is not that complicated as the first one. Cyclic electron flow is a short circuit that only uses photosystem I and generates extra ATP to satisfy the higher demand in the Calvin Cycle. Cyclic Electron Flow Photoexcited electrons from PS I cycle back from ferredoxin (Fd) to the cytochrome complex, then via a plastocyanin molecule (Pc) to a P700 chlorophyll in the PS I reaction- center complex. No oxygen and NADPH produced from this process. On the other hand, cyclic flow does generate ATP. Cyclic Electron Flow The products of the light dependent reactions (ATP and NADPH) are used in the light independent reactions. The oxygen is not used in the rest of the photosynthetic process. Calvin Cycle Calvin Cycle The Calvin cycle, also called as the dark reaction, is where carbon fixation takes place. Carbon fixation is the process of turning atmospheric carbon dioxide into glucose. As its other name suggests, Calvin cycle does not require direct light to take place. It occurs in the stroma that uses the products of light reaction (ATP and NADPH) to make sugar from carbon fixation. Calvin Cycle The Calvin cycle, also called as the dark reaction, is where carbon fixation takes place. Carbon fixation is the process of turning atmospheric carbon dioxide into glucose. As its other name suggests, Calvin cycle does not require direct light to take place. It occurs in the stroma that uses the products of light reaction (ATP and NADPH) to make sugar from carbon fixation. Carbon Fixation In the stroma, in addition to CO2, two other chemicals are present to initiate the Calvin cycle: an enzyme abbreviated RuBisCO, and the molecule ribulose bisphosphate (RuBP). RuBP has five atoms of carbon and a phosphate group on each end. It produces an unstable six-carbon intermediate which immediately breaks down into two molecules of the three-carbon compound phosphoglycerate (PGA). A total of 3 molecules of CO2 must be fixed this way in order to produce one molecule of the six-carbon sugar glucose. Reduction The energy from ATP and the reducing power of NADPH (both produced during the light-dependent reactions) is now used to convert the molecules of PGA to glyceraldehyde-3-phosphate (G3P), another three-carbon compound. For every six molecules of CO2 that enter the Calvin cycle, two molecules of G3P are produced. Most of the G3P produced during the Calvin cycle - 10 of every 12 G3P produced - are used to regenerate the RuBP in order for the cycle to continue. Some of the molecules of G3P, however, are used to synthesize glucose and other organic molecules. Two molecules of the three-carbon G3P can be used to synthesize one molecule of the six-carbon sugar glucose. Regeneration of the CO2 acceptor (RuBP) Ten molecules of the three-carbon compound G3P eventually form six molecules of the four-carbon compound ribulose phosphate (RP). Each molecule of RP then becomes phosphorylated by the hydrolysis of ATP to produce ribulose bisphosphate (RuBP), the starting compound for the Calvin cycle. C3, C4, and CAM Plants Photorespiration 85% of plants use the Calvin cycle as the standard mechanism of carbon fixation. These plants, including rice, wheat, soybeans, and all trees, are called the C3 plants. However, C3 plants have the disadvantage during hot dry conditions. When it is hot and the atmosphere is dry, their photosynthetic efficiency suffers because of a process called photorespiration, a wasteful pathway that competes with the Calvin cycle. Photorespiration When the CO2 concentration in the chloroplasts drops below about 50 ppm, the catalyst RuBisCo that helps to fix carbon begins to fix oxygen instead. This is highly wasteful of the energy that has been collected from the light, and causes the RuBisCo to operate at perhaps a quarter of its maximal rate. C4 and CAM In some plant species, alternative modes of carbon fixation have evolved to reduce photorespiration and maximize the Calvin cycle — even in dry, arid climates. C4 photosynthesis and Crassulacean acid metabolism (CAM) are the two most common of these photosynthetic adaptations. C4 Photosynthesis In C4 plants, the light- dependent reactions and the Calvin cycle are physically separated. The light- dependent reactions occur in the mesophyll cells and the Calvin cycle occurs in special cells around the leaf veins called bundle-sheath cells.  Single Cell C4 Photosynthesis First detected in Borszczowia aralocaspica and followed by a related species Bienertia cycloptera, both belonging to Family Chenopodiaceae Both able to perform C4 PS within a single chlorenchyma cell by intracellular partitioning of enzymes and organelles in two compartments  A B Scanning electron microscopy and immunolocalization of rubisco in (A) B. aralocaspica and (B) B. cycloptera C3 Rice to C4 Rice ? CAM Photosynthesis (Crassulacean Acid Metabolism) Many plants that are adapted to dry conditions, such as cacti and pineapples, use the Crassulacean Acid Metabolism (CAM) pathway to reduce photorespiration. Its name comes from the plant family, the Crassulaceae, where scientists first found the pathway. CAM Photosynthesis (Crassulacean Acid Metabolism) Instead of separating light-dependent reactions and using CO2 in the Calvin cycle in space, CAM plants separate these processes in time. CAM plants open their stomata at night, allowing CO2 to spread to the leaves. This CO2 is fixed to oxaloacetate by PEP carboxylase and then converted to malate or another type of organic acid. CAM Photosynthesis (Crassulacean Acid Metabolism) During the day, their stomata are closed but they still can photosynthesize. This is made possible by the organic acids being transported out of the vacuole and broken down to release CO2, which enters the Calvin cycle. Crassulacean acid metabolism