BI110 Lecture 13 Photosynthesis PDF

BI110 LECTURE 13 AND 14 Chapter 6 - Photosynthesis  Photosynthesis Photosynthesis is the use of light energy to convert carbon from CO2 gas into an organic form. Oxygen, generated from the oxidation of H2O during the process, is released Provides the source as a by-product....

BI110 LECTURE 13 AND 14 Chapter 6 - Photosynthesis  Photosynthesis Photosynthesis is the use of light energy to convert carbon from CO2 gas into an organic form. Oxygen, generated from the oxidation of H2O during the process, is released Provides the source as a by-product. of all the food we eat, through direct and indirect consumption O2 CO2 + H2O  Metabolic Classification of Organisms AllAllorganisms organisms Phototrophs Phototrophs Chemotrophs Chemotrophs Energy source Autotrophs Autotrophs Heterotrophs Heterotrophs Autotrophs Autotrophs Heterotrophs Heterotrophs Carbon source Cyanobacteria Heliobacteria Sulfur-oxidizing bacteria Most bacteria Examples Vascular plants Most green non-sulfur Hydrogen Animals bacteria bacteria  Autotrophs and Heterotrophs Autotrophs Organisms that make required organic (food) molecules from inorganic sources such as CO2 and water; self-feeding Heterotrophs Consumers and decomposers which need a source of organic (food) molecules to survive Photoautotrophs Autotrophs that use light as the energy source to make organic molecules by photosynthesis  Photoautotrophs: Primary Producers of the Earth Photosynthetic organisms: Convert sunlight energy into chemical energy Use energy to assemble complex organic molecules from inorganic raw materials The organic molecules are then used as energy sources (but also used as energy source by other organisms) Fig. 6.1, p. 126 Energy Flow  Two Stages of Photosynthesis Light reactions: light energy absorbed by pigment Calvin cycle: molecules is transformed into ATP and NADPH; O2 NADPH and ATP that is produced as a result of the oxidation of produced during the water is released as a by-product. light reactions provide energy and reducing power to fix [Insert Fig. 6.2 on p. 127] carbon from CO2 and convert it into carbohydrates Fig. 6.2, p. 127 Photosynthesis is a Redox Reaction The synthesis of Oxidation reactions are reactions carbohydrates from in which a molecule loses CO2 requires an input electrons and releases energy. of energy, which comes from sunlight. Light 6 + 12 6 Reduction reactions are reactions in which a molecule acquires electrons and gains energy.  Chloroplasts In eukaryotes (higher plants, algae), photosynthesis takes place in chloroplasts Fig. 6.3, p. 127  Chloroplast Structure Surrounded by two membranes: outer and inner membranes Separated by intermembrane space The main aqueous compartment is called the stroma Location of carbohydrate synthesis Thylakoid membranes Location of photosynthetic pigments and electron transport chain A complex of flattened, closed sacs Stacks of membranes called grana Tubular lamellae connect the grana The soluble compartment enclosed by thylakoids called the thylakoid lumen Photosynthesis impacts global CO2 levels Figure 6.4 in textbook shows that chlorophyll/chloroplasts are visible from space! Check out the YouTube video, NASA: A Year in the Life of Earth’s CO2, to see how photosynthesis impacts global atmospheric CO2 levels You can see CO2 levels increase during the late fall and through the winter in the northern hemisphere, and decrease starting in the spring and through the summer  Two Stages of Photosynthesis Light reactions—capture Calvin cycle—NADPH of light energy by pigment and ATP are consumed molecules and energy and CO2 is fixed into used to synthesize both carbohydrates ATP and NADPH [Insert Fig. 6.2 on p. 127] Fig. 6.2, p. 127  Light and the Electromagnetic Spectrum Light is the ultimate source of energy, sustaining virtually all organisms. The Sun converts matter to energy, releasing it as electromagnetic radiation. The range of wavelengths of electromagnetic radiation is called the electromagnetic spectrum.  What is Light? Portion of electromagnetic spectrum that humans can detect with their eyes Electromagnetic spectrum Forms of radiant energy that differ in wavelength (horizontal distance between crests of successive waves) Various forms range from radio waves (10 metres to kilometres) to gamma rays (10−2 to 10−6 nm) Figure 6.5a  Visible Light and Photons Visible light has wavelengths between about 700 nm (red light) and 400 nm (blue light) We see the entire spectrum combined together as white light Energy in a unit of light (photon) is inversely proportional to wavelength—the shorter the wavelength, the greater the energy of the photon Figure 6.5b  Light Interacts with Matter When photons of light hit an object, 1 of 3 things can happen. The photon can be: reflected transmitted absorbed To be used as energy, light must be absorbed - the energy of the photon is transferred to an electron within a molecule. The energy transfer switches the electron from a grounded state to an excited state.  Pigments Absorb Photons Pigments—molecules that absorb photons of specific wavelengths Critical light absorption feature: a region where carbon atoms are covalently bonded to each other with alternating single and double bonds (conjugated system) Fig. 6.4, p. 137  Pigments Absorb Photons (cont’d) Differences in the arrangement of conjugated systems and different chemical structure explain why each type of pigment absorbs light of only certain wavelengths. A pigment’s colour is the result of photons of light it does not absorb. Fig. 6.5, p. 138  Pigments A major class of molecules that are very efficient at absorbing visible light are pigments because their structure results in a number of excitable electrons Chapter 1 Electromagnetic spectrum High energy Low energy Gamma X-rays Ultra- Visible Infrared Micro- Radio rays violet light waves waves 400 nm 700 nm Chlorophyll is a Absorption photosynthetic pigment  Fates of an Excited-State Electron 1. Excited electron returns to its ground state, releasing energy as heat or as light of a longer wavelength (fluorescence) Fig. 6.6, p. 129  Fates of an Excited-State Electron 1 2. Energy from excited electron in one pigment molecule is transferred to a neighbouring pigment molecule: inductive resonance Energy transfer excites a second pigment and the first pigment returns to its ground state Fig. 6.6, p. 129  Fates of an Excited-State Electron 3. Excited-state electron itself is transferred to nearby electron- accepting molecule  photoreduction Fig. 6.6, p. 129  Fates of an Excited-State Electron All three possibilities occur following light absorption by photosynthetic pigments Fig. 6.6, p. 129  Three Fates of an Excited-State Electron 1. Excited electron returns to its ground state, releasing energy as heat or as light of a longer wavelength (fluorescence) 2. Energy from excited electron in one pigment molecule is transferred to a neighbouring pigment molecule Transfer excites second pigment, returning first pigment to its ground state 3. Excited-state electron itself is transferred to nearby electron-accepting molecule  Chlorophyll and Carotenoid Chlorophylls are the major photosynthetic pigments in plants, green algae, and cyanobacteria Chlorophyll a is oxidized and donates electron to primary electron acceptor Chlorophyll b and carotenoids are accessory pigments  Donate excitation energy to chlorophyll a via inductive resonance Fig. 6.7, p. 130 Chlorophyll and Carotenoids Light absorbed by carotenoids and chlorophylls, acting in combination, drives the reactions of photosynthesis. The amount of light of different wavelengths that is absorbed by a pigment is its absorption spectrum. The effectiveness of light of each wavelength in driving photosynthesis produces a graph called the action spectrum of photosynthesis. Fig. 6.11 Engelmann’s Experiment (1883) Theodor Engelmann used a glass prism to break light into a spectrum of colours—cast across a microscope slide with a strand of algae and aerobic bacteria. Bacteria grew best where algae released oxygen in greatest quantity —in areas of blue, violet, and red light. Engelmann constructed an action spectrum for wavelengths of light, showing the effects of each colour Engelmann’s Action Spectrum on photosynthesis. Fig. 6.12  Two Stages of Photosynthesis Light reactions—capture Calvin cycle—NADPH of light energy by pigment and ATP are consumed molecules and energy and CO2 is fixed into used to synthesize both carbohydrates ATP and NADPH [Insert Fig. 6.2 on p. 127] Fig. 6.7  Major Components of a Photosystem Antenna complex Absorbs light energy: Chlorophyll a and b Carotenoids Energy transferred to Reaction Center chlorophyll a molecules [Insert Fig. 6.10 on p. 131] via inductive resonance Reaction Center Pair of special chlorophyll a molecules of reaction center are bound by proteins: P680 in PSII P700 in PSI Primary electron acceptor: Pheophytin in PSII Fig. 6.13  Photosystems I and II Photosystems composed of many pigments and proteins Special chlorophyll a molecules in the reaction centres can be oxidized (photoreduction) Photosystem II Special chlorophyll a molecules in the reaction centre are called P680 (P = pigment; 680 denotes wavelength of max absorbance for the reaction centre) Photosystem I Specialized chlorophyll a molecules in the reaction centre are called P700  Two Photosystems Photosystem I Photosystem II H 2O NADP+ + H+ e– e– e– e– NADPH O2 + H + The two photosystems are Two systems are necessary to part of and are connected by provide enough energy to pull the photosynthetic electron electrons from water and then transport chain use them to reduce NADP+.  Photosystem II e- in P680 raised from ground state to excited state  P680* P680* oxidized to P680+ by primary e- acceptor Pheophytin (Pheo) Pheo transfers e- to PQ Shuttles e- to cytochrome complex P680 is reformed when P680+ gains an e- from oxidation of H2O Water splitting complex Also see Figure 6.12 on p. 133 (section 6.3b)  Photosynthetic Electron Transport and ATP Synthesis Electrons released by oxidation of the reaction centre chlorophylls of photosystem II (P680) are passed along an electron transport chain This photosynthetic electron transport chain (the light reactions) uses energy of light absorbed by photosystem II and photosystem I to generate reducing power in the form of NADPH (high energy electron carrier), and forms a H+ gradient that can be used to generate ATP.  Linear Electron Transport 1 Oxidation of P680 in Photosystem II Absorption of light by PS II results in formation of P680* P680* gets oxidized to P680+ by the primary e- acceptor Pheophytin (Pheo) Fig. 6.14  Linear Electron Transport 2 Oxidation-reduction of Plastoquinone Pool The Electron given up by chlorophyll to Pheo is transferred to plastoquinone (PQ); the electron “hole” in P680 chlorophyll is replaced by the oxidation of water PQ also takes a H+ from stroma, migrates through lipid bilayer Donates e- to cytochrome b6/f complex Releases H+ into lumen Fig. 6.14  Linear Electron Transport 3 Electron transfer from cytochrome complex to PC The cytochrome complex transfers electrons to plastocyanin (PC) PC shuttles e- to Photosystem I Fig. 6.14  Linear Electron Transport 4 Oxidation-Reduction of P700 in PS I Absorption of light by PS I results in formation of P700* Oxidation of P700* to P700+ by the primary e- acceptor of PS I e- “hole” in P700+ chlorophyll is replaced by e- donated by PC Fig. 6.14  Linear Electron Transport 5 Electron Transfer to NADP+ by Ferredoxin e- from PSI is transferred to ferredoxin (iron-sulfur protein) e- transferred to NADP+ (Final e- acceptor) via Ferredoxin NADP+ gets reduced to NADPH by NADP+ reductase  Chemiosmotic Synthesis of ATP (low pH) stroma Proton-motive force established across thylakoid membrane used to synthesize ATP by chemiosmosis and ATP Synthase H+ flow from thylakoid lumen into stroma drives synthesis of ATP Fig. 6.14  Energy Levels in Thylakoid Membrane [Insert Fig. 6.12 on p. 133] Fig. 6.15 Absorption of light energy by PS II A second input of light energy by PS I allows electrons pulled from water produces electron donor molecules to enter the photosynthetic ETC. capable of reducing NADP+.  Stoichiometry How many photons need to be absorbed by the photosynthetic apparatus to produce a single molecule of O 2? 2H2O → 4H+ + 4e- + O2  Stoichiometry To get 1 electron through the electron transport chain from photosystem II to NADP+ takes 2 photons of light (i.e. 1 photon absorbed by each photosystem) 2H2O → 4H+ + 4e− + O2 Therefore, for 4 electrons (or one molecule of O2), a total of 8 photons of light need to be absorbed -- 4 by each photosystem Cyclic Electron Transport Photosystem I can also operate independently of photosystem II by using cyclic electron transport [Insert Fig. 6.13 on p. 135] Keeps moving protons across thylakoid membrane without involvement of photosystem II Fig. 6.13, p. 135  Cyclic Electron Transport Electron flow from photosystem I to ferredoxin is not always followed by electron donation to the NADP + reductase complex Instead, reduced ferredoxin can donate electrons back to the plastoquinone pool, which gets continually reduced and oxidized Energy absorbed from light is therefore converted into chemical energy of ATP but not NADPH as a result of cyclic electron transport Calvin cycle requires more ATP than NADPH Additional ATP provided by cyclic electron transport & chemiosmosis  Two Stages of Photosynthesis Light reactions—capture Calvin cycle—NADPH of light energy by pigment and ATP are consumed molecules and energy and CO2 is fixed into used to synthesize both carbohydrates ATP and NADPH [Insert Fig. 6.2 on p. 127] Fig. 6.2, p. 127  Calvin Cycle The Calvin cycle is a metabolic pathway that reduces CO2 and converts it into organic substances. It is an anabolic, endergonic process.  NADPH provides electrons and hydrogen  ATP provides additional energy Carbon fixation involves capturing CO2 molecules with the key enzyme Rubisco (RuBP carboxylase/oxygenase)  Fixation: CO2 added to Calvin Cycle RuBP to produce two 3PGA molecules (3- phosphoglycerate) molecules  catalyzed by Rubisco [Insert Fig. 6.14 on p. 135] Reduction: NADPH Regeneration: and ATP used to Remaining G3P convert 3PGA into molecules are G3P, a higher energy used to regenerate molecule used to build the starting sugars material RuBP Fig. 6.14, p. 135 Glyceraldehyde 3-Phosphate (G3P) Starting point for synthesis of many organic molecules:  Glucose  Sucrose Disaccharide of glucose linked to fructose Main form for photosynthetic products that circulate among cells  Starch  Cellulose  Amino acids  Fatty acids and lipids  Proteins  Nucleic acids  Rubisco Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase): Most abundant protein on Earth Provides the source of organic molecules for most of the world’s organisms 100 billion tons of CO2 are converted into carbohydrates each year Represents 50% or more of the total protein in leaves  Rubisco Composed of: 8 Large Subunits (LSU) chloroplast genome 8 Small Subunits (SSU) nuclear genome Requires coordinated gene expression of two genomes. Rubisco has Carboxylase and Oxygenase Activity The oxygenase activity results in a net loss of carbon. Because oxygenase activity consumes O2 and releases CO2, the metabolic pathway it leads to is also called photorespiration. Photorespiration Fig. 6.16, p. 138  Rubisco Oxygenase Activity O2 acts as a competitive inhibitor of Rubisco Ancient enzyme, developed when very little O2 in the atmosphere Rubisco active site has greater affinity for CO2 than O2 Atmosphere is 21% O2, 0.04 % CO2 At moderate temperatures carboxylation 75%, oxygenation 25% Some species have evolved mechanisms to reduce the oxygenation reaction Increase ratio of CO2/O2 where Calvin cycle takes place Fig. 7-16, p. 153 Carbon-Concentrating Mechanisms in Algae Aquatic environments Concentration of CO2 dissolved in water is well below what is needed to saturate the Rubisco active site Experimentally, addition of CO2 to phytoplankton does not usually lead to increase in rate of photosynthesis Algae pump carbon dioxide into their cells Carbon-Concentrating Mechanisms in Algae Some algae actively pump bicarbonite anion, the most abundant form of inorganic carbon dissolved in water, into their cells so that the concentration of CO2 near Rubisco is higher than would otherwise be possible with simple diffusion Fig. 6.17, p. 138  Dilemma of Plants in Hot Dry Climates Terrestrial plants, especially those in hot dry climates, face problems of photorespiration and water loss Leaf surface is covered with waxy cuticle to prevent water loss; but also prevents rapid diffusion of gases into leaf Stomata regulate gas exchange Dilemma of plants in hot dry climates: Need to open stomata to let in CO2, but need to keep them closed to conserve water Fig. 7-18, p. 154 Photorespiration and Temperature [Insert Table 6.1 on p. 139] High temperatures increase photorespiration Table 6.1, p. 139  C4 Photosynthesis Some plants have evolved the C pathway to increase the 4 concentration of CO2 relative to O2 near Rubisco so that photorespiration is minimized In the C cycle, CO is combined with a 3-carbon 4 2 molecule, phosphoenolpyruvate (PEP), to produce a 4- carbon intermediate The Calvin cycle is called the C cycle because a 3- 3 carbon intermediate is produced first (3PGA)  C4 Photosynthesis The enzyme PEP carboxylase that binds CO2 does not have any oxygenase activity Later, when O2 concentrations are low, the 4-carbon molecule is oxidized to release CO2 CO2 then enters the Calvin cycle by binding to Rubisco C4 and the Calvin Cycle [Insert Fig. 6.19 on p. 140] Fig. 6.19, p. 140 C4 in Different Locations Some C4 plants run the C4 and Calvin cycles in different cells (spatial separation of the two pathways) CO2 is captured by PEP carboxylase in mesophyll cells close to the surface of leaves, since PEP carboxylase is not affected by high O2 concentrations The 4-carbon intermediate is then transported to bundle sheath cells deeper in the leaf where CO2 is released There, O2 is less abundant and CO2 is more concentrated, so Rubisco’s oxygenase activity is minimized (less photorespiration)  Crassulacean Acid Metabolism (CAM) Some C plants run the C and Calvin 4 4 cycles at different times (instead of in different cells) to avoid photorespiration: temporal separation In crassulacean acid metabolism (CAM), certain cacti and succulent desert plants capture CO2 at night with the C4 cycle, but run the Calvin cycle during the day when sunlight is available (and therefore more ATP & NADPH are being generated) Crassulacean Acid Metabolism (CAM) CAM plants open their leaf stomata during the night to capture CO2 using the C4 cycle 4-carbon intermediates like malate accumulate and are stored in the large cell vacuole During the day, plants close their stomata to conserve water and malate is oxidized to release CO2 inside the chloroplasts CO2 concentrations are high inside the chloroplast and O 2 concentrations are relatively low, so photorespiration is minimized C4 and CAM Pathways C4 plants spatially separate the C4 pathway and the Calvin cycle CAM plants temporally separate the C4 pathway and the Calvin cycle Fig. 6.20, p. 141 Photosynthesis and Cellular Respiration Photosynthesis occurs in cells of plants that contain chloroplasts Cellular respiration occurs in all cells Overall reactions of the two processes are essentially the reverse of each other—with the reactants of one being the products of the other Fig. 6.21, p. 142