BIOL 1020 – CHAPTER 10 LECTURE NOTESChapter 10: PhotosynthesisI. Organisms can be classified based on how they obtain energy and how they obtain carbonA. energy source1. chemotrophs can only get energy directly from chemical compounds2. phototrophs can get energy directly from light (these organisms can use chemical compounds as energy sources as well)B. carbon source1. autotrophs can fix carbon dioxide, thus they can use CO2 as a carbon source2. heterotrophs cannot fix CO2; they use organic molecules from other organisms as a carbon sourceC. combined, these lead to 4 possible groups:1. photoautotrophs – carry out photosynthesis (use light energy to fix CO2, storing energy in chemical bonds of organic molecules); includes green plants, algae, and some bacteria2. photoheterotrophs – use light energy but cannot fix CO2; only nonsulfur purple bacteria3. chemoautotrophs – obtain energy from reduced inorganic molecules and use some of it to fix CO2; some bacteria4. chemoheterotrophs – use organic molecules as both carbon and energy sources; dependent completely on other organisms for energy capture and carbon fixation; includes all animals, all fungi, most protests, and most bacteriaII. The electromagnetic spectrum and visible lightA. visible light is a form of electromagnetic radiationB. electromagnetic radiation consists of particles or packets of energy (photons) that travel as waves1. amount of energy carried is inversely proportional to wavelength (distance from one wave peak to another)2. spectrum ranges from short wavelength/high energy gamma rays to long wavelength/low energy radio wavesC. the portion of the spectrum visible to humans (thus what we call visible light) ranges from higher-energy violet at 380 nm tolower-energy red at 760 nm; between lie all the colors of the rainbowD. molecules can absorb photons, thus becoming energized; typically, an electron absorbs the energy1. high energy: electron can be freed from the atom it was bound to (ionization)2. moderate energy (of correct amount): electron moves to a higher-energy orbital-electron can then be removed from the atom, going to an acceptor molecule-electron can return to a lower energy level, emitting a photon (fluorescence) or a series of photons (mostly infrared, experienced as heat)-ground state – when all electrons in a atom fill only the lowest possible energy levelsIII. ChloroplastsA. in photosynthetic eukaryotes (plants and algae), photosynthesis occurs in chloroplastsB. chloroplasts have both an inner and outer membrane1. stroma – fluid-filled region inside the inner membrane2. thylakoids – disklike membranous sacs found in stroma (interconnected with each other and inner membrane)3. thylakoid lumen – fluid-filled region inside a thylakoid4. granum – stack of thylakoids (plural: grana)C. chlorophyll, the main light-harvesting molecule, is found in the thylakoid membrane1. chlorophyll has a porphyrin ring and hydrocarbon side chain2. light energy is absorbed by the ring3. chlorophyll-binding proteins associate with chlorophyll in the membrane4. chlorophyll has several forms; in plants, typically chlorophyll a (chl a) initiates photosynthesisD. accessory pigments are also found in the thylakoid membrane1. pigments are compounds that absorb light; we see them as the main color of light that they do not absorb well (thus they scatter those colors or reflect them back)2. all pigments have an absorption spectrum3. chl a, a green pigment, absorbs violet-blue and red light4. several accessory pigments, with absorption spectra that differ from chl a, aid in photosynthesis-chl b is the main accessory pigment; a slight difference in the ring shifts its absorption spectrum-carotenoids are important yellow and orange accessory pigments-accessory pigments can transfer captured energy to chl a-they also help protect chl a and other compounds from excess light energy (high light intensity can cause damage)E. the relative rate of photosynthesis for a given radiation wavelength is an action spectrum1. the action spectrum looks similar to the absorption spectrum of chl a, but is augmented by the absorption spectrum of the accessory pigments2. blue and red light are most effective for photosynthesis3. action spectra can vary depending on speciesF. photosynthetic prokaryotes have plasma membrane folds that act like thylakoid membranesIV. Photosynthesis overviewA. photosynthesis converts energy from light into stored energy in chemical bonds1 of 4BIOL 1020 – CHAPTER 10 LECTURE NOTESB. in the process, CO2 is fixed and used in synthesizing carbohydratesC. overall reaction: 6 CO2 +12 H2O à C6H12O6 + 6 O2 + 6 H2O1. water is on both sides because it is consumed in some steps and produced in others; overall, there is a net use of water2. hydrogen atoms are transferred from water to carbon dioxide; yet another redox reactionD. usually divided into light reactions and the C3 cycle; more details on these later, but in summary:1. light reactions occur in the thylakoids; they capture light energy and consume water, producing O2; energy is placed in ATP and NADPH in the stroma2. the C3 cycle occurs in the stroma; it consumes CO2 and energy (proved by ATP and NADPH), producing carbohydratesE. in many ways this is the reverse of aerobic respirationV. The light reactions of photosynthesisA. overall:12 H2O + 12 NADP+ + 18 ADP + 18 Pi + light energy à 6 O2 + 12 NADPH + 12 H+ + 18 ATP + 18 H2OB. the overall equation takes into account the amount of NADPH and ATP needed to create one molecule of glucoseC. light is captured in photosystems that contain antenna complexes and a reaction center1. there are two types, Photosystem I and Photosystem II2. antenna complexes are highly organized arrangements of pigments, proteins, and other molecules that capture light energy3. energy is transferred to a reaction center where electrons are actually moved into electron transport chains-Photosystem I reaction center has a chl a absorption peak at 700 nm (P700)-Photosystem II reaction center has a chl a absorption peak at 680 nm (P680)4. chlorophyll molecule + light energy à an excited electron in the chlorophyll5. the excited electron is captured by a carrier in the photosynthetic electron transport chain, thus reducing the carrier andoxidizing the chlorophyll molecule (a redox reaction)6. the electron can then be transferred down the electron transport chain, with energy harvest possibleD. noncyclic electron transport produces ATP