Ocea 130/230Ocean Processes and Ecology PhotosynthesisPhotosynthetic Light ReactionsA. Oxidation-Reduction Reactions:3Fe + 2O2  Fe3O4 (oxidation of iron to form rust)Fe3O4 + 2C  3Fe + 2CO2 (reduction of iron by heating with carbon)• Reduction is the removal of oxygen, the addition of electrons, or the additionof hydrogen atoms; conversely, oxidation is the addition of oxygen, theremoval of electrons, or the removal of hydrogen atoms.• In phytoplankton, these oxidation-reduction reactions are catalyzed by thepresence of photosynthetic pigmentsChlorophyll does NOT by itself lead to a chemical reaction---cooked spinach isstill green, but it is no longer capable of photosynthesizing• In plants, we often refer to photosynthesis as:6 CO2 + 6 H20  C6H12O6 +6 O2• Which can be broken down into two separate reactions:2 H20 + Light  4 H+ + 4e- + O2 (oxidation of H2O)CO2 + 4H+ + 4e-  CH2O + H2O (reduction of carbon)• These are the Light Reactions and Dark Reactions• All photosynthetic organisms (except for some bacteria) use oxygenicphotosynthesis, meaning they produce oxygen. All have chlorophyll a as theprimary reaction pigment, in the form of PS I and PSII (but not all algae useChl a as the primary antenna pigment)B. History of Photosynthetic Unit• Early 20th Century, realized that Chlorophyll was a catalyst (wasn’t destroyedduring the reaction), and thought that it somehow absorbs light, produces O2,and fixes CO2• 1932, Emerson and Arnold set up an experiment with a bunch of equipmentto give very short, very bright flashes:• Allowed the dark time between flashes to varyOcea 130/230Ocean Processes and Ecology Photosynthesis• Long time between flashes (long dark period), O2 evolution isindependent of temperature• Short time interval, strongly temperature dependent (enzymesinvolved)• Took about 2500 chl molecules to produce O2 (turns out it’s between1500-2500 on average)• Determined 3 things (VERY IMPORTANT):-Light and Dark reactions-High light, dark reactions (enzymes) are limited-Many chlorophyll molecules are required• Von Warburg (1920s)• Determined that the O2 produced requires CO2 to be consumed• According to his measurements, it takes 4 electrons to carry outthis reaction (so each O2 molecule requires 4 electrons, orphotons)• Based on this calculation, Von Warburg determined that thequantum yield for O2 production is 0.25 (1 oxygen moleculerequires 4 photons, so the yield based on quanta is 1/4)• So, we know chlorophyll is involved in photosynthesis, and it takes at least 4electrons to produce O2. This reaction has light and dark reactions, andinvolves a lot of chl molecules• 1943, Emerson and Lewis asked what the other pigments (besides Chl a)were doing…• Shined other color lights on algae, discovered it’s not as efficient• When they shined far-red light, very inefficient (red drop)• Shine blue AND red light, more efficient than either one by itself• Means there must be TWO independent light reactions (PSII, PSI)• PSI does NOT produce oxygen!• So, Von Warburg was technically correct in that it takes 4 electrons toproduce O2, but it takes 8 electrons to complete photosynthesis• In Summary, we now know that algae (and plants, and bacteria) achievephotosynthesis by using a group of pigment molecules working together…theentire functional unit (the minimum collection of pigments, etc. needed tophotosynthesize) is made up of a collection of pigments and biochemicalreactants:PSU – Photosynthetic Unit, the functional unit of photosynthesisPS II – Photosystem II (also called the Reaction Center), where Oxygen isproducedPSI – Photosystem I, necessary to complete photosynthesisOcea 130/230Ocean Processes and Ecology PhotosynthesisAntenna Pigments – all the rest of the pigments associated with the PSULight Harvesting Complex – the physical structure of the antenna pigmentsC. Quantum Yields of Photosynthesis• Quantum Yield is simply the efficiency with which a reaction proceeds….howfast does the reaction go as a function of the number of absorbed photons?• Moles of photons absorbed per moles product• NOT going to be equivalent for different products (for example, quantumyield of oxygen doesn’t have to equal the quantum yield of CO2 fixation)• Why did the Von Warburg Experiment come up with the wrong answer?• He was a feared Nobel Laureate at that time, so nobody wanted toquestion his results—he could ruin your career!• Von Warburg used CO2 starved Chlorella (a green alga),and whitelight…so he was actually shining blue and red light on the algal culture• Came up with quantum yield for O2 of 0.25, meaning you need toabsorb 4 electrons for each O2 produced• Emerson in 1950s, said it was 0.10 to 0.125 (or about 8 electrons)D. Effective Absorption Cross-Sections• If you flash an algal culture with bright light, the oxygen produced increases ina Poisson Function, which can be described as:Y/Ymax = 1-e(-σE)Where Y is the yield (oxygen production), sigma is the slope, and Eis the flash energy• The “effective” cross section means it doesn’t have anything to do with thephysical size…it’s a theoretical measurement, that makes it easier tomeasure• Quantum Yield of Photosynthesis is the relative ratio of the cross-section ofPSII to the entire PSU• So, if the ONLY reaction is absorption by PSII and PSI, maximum yield is0.125Sigma PSUSigma PSIISigma PSIOcea 130/230Ocean Processes and Ecology PhotosynthesisE. Physical Structure of Photosynthesis• To make photosynthesis work, we must have the PSU embedded in amembrane, which provides physical support, access to chemicals (CO2,nutrients, etc) and sets up a gradient for the products (to keep the energysources and sinks separated)• The cellular organelle responsible for photosynthesis is the chloroplast• The thylakoid is the structural unit of photosynthesis.• Both photosynthetic prokaryotes and eukaryotes have these flattenedsacs/vesicles containing photosynthetic chemicals.• Only eukaryotes have chloroplasts with a surrounding membrane.• Thylakoids are stacked like pancakes in stacks known collectively as grana.• The areas between grana are referred to as stroma• Although this is all very complicated, it’s not really well known why the structureis so convoluted…for our purposes, it’s enough to know that the PSUs areembedded in a cellular organelle• A chemical gradient