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LECTURE 7 PHOTOSYNTHESIS & CARBON FIXATION From lec. 2, Kluyver and van Niel proposed that all photosynthetic reactions could be summarized with: Photosynthesis is the conversion of light energy to chemical energy, usually using only CO2 as a carbon source. Photosynthesis exists in all 3 domains of life, although eukaryotes are photosynthetic only via the presence of symbiotic organelles. CO2 + 2 H2A ----> CH2O (cell material) + H2O + 2 A CO2 is being reduced Phototrophy is wide spread in bacteria and appears to be a very ancient trait --- H2A is being oxidized fossil stromatolites in rock over 3 billion years old show photosynthetic prokaryotes. Fig. 10.14. Modern day stromatolites in Australia Table 6.9 Five main groups of Bacteria are photosynthetic: • Purple bacteria (sulfur and non-sulfur) • Green sulfur bacteria • Green nonsulfur bacteria • Heliobacteria • Cyanobacteria 2 parts to photosynthesis: • light reactions - light energy is trapped by molecules and converted to chemical energy (ATP + NADPH) • dark reactions - this chemical energy is used to reduce CO2. Photosynthesis is perhaps the most important biological process on the planet. •Produces most organic carbon on Earth •These organisms completely transformed the planet. The earliest photosynthesis was anoxygenic - did not produce O2 gas. Oxygenic photosynthesis created our atmosphere, including the ozone layer which allowed life to evolve onto land. Need molecule that is capable of absorbing light. **CHLOROPHYLLS** Similar in structure to cytochromes from the ETC, but instead of iron in their center, they have a magnesium atom. Photosynthetic bacteria also have chlorophylls, though there is a much greater diversity in the types of molecules. Purple bacteria - chlorophylls a and b Green sulfur bacteria - chlorophylls c, d, and e Green nonsulfur bacteria - chlorophyll cS Heliobacteria - chlorophyll g For example, purple bacteria have bacteriochlorophyll a… In prokaryotes, the chlorophylls are intergrated into internal membrane systems in different places in different bacteria: • purple bacteria - invagination of the membrane • heliobacteria - the membrane itself • green bacteria - both the membrane and in membrane-enclosed structures called chlorosomes In addition to chlorophylls, organisms may have other pigments such as carotenoids and phycobiliproteins. These accessory pigments absorb light in other areas of the spectrum and can transfer the trapped energy to chlorophyll. • cyanobacteria - thylakoid membranes Chlorophylls and accessory pigments are arranged in arrays called antennas (fig. 6.26). Fig. 6.26. Light energy being used to excite an electron Remember that as electrons flow down an electrochemical gradient, energy is released. A similar process occurs in photosynthetic organisms, i.e. they use membrane-bound proteins to pass electrons down a gradient, generating a proton motive force. This is called photophosphorylation. Because electrons are being driven out of chlorophyll using light energy, they must be replaced. • oxygenic photosynthesis - the source is water. O2 is produced as a byproduct. Oxygenic Photosynthesis Eukaryotes, cyanobacteria and some other bacteria Use water as an electron source and produce oxygen as a final product. • Anoxygenic photosynthesis - the source is reduced sulfur compounds, organic compounds, hydrogen gas or Fe(II) Oygenic photosynthesis involves 2 distinct, but interconnected reactions: The products of these light reactions then are: • Photosystem I - chlorophyll P700 and absorbs light at long wavelengths (far red). • ATP (non-cyclic photophosphorylation) • Photosystem II - chlorophyll P680 and absorbs light at shorter wavelengths (near red). •O2 (from splitting of water) • NADH (Where the electrons end up..) Fig. 6.27. The path of electron flow looks like a Z turned on its side, so it is often called the “Z scheme”. The process we just talked about was oxygenic photosynthesis and was used by cyanobacteria and eukaryotes. Other bacteria use anoxygenic photosynthesis, i.e. harvest light energy to synthesize ATP without water as an electron source (no O2 is produced). In these organisms, light energy also excites a pigment molecule. Electrons also cascade down a series of molecules, to drive photophosphoryllation to generate ATP. If enough carbon is present - cyclic electron flow = cyclic photophosphorylation Green Sulfur Bacteria Use H2S or S2O32- as electron donors, thus no O2 produced, often do cyclic photophosphorylation. Table 6.9 The most common way that organisms fix carbon is via the Calvin cycle. This is used by cyanobacteria, purple bacteria, algae, and some Archaea. See fig. 6.28 At the end of one turn of the Calvin cycle, we have: • regenerated our 1,5-ribulose biphosphate Those were all light reactions. The next steps, called the dark reactions, involve the fixation of inorganic carbon using the ATP and NADH generated in the light reactions. In order to form 6-carbon sugars such as glucose or fructose, the Calvin cycle must turn 6 times. The incorporation of one CO2 into an organic compound requires 3 ATP and 2 NADPH. Therefore, the formation of glucose requires 18 ATP and 12 NADPH. • incorporated 1 CO2 • freed a carbon for biosynthesis Thus, we can sum up the equation for the formation of glucose as follows: 6 CO2 + 12 NADPH + 18ATP -> C6H12O6 + 12 NADP+ + 18 ADP Green sulfur bacteria do not use the Calvin cycle. They just run the TCA cycle backwards - called reductive TCA.