Photosynthesis

Photoautotrophic productivity largely refers to carbon assimilation by photosynthetic bacteria and the chloro-plast in eukaryotic organisms. In most photosynthetic bacteria, and in the chloroplast, the biochemistry of photosynthesis is nearly identical, reflecting a common origin in bacterial species that first assembled the photo-synthetic apparatus some 3 billion years ago. The major components of this biochemistry are light-harvesting protein complexes, electron-transport carriers, an ATP synthase, and soluble enzymes that assimilate CO2 and synthesize carbohydrates (Figure 1).

Photosynthesis begins with light absorbance by pigments embedded in protein complexes that float within

complex

Figure 1 A schematic of oxygen-evolving photosynthesis. Light is absorbed by pigments associated with photosystem II (PSII) and photosystem I (PSI) which excite electrons that leave a special pair of chlorophylls and pass through a series of electron-transport carriers, eventually reducing NADP to NADPH. Protons transported across the membrane in the reduction and oxidation of plastoquinone (PQ) are used to form ATP from ADP and phosphate. ATP and NADPH power the conversion of PGA to carbohydrates (hexoses, and then starch and sucrose). Arrows indicate the path of electron flow. OEC, oxygen-evolving complex; CYT B, cytochromeb6f complex; PC, plastocyanin; Fd, ferridoxin; NADP red., NADP + reductase.

complex

Figure 1 A schematic of oxygen-evolving photosynthesis. Light is absorbed by pigments associated with photosystem II (PSII) and photosystem I (PSI) which excite electrons that leave a special pair of chlorophylls and pass through a series of electron-transport carriers, eventually reducing NADP to NADPH. Protons transported across the membrane in the reduction and oxidation of plastoquinone (PQ) are used to form ATP from ADP and phosphate. ATP and NADPH power the conversion of PGA to carbohydrates (hexoses, and then starch and sucrose). Arrows indicate the path of electron flow. OEC, oxygen-evolving complex; CYT B, cytochromeb6f complex; PC, plastocyanin; Fd, ferridoxin; NADP red., NADP + reductase.

internal membrane systems of bacterial cells and chlor-oplasts. The light energy is passed from the pigments to a pair of chlorophyll molecules in the inner core of the light-harvesting apparatus. This inner core is termed photosystem II (PSII). Electrons in the pair of chlorophyll molecules in the PSII core are excited by the absorbed light to a higher energy state, which allows them to flow through a series of membrane-bound electron carriers to a second pigment-protein complex termed photosystem I (PSI; it was discovered and named before PSII). At PSI, the electrons reduce a second special pair of chlorophyll molecules. Light energy is absorbed by pigments associated with the PSI complex and this energy re-excites the electrons to a higher energy state. The excited electrons are then passed to NADP+, forming NADPH, in a series of oxidation-reduction reactions involving intermediate electron carriers. To replace the electrons passed from PSII to NADP+, electrons are pulled from water, splitting it into protons and O2. The O2 is released into the atmosphere as a waste product. Electron flow from PSII to PSI also moves protons across the internal membrane system, creating a proton gradient which powers the formation of ATP by an enzyme termed the ATP synthase (Figure 1).

The ATP and NADPH formed by the light-driven reactions of photosynthesis provide the energy to assimilate CO2 and convert it to energy-rich carbon compounds such as sugars, starches, lipids, and amino acids (Figure 1). The initial step in carbon assimilation is the fixation of CO2, catalyzed by the enzyme ribulose bisphosphate carboxylase/oxygenase (Rubisco). Rubisco catalyzes the reaction of CO2 with the five-carbon sugar ribulose bisphosphate (RuBP). The product of RuBP car-boxylation is two three-carbon compounds termed phosphoglycerate (PGA). For every 12 PGA molecules formed, 10 are processed back to RuBP to sustain CO2 fixation. The remaining two PGA molecules are processed to hexoses in a series of reactions that consume NADPH and ATP. Hexoses (principally glucose and fructose) are important metabolic intermediates that can be used to form storage pools of carbohydrate (mainly starches, which are polymers of glucose), or sucrose (a disaccharide of glucose and fructose) and other disac-charides for transport to the rest of the organism. Because sucrose is the most widely used transport sugar by higher plants, it is considered by plant biologists to be the end product of photosynthesis. A simple form of the reaction sequence for photosynthetic carbon assimilation of CO2 to sucrose is

! 1C12H22O11 (sucrose) + 12O2

In addition to carbon assimilation, autotrophy involves the assimilation of inorganic mineral nutrients required to produce new biomass. The most important inorganic nutrients are nitrogen in the form of nitrate and ammonia, phosphorus as phosphates, sulfur as sulfates, and various cations (potassium, calcium, magnesium, and iron). Large expenditures of carbon and energy are required for nutrient assimilation, both to pump the elements across membranes from the external environment, and to build and maintain assimilatory structures such as roots in higher plants. In land plants, 30-50% of the photosynthetic output is required for root growth and function. Once acquired, substantial energy is then needed to convert inorganic nutrients into useable organic forms. Assimilation of ammonia into amino acids requires one ATP and one NADH equivalent per ammonia. Nitrate assimilation to amino acids requires the energy equivalent of 14 ATP molecules, while sulfate assimilation into amino acids requires three ATP and five NADH equivalents per sulfate molecule. In organisms that fix nitrogen either directly (cyanobacteria) or indirectly via a symbiosis with bacteria (advanced plants such as legume species and alder trees), 16 ATP and 8 NADPH equivalents are required to assimilate one N2 molecule to two ammonia molecules.

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