forms that can be rapidly synthesized and degraded and may buffer the intracellular sugar concentrations. The intraradical hexose enters the oxidative pentose phosphate pathway, which produces pentose for nucleic acids. Lipid biosynthesis also occurs in the intraradical mycelium. Lipids are then stored or exported to extraradical hyphae where they may be stored or metabolized. The breakdown of lipids into hexoses, known as gluconeogenesis, occurs in the extraradical mycelium. Approximately 25% of the carbon translocated from the plant to the fungi is stored in the extraradical hyphae. Up to 20% of the host plant's carbon may be transferred to the AM fungi. This represents the host plant's considerable carbon investment in mycorrhizal network and contribution to the below-ground organic carbon pool. Increasing the plant's carbon supply to the AM fungi increases uptake and transfer of phosphorus from fungi to plant Likewise, phosphorus uptake and transfer is lowered when the photosynthate supplied to the fungi is decreased. Species of AMF differ in their abilities to supply the plant with phosphorus. In some cases, arbuscular mycorrhizae are poor symbionts, providing little phosphorus while taking relatively high amounts of carbon. The main benefit of mycorrhizas to plants has been attributed to increased uptake of nutrients, especially phosphorus. This may be due to increased surface area in contact with soil, increased movement of nutrients into mycorrhizae, a modified root environment, and increased storage. Mycorrhizas can be much more efficient than plant roots at taking up phosphorus. Phosphorus travels to the root or via diffusion and hyphae reduce the distance required for diffusion, thus increasing uptake. The rate of phosphorus flowing into mycorrhizae can be up to six times that of the root hairs. In some cases, the role of phosphorus uptake can be completely taken over by the mycorrhizal network, and all of the plant's phosphorus may be of hyphal origin. Less is known about the role of nitrogen nutrition in the arbuscular mycorrhizal system and its impact on the symbiosis and community. While significant advances have been made in elucidating the mechanisms of this complex interaction, much investigation remains to be done. Mycorrhizal activity increases the phosphorus concentration available in the rhizosphere. Mycorrhizae lower the root zone pH by selective uptake of NH4+ (ammonium-ions) and by releasing H+ ions. Decreased soil pH increases the solubility of phosphorus precipitates. The hyphal NH4+ uptake also increases the nitrogen flow to the plant as the soil's inner