sap. When fully matured, they have no nucleus, and only a handful of organelles. This allows them to be highly specified, and very efficient at transport, since they are not taking any of the solutes they are transporting. These cells are connected to form the full tube by their plasmodesmata. From here, the solutes traveling through the phloem can move either as a symplast, or apoplast. The loading and unloading of phloem sap is done mainly by pressure flow, and relies on loading of the cells and unloading of the cells happening at the same time to maintain the turgor pressure of the system. Sap components There are two different types of sap that are in a plant. These types are xylem and phloem sap, both differing in their compositions. Sap that is transported in the phloem is mainly made of water. The second most abundant substance is sucrose. One study found that the rice plant Oryza sativa had a sucrose concentration of 570 nm, but the sucrose concentration is unique to each organism. Another important component of phloem sap is nitrogen. Nitrogen is usually not being transported in its ionic form. Instead it is incorporated in amino acids such as glutamate and aspartate. Hormones, inorganic ions, RNA, and proteins are found in the phloem sap as well. Xylem sap is mostly made of water. This is because one of the main roles of xylem is to transport water and inorganic nutrients throughout the plant. Water is not the only thing that makes up xylem sap though. Xylem sap contains long-distance signaling hormones, proteins, enzymes, and transcription factors. One study found that proteins transported in this sap can be as large as 31 kDa. References Plant physiology