Phloem transports sugars and amino acids dissolved in water. The xylem transports water and minerals from the roots up the plant stem and into the leaves. In a mature flowering plant or tree, most of the cells that make up the xylem are specialised cells called vessels. Transport in the xylem is a physical process.
It does not require energy. Phloem moves sugar that the plant has produced by photosynthesis to where it is needed for processes such as:.
Transport in the phloem is therefore both up and down the stem. Transport of substances in the phloem is called translocation. In order to track down the function of the SWEET proteins, they switched off the corresponding genes in a series of plants. This enabled them to discover that, when their SWEETs do not work, plants have a considerably higher sucrose content in their leaves. This discovery represents an important development for plant breeding as, in many cases, the parts of plants used by humans, like seeds and tubers, do not form any carbohydrates themselves but are, instead, supplied by the leaves.
Moreover, the SWEETs provide a promising starting point for the protection of plants against pest infestations. For this reason, the scientists would now like to clarify the role of these transporters in pest infestation in greater detail.
The researchers suspect, moreover, that the corresponding pump proteins have a similar function in humans and animals. If confirmed, this would constitute a very important discovery for diabetes and obesity research, as the identity of the protein responsible for the transport of carbohydrate from the intestine into the blood and from liver cells is not yet known.
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The cotransport of a proton with sucrose allows movement of sucrose against its concentration gradient into the companion cells. From the companion cells, the sugar diffuses into the phloem sieve-tube elements through the plasmodesmata that link the companion cell to the sieve tube elements. Phloem sieve-tube elements have reduced cytoplasmic contents, and are connected by a sieve plate with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap.
Phloem is comprised of cells called sieve-tube elements. Phloem sap travels through perforations called sieve tube plates. Neighboring companion cells carry out metabolic functions for the sieve-tube elements and provide them with energy. Lateral sieve areas connect the sieve-tube elements to the companion cells. Image credit: OpenStax Biology. This increase in water potential drives the bulk flow of phloem from source to sink.
Unloading at the sink end of the phloem tube can occur either by diffusion , if the concentration of sucrose is lower at the sink than in the phloem, or by active transport , if the concentration of sucrose is higher at the sink than in the phloem. If the sink is an area of active growth, such as a new leaf or a reproductive structure, then the sucrose concentration in the sink cells is usually lower than in the phloem sieve-tube elements because the sink sucrose is rapidly metabolized for growth.
If the sink is an area of storage where sugar is converted to starch, such as a root or bulb, then the sugar concentration in the sink is usually lower than in the phloem sieve-tube elements because the sink sucrose is rapidly converted to starch for storage. But if the sink is an area of storage where the sugar is stored as sucrose, such as a sugar beet or sugar cane, then the sink may have a higher concentration of sugar than the phloem sieve-tube cells.
In this situation, active transport by a proton-sucrose antiporter is used to transport sugar from the companion cells into storage vacuoles in the storage cells.
Sucrose is actively transported from source cells into companion cells and then into the sieve-tube elements. This reduces the water potential, which causes water to enter the phloem from the xylem. Within the phloem, sugars travel from areas of high osmotic concentration and high water pressure, called sources, to regions of low osmotic concentration and low water pressure, called sinks.
Osmotic concentration refers the concentration of solutes, or sugars in this case; where the concentration of solutes is highest, so is the osmotic concentration. The nutrient-rich regions that supply sugars for the rest of the plant are called the sources.
Sources include the leaves, where sugar is generated through photosynthesis. When they are high in supplies, the nutrient storage areas, such as the roots and stems, can also function as sources. In the sources, sugar is moved into the phloem by active transport, in which the movement of substances across cell membranes requires energy expenditure on the part of the cell.
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