Difference between revisions of "Phloem" - New World Encyclopedia
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Unlike [[xylem]] (which is composed primarily of dead cells), the phloem is composed of still-living cells that transport sap. Phloem sap is rich in sugar and is made in photosynthetic areas. The sugars are transported to non-photosynthetic parts of the plant, such as the roots, or into storage structures, like tubers or bulbs. | Unlike [[xylem]] (which is composed primarily of dead cells), the phloem is composed of still-living cells that transport sap. Phloem sap is rich in sugar and is made in photosynthetic areas. The sugars are transported to non-photosynthetic parts of the plant, such as the roots, or into storage structures, like tubers or bulbs. | ||
| − | + | The movement in phloem is variable, whereas in xylem cells it is unidirectional (upward). Phloem sap moves from a ''sugar source'' to ''sugar sink''. A sugar source is any part of the plant that is producing sugar by photosynthesis or releasing sugar by breaking down starch. Leaves are the main source of sugar. Sugar sinks are storage organs that consume water or sugar. Developing [[seed]]-bearing organs (such as [[fruit]]) are always sinks. Storage organs, including tubers and bulbs, can be a source or a sink depending on the time of year. During the plant's growth period, usually during the spring, storage organs break down, providing sugar for sinks in the plant's many growing areas. After the growth period, storage organs store carbohydrates, becoming sinks. Because of this multi-directional flow, coupled with the fact that sap cannot move easily between adjacent sieve-tubes, it is not unusual for sap in adjacent sieve-tubes to be flowing in opposite directions. | |
| − | + | In 1930, German plant physiologist Ernst Munch proposed the '''Pressure flow hypothesis''' to explain the mechanism of phloem [[translocation]]. A high concentration of organic substance inside [[Cell (biology)|cell]]s of the phloem at a source, such as a [[leaf]], creates a [[diffusion|diffusion gradient]] that draws water into the cells. Movement occurs by bulk flow; | |
While movement of water and minerals through the xylem is driven by negative pressures (tension) most of the time, movement through the phloem is driven by positive [[hydrostatic pressure]]s. This process is termed ''translocation'', and is accomplished by a process called ''phloem loading'' and ''unloading''. Cells in a sugar source "load" a sieve-tube element by [[active transport|actively transporting]] solute molecules into it. This causes water to move into the sieve-tube element by [[osmosis]], creating pressure that pushes the sap down the tube. In sugar sinks, cells actively transport solutes ''out'' of the sieve-tube elements, producing the exactly opposite effect. | While movement of water and minerals through the xylem is driven by negative pressures (tension) most of the time, movement through the phloem is driven by positive [[hydrostatic pressure]]s. This process is termed ''translocation'', and is accomplished by a process called ''phloem loading'' and ''unloading''. Cells in a sugar source "load" a sieve-tube element by [[active transport|actively transporting]] solute molecules into it. This causes water to move into the sieve-tube element by [[osmosis]], creating pressure that pushes the sap down the tube. In sugar sinks, cells actively transport solutes ''out'' of the sieve-tube elements, producing the exactly opposite effect. | ||
Revision as of 16:57, 19 July 2006
In vascular plants (all those except mosses and their relatives), phloem is the living tissue that carries sugar and organic nutrients throughout the plant. The other type of transport tissue in plants is xylem, which transports water. In trees, the phloem and other tissues make up the bark, hence its name, derived from the Greek word for "bark".
Structure
Sap is transported through phloem in elongated tubes, called sieve tubes, formed by chains of living cells called sieve tube members. The sieve-tube cells lack a nucleus, ribosomes, and a distinct vacuole. In angiosperms, at the end wall of sieve-tube members are pores, called sieve plates, through which phloem sap flows.
Beside each sieve-tube member is a companion cell, which connects to sieve-tube cells by many channels, or plasmodesmata, in the cell wall. Companion cells carry out all of the cellular functions of a sieve-tube element, and the nucleus and ribosomes of a companion cell may serve one or more adjacent sieve-tube cells.
Function
Unlike xylem (which is composed primarily of dead cells), the phloem is composed of still-living cells that transport sap. Phloem sap is rich in sugar and is made in photosynthetic areas. The sugars are transported to non-photosynthetic parts of the plant, such as the roots, or into storage structures, like tubers or bulbs.
The movement in phloem is variable, whereas in xylem cells it is unidirectional (upward). Phloem sap moves from a sugar source to sugar sink. A sugar source is any part of the plant that is producing sugar by photosynthesis or releasing sugar by breaking down starch. Leaves are the main source of sugar. Sugar sinks are storage organs that consume water or sugar. Developing seed-bearing organs (such as fruit) are always sinks. Storage organs, including tubers and bulbs, can be a source or a sink depending on the time of year. During the plant's growth period, usually during the spring, storage organs break down, providing sugar for sinks in the plant's many growing areas. After the growth period, storage organs store carbohydrates, becoming sinks. Because of this multi-directional flow, coupled with the fact that sap cannot move easily between adjacent sieve-tubes, it is not unusual for sap in adjacent sieve-tubes to be flowing in opposite directions.
In 1930, German plant physiologist Ernst Munch proposed the Pressure flow hypothesis to explain the mechanism of phloem translocation. A high concentration of organic substance inside cells of the phloem at a source, such as a leaf, creates a diffusion gradient that draws water into the cells. Movement occurs by bulk flow;
While movement of water and minerals through the xylem is driven by negative pressures (tension) most of the time, movement through the phloem is driven by positive hydrostatic pressures. This process is termed translocation, and is accomplished by a process called phloem loading and unloading. Cells in a sugar source "load" a sieve-tube element by actively transporting solute molecules into it. This causes water to move into the sieve-tube element by osmosis, creating pressure that pushes the sap down the tube. In sugar sinks, cells actively transport solutes out of the sieve-tube elements, producing the exactly opposite effect.
Organic molecules such as sugars, amino acids, certain hormones, and even messenger RNAs are transported in the phloem through sieve tube elements.
Origin
The phloem originates, and grows outwards from, meristematic cells in the vascular cambium. Phloem is produced in phases. Primary phloem is laid down by the apical meristem. Secondary phloem is laid down by the vascular cambium to the inside of the established layer(s) of phloem.
Nutritional use
Phloem of pine trees has been used in Finland as a substitute food in times of famine, and even in good years in the northeast, where supplies of phloem from earlier years helped stave off starvation somewhat in the great famine of the 1860s . Phloem is dried and milled to flour (pettu in Finnish) and mixed with rye to form a hard dark bread (Vanharanta 2002). Since the late 1990s, pettu has again become available as a curiosity, and some have made claims of health benefits (Mursu 2005; Vanharanta 1999).
Girdling
Because phloem tubes sit on the outside of the xylem in most plants, a tree or other plant can be effectively killed by stripping away the bark in a ring on the trunk or stem. With the phloem destroyed, nutrients cannot reach the roots and the tree/plant will die. Trees located in areas with animals such as beavers are vulnerable the beavers chew off the bark at a fairly precise height. This process is known as girdling, and is used in agricultural purposes. For example, enormous fruits and vegetables seen at fairs and carnivals are produced via girdling. A farmer would place a girdle at base of a large branch, and remove all but one fruit/vegetable from that branch. Thus, all the sugars manufactured by leaves on that branch have no sinks to go to but the one fruit/vegetable which thus expands to many times normal size.
See also
ReferencesISBN links support NWE through referral fees
- Campbell, N. A., and J. B. Reece. 2002. Biology (6th edition). San Francisco, CA: Benjamin Cummings
- Levine, J. S., and K. R. Miller. 1991. Biology: Discovering Life. Lexington, MA: D. C. Heath and Company
- Mursu, J., et al. 2005. Polyphenol-rich phloem enriches the resistance of total serum lipids to oxidation in men. Journal of Agricultural and Food Chemistry. 53(8):3017-22.
- Vanharanta M., et al. 2002. Phloem fortification in rye bread elevates serum enterolactone level. European Journal of Clinical Nutrition 56(10):952-7.
- Vanharanta, M., S. Voutilainen, T. A. Lakka, M. van der Lee, H. Adlercreutz, and J. T. Salonen. 1999. Risk of acute coronary events according to serum concentrations of enterolactone: a prospective population-based case-control study. Lancet 354:2112-2115.
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