In vascular plants, phloem is the living tissue that carries organic nutrients, particularly sucrose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word φλοιος (phloios) for "bark". The phloem is mainly concerned with the transport of glucose and starch made during photosynthesis. For other uses, see Plant (disambiguation). ... Biological tissue is a collection of interconnected cells that perform a similar function within an organism. ... Sucrose (common name: table sugar, also called saccharose) is a disaccharide (glucose + fructose) with the molecular formula C12H22O11. ... The coniferous Coast Redwood, the tallest tree species on earth. ... For other meanings of bark, see Bark (disambiguation). ... The leaf is the primary site of photosynthesis in plants. ...

Structure

Phloem tissue consists of less specialised and nucleate parenchyma cells, sieve-tube cells, and companion cells (in addition albuminous cells, fibers and sclereids). Parenchyma is a term used to describe a bulk of a substance. ...

Sieve Tubes

The sieve-tube cells lack a nucleus, have very few vacuoles, but contain other organelles such as ribosomes. The endoplasmic reticulum is concentrated at the lateral walls. Sieve-tube members are joined end to end to form a tube that conducts food materials throughout the plant. The end walls of these cells have many small pores and are called sieve plates and have enlarged plasmodesmata. Plasmodesmata (Singular, plasmodesma) are small cell junctions in a plant cell which connect the cytoplasm of adjacent plant cells, forming a circulatory and communication system connecting the cells in plant tissue. ...

Companion Cells

The survival of sieve-tube members depends on a close association with the companion cells. All of the cellular functions of a sieve-tube element are carried out by the (much smaller) companion cell, a typical plant cell, except the companion cell usually has a larger number of ribosomes and mitochondria. This is because the companion cell is more metabollically active than a 'typical' plant cell. The cytoplasm of a companion cell is connected to the sieve-tube element by plasmodesmata. Plant cell structure Plant cells are quite different from the cells of the other eukaryotic kingdoms organisms. ... Figure 1: Ribosome structure indicating small subunit (A) and large subunit (B). ... In cell biology, a mitochondrion is an organelle found in the cells of most eukaryotes. ... Organelles. ...

There are three types of companion cell.

1. Ordinary companions cells - which have smooth walls and few or no plasmodesmata connections to cells other than the sieve tube.
2. Transfer cells - which have much folded walls that are adjacent to non-sieve cells, allowing for larger areas of transfer. They are specialised in scavenging solutes from those in the cell walls which are actively pumped requiring energy.
3. Intermediary cells - which have smooth walls and numerous plasmodesmata connecting them to other cells.

The first two types of cell collect solutes through apoplastic (cell wall) transfers, whilst the third type can collect solutes symplastically through the plasmodesmata connections. Within a plant, the apoplast is the free diffusional space outside the plasma membrane. ... Apoplast and symplast are two related concept in plant biology. ...

Function

Unlike xylem (which is composed primarily of dead cells), the phloem is composed of still-living cells that transport sap. The sap is a water-based solution, but rich in sugars made by the photosynthetic areas. These sugars are transported to non-photosynthetic parts of the plant, such as the roots, or into storage structures, such as tubers or bulbs. In vascular plants, xylem is one of the two types of transport tissue in plants, phloem being the other one. ... Sap exuding (gummosis) from the stem of a koa tree, probably in response to surface damage Sap is the fluid carried in tubules inside a plant, circulating to distribute food and water to various parts of the plant. ... This article is about sugar as food and as an important and widely-traded commodity. ... For fungal genus, see tuber (genus). ...

The Pressure flow hypothesis was a hypothesis proposed by Ernst Munch in 1930 that explained the mechanism of phloem translocation^[1]. 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; phloem sap moves from sugar sources to sugar sinks by means of turgor pressure. A sugar source is any part of the plant that is producing or releasing sugar. During the plant's growth period, usually during the spring, storage organs such as the roots are sugar sources, and the plant's many growing areas are sugar sinks. The movement in phloem is bidirectional, whereas, in xylem cells, it is unidirectional (upward). Ernst Munch (1876-1946) was a German plant physiologist who proposed the pressure flow hypothesis. ... Chromosomal translocation of the 4th and 20th chromosome. ... Drawing of the structure of cork as it appeared under the microscope to Robert Hooke from Micrographia which is the origin of the word cell being used to describe the smallest unit of a living organism Cells in culture, stained for keratin (red) and DNA (green) The cell is the... Look up foliage in Wiktionary, the free dictionary. ... diffusion (disambiguation). ... Turgor (also called turgor pressure or osmotic pressure) is the pressure that can build in a space that is enclosed by a membrane that is permeable to a solvent of a solution such as water but not to the solutes of the soluton. ... For other uses, see Root (disambiguation). ...

After the growth period, when the meristems are dormant, the leaves are sources, and storage organs are sinks. Developing seed-bearing organs (such as fruit) are always sinks. Because of this multi-directional flow, coupled with the fact that sap cannot move with ease between adjacent sieve-tubes, it is not unusual for sap in adjacent sieve-tubes to be flowing in opposite directions. A meristem is a tissue in plants consisting of undifferentiated cells (meristematic cells) and found in zones of the plant where growth can take place. ... Look up foliage in Wiktionary, the free dictionary. ... A ripe red jalapeño cut open to show the seeds For other uses, see Seed (disambiguation). ... For other uses, see Fruit (disambiguation). ...

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. Hydrostatic pressure is the pressure exerted by a fluid due to its weight. ... Sodium-Potassium pump, an example of Primary active transport secondary active transport Active transport (sometimes called active uptake) is the mediated transport of biochemicals, and other atomic/molecular substances, across membranes. ... Osmosis is the net movement of water across a partially permeable membrane from a region of high solvent potential to an area of low solvent potential, up a solute concentration gradient. ...

Some plants however appear not to load phleom by active transport. In these cases a mechanism known as the polymer trap mechanism was proposed by Robert Turgeon^[2]. In this case small sugars such as sucrose move into intermediary cells through narrow plasmodesmata, where they are polymerised to raffinose and other larger oligosaccharides. Now they are unable to move back, but can proceed through wider plasmodesmata into the sieve tube element. Raffinose is a complex carbohydrate, a trisaccharide composed of galactose, fructose and glucose. ... The term “oligosaccharide” refers to a short chain of sugar molecules (“oligo” means “few” and “saccharide” means “sugar. ...

The symplastic phloem loading is mostly confined to plants in tropical rain forests and is seen as more primitive. The actively transported apoplastic phloem loading is viewed as more advanced, as it is found in the later evolved plants, and particularly in those in temperate and arid conditions. This mechanism may therefore have allowed plants to colonise the cooler locations.

Organic molecules such as sugars, amino acids, certain hormones, and even messenger RNAs are transported in the phloem through sieve tube elements. 3D (left and center) and 2D (right) representations of the terpenoid molecule atisane. ... Phenylalanine is one of the standard amino acids. ... Norepinephrine A hormone (from Greek όρμή - to set in motion) is a chemical messenger from one cell (or group of cells) to another. ... The life cycle of an mRNA in a eukaryotic cell. ... The names Sieve tube element and Sieve tube member are synonyms and are used to describe a certain type of elongated cell in phloem tissue. ...

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. A meristem is a tissue in plants consisting of undifferentiated cells (meristematic cells) and found in zones of the plant where growth can take place. ... The vascular cambium is a lateral meristem: The vascular cambium is the source of both the secondary xylem (inwards) and the secondary phloem (outwards), and hence is located between these tissues in the stem. ... It has been suggested that this article or section be merged with Meristem. ... The vascular cambium is a lateral meristem: The vascular cambium is the source of both the secondary xylem (inwards) and the secondary phloem (outwards), and hence is located between these tissues in the stem. ...

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. Recently, pettu has again become available as a curiosity, and some have made claims of health benefits. For other uses, see Pine (disambiguation). ... <nowiki>Insert non-formatted text hereBold text</nowiki>A famine is a social and economic crisis that is commonly accompanied by widespread malnutrition, starvation, epidemic and increased mortality. ... This article does not cite any references or sources. ... Binomial name Secale cereale M.Bieb. ...

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 since 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. In vascular plants, xylem is one of the two types of transport tissue in plants, phloem being the other one. ... It has been suggested that this article or section be merged with Carbon capture and storage. ...

See also

• Xylem
• Apical dominance

In vascular plants, xylem is one of the two types of transport tissue in plants, phloem being the other one. ... Many conifers show particularly strong apical dominance, strongest of all in the family Araucariaceae, showing a single erect central trunk with strongly differentiated horizontal branching. ...

References

1. ^ Münch, E (1930). "Die Stoffbewegunen in der Pflanze". Verlag von Gustav Fischer, Jena: 234. 
2. ^ Turgeon, R (1991). "Symplastic phloem loading and the sink-source transition in leaves: a model". VL Bonnemain, S Delrot, J Dainty, WJ Lucas, (eds) Recent Advances Phloem Transport and Assimilate Compartmentation.