In
vascular plantVascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the ferns, clubmosses, flowering plants, conifers and other gymnosperms...
s,
phloem is the living
tissueTissue is a cellular organizational level intermediate between cells and a complete organism. Hence, a tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function...
that carries organic
nutrientA nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. Nutrients are the substances that enrich the body. They build and repair tissues, give heat and energy, and regulate body processes...
s (known as photosynthate), particularly
sucroseSucrose, commonly called table sugar, is a moosaccharide of glucose and fructose with the molecular formula C12H22O11. This white, odorless, crystalline powder has a pleasing, sweet taste. It is best known for its role in human nutrition...
, a sugar, to all parts of the plant where needed. In
treeA tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...
s, the phloem is the innermost layer of the
barkBark is the outermost layers of stems and roots of woody plants. Plants with bark include trees, woody vines and shrubs. Bark refers to all the tissues outside of the vascular cambium and is a nontechnical term. It overlays the wood and consists of the inner bark and the outer bark. The inner...
, hence the name, derived from the
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
word (
phloos) "bark". The phloem is concerned mainly with the transport of soluble organic material made during
photosynthesisPhotosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea...
. This is called translocation.
Structure
Phloem tissue consists of less specialized and nucleate
parenchymaParenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, from Greek parenkhuma, visceral flesh, from parenkhein, to pour in beside : para-, beside + en-, in + khein, to pour.- In animals :The parenchyma are the...
cells,
sieve-tube cells, and
companion cells (in addition
albuminous cells, fibres and
sclereids).
Sieve tubes
The sieve-tube cells lack a
nucleusIn cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as...
, have very few
vacuolethumb|400px|Plant cell structurethumb|400px|Animal cell structureA vacuole is a membrane organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells...
s, but contain other organelles such as
ribosomeRibosomes are complexes of RNA and protein that are found in all cells. The ribosome is part of the mechanism that translates the DNA sequence into the protein sequence. Ribosomes from bacteria, archaea and eukaryotes , have significantly different structure and RNA...
s. The
endoplasmic reticulumThe endoplasmic reticulum is an eukaryotic organelle that forms an interconnected network of tubules, vesicles, and cisternae within cells. The lacey membranes of the endoplasmic reticulum were first seen by Keith R. Porter, Albert Claude, and Ernest F...
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
plasmodesmataPlasmodesmata are microscopic channels which traverse the cell walls of plant cells and some algal cells enabling transport and communication between them. Species that have plasmodesmata include members of the Charophyceae, Charales and Coleochaetales , as well as all embryophytes, better known...
.
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 cellPlant cells are eukaryotic cells that differ in several key respects from the cells of other eukaryotic organisms. Their distinctive features include:...
, 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
cytoplasmThe cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the rest of the cytoplasm by biological membranes. The contents of the cell nucleus...
of a companion cell is connected to the sieve-tube element by plasmodesmata.
There are three types of companion cell.
- Ordinary companions cells - which have smooth walls and few or no plasmodesmata connections to cells other than the sieve tube.
- Transfer cells
Transfer cells are specialized parenchyma cells that have an increased surface area, due to infoldings of the plasma membrane. They facilitate the transport of sugars from a sugar source, mainly leaves, to a sugar sink, often developing fruits....
- 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 that are actively pumped requiring energy.
- Intermediary cells - which have smooth walls and numerous plasmodesmata connecting them to other cells.
The first two types of cell collect solutes through
apoplastWithin a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the Casparian strip in roots, air spaces between plant cells and the cuticula of the plant....
ic (cell wall) transfers, whilst the third type can collect solutes via the
symplastThe symplast of a plant is the inner side of the plasma membrane in which water can freely diffuse.The plasmodesmata allow the direct flow of small molecules such as sugars, amino acids, and ions between cells...
through the plasmodesmata connections.
Function
Unlike
xylemIn vascular plants, xylem is one of the two types of transport tissue, phloem being the other. The word "xylem" is derived from classical Greek ξυλον , "wood", and indeed the best-known xylem tissue is wood, though it is found throughout the plant...
(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
sugarSugar is a class of edible crystalline substances, mainly sucrose, lactose, and fructose. Human taste buds interpret its flavor as sweet. Sugar as a basic food carbohydrate primarily comes from sugar cane and from sugar beet, but also appears in fruit, honey, sorghum, sugar maple , and in many...
s 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
tuberTubers are various types of modified plant structures that are enlarged to store nutrients. They are used by plants to overwinter and regrow the next year and as a means of asexual reproduction. Two different groups of tubers are: stem tubers, and root tubers.- Stem tubers :A stem tuber forms from...
s or bulbs.
The
Pressure flow hypothesis was a hypothesis proposed by
Ernst MunchErnst Münch was a German plant physiologist who proposed the Pressure Flow Hypothesis in 1926.He worked in Aschaffenburg and Munich with Robert Hartig. He worked in a number of fields including Forest pathology, resin production, and fungi...
in 1930 that explained the mechanism of phloem translocation. A high concentration of organic substance inside
cellThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos...
s of the phloem at a source, such as a
leafIn botany, a leaf is an above-ground plant organ specialized for photosynthesis. For this purpose, a leaf is typically flat and thin. There is continued debate about whether the flatness of leaves evolved to expose the chloroplasts to more light or to increase the absorption of carbon dioxide. In...
, creates a
diffusion gradientMolecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of material...
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
rootIn vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s 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).
After the growth period, when the
meristemA meristem is the tissue in all plants consisting of undifferentiated cells and found in zones of the plant where growth can take place....
s are dormant, the
leavesIn botany, a leaf is an above-ground plant organ specialized for photosynthesis. For this purpose, a leaf is typically flat and thin. There is continued debate about whether the flatness of leaves evolved to expose the chloroplasts to more light or to increase the absorption of carbon dioxide. In...
are sources, and storage organs are sinks. Developing
seedA seed , referred to as a kernel in some plants, is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
-bearing organs (such as
fruitThe term fruit has different meanings dependent on context, and the term is not synonymous in food preparation and biology. Fruits are the means by which flowering plants disseminate seeds, and the presence of seeds indicates that a structure is most likely a fruit, though not all seeds come from...
) 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.
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 transportingActive transport is the mediated process of moving particles across a biological membrane against a concentration gradient. If the process uses chemical energy, such as from adenosine triphosphate , it is termed primary active transport. Secondary active transport involves the use of an...
solute molecules into it. This causes water to move into the sieve-tube element by
osmosisOsmosis is the diffusion of water through a semi-permeable membrane. More specifically, it is the movement of water across a semi-permeable membrane from an area of high water potential to an area of low water potential...
, 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.
Some plants however appear not to load phloem by active transport. In these cases a mechanism known as the polymer trap mechanism was proposed by Robert Turgeon. In this case small sugars such as sucrose move into intermediary cells through narrow plasmodesmata, where they are polymerised to
raffinoseRaffinose is a trisaccharide composed of galactose, fructose, and glucose. It can be found in beans, cabbage, brussels sprouts, broccoli, asparagus, other vegetables, and whole grains. Raffinose can be hydrolyzed to D-galactose and sucrose by the enzyme α-galactosidase , an enzyme not found in the...
and other larger oligosaccharides. Now they are unable to move back, but can proceed through wider plasmodesmata into the sieve tube element.
The symplastic phloem loading (polymer trap mechanism above) is confined mostly 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
moleculeA molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense...
s such as sugars,
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
s, certain
hormoneA hormone is a chemical released by one or more cells that affects cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. It is essentially a chemical messenger that transports a signal from one cell to another. All multicellular organisms...
s, and even
messenger RNAMessenger ribonucleic acid is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino...
s are transported in the phloem through
sieve tube elementIn plant anatomy, sieve vascular tissue tube elements, also called sieve tube members, are a type of elongated parenchyma cells in phloem tissue. At the ends these cells are connected with other sieve elements, and together they constitute the sieve tube...
s.
Girdling
Because phloem tubes sit on the outside of the
xylemIn vascular plants, xylem is one of the two types of transport tissue, phloem being the other. The word "xylem" is derived from classical Greek ξυλον , "wood", and indeed the best-known xylem tissue is wood, though it is found throughout the plant...
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 can be used for 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
sinksA carbon sink is a natural or manmade reservoir that accumulates and stores some carbon-containing chemical compound for an indefinite period.The main natural sinks are:*Absorption of carbon dioxide by the oceans*Photosynthesis by plants and algae...
to go to but the one fruit/vegetable, which thus expands to many times normal size.
Origin
The phloem originates, and grows outwards from,
meristemA meristem is the tissue in all plants consisting of undifferentiated cells and found in zones of the plant where growth can take place....
atic cells in the
vascular cambiumThe vascular cambium is a lateral meristem in the vascular tissue of plants. The vascular cambium is the source of both the secondary xylem and the secondary phloem , and is located between these tissues in the stem and root...
. Phloem is produced in phases.
Primary phloem is laid down by the apical meristem.
Secondary phloem is laid down by the
vascular cambiumThe vascular cambium is a lateral meristem in the vascular tissue of plants. The vascular cambium is the source of both the secondary xylem and the secondary phloem , and is located between these tissues in the stem and root...
to the inside of the established layer(s) of phloem.