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From Wikipedia

Phloem

In vascular plants, phloem is the living tissue that carries organic nutrients (known as photosynthate), 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 φλόος (phloos) "bark". The phloem is concerned mainly with the transport of soluble organic material made during photosynthesis. This is called translocation.

Structure

Phloem tissue consists of less specialized and nucleate parenchymacells, sieve-tube cells, and companion cells (in addition albuminous cells, fibres andsclereids).

Sieve tubes

The sieve-tube cells lack a nucleus, have very few vacuoles, but contain other organelles such as ribosomes. The sieve tube is an elongated rank of individual cells, called sieve-tube members, arranged end to end. 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.

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 metabolically active than a 'typical' plant cell. The cytoplasm of a companion cell is connected to the sieve-tube element by plasmodesmata.

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 that 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 via the symplast through the plasmodesmata connections.

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.

The Pressure flow hypothesis was a hypothesis proposed by Ernst Munch in 1930 that explained 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; phloem sap moves from sugar sources to sugar sinks by means of turgor pressure gradient. 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 multidirectional, whereas, in xylem cells, it is unidirectional (upward).

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.

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.

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 intermed


From Yahoo Answers

Question:

Answers:Xylem transports water and inorganic solutes, and phloem transports organic solutes (mainly sucrose) in water. Also, xylem is composed of dead cells, whereas phloem is composed of living cells.

Question:Hi I have a science test tomorrow and I stupidly haven't studied. It is about plants and I just wanted to know the simple difference between a phloem and a xylem. Nothing too fancy, just a very basic deffinition of each please!

Answers:Both xylem and phloem are vascular tissues found in a plant. Xylem is a tubular structure which is responsible for water transport from the roots towards all of the parts of the plant. Phloem is also a tubular stucture which, on the other hand, is responsible for the transportation of food and other nutrients needed by plant. Xylem: Hard wall cells transport water and mineral nutrients in a kind of tissue called Xylem. Phloem: Relatively soft -walled cells transport organic nutrients in a kind of tissue called Phloem. Hope I helped! Good luck on your test!

Question:i have a bio final, and i find it hard to identify them.

Answers:Both xylem and phloem are vascular tissues found in a plant. Xylem is a tubular structure which is responsible for water transport from the roots towards all of the parts of the plant. Phloem is also a tubular stucture but is responsible for the transportation of food and other nutrients needed by plant. Xylem: Hard wall cells transport water and mineral nutrients in a kind of tissue called Xylem. Phloem: Relatively soft -walled cells transport organic nutrients in a kind of tissue called Phloem. way I remembered it, was Xylem inside, near the center, Phloem near the outside.

Question:I took my bio final today and one question showed a cross section of a leaf and poitned to a vein and said "What kind of cells are found here?". Using process of elimination, I narrowed it down to "Parenchyma", and "Xylem and Phloem". I ended up choosing xylem and phloem, but those arent types of cells per se, so that's why I'm asking this.

Answers:xylem (e.g. a vessel element) is usually sclerenchyma (thick, lignified walls and no cytoplasm - dead at maturity). xylem rays in wood do contain parenchyma cells, but these are not the main conducting cells.

From Youtube

Sugar We're Goin' Down (Phloem) :A Biology song I made for a Biology Project about Xylem and Phloem. Original: www.youtube.com Lyrics Did you get what you bargained for yet? Ive been dyin because I have no food, to survive, Cause I cant photosynthesize. Water you got, sent to you through transpiration, It went up through the xylem, Cohesion helped it along. (It went up through the xylem, cohesion helped it along) Tracheids, vessel elements, Water goes up them, and I need you man, to reward me..... Leaves, Leaves, sugar source of the trees, Your sugar is going down Phloem. Cause a sugar sink would enjoy it, If you send it to him to use it or store it. Leaves, Leaves, sugar source of the trees, Your sugar is going down Phloem. Cause a sugar sink would enjoy it, If you send it to him to use it or store it. Did you get what you bargained for yet? Ive been waitin for tasty sugar to supply me Wishin those sieve-tubes leave sap at my door Youve really messed up, please send some sugar to me, Or Ill stop sending water, youll realize that you have done wrong, Your guard cells, would lose firmness. Your stomata would close, no way to undergo, photosynthesis. Leaves, Leaves, sugar source of the trees, Your sugar is going down Phloem. Cause a sugar sink would enjoy it, If you send it to him to use it or store it. Leaves, Leaves, sugar source of the trees, Your sugar is going down Phloem. Cause a sugar sink would enjoy it, If you send it to him to use it or store it.