Navigation Links
Phloem


In vascular plants, phloem is the tissue that carries organic nutrients, particularly sucrose. In trees, the phloem is part of the bark, hence the name, derived from the Greek word for "bark". See also xylem.

Phloem sap moves from sugar sources to sugar sinks. 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, wheras 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 with ease move between adjacent sieve tubes, it is not unusual for sap in adjacent sieve tubes to be flowing in opposite directions.

Phloem tissue consists of sieve-tube members and companion cells . The sieve-tube elements are large, cylindrical cells, with large pores in the cell wall at either end. They are almost entirely dead, and have no organelles. All of their functions of a sieve-tube element are carried out by its (much smaller but quite living) companion cell.

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 .

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.


'"/>


(Date:7/11/2014)... Janeiro, Brazil- In the brains of all vertebrates, ... allows an electric or chemical signal to be ... synapses, which are the most abundant type of ... formation is crucial for learning, memory, perception and ... synapses critical for brain function. For instance, ...
(Date:7/10/2014)... -- Aware, Inc. (NASDAQ: AWRE ), a leading ... June 26, 2014 that its Board of Directors had ... or approximately $40 million in total.  The Board established ... payment date of July 24, 2014.  The Company was ... ex-dividend date for this special cash dividend.  An ex-dividend ...
(Date:7/10/2014)... -- NXT-ID, Inc., (OTCQB: NXTD) a biometric authentication company focused on the ... was interviewed on July 1 st on the ... . Mr. Pereira discusses the company,s next generation smart wallet ... the Wocket™ aims to replace a traditional wallet with a ... tells Gino he has never heard of the Wocket ...
Breaking Biology News(10 mins):Blame it on the astrocytes 2NASDAQ Sets Ex-Dividend Date for Aware, Inc.'s Special Cash Dividend of $1.75 Per Share 2Security and Your Money: Tampa Bay Radio Host Cedric Harris Talks to NXT-ID CEO Mr. Gino Pereira about the Wocket Smart Wallet 2Security and Your Money: Tampa Bay Radio Host Cedric Harris Talks to NXT-ID CEO Mr. Gino Pereira about the Wocket Smart Wallet 3Security and Your Money: Tampa Bay Radio Host Cedric Harris Talks to NXT-ID CEO Mr. Gino Pereira about the Wocket Smart Wallet 4
... carried a gene that increases your risk for Alzheimer,s disease? And ... damage not when you,re old but when you,re young? ... strong genetic component to the development of late-onset Alzheimer,s. In 1993, ... a quarter of us that triples the risk for getting ...
... ARLINGTON, VA (May 3, 2011) Cancer studies from ... unveiled at the American Association of Pharmaceutical Scientists, (AAPS) ... May 16 -Wednesday, May 18 at the Hilton San ... Oral Vaccine for Cervical Cancer Cervical cancer ...
... overcome their evolutionary legacies to become much better at ... of energy that can power vehicles in the near ... solar scientists has found. A U.S. Department of ... 18 scientists to compare the efficiency of plants and ...
Cached Biology News:Alzheimer's risk gene disrupts brain's wiring 50 years before disease hits 2Alzheimer's risk gene disrupts brain's wiring 50 years before disease hits 3AAPS national biotechnology conference to highlight breakthrough cancer treatments 2NREL's multi-junction solar cells teach scientists how to turn plants into powerhouses 2NREL's multi-junction solar cells teach scientists how to turn plants into powerhouses 3
Other biology definitionOther Tags