Phloem
Say it: FLOH-em
Phloem is the living vascular tissue of plants that carries sugars and other organic nutrients from leaves to the parts that use or store them.
Also: bast
Definition
What is phloem?
Phloem is the plant vascular tissue that distributes the products of photosynthesis, mainly the sugar sucrose, along with amino acids, hormones and signaling molecules. Its conducting cells, sieve-tube elements, are joined end to end into sieve tubes, with porous end walls called sieve plates. Unlike the water-conducting cells of xylem, they are alive at maturity, although they have lost their nucleus and most organelles. Each one depends on a neighboring companion cell, which carries out much of its metabolism and is linked to it by many tiny channels.
Phloem moves sap from sources, such as mature leaves, to sinks, such as roots, fruits, seeds and growing shoots. The accepted explanation is the pressure-flow model. Sucrose is actively loaded into sieve tubes at the source, which costs energy; water then enters by osmosis from nearby xylem, building pressure that pushes the sap along. At the sink, sucrose is unloaded and water leaves, keeping pressure low at that end. Because sinks change as a plant grows, phloem can carry material up or down.
The name comes from the Greek for bark, because in woody stems phloem lies just inside the bark.
Word parts
Where does the word “phloem” come from?
- phloe-
- barkGreek phloos
Practice it
Where you'll meet it
Quizzes that use it
Related
Terms that go with it
- XylemXylem is the vascular tissue of plants that carries water and dissolved minerals upward from the roots to the stems and leaves.
- PhotosynthesisPhotosynthesis is the process in which plants, algae and cyanobacteria capture light energy and store it in sugar built from carbon dioxide and water.
- OsmosisOsmosis is the diffusion of water across a selectively permeable membrane, from the side with fewer dissolved solutes to the side with more.
- Active transportActive transport is the movement of a substance across a cell membrane against its concentration gradient, powered by energy, usually from ATP.
- CarbohydrateA carbohydrate is a sugar or a chain of sugars, such as starch or cellulose, built from carbon, hydrogen and oxygen and used for energy and structure.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- OpenStax Biology 2e, 30.2 Stems
- OpenStax Biology 2e, 30.5 Transport of Water and Solutes in Plants
- Omid et al. (2008), Long-distance trafficking of macromolecules in the phloem, Plant Signaling & Behavior
Last updated
Spotted a mistake? Tell us and we'll correct it.