Describe current theories about processes responsible for the movement of materials through plants in xylem and phloem tissues

Xylem:
-Function and structure:
-dead, hollow cells joined end to end
-reinforced with rings and spirals of lignin
-carries water and dissolved minerals UP from roots to leaves
-passive transport
-Cause of transport of substances:
-transpiration: the evaporation of water from leaves; stomates must be open for this to occur
-cohesion: when a substance (water in this case) is strongly attached to itself. This causes a pulling effect when transpiration occurs.
-adhesion: when a substance is strongly attracted to other substances. e.g. glass with water. This assists in the transpirational stream because water 'climbs' against gravity along narrow tubes (capillaries)
-transpirational stream: when water is pulled upwards by transpiration and cohesion

Phloem:
-Function and structure:
-living cells joined end to end by perforated ends (sieve plates) so each is open to the next.
-phloem cell- filled with cytoplasm; companion cell- has mitochondria to provide energy for active transport
-flows in either direction; if photosynthesising: phloem carries nutrients to storage sites in root or stem. if not photosynthesising, phloem carries nutrients to leaves/flower/fruit
-carries nutrients (mainly glucose)
-Cause of transport of substances:
-translocation: the transport of dissolved minerals depending on pressure
-sugars get carried in & out of phloem tubes by active transport
-water gets carried in & out phloem tubes by osmosis
-sugar solution moves to destination due to pressure differential

	Created by shaycrystal4 almost 12 years ago

Biology (MAINTAINING A BALANCE) Note on 2. Plants and animals transport dissolved nutrients and gases in a fluid medium, created by shaycrystal4 on 31/10/2013.