Plant Science
9.2 Transportation in the Phloem of plants
Nature of Science
 Developments in scientific research follow
improvements in apparatus – experimental methods
for measuring phloem transport rates using aphid
stylets and radioactively-labeled carbon dioxide
were only possible when radioisotopes became
available.
Understandings
 Plants transport organic compounds from sources to
sinks
 Incompressibility of water allows transport along
hydrostatic pressure gradients
 Active transport is used to load organic compounds into
phloem sieve tubes at the source
 High concentrations of solutes in the phloem at the
source lead to water uptake by osmosis
 Raised hydrostatic pressure causes the contents of the
phloem to flow towards sinks
Applications and Skills
 Applications: Structure – function relationships of
phloem sieve tubes
 Skill: Identification of xylem and phloem in
microscope images of stem and root
 Skill: Analysis of data from experiments measuring
phloem transport rates using aphid stylets and
radioactively-labeled carbon dioxide.
Utilized Skills
 Membrane transport
 Properties of water
Recap on XYLEM
 Single direction transportation
 Water and minerals
 Water travels by cohesion and adhesion
 Transfer water to leaves on top of the plant
So… What on earth is a
PHLOEM?
Phloem structure
Phloem in a tree…
Differences between Xylem
and Phloem
Why Xylem and Phloem
are important?
 The “Blood vessels” of vascular plants
 Only found in vascular plants (e.g. angiosperm)
 Transportation of important materials for plant
growth and life
Procedure of Phloem
Transportation
 Organic molecules (amino acids and sugars) move
from their sources (e.g. photosynthesis, storage
organs) into the tube system of phloem
 Sugars are transported as sucrose (because it is
soluble but metabolically inert) in the fluid of the
phloem (called the sap)
 They are actively loaded into the phloem by
companion cells, creating a high concentration
which draws water from the xylem via osmosis
(passive)
Procedure of Phloem
Transportation (2)
 The sap volume and pressure consequently increase
to create mass flow which drives the sap along the
phloem
 The organic molecules are actively unloaded by
companion cells and stored in the sink (fruits, seeds,
roots)
 Sucrose is stored as starch (insoluble), while the
water in the phloem is released (now that solute
concentration is low) and returned to the xylem
In case you haven’t catch what
I’ve said, here is a diagram
Phloem Loading
In case you are wondering
how the water travels…
 Hydrostatic pressure gradients
 Hydrostatic pressure is pressure in a liquid
 High concentrations of amino acids in the sieve
tubes lead to water uptake by osmosis and high
hydrostatic pressure
 Therefore, a pressure gradient that makes sap inside
phloem sieve tubes flow from sources to sink
Useful Links
 https://www.youtube.com/watch?v=xGCnuXxbZ
Gk
 https://www.youtube.com/watch?v=h9oDTMXM
7M8
 http://highered.mheducation.com/sites/983409233
9/student_view0/chapter38/animation__phloem_loading.html