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Topic 9 Plant Biology
IB Biology
9.2 Transport 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-labelled carbon dioxide
were only possible when radioisotopes became available.
UNDERSTANDING STATEMENTS

Plants transport organic compounds from sources to sinks.
o Phloem =
o Composed of sieve tubes which are made of columns of cells called sieve tube cells. These cells
are closely associated with ____________________________________________.
o Transports organic compounds around the plant, which is called
____________________________________.
o Phloem moves material from source to sink
 SOURCE =
 SINK =
o Sometimes sinks turn into sources, or vice versa. For this reason the tubes in phloem must be able
to transport biochemicals in either direction
Sources
Sinks

Active transport is used to load organic compounds into phloem sieve tubes at the source.
o ___________________________________ is the most prevalent solute in ________________________________.
Sucrose is not as readily available for plant tissues to metabolize directly in respiration and therefore
makes a good transport form of carbohydrate as it will not be metabolized during transport.
o Phloem loading – the process by which plants bring sugar into the phloem.
o Plants differ in this mechanism:
o _____________________________________________________ - In some species, a significant amount
travels through cell walls from mesophyll cells to the cell walls of companion cells, and
sometimes sieve cells, where a sucrose transport protein then actively transports the sugar in. A
concentration gradient of sucrose is established by active transport. H+ ions are actively
transported out of the companion cell from surrounding tissues using ATP as an energy source.
The build-up of H+ then flows down its concentration gradient through a co-transport protein.
The energy released is used to carry sucrose into the companion cell-sieve tube complex.
o
_______________________________ – much of the sucrose travels between cells through
connections between cells called plasmodesmata.
Once the sucrose reaches the
companion cell it is converted to an oligosaccharide to maintain the sucrose concentration
gradient.



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.
Incompressibility of water allows transport by hydrostatic pressure gradients.
o The buildup of sucrose and other carbohydrates draws water into the companion cell through
osmosis (from xylem). The rigid cell walls combined with the incompressibility of water result in a
build-up of pressure. Water will flow from this area of high pressure to an area of low pressure.
o At the sink end, sucrose is withdrawn from the phloem and either utilized as an energy source or
converted into starch. The loss of solute causes a reduction in osmotic pressure and the water
that carried the solute to the sink is then drawn back in to the transpiration stream in the xylem.
APPLICATION:
Structure-function relationships of phloem sieve tubes
SKILL:
Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactivelylabelled carbon dioxide. Complete DBQ on pg 418.
SKILL:
Identification of xylem and phloem in microscope images of stem and root.