Download 9.2 Transport in Phloem

Topic 9.2 – Transport in the phloem of plants
Essential Idea:
Structure and function
are correlated in the
phloem of plants.
Skill: Identification of xylem and phloem in microscopic images of cell and root.
Indicate the major vascular
structures in the figures below.
Skill: Identification of xylem and phloem in microscopic images of cell and root.
Stem Cross-Section
Notice the difference between vascular tissue that
is located in stems and roots.
Root Cross-Section
Application: Structure – function relationship of phloem sieve tubes.
The function of phloem is to carry sap
throughout a plant.
Phloem tissue has several key adaptations to
effective sap movement throughout the plant.
Sieve tubes have
interconnected cytoplasm,
which are connected by pores
in the sieve plates.
Companion cells are paired up
with sieve tubes and serve to
“load” the phloem with solutes
(sugars)
Sieve plates allow free
movement of material from one
phloem cell to another, even
more so than plasmodesmata
Plasmodesmata allow for
transfer of materials between
plant cells. Companion cells
only have them with sieve tubes
Plasmodesmata
Application: Structure – function relationship of phloem sieve tubes.
Identify unique structures in the
phloem, shown here in orange.
What do you think the structurefunction relationship of the sieve plate
is?
How about the Companion cell?
Here is a side view of a corn stem.
Can you pick out the xylem and
phloem from this image?
Plants transport organic compounds from sources to sinks.
Source: Site of Production or storage
Sink: Destination/site of use
Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks.
Skill: Analysis of data from
experiments measuring
phloem transport rates
using aphid stylets and
radioactively-labelled
carbon dioxide.
But how fast? And how
do scientists know what
is moving?
Why do aphids insert
their stylet into the
phloem of a plant? Why
not the xylem?