Download Topic 8 Metabolism, Cell Respiration, and Photosynthesis

Topic 9 Plant
Biology
9.2 Transport in the Phloem
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.
Translocation occurs from source to sink

Plants transport organic compounds from sources to
sinks.
o
Phloem = vascular tissue that transports sugar.
o
Composed of sieve tubes which are made of
columns of cells called sieve tube cells. These
cells are closely associated with companion
cells.
o
Transports organic compounds around the plant,
which is called phloem translocation
o
Phloem moves material from source to sink
o
o
o
SOURCE = place that has a surplus of organic
compounds (ie. Leaves)
SINK = place that needs organic compounds (ie. Roots)
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
Photosynthetic Tissues
• Mature green leaves
• Green stems
Storage organs that are
unloading their stores:
• Storage tissues in
germinating seeds
• Tap roots or tubers at
the start of the growth
season
Sinks
Roots that are growing or
absorbing mineral ions using
energy from cell respiration.
Parts of the plant that are
growing or developing food
stores:
• Developing fruits
• Developing seeds
• Growing leaves
• Developing tap roots or
tubers
Phloem Loading
• Active transport is used to load organic compounds into phloem
sieve tubes at the source.
o
o
o
Sucrose is the most prevalent solute in phloem sap.
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.
Phloem loading – the process by which plants bring
sugar into the phloem.
Two mechanisms…
Phloem Loading
o
Apoplast route - 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.
Phloem Loading
o Symplast route – 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.
Pressure and water potential differences
play a role in translocation
 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
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.
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 soute to the sink is then drawn back in to the
transpiration stream in the xylem.
• https://www.youtube.com/watch?v
=MxwI63rQubU
Application
• Structure-function relationships of
phloem and sieve tubes.
o
o
o
Composed of sieve tubes, which are
composed of columns of specialized
cells called sieve tube cells. These cells
are living although they have reduced
cytoplasms and no nucleus.
Sieve tube cells are closely associated
with companion cells.
Companion cells perform many of the
genetic and metabolic functions of the
sieve tube cell and maintain viability.
Lots of mitochondria in companion cell
to support active transport of sucrose.
Infolding of the plasma membrane
increases phloem loading capacity.
Plasmodesmata connect the cytoplasm
of companion cells with the sieve tube
cells
Skill
• Analysis of data from experiments measuring phloem transport
rates using aphid stylets and radioactively labelled carbon
dioxide.
****Complete DBQ on pg 418
Skill
• Identification of xylem and phloem in microscope images of
stem and root.