Transcript Transport in Multicellular Plants

Ch. 7 Part 3: Translocation
Transport in Phloem
 Translocation
movement of ORGANIC substances in
phloem tissue
 Main substances moved (in solution):

ASSIMILATES (substances that have been made by plants)
 Amino
acids
 Sucrose
CLICK!
Phloem Tissue Cells
 Sieve
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Tube Elements
LIVING cells
Contain cytoplasm and a few
organelles
NO nucleus
Walls are made of Cellulose (no
lignin)
 Companion
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
cells
Associated with sieve tube
elements
Contains cytoplasm, many
organelles(including nucleus
and mitochondria)
Sieve Tube Elements
 Cell
wall with cellulose
 Cytoplasm
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mitochondria
NO nucleus
A few other organelles
Creates SPACE for
movement of phloem
sap
 Sieve
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plate
Where the end walls of
2 sieve elements meet
Perforated END WALL
Allows mass flow of
phloem sap through
sieve pores
Contents of Sieve Tube Elements

Phloem sap
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Table shows composition of
phloem sap in castor oil plant
(composition varies depending on
plant species)
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Easy to collect because it does
not behave like normal sap
when a sieve tube is cut
Normal sap Phloem sap hard
to analyze
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Contents of sieve tubes under
HIGH pressure
When sieve tube is cut, pressure
is released
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Rush of phloem sap to cut
Sieve plate blocks flow (good
way to prevent loss of sap)
Sieve element secretes
carbohydrate CALLOSE to seal cut
(“clotting”)
Use of Aphids to analyze sap
 Stylets
(tubular
mouth parts) inserted
into phloem
 Pressure not
disrupted and sap
flows into stylet (due
to small diameter of
stylet)
How Assimilates Move Through Pores
 SOURCE
part of plant
where assimilates (a.a. &
sucrose) ENTER the phloem

a plant cell with a high
concentration of sugars and other
solutes,
such as a leaf cell
 SINK
 part of plant where
assimilates LEAVE the
phloem


a plant cell with a low concentration of
sugars
sugars may be converted to
starch for storage or used rapidly for
energy or as building blocks of other
carbohydrates
Leaf source & Roots
 sink
 Ex.
Translocation in Phloem
 Phloem


sap moves by mass flow
Much faster than diffusion
Mass flow relies on pressure
 Pressure
difference in xylem created by soil (root
pressure) and air (transpiration)…NO energy needed
 Pressure difference in phloem requires ENERGY

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Created by ACTIVE LOADING process
Mass Flow in Phloem  ACTIVE process
 Active
loading of sucrose  decreases water
potential  water rushes in via osmosis  creates
pressure gradient (hydrostatic/turgor pressure aka
pressure potential)

Mass Flow in Xylem  PASSIVE process
TRANSLOCATION

Requires Energy

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Cell respiration in Companion cells
at source provide ATP used to fuel
active transport of sucrose INTO
companion cell
Sucrose Loading
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Increases concentration of sucrose
in companion cell by:
Hydrogen ion pump moves H+ OUT
of cell and INTO cell wall, creating
HIGH concentration of H+ ions
H+ ions want to come back into
companion cell but they must use
FACILITATE DDIFFUSION

SYMPORT co-transporter
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Carries H+ and sucrose into companion
cell
Allows sucrose to move down
concentration gradient via diffusion
INTO PHLOEM SIEVE ELEMENT
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With all this sucrose in companion
cell and SE, water potential has
DECREASED in these guys
Water flows into companion cell and
SE, moving down gradient
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At Sink:
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Sucrose diffuses OUT of
phloem sieve element and
DOWN a concentration
gradient into a cell that is
using sucrose
Water potential gradient
produced
Water diffuses OUT of
phloem sieve element
 Water
is ADDED at the source
 Water is LOST at the sink
 Creates higher HYDROSTATIC
PRESSURE inside phloem
sieve element at the SOURCE
than at the sink
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Phloem sap moves by MASS
FLOW down pressure gradient,
through phloem sieve
elements  through sieve
pores
From source to sink
Unloading of Sucrose
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Occurs at tissue that requires sucrose
Sucrose moves out of phloem into tissues using symplast
and apoplast pathways, utilizing ATP
Once in tissue, sucrose converted into other molecules
(maintaining low sucrose gradient in tissue, so sucrose
continues to enter)
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Maintains concentration gradient
Enzymes involved
Invertase
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Hydrolyses sucrose to glucose and fructose
Comparing Xylem and Phloem
Potometer
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A potometer is a device that
measures the rate at which a plant
draws up water
Plant draws up water as it loses it
by transpiration
Measure the rate of transpiration
based on rate water is lost
Apparatus must be air tight and
water tight…NO LEAKS
Basic elements of a potometer
are:
A plant cutting
 A calibrated pipette to measure
water loss
 A length of clear plastic tubing
 An air-tight seal between the plant
and the water-filled tubing
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• The rate of transpiration is
measured as the amount of
water lost/ square meter/
minute
• Water evaporates through
the many stomata on the
leaf surface
• the rate of transpiration
is directly related to the
surface area
• To arrive at the rate of
transpiration, calculate the
leaf surface area of each
plant
Micrographs of Plant Stem
 Click
here
Infographic Posters on Ch. 7
 Groups
of 2-3 people
 Minimal text, more
pics, diagrams and
graphs
 Must cover all major
points in chapter


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