Transport in plants · Transport in plants occurs on three levels: (1) The uptake and loss of water...
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TRANSPORT IN PLANTS 2/22/2014 LECTURE 4
Transcript of Transport in plants · Transport in plants occurs on three levels: (1) The uptake and loss of water...
TRANSPORT IN
PLANTS
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LECTURE 4
SUBTOPICS
The Transpiration Stream
The Anatomy Of Vascular Tissues
Solute Transport
Translocation Of Food
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Transport in plants occurs on three levels:
(1) The uptake and loss of water and solutes by individual cells
(2) Short-distance transport
of substances from cell to
cell at the level of tissues
or organs
(3) Long-distance transport
of sap within xylem and
phloem at the level of
the whole plant.
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TRANSPORT IN PLANTS
Cell membrane is selectively permeable.
Passive Transport:
- Simple Diffusion
- Osmosis
- Facilitated diffusion (transport proteins)
Active Transport: need ATP energy
- Chemiosmosis (proton pump)
- Charge Gradient
CELL TRANSPORT PROCESSES 2
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Simple Diffusion
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OSMOSIS
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Facilitated Diffusion 2
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CHEMIOSMOSIS 2
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Uses the attractive-repulsive properties of
ions to move other ions across membranes
Charge Gradient 2
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SHORT-DISTANCE TRANSPORT
Involves simple diffusion, osmosis and active
transport.
Routes
1. Cell-to-Cell Across Cell Membranes
2. Symplast (involves cytoplasm and
plasmodesmata)
3. Apoplast (transport through porous cell
walls)
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Short-Distance Transport 2
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LONG-DISTANCE TRANSPORT
Involves transpiration
and root pressure.
Continuous tube of water
depends upon water
cohesion and adhesion.
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The Transpiration Stream
Water is taken in from the soil via the roots.
These are covered in millions of tiny root
hair cells which have a large surface area so
that the plant can absorb enough water from
the soil.
Water is absorbed into the roots by osmosis.
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The Transpiration Stream
Water travels from the roots, up the plant, to the
leaves.
The water is carried in tubes called xylem vessels.
Xylem vessels have very narrow diameters - they
are microscopic capillary tubes, so that capillarity
helps to suck water up the plant.
Xylem vessels are made of hollow, dead cells and
in plant stems and leaves are grouped together
with phloem vessels in vascular bundles (vascular
means vein-like).
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Transpiration 2
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The Transpiration Stream
On the underside of the leaves are tiny holes
called stomata which allow the plant to breathe.
When the water reaches the leaves it evaporates
through the stomata.
As the water evaporates, more water is drawn up
the xylem by suction pressure.
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The Transpiration Stream
But plants also need to have water evaporate
from their leaves to keep the transpiration
stream flowing and cool themselves down.
So they need to strike a balance between
water gain and water loss.
Plants control the amount of water lost
through transpiration by opening and closing
their stomata.
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The Transpiration Stream
Two guard cells surrounding each stoma
can open and close it by becoming more or
less turgid.
The holes are opened to allow carbon
dioxide and oxygen to pass in and out, and
are closed to reduce loss of water.
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Stomatal Opening and Closing
Guard Cells Mediate Transpiration 2
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Stomatal Opening and Closing
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Stomatal Opening and Closing
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The Transpiration Stream
A plant's transport system is made up of two
types of tubes - strong, thick pipes called
xylem vessels, and thinner tubes called phloem
vessels.
The cells of these vessels are modified to make
them suited to performing their special
functions.
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The Transpiration Stream 2
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The Transpiration Stream
Together xylem and phloem form the vascular
tissue, often also referred to as the vascular
bundle.
The two types of vessel are always found
together, but they occupy slightly different
locations in the root and the stem.
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Vascular tissue:
Runs continuous throughout the plant
Transports materials between roots and shoots.
Xylem transports water and dissolved minerals upward
from roots into the shoots. (Water the xylem)
Phloem transports food from the leaves to the roots
and to non-photosynthetic parts of the shoot system.
(Feed the phloem)
Vascular Tissue 2
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The water conducting elements of xylem are the
tracheids and vessel elements.
Xylem 2
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XYLEM
Tracheids
Characteristics
Sharpened elongated cells
Connect to each other through pits
Secondary cell walls strengthened with lignin
Dead at functional maturity
Functions
Transport of water plus dissolved minerals
Support
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Water conducting cells of the xylem 2
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XYLEM
Vessel elements
Characteristics
Shorter and wider than tracheids
Possess thinner cell walls than tracheids
Arranged end-to-end to form long micropipes
Dead at functional maturity
Functions
Transport of water plus dissolved minerals
Support
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Food and minerals move through tubes
formed by chains of cells, sieve-tube
members:
Sieve plates
Companion cell
Phloem 2
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PHLOEM
Sieve-tube members
Characteristics
Living cells arranged end-to-end to form food-
conducting cells of the phloem
Lack lignin in their cell walls
Mature cells lack nuclei and other cellular organelles
Alive at functional maturity
Functions
Transport products of photosynthesis
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Food conducting cells of the phloem 2
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PHLOEM
Companion cells
Characteristics
Living cells adjacent to sieve-tube members
Connected to sieve-tube members via plasmodesmata
Functions
Support sieve-tube members
May assist in sugar loading into sieve-tube members
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The Transpiration Stream
The table below will help you remember
the differences between the two types of
vessel:
Xylem Phloem
Made of Dead cells Living cells
Cell wall Thickness Thick thin
Cell wall material Lignin Cellulose
Permeability of
cell wall
Impermeable Permeable
Cross walls? Absent Perforated cross walls
called sieve plates
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The Transpiration Stream
Water is taken up the plant from the roots to
the leaves (for photosynthesis and
transpiration) - in xylem vessels .
Minerals dissolved in the water are taken up
the plant to the shoots and leaves - in xylem
vessels.
Food (the product of photosynthesis) is
taken from the leaves and moved up and
down the plant to any part which needs it
(for growth or for storage) - in phloem
vessels.
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Solute Transport
Definition:
Solute transport in plants, translocation,
primarily occurs in the phloem, but it can
occur in the xylem.
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Solute Transport
Solute Transport in the Xylem:
Water and dissolved ions are primarily transported
in the xylem and move via the transpiration stream
in vessels/tracheids.
Xylem sap may also contain organic materials,
usually in relatively low concentration (with a
notable exception being some trees sap in the
spring which is comprised of 2% or more sucrose).
Substances move at different rates depending on
matrix effects, metabolic needs, etc.
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Solute Transport
Solute Transport in the Phloem
A. Phloem is difficult to study in plants because:
1. The transport cells/tissue in plants are small
(microscopic) in comparison to the transport
structures in animals;
2. There is a very rapid response of the phloem to
wounding (injury);
3. Transport in plants is intracellular (vs.
extracellular in animals); and
4. The transport cells are alive.
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Solute Transport
Solute Transport in the Phloem:
B. Phloem is the primary transport tissue for
photosynthates (photoassimilates, or
simply stated - organic materials).
Radiotracer studies in which leaves are
briefly exposed to 14C-labeled carbon
dioxide show that radioactive
photosynthates are localized in the
phloem.
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Solute Transport
Solute Transport in the Phloem:
C. Phloem Content.
Analysis - early studies to determine the
content of the phloem involved cutting into
the plant and analyzing the contents of the
sap that was recovered.
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Solute Transport
Solute Transport in the Phloem:
C. Phloem Content:
Substance % of the
sap
Examples
Carbohydrates 16-25 The major organic transport
materials are sucrose, stachyose ,
raffinose
Amines/amides 0.04-4 asparagine, glutamine, aspartic
acid, ureides like ureas, citrulline,
allantoin and allantoic acid
Other organic
substances
ATP, hormones, sugar alcohols like
sorbitol (apple, pear, prune) and
mannitol (mangrove, olive)
Inorganic
substances
magnesium and potassium
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Solute Transport
Direction of phloem transport
Information from several types of experiments
demonstrated that:
Plants transport substances in the phloem downward
toward the roots.
Substances in the phloem are transported downward
towards the roots or upwards toward the shoot
meristem.
The typical direction of transport is downward from the
primary source (leaves) to the major sink (roots).
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Solute Transport
Direction of phloem transport
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TRANSLOCATION OF FOOD
Food and other organic substances (e.g., Some plant
hormones and even messenger RNAs manufactured
in the cells of the plant are transported in the
phloem.
I. Sugars (usually sucrose),
II. Amino acids, and
III. Other organic molecules.
Enter the sieve tubes through plasmodesmata.
Once within the sieve tubes, these molecules can be
transported either up or down to any region of the
plant.
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Sugar Loading
into Sieve-tube
Members
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TRANSLOCATION OF FOOD
MECHANISM OF TRANSLOCATION OF FOOD
THROUGH THE PHLOEM
Translocation through the phloem is dependent on
metabolic activity of the phloem cells (in contrast to
transport in the xylem).
I. Chilling its petiole slows the rate at which food
is translocated out of the leaf.
II. Oxygen lack also depresses it.
III. Killing the phloem cells puts end to it.
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THE PRESSURE-FLOW HYPOTHESIS
The best-supported theory to explain the
movement of food through the phloem is
called the pressure-flow hypothesis.
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PRESSURE-FLOW THEORY
The pressure-flow theory explains how sap
moves in a plant from source to sink:
Sugars begin at a source and are pumped
into phloem tube cells.
Osmosis moves water into the cells and
raises pressure.
Pressure moves the sap.
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PRESSURE FLOW 1
The leaf is a source of
sugar, since it makes
sugar by photosynthesis.
Glucose and fructose
made by photosynthesis
are linked to make
sucrose.
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PRESSURE-FLOW 2
Active transport
is used to load
sucrose into
phloem tubes
against a
diffusion gradient.
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PRESSURE-FLOW 3
The high concentration
of sucrose in the sieve
tube cells of the phloem
causes water to move in
by osmosis, which
raises pressure and
causes the sap to move.
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PRESSURE-FLOW 4
A developing fruit is one
example of a sink.
Sucrose may be actively
transported out of phloem
into the fruit cells.
In a root, sucrose is
converted into starch,
which keeps sugar moving
in by diffusion.
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PRESSURE-FLOW 5
As the sugar
concentration drops in
the sieve tube cells,
osmosis moves water
out of the tube.
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PRESSURE-FLOW 6
As water moves out
by osmosis, the
pressure in the sieve
tube cells drops.
The pressure
difference along the
column of sieve tube
cells keeps the sap
flowing.
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PRESSURE-FLOW: REVIEW
Thus it is the
pressure gradient
between "source"
(leaves) and "sink"
(shoot and roots) that
drives the contents of
the phloem up and
down through the
sieve tubes.
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THE PRESSURE-FLOW 2
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THE PRESSURE-FLOW 2
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Interaction
Between Xylem
and Phloem
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