Transport in Plants |
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Topic Notes |
Additional Support Materials i.e. animations, quizzes, pictures, worksheets |
Factors affecting transpiration Mineral Ion transport in plants
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Concept
Map on Transport in Plants
Plant
Transport Mechanisms
Concept
Map on Transport in Plants
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Plants
don’t have a circulatory system like animals, but they do have a sophisticated
transport system for carrying water and dissolved solutes to different parts of
the plant, often over large distances.
Stem
Structure
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Root
Structure
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Xylem
Tissue Xylem
tissue is composed of dead cells joined together to form long empty
tubes. Different kinds of cells form wide and narrow tubes, and the end
cells walls are either full of holes, or are absent completely. Before
death the cells form thick cell walls containing lignin, which is
often laid down in rings or helices, giving these cells a very
characteristic appearance under the microscope. Lignin makes the xylem
vessels very strong, so that they don’t collapse under pressure, and
they also make woody stems strong. |
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Phloem
Tissue
Phloem
tissue is composed of sieve tube cells, which form long columns
with holes in their end walls called sieve plates. These cells
are alive, but they lose their nuclei and other organelles, and their
cytoplasm is reduced to strands around the edge of the cells. These cytoplasmic
strands pass through the holes in the sieve plates, so forming
continuous filaments. The centre of these tubes is empty. Each sieve
tube cell is associated with one or more companion cells, normal
cells with nuclei and organelles. These companion cells are connected to
the sieve tube cells by plasmodesmata, and provide them with
proteins, ATP and other nutrients. |
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Vast
amounts of water pass through plants. A large tree can use water at a rate of 1
dm³ min-1. Only 1% of this water is used by the plant cells for
photosynthesis and turgor, and the remaining 99% evaporates from the leaves and
is lost to the atmosphere. This evaporation from leaves is called transpiration.
The
movement of water through a plant can be split into three sections: through the
roots, stem and leaves:
Water
moves through the root by two paths:
The
Symplast pathway
consist of the living cytoplasms of the cells in the root (10%). Water is
absorbed into the root hair cells by osmosis, since the cells have a lower
water potential that the water in the soil. Water then diffuses from the
epidermis through the root to the xylem down a water potential gradient. The
cytoplasms of all the cells in the root are connected by plasmodesmata
through holes in the cell walls, so there are no further membranes to cross
until the water reaches the xylem, and so no further osmosis.
The Apoplast pathway consists of the cell walls between cells (90%). The cell walls are quite thick and very open, so water can easily diffuse through cell walls without having to cross any cell membranes by osmosis. However the apoplast pathway stops at the endodermis because of the waterproof casparian strip, which seals the cell walls. At this point water has to cross the cell membrane by osmosis and enter the symplast. This allows the plant to have some control over the uptake of water into the xylem.
The
uptake of water by osmosis actually produces a force that pushes water up the
xylem. This force is called root pressure, which can be measured by
placing a manometer over a cut stem, and is of the order of 100 kPa (about 1
atmosphere). This helps to push the water a few centimetres up short and young
stems, but is nowhere near enough pressure to force water up a long stem or a
tree. Root pressure is the cause of guttation, sometimes seen on wet
mornings, when drops of water are forced out of the ends of leaves.
The
xylem vessels form continuous pipes from the roots to the leaves. Water can move
up through these pipes at a rate of 8m h-1, and can reach a height of
over 100m. Since the xylem vessels are dead, open tubes, no osmosis can occur
within them. The driving force for the movement is transpiration in the leaves.
This causes low pressure in the leaves, so water is sucked up the stem to
replace the lost water. The column of water in the xylem vessels is therefore
under tension (a stretching force). Fortunately water has a high tensile
strength due to the tendency of water molecules to stick together by
hydrogen bonding (cohesion), so the water column does not break under the
tension force. This mechanism of pulling water up a stem is sometimes called the
cohesion-tension mechanism.
The
very strong lignin walls of the xylem vessels stops them collapsing under the
suction pressure, but in fact the xylem vessels (and even whole stems and
trunks) do shrink slightly during the day when transpiration is maximum.
The
xylem vessels ramify in the leaves to form a branching system of fine vessels
called leaf veins. Water diffuses from the xylem vessels in the veins
through the adjacent cells down its water potential gradient. As in the roots,
it uses the symplast pathway through the living cytoplasm and the apoplast
pathway through the non-living cell walls. Water evaporates from the spongy
cells into the sub-stomatal air space, and diffuses out through the stomata.
Evaporation
is endothermic and is driven by solar energy, which is therefore the ultimate
source of energy for all the water movements in plants:
The
rate of transpiration can be measured in the lab using a potometer
(“drinking meter”):
A potometer actually measures
the rate of water uptake by the cut stem, not the rate of transpiration; and
these two are not always the same. During the day plants often transpire more
water than they take up (i.e. they lose water and may wilt), and during the
night plants may take up more water than they transpire (i.e. they store water
and become turgid). The difference can be important for a large tree, but for a
small shoot in a potometer the difference is usually trivial and can be ignored.
The potometer can be used to
investigate how various environmental factors affect the rate of transpiration.
Light.
Light stimulates the stomata to open allowing gas exchange for
photosynthesis, and as a side effect this also increases transpiration. This
is a problem for some plants as they may lose water during the day and wilt.
Temperature.
High temperature increases the rate of evaporation of water from the spongy
cells, and reduces air humidity, so transpiration increases.
Humidity.
High humidity means a higher water potential in the air, so a lower water
potential gradient between the leaf and the air, so less evaporation.
Air
movements. Wind blows away
saturated air from around stomata, replacing it with drier air, so
increasing the water potential gradient and increasing transpiration.
Many
plants are able to control
their stomata, and if they are losing too much water and their cells are
wilting, they can close their stomata, reducing transpiration and water loss. So
long periods of light, heat, or dry air could result in a decrease in
transpiration when the stomata close.
Plants
in different habitats are adapted to cope with different problems of water
availability.
Mesophytes
plants adapted to a habitat with adequate water
Xerophytes
plants adapted to a dry habitat
Halophytes
plants adapted to a salty habitat
Hydrophytes
plants adapted to a freshwater habitat
Some
adaptations of xerophytes are:
Adaptation |
How it works |
Example |
thick
cuticle |
stops
uncontrolled evaporation through leaf cells |
most dicots |
small leaf
surface area |
less area
for evaporation |
conifer
needles, cactus spines |
low stomata
density |
fewer gaps
in leaves |
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stomata on
lower surface of leaf only |
more humid
air on lower surface, so less evaporation |
most dicots |
shedding
leaves in dry/cold season |
reduce
water loss at certain times of year |
deciduous
plants |
sunken
stomata |
maintains
humid air around stomata |
marram
grass, pine |
stomatal
hairs |
maintains
humid air around stomata |
marram
grass, couch grass |
folded
leaves |
maintains
humid air around stomata |
marram
grass, |
succulent
leaves and stem
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stores
water |
cacti |
extensive
roots |
maximise
water uptake |
cacti |
Ions
are absorbed from the soil by both passive and active transport. Specific ion
pumps in the membranes of root hair cells pump ions from the soil into the
cytoplasms of the epidermis cells. Two lines of evidence indicate that active
transport is being used:
The concentrations of ions inside root
cells are up to 100 time greater than in the soil, so they are being
transported up their concentration gradient.
If respiratory inhibitors such as cyanide are applied to
living roots, ion uptake is greatly reduced, since there is no ATP being
made to drive the membrane pumps. Any remaining uptake must be passive.
The
active uptake of ions is partly responsible for the water potential gradient in
roots, and therefore for the uptake of water by osmosis.
Ions
diffuse down their concentration gradient from the epidermis to the xylem. They
travel up the xylem by mass flow as the water is pulled up the stem (in other
words they are simply carried up in the flow of the xylem solution). In the
leaves they are selectively absorbed into the surrounding cells by membrane
pumps.
The
phloem contains a very concentrated solution of dissolved solutes, mainly
sucrose, but also other sugars,
amino acids, and other metabolites. This solution is called the sap, and
the transport of solutes in the phloem is called translocation.
Unlike
the water in the xylem, the contents of the phloem can move both up or down a
plant stem, often simultaneously. It helps to identify where the sugar is being
transported from (the source), and where to (the sink).
During the summer sugar is mostly transported from the
leaves, where it is made by photosynthesis (the source) to the roots, where
it is stored (the sink).
During the spring, sugar is often transported from the
underground root store (the source) to the growing leaf buds (the sink).
Flowers and young buds are not photosynthetic, so sugars can
also be transported from leaves or roots (the source) to flowers or buds
(sinks).
Surprisingly,
the exact mechanism of sugar transport in the phloem is not known, but it is
certainly far too fast to be simple diffusion. The main mechanism is thought to
be the mass flow of fluid up the xylem and down the phloem, carrying dissolved
solutes with it. Plants don’t have hearts, so the mass flow is driven by a
combination of active transport (energy from ATP) and evaporation (energy from
the sun). This is called the mass flow theory, and it works like this:
Sucrose
produced by photosynthesis is actively pumped into the phloem vessels by the
companion cells.
This
decreases the water potential in the leaf phloem, so water diffuses from the
neighbouring xylem vessels by osmosis.
This
is increases the hydrostatic pressure in the phloem, so water and dissolved
solutes are forced downwards to relieve the pressure. This is mass flow:
the flow of water together with its dissolved solutes due to a force.
In
the roots the solutes are removed from the phloem by active transport into
the cells of the root.
At
the same time, ions are being pumped into the xylem from the soil by active
transport, reducing the water potential in the xylem.
The
xylem now has a lower water potential than the phloem, so water diffuses by
osmosis from the phloem to the xylem.
Water
and its dissolved ions are pulled up the xylem by tension from the leaves.
This is also mass flow.
This
mass-flow certainly occurs, and it explains the fast speed of solute
translocation. However there must be additional processes, since mass flow does
not explain how different solutes can move at different speeds or even in
different directions in the phloem. One significant process is cytoplasmic
streaming: the active transport of molecules and small organelles around
cells on the cytoskeleton.
1.
Puncture Experiments
If the phloem is
punctured with a hollow tube then the sap oozes out, showing that there
is high pressure (compression) inside the phloem (this is how
maple syrup is tapped). If the xylem is punctured then air is sucked in,
showing that there is low pressure (tension) inside the xylem.
This illustrates the main difference between transport in xylem and
phloem: Water is pulled up in the xylem, sap is pushed
down in the phloem.
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2.
Ringing Experiments
Since
the phloem vessels are outside the xylem vessels, they can be
selectively removed by cutting a ring in a stem just deep enough to cut
the phloem but not the xylem. After a week there is a swelling above the
ring, reduced growth below the ring and the leaves are unaffected. This
was early evidence that sugars were transported downwards in the phloem. |
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3.
Radioactive Tracer Experiments
Radioactive
isotopes can be used trace precisely where different compounds are being
transported from and to, as well as measuring the rate of transport. The
radioactivity can be traced using photographic film (an autoradiograph)
or a GM tube. This techniques can be used to trace sugars, ions or even water.
In
a typical experiment a plant is grown in the lab and one leaf is exposed for a
short time to carbon dioxide containing the radioactive isotope 14C.
This 14CO2 will be taken up by photosynthesis and the 14C
incorporated into glucose and then sucrose. The plant is then frozen in liquid
nitrogen to kill and fix it quickly, and placed onto photographic film in the
dark. The resulting autoradiograph shows the location of compounds containing 14C.
This
experiment shows that organic compounds (presumably sugars) are transported
downwards from the leaf to the roots. More sophisticated experiments using
fluorescently labelled compounds can locate the compound specifically to the
phloem cells.
4.
Aphid Stylet Experiments
Aphids, such as
greenfly, have specialised mouthparts called stylets, which they
use to penetrate phloem tubes and sup of the sugary sap therein. If the
aphids are anaesthetised with carbon dioxide and cut off, the stylet
remains in the phloem so pure phloem sap can be collected through the
stylet for analysis. This surprising technique is more accurate than a
human with a syringe and the aphid’s enzymes ensure that the stylet
doesn’t get blocked. |
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Last updated 20/06/2004