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Unit 6 – Transport in Flowering Plants 6.1 – Water and ion uptake, 6.2 – Transpiration and translocation
SUFEATIN SURHAN BIOLOGY MSPSBS 2009
SYLLABUS CHECKLIST
Candidates should be able to:
a) relate the structures and functions of root hairs to their surface area and to water and ion uptake;
b) state that transpiration is the loss of water vapour from the leaves to the stomata;
c) describe: how water vapour loss is related to cell surfaces, air spaces and stomata the effects of variation of temperature, humidity and light intensity on transpiration rate how wilting occurs;
d) investigate, using a suitable stain, the pathway of water in a cut stem;
e) explain the movement of water through the stem in terms of transpiration pull;
f) identify xylem and phloem tissues as seen in transverse sections of unthickened,
herbaceous, dicotyledeneous roots, stems and leaves;
g) state the functions of xylem and phloem.
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For plants, there are some advantages in large size.
They can, for example, compete more readily for
light. As a result, many trees are tall,
some exceeding 100m.
The leaves, as a site for photosynthesis, must be in
these aerial parts to obtain light.
The water, so essential for photosynthesis, is
however, collected by the roots which may be some
considerable distance beneath the soil surface. An
efficient means of transporting this water and
certain minerals to the leaves is necessary.
The sugars formed as a result of photosynthesis in
the leaves must be transported in the opposite
direction for cellular respiration in the roots.
The transport system in plants is known as the
vascular system.
The Vascular System
The vascular system of plants consists of vascular
tissues; the xylem and the phloem.
The xylem and phloem extend uninterrupted from
the roots to the stem and the leaves.
Xylem
The xylem is composed mainly of vessels. These are
long hollow tubes stretching continuously from root
to leaf formed by cells joined end to end, without
any cross walls.
Xylem cells are dead cells which contain no
protoplasm and the walls are strengthened by lignin
deposits.
Lignin is a tough, complex, organic compound that
gives a plant its woody characteristic. Cellulose and
lignin both provide strength and keep the plant
upright.
Xylem vessels conduct a mixture of water and
solutes called xylem sap.
The different types of xylem vessels:
The functions of the xylem include:
Transporting water and mineral salts from the
roots to the stems and leaves.
To provide support for plants.
Phloem
The phloem is composed mainly of sieve tubes and
companion cells.
A sieve tube is made up of a single row of elongated
cells with perforated cell walls (sieve plates).
Phloem cells are living cells (however, no nuclei in
mature cells) and are thin-walled.
Transport of food (in the form of sucrose and amino
acids) occurs by diffusion and active transport.
Companion cells have nuclei and assist sieve tubes
in their transport function.
The function of the phloem vessel is to conduct
manufactured food mainly from leaves to all other
parts of the plant – this is known as translocation.
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Positions of xylem and phloem in the root
of a dicotyledon (Transverse section)
Actual tissue structure:
Schematic diagram:
Positions of xylem and phloem in the stem
of a dicotyledon (Transverse section)
Actual tissue structure:
Schematic diagram:
Positions of xylem and phloem in the leaf
of a dicotyledon (Transverse section)
Actual tissue structure:
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Functions of the main tissues in the stems
and roots
Type of tissue: Epidermis.
Description:
A thin outer layer of living cells.
Functions:
Maintains the shape of the whole structure.
Protects against bacterial or fungal
infections.
Some differentiates into projections called
root hair cells.
Type of tissue: Cortex.
Description:
Known also as packing tissues.
It has several layers of relatively large thin-
walled living cells located between the
epidermis and pith.
Functions:
Cells are highly permeable to water and
dissolved solutes.
Intercellular spaces in this layer allow
oxygen to diffuse into the stem or root for
aerobic respiration.
Cells may be used to store starch in the
form of granules.
Type of tissue: Pith.
Description:
Central part of stem.
May be absent in the stem of some plants
(hollow).
Function:
Storage of starch grains.
Root systems of flowering plants
There are two types of root systems in flowering
plants: Fibrous root system and Tap root system.
Structure of a root
The root is divided into several regions:
The root cap protects the root as it grows
through the soil.
The meristematic region consists of small young
cells that are actively dividing by mitosis to
form new identical cells.
The elongation region is where the new cells
elongate to increase the length of the roots.
The maturation region has minute projections
called root hairs several hundred per cm2 which
increases the surface area for rapid and
efficient uptake of water and mineral salts.
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Movement of water and ions into the root
Soil water enters the root hairs by osmosis.
From the root hairs, it moves inwards from cell to
cell by osmosis until it reaches the xylem vessels in
the root.
It is transported from the root to the stem and
leaves in the xylem by root pressure, capillary
action and transpiration pull (main factor).
From the above diagram:
The soil water is at a higher water potential
than the cell sap in the root hair cell. Hence,
soil water is drawn into the root hair cell by
osmosis (along the water potential gradient).
Cell sap of A becomes more dilute, that is, it
has a higher water potential than that of B.
Water is drawn from A into B by osmosis. The
cell sap of B becomes more dilute than that of
C.
Water then enters C. This process of a water
potential gradient being created and water
being drawn inwards from root cell to cell
increases the cell’s turgor pressure.
Due to this pressure water is forced out through
the cell right through the cortex of the root to
the xylem vessels.
In the xylem vessel, water moves in the same way
that a drink moves up a straw when you suck it.
When you suck a straw, the pressure at the top of
the straw is reduced. This causes the liquid at the
bottom of the straw, which is at a higher pressure
to flow up the straw into your mouth.
In the same way, water in the xylem vessel flows up
the xylem vessels to the top of the plant.
The pressure at the top of the plant is reduced by
transpiration.
The water tension developed in the vessels by a
rapidly transpiring plant is thought to be sufficient
to draw water through the root of the soil.
Experimental evidence proving that xylem
transports water:
Mineral salts are dissolved in soil water and exist
in the form of ions. They are often present in low
concentration.
They enter the root hairs mainly by active
transport. Special carrier molecules in the cell
membrane carry the ions into the cell against the
concentration gradient.
Energy used during this transport is supplied by the
mitochondria in the root.
Mineral salts are transported from the roots to the
stem and leaves.
Adaptation of root hair cell to absorption
Root hair is narrow and long. This increases the
surface area to volume ratio which increases the
rate of absorption of water and mineral salts.
The cell sap contains sugars, amino acids and
salts. This makes the cell sap more concentrated
than the soil solution and it is prevented from
leaking out by the plasma membrane. Water can
therefore enter the root hairs by osmosis.
The root hair cell respires to provide energy for
active transport.
Transport of water and mineral salts up
the stem
Three forces cause water to rise up a plant. They
are:
(A) Root pressure.
(B) Capillarity.
(C) Transpiration pull.
(A) Root pressure
Water from the soil enters the root hair cells via
osmosis as a result of osmotic gradient between the
more concentrated root hair cell sap and dilute soil
water.
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Water is transported across the cortex of the root to
the vascular bundle in the centre via osmosis also
due osmotic gradient across the cortex.
The continuous inward movement of water into the
xylem causes a hydrostatic pressure in xylem called
root pressure, forcing water up the root towards the
stem.
An experiment showing root pressure:
Capillarity
Once in the xylem of the stem, water is carried
upwards by a second force called capillarity.
Capillarity is the movement of a thin column of
water upward through narrow xylem tubes.
Capillarity depends on two forces:
1. Adhesive forces between water molecules and
xylem vessel walls and
2. Cohesive forces between water molecules
(attraction between water molecules).
Capillarity can be demonstrated by dipping the end
of capillary tubing in water.
Xylem vessels have a microscopic bore (diameter),
which can be responsible for carrying water 20 cm
or more up a plant.
Transpiration pull
Transpiration is the loss of water vapour from the
aerial parts of a plant, especially through the
stomata of the leaves.
It creates a suction force known as ‘transpiration
pull’.
Mesophyll cells take up water from xylem to replace
those lost.
Water in xylem is drawn up to the leaf to replace
the water that has been taken.
Transpirational pull in particular relies on cohesive
forces of water molecules as they move up the
plant.
These cohesive forces ensure that a long, thin,
continuous column of water and mineral salts in
xylem vessels travels up the plant to the leaves.
This is known as the transpiration stream.
Out of these three forces, root pressure and
transpiration pull provide the most force to draw
water up the tree.
Transpiration stream - the flow of water through the
plant from root to leaf:
Water movement from root hair to leaf – a
summary:
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A summary of the transport of water into the root of
a plant, up the stem and into the atmosphere:
Importance of transpiration
Transpiration enables water and mineral salts to be
transported to mesophyll cells of the leaves for
photosynthesis to manufacture food.
Transpiration allows water to flow through the
plant, making the cells turgid, thus providing
support.
Transpiration removes latent heat from plant
(cooling effect).
Factors affecting transpiration rate
There are two types of conditions which affect the
rate of transpiration:
1. The internal conditions; which are the structural
features of the leaves of the plant.
2. The external conditions; which are the
conditions of the surrounding / environment.
The internal conditions:
Factor Fast rate of
transpiration Slow rate of transpiration
Number of stomata in
a leaf High Low
Thickness of cuticle in a leaf
Thin Thick particularly in lower surface
Surface area of
leaf
Large (broad leaf)
- more exposure to sunlight
- traps more light and heat energy
Small (narrow leaf)
- less exposure to sunlight
- traps less light and heat energy
The external conditions:
1. Humidity (Amount of water vapour in the air).
Slow rate of transpiration Fast rate of transpiration
Humid / high humidity:
- High amount of water vapour both in the air spaces inside the leaf and in the air outside the leaf (saturated)
- Concentration gradient for water vapour becomes low
- Less water vapour diffuses out of the leaf in a unit time (slow transpiration rate)
Dry / Low humidity:
- Less amount of water vapour in the air outside the leaf (unsaturated)
- High amount of water vapour in the air spaces inside the leaf (saturated)
- This creates a steep concentration gradient for water vapour in the air spaces inside the leaf and in the air outside
- More water vapour diffuses out of the leaf in a unit time (fast transpiration rate)
2. Temperature.
Slow rate of transpiration Fast rate of transpiration
Low
- Water molecules have less kinetic energy
- Slower evaporation rate
- Also, warm air can hold more water vapour than cold air
High
- Water molecules gain kinetic energy
- Faster evaporation rate
- Exception: At very high temperatures, stomata close to prevent excessive water loss
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3. Air movement.
Slow rate of transpiration Fast rate of transpiration
Still air
- Air surrounding the leaf becomes saturated
- High amount of water vapour both in the air spaces inside the leaf and in the air outside the leaf
- Concentration gradient for water vapour becomes low
- Less water vapour diffuses out of the leaf in a unit time (slow transpiration rate)
Windy
- Wind carries away water vapour from the surface of the leaf (outside the stomata)
- This means there is less water vapour outside the stomata (unsaturated, dry air)
- There is a steep concentration gradient for water vapour between the air spaces in the leaf and the air outside the leaf
- More water vapour diffuses out of the leaf in a unit time (fast transpiration rate)
- Exception: At very high wind velocities, stomata close to prevent excessive water loss
4. Light intensity.
*This factor does not affect the rate of
evaporation but affects the size of stomata
through which the water vapour passes.
Slow rate of transpiration Fast rate of transpiration
Low or dark (at night)
- Stomata close
High (during the day)
- Stomata open
- Exception: Stomata close at very high light intensities to protect the leaves from damage by UV rays
Experiments involving transpiration
For transpiration experiments, two chemicals are
usually used:
1. Cobalt chloride paper: It changes from blue to
pink in the presence of water.
2. Anhydrous copper sulphate: It changes from
white to blue in the presence of water.
To demonstrate transpiration from the
surface of a leaf
PROCEDURE:
1. A Piece of dry cobalt chloride paper is placed on
either side of a leaf attached to a plant.
2. It is secured in place by rubber bands as shown in
the figure above.
3. The glass slides prevent the cobalt chloride papers
from coming in contact with the water vapour in the
air.
4. After a short while, note the colour of the papers.
OBSERVATION:
After 10 minutes, the cobalt chloride paper on the
upper surface was still blue and that on the lower
surface had turned pink. This experiment shows that
transpiration is higher on the lower surface of the leaf
compared to the upper surface.
To measure the rate of transpiration
using a Potometer
A potometer can be used to compare the rates of
transpiration under different conditions.
A potometer consists of a bottle fitted with a three
holed rubber stopper.
The holes carry:
1. A funnel with a long stem
2. A graduated bent capillary tube
3. A leafy shoot
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The rate of water uptake by a cut leafy shoot can be
determined by measuring the distance travelled by
the air bubble in the capillary tube in a given time
interval.
Initial reading on capillary tube = a cm3
Final reading on capillary tube = b cm3 (after t mins)
Rate of transpiration = (b – a) cm3
t mins
The rate of water uptake is only approximately
equal to the transpiration rate since some water
may be utilised by the plant for other processes
such as photosynthesis and maintenance of turgidity
of plant cells.
To measure the rate of transpiration by
potometer (weighing method)
The difference between the weight of the flask and
plant at the beginning and at the end of the
experiment gives the weight of the water loss by
evaporation.
The total volume of water added to the flask to
restore the water level is the amount of water
absorbed.
Wilting
When a plant loses more water (as water vapour)
through its leaves than it absorbs through its roots;
wilting will occur.
Wilting can be caused by lack of water in the soil,
low humidity, windy condition and high
temperature. When these occur:
1. The turgor pressure of the plant cells
decreases;
2. The plant cells then becomes flaccid;
3. The leaves droop and the stem becomes limp
(for herbaceous plant).
Since the guard cells also become flaccid during
wilting, stomata are closed causing less inward
diffusion of carbon dioxide. Hence, rate of
photosynthesis decreases as a result of wilting.
Wilting of non-woody stem:
Xerophytes
Xerophytes are plants that live in conditions where
water is scarce.
These plants include those in hot, dry, desert
conditions and plants deprived of water during
winter when the soil water freezes.
Xerophytic adaptations enable a plant to reduce its
water loss in a number of ways:
Feature Effect/s
Small or rolled or spiny or no
leaves
Reduced surface area exposed for evaporation of water
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Rolled or hairy leaves
Traps water vapour outside the stomata (hairs) / within the enclosed space (rolled leaves)
Low concentration gradient for water vapour
Less water vapour diffuses out of leaves = low transpiration rate
Thick layer of waxy cuticle covering the
leaves
Provides thicker waterproof barrier to reduce water loss through evaporation from the surfaces of the leaves
Reduced number of stomata in
leaves
Less water vapour lost
Sunken stomata
Water vapour is trapped within the pockets / pits
Creates moist air (high humidity, saturated) outside the stomata
Low or no concentration gradient for water vapour
Less water vapour diffuses out of leaves = low transpiration rate
Translocation
All products of photosynthesis are distributed by
phloem cells to non-photosynthetic parts of the
plant.
The entire mass of solutes and water is transported,
either along the concentration gradient or against
it. This movement of synthesized products is called
translocation.
Translocation is rapid and much faster than simple
diffusion. The products are moved to other regions
of the plant for utilization or storage.
These regions include:
a) All living cells that respire,
b) Actively growing regions, e.g. root tip and stem
apex,
c) Flowers, fruits and seeds,
d) Storage organs, mainly in the roots.
Translocation in plants can be affected (slowed
down) if:
a) The temperature is lowered,
b) There is no oxygen supply and
c) There are poisons are present (inhibit enzymes
during respiration).
Translocation results in both downwards (to roots
and storage organs) and upwards (to fruits and
growing buds) transport of substances.
Process of translocation occurs in the following
ways:
1. Carbohydrates made from photosynthesis are
converted first into sucrose (an inert sugar).
2. Sucrose and amino acids are actively
transported into phloem sieve tubes by
companion cells (energy in the form of ATP from
aerobic respiration).
3. These are then transported from one sieve tube
cell into another through the pores in the sieve
plates.
Evidence that phloem translocate organic
food
Using radio isotopes in translocation studies
Carbon dioxide labeled with radioactive carbon
(14CO2) is fed to a green leaf in the presence of
light.
This radioactive carbon would then turn into
radioactive labelled sugars which would then be
transported about the plant.
After a short while, sections can be cut off the
stem.
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If the sections are placed on an X-ray photographic
film, this will show that all radioactivity is in the
phloem tubes. This means that the food that the
leaves have made is being transported in the
phloem.
Using Bark ringing experiments
The removal of a ring of bark from around the trunk
of a woody plant takes away all tissues external to
the xylem.
When the ring was cut, sugars from the leaves
(moving to the roots) accumulated in the outer
stem, above the ring.
The tree did not wilt showing that xylem was not
harmed. However, the plant eventually died
because the roots were deprived of sugars. This
shows that photosynthetic products such as sugars
are translocated in phloem cells.
Using aphids in translocation studies
Aphids are insects that feed on plant juices. They
have special mouthparts called proboscis or stylets
that can penetrate the leaf or stem.
The stylets are inserted into the phloem tubes of
the plant and via these structures, the plant juices
or sap in the phloem tubes are sucked up.
A feeding aphid can be anesthetised with carbon
dioxide and the mouthparts cut off.
The sap when analysed contains sugars, amino acids
and other organic materials.