TRANSPORT IN FLOWERING PLANTS
Plants need a transport system for the following reasons:
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To carry water and mineral salts from the roots to other parts of the plant
To transport manufactured foods from the leaves to other parts of the plant
To transport hormones from sites of synthesis to sites of usage
EXTERNAL AND INTERNAL STRUCTURE OF ROOTS AND STEMS.
Arrangement of Vascular Tissues in Dicots and Monocots
(a) Cross-section of a Dicot Root
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Xylem is located in the centre and is star-shaped
Phloem is located between the „arms‟ of the xylem
(b) Cross-section of a Monocot Root
(c) Cross-section of a Dicot Stem
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(d) Cross section of a monocot
stem
(e) Longitudinal section of a root
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(f) Longitudinal section of a stem
The transport system in flowering plants is called the vascular system (Vascular bundles). This
consists of xylem and phloem which are closely associated with a meristematic tissue called
cambium.
 Xylem conducts water and mineral salts from the roots to other parts of the plant and
supports the plant mechanically.
 Phloem transports manufactured foods from the leaves to other parts of the plant.
 Cambium carries out cell division to produce new cells, including xylem and phloem
cells
 Epidermis is the outer layer and protects inner tissues from mechanical damage and
entry of pathogens. In most stems, it has pores called lenticels for gaseous exchange
between the stem tissues and the surrounding environment.
 Root hairs absorb water and mineral salts from the soil
 Cortex serves as a passage for water and mineral salts in roots. In some plants, it serves
as a storage tissue in roots. In stems, it helps to maintain the firmness and shape of the
stem (mechanical support) through its three layers namely collenchyma, sclerenchyma
and parenchyma.
 Endodermis is the innermost layer of the cortex. It consists of a single layer of cells
which contain starch grains. This layer stains blue black with iodine solution. It actively
controls the movement of water and mineral salts into the vascular tissue.
 Pericycle is a single layer of cells located immediately below the endodermis. It consists
of meristematic cells which give rise to lateral roots.
 Pith forms the central region of the stem in dicotyledonous plants. It consists of loosely
packed parenchyma cells whose function is storage of water and food substances. The
pith is absent in stems of monocotyledons such as maize.
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ABSORPTION OF WATER AND UPTAKE OF MINERAL SALTS BY ROOTS
Water and mineral salts are absorbed by root hairs which are found near the tips of roots. A
root hair cell absorbs water by osmosis and mineral salts by active transport.
(a)ABSORPTION OF WATER BY ROOTS
In the soil, water exists as a film which surrounds the root hair cells. The root hair cells absorb
water from the soil by osmosis. The cell sap in the vacuole of the root hair cell has a high
concentration of dissolved mineral salts and sugars. The presence of these solutes makes the
cell sap hypertonic to the water found between the soil particles. A concentration gradient
therefore exists between the soil water and the root hair cells. Due to the concentration
gradient, water molecules move from the soil through the selectively permeable membrane of
the root hair cells by osmosis.
(b)UPTAKE OF MINERAL SALTS BY ROOTS
Mineral salts are absorbed as ions into the root hairs of a plant from the surrounding soil for
land plant or water for aquatic plant .The concentration of salts in the cell sap of the root hair
cells is usually higher than that in the soil or water. Mineral ions are taken up by active
transport against their concentration gradient using energy. Once the mineral ions are
absorbed into the xylem, they are transported in dissolved form to other parts of the plant.
MOVEMENT OF WATER AND TRANSPORT OF MINERAL SALTS FROM THE
ROOTS TO THE LEAVES.
(a) Movement of water from the roots to the leaves.
(i)
Movement of Water from the root hairs to the xylem
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(ii)
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After being absorbed, water moves from the root hair cell to the xylem using three
possible routes namely apoplast (from cell wall to cell wall), symplast (from
cytoplasm to cytoplasm)and vacuolar route (from vacuole to vacuole). The
movement of water from cell to cell is due to osmosis and transpiration pull.
Movement of water up the Plant
Water moves up the plant through xylem vessels in a continuous stream known as
the transpiration stream. The forces responsible for movement of water in
the transpiration stream are transpiration, capillarity, root pressure and
guttation.
Transpiration (the diffusion of water vapour from plant leaves to the
atmosphere through stomata). This creates a suction force that pulls water up the
xylem vessels.
Capillarity: This is the movement of water into narrow tubes or openings as a
result of cohesion (attractive forces between molecules of the same kind e.g. water to
water) and adhesion (attractive forces between molecules of different substances e.g.
water molecules and wall of xylem vessel).
Root Pressure: This is the pressure created in xylem vessels due to osmotic gain of
water by the roots. This pushes water up the xylem vessels.
Guttation: the loss of water drops from the tips and margins of leaves through
openings called hydathodes. This creates a suction force that pulls water up the
xylem vessels.
(b)Transport of mineral salts from the roots to the leaves.
As water flows from the roots to the leaves of a plant in the transpiration stream, it carries with
it dissolved mineral ions which have been absorbed into the plant through the roots. In this way,
mineral ions are distributed throughout the plant.
TRANSLOCATION
This is the movement of manufactured food from the source (point of origin/ manufacture) to the
sink sites (the sites of usage or storage) through phloem. The organic solutes mainly include
sucrose and amino acids dissolved in water. In most cases the leaves are the sources. In some
cases, storage sites may also act as sources e.g. when food from a tuber is being translocated to
points of growth.
The processes involved in the movement of manufactured food included diffusion, active
transport and cytoplasmic streaming (movement of materials from the cytoplasm of one cell to
the next cytoplasm of another cell).
TRANSPIRATION
This is the diffusion of water vapour from leaves to the atmosphere through stomata. In leaves of
most plants, there are more stomata on the under-side than on the upper-side. The water moves
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from the xylem vessels to the mesophyll cells by osmosis then it evaporates from the surfaces
of the mesophyll cells into the air spaces and finally diffuses out of the air spaces to the
atmosphere through the stomata.
Each stoma is surrounded by two guard cells. Changes in the turgidity of the guard cells lead to
opening and closing of stomata. When the guard cells gain water, they become turgid and the
stomata open. When water leaves guard cells, the stomata close
Excessive transpiration can lead to plasmolysis of plant cells causing wilting of the plant.
Wilting is the sagging of delicate plant parts such as leaves, flowers and young stems due loss
of water. Temporary wilting is wilting that can be reversed by supplying a plant with water.
Permanent wilting cannot be reversed even if a plant is supplied with water but leads to death
of the plant. A plant undergoes wilting when the rate of transpiration is higher than the rate of
water uptake.
Factors that affect the rate of transpiration
These include temperature, humidity, light intensity and wind.
Temperature: this is the degree of hotness or coldness of a substance. The higher the
temperature, the higher the transpiration rate. This is because high temperatures increase the
kinetic energy of the water molecules making them diffuse faster out of the leaf.
Humidity: this is the amount of water vapour in the atmosphere. The higher the humidity, the
lower the transpiration rate because high humidity lowers the concentration gradient between
the leaf and the atmosphere.
Light Intensity: This is the brightness or dimness of light. The higher the light intensity, the
higher the transpiration rate because high light intensity causes opening of the stomata.
Wind: wind is moving air. The higher the wind speed, the greater the rate of transpiration.
When the air is still, a layer of water vapour forms over the leaf and reduces the transpiration
rate. But when there is wind, this layer of vapour is blown away thereby increasing the
diffusion rate.
Adaptive features of a leaf to reduce excess loss of water.
Plants can be adapted to reduce the rate of transpiration by having xeromorphic features
which include the following:
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Presence of a thick waxy cuticle. Reduces loss of water by transpiration
Sunken stomata. Reduces loss of water by transpiration
Reduced size of leaves (needle-shaped leaves)/reduced number of stomata. This
minimizes the surface area over which transpiration takes place in the leaf thus reducing
water loss by transpiration
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Shedding of leaves. Some plants shed their leaves during prolonged drought season to
reduce water loss by transpiration. Shading of leaves therefore, reduces the surface area
for transpiration.
Presence of hairs on the lower side of the leaf. Reduces loss of water by transpiration
Leaves can roll up when water is scarce. Reduces loss of water by transpiration
Significance (importance) of transpiration in plants
(i)
Water cycle. Transpiration is important in the maintenance of the water cycle. Plants
absorb ground water and releases it into the atmosphere inform of water vapour.
Water vapour condenses to form clouds which later precipitate to form rainfall.
(ii)
Cooling of the plant. As water evaporates from the plant, it carries away latent heat
of vapourization hence leaving the plant cool.
(iii) Absorption of mineral ions. As water evaporates from the leaves, more is absorbed
from the soil. This creates a pull that draws more water containing dissolved mineral
ions by the roots
(iv)
Provides water for photosynthesis. Some water is not lost to the atmosphere but is
used during photosynthesis.
Measuring the Rate of Transpiration
This can be measured using an instrument called the potometer.
Diagram of potometer
When using the potometer it is assumed that water uptake is equal to water loss through
transpiration. The distance moved by the air bubble/meniscus, the cross sectional area of the
capillary tube and the time taken need to be known in order to calculate the transpiration rate
using the following formula:
Rate of transpiration = Distance moved by meniscus X Cross sectional area of tube
Time taken
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Example
A student used a potometer to measure the transpiration rate of a leafy shoot of a plant. The
water meniscus moved 30 cm in 30 minutes. If the cross-sectional area of the capillary tube
was 0.25 cm2.
(a) What was the transpiration rate of the shoot?
(b) State two precautions that were taken by the student in setting up the potometer
(c) Suggest the changes that would be observed in the air-water meniscus under the following
conditions. Give reason for your answer in each case.
(i)
Removal of all leaves from the leafy shoot
(ii)
Placing the potometer in the school playing field on sunny, windy day.
Solution
(a)
Rate of transpiration= Distance moved by meniscus X Cross-sectional area of tube
Time taken
= 30 cm X 0.25 cm2
= 0.25 cm3/ minute
30 minutes
(b) 1.applying grease/ petroleum jelly between the rubber bung and the leafy shoot to prevent
entry of air into the xylem vessel
2.The leafy shoot was cut and fixed under water to prevent blockage of the narrow xylem
vessels by air bubbles
(c) (i) The movement of air-water meniscus would stop since there would be no loss of water
by transpiration
(ii) The air – water meniscus would move rapidly as the rate of transpiration would increase
due to increases in temperature and wind speed
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