Unit 8 Transport System in Plants
Key Unit Competence
To be able to describe the structure of the transport tissues in plants and the mechanismsby which substances are moved within the plant.
LEARNING OBJECTIVES
At the end of this unit, the learner will be able to:
• Recall that plants have two transport tissues: xylem and phloem.
• Appreciate the importance of transport systems in plants.
• Observe, draw and label, from prepared slides, plan diagrams of transverse sections of stems,
roots and leaves of herbaceous dicotyledonous plants to show the tissues in correct proportion.
• Draw and label, from prepared slides, the cells in roots, stems and leaves using transverse and
longitudinal sections.
• Recognise, from prepared slides, using the light microscope to draw and label the structure
of xylem vessel elements, phloem sieve tube elements and companion cells.
• Acknowledge that plants do not have systems for transporting oxygen and carbon dioxide.
Instead, these gases diffuse through air spaces within stems, roots and leaves.
• Show resilience when setting apparatus and making observations using microscopes and
solutions of different concentration to ensure improved reliability.
• Relate the structure of xylem vessel elements, phloem sieve tube elements and companion
cells to their functions.
• Explain the movement of water between plant cells, and between them and their environment,
in terms of water potential.
• Recall the term transpiration and understand that transpiration is an inevitable consequence
of gas exchange in plants.
• Experimentally investigate and explain the factors that affect transpiration rate using simple
potometers, leaf impressions, epidermal peels, and grids for determining surface area.
• Make annotated drawings, using prepared slides of cross-sections, to show how leaves of
xerophytes are adapted to reduce water loss by transpiration.
• Explain how hydrogen bonding is involved with the movement of water in the xylem by
cohesion-tension in transpiration pull and adhesion to cellulose cell walls.
• State that assimilates, such as sucrose and amino acids, move between sources and sinks in
phloem sieve tubes.
• Explain how transport systems in plants move substances from where they are absorbed or
produced to where they are stored or used.
• Show concern when selecting crop plants to reflect adaptations to environments e.g., where
they grow well, and when under water or not under water stress.
• Explain how sucrose is loaded into phloem sieve tubes by companion cells using proton
pumping and the co-transporter mechanism in the cell surface membranes.
• Explain mass flows in phloem sap down a hydrostatic pressure gradient from source to sink.• Carry out an investigation to demonstrate mass flow hypothesis.
INTRODUCTORY ACTIVITY
Have you ever thought about how plants get water and minerals from roots or how does
food prepared by leaves reach other parts of plants? What kind of transport system allows
the passage of water and food to various parts of plants? Research on these and discussyour findings in the class.
8.1 NEED FOR A TRANSPORT SYSTEM
ACTIVITY 1
Have you ever thought about how plants get water and minerals from roots or how does
food prepared by leaves reach other parts of plants? What kind of transport system allows
the passage of water and food to various parts of plants? Research on these and discuss
your findings in the class.
Water is the most abundant constituent of all living things on this earth. Plant tissues contain
more than 70 per cent water. Water content in cells affects many metabolic activities inside
the plant systems. Quantity of water in plant cells is maintained because these have evolved
various mechanisms to take up water from the soil through efficient conducting systems. The
water content in different plant parts is variable for example the amount of water in root cells
has been found to be different from that of fruits, stem and seed in sunflower plants. Differentplant species also have different water content expressed as percentage of fresh weight in their
plant parts. Plants suffer continuous water loss through the aerial parts by transpiration and
evaporation. However, these sustain continuous water loss from aerial parts by maintaining
an efficient transport system and uptake of water from the soil. It is interesting to study how
plants take up water from the soil and transport it to the aerial parts.
Plants have a unique mechanism for transport of water, nutrients and food. Water is taken up
by the roots and transported along with minerals to other parts of the plant. Along with water,
many nutrient elements that are essential for the growth of the plants are also taken up from
the soil. The food manufactured in leaves is similarly translocated from the source to the other
parts of the plant. The transport of food is also carried out by the conducting tissue. The unitdwells on various aspects of the transport in plants.
8.2 TRANSPORT TISSUES
To understand the movement of various substances in plants, one needs to recall what substances
are transported in plant systems and what structures are involved in the transport. Vascular
system in flowering plants or angiosperms is highly evolved. It is represented by complex
tissue system having xylem and phloem elements. There is a division of labour between thetwo tissue complexes.
Xylem is involved in uptake of water and minerals. Phloem is involved in uptake of food
material. These complex tissues are composed of various cell types that play very importantrole in the transport of water, mineral elements and photosynthates
8.3 TRANSPORT OF WATER AND MINERALS
8.3.1 Structure of Xylem Tissue
ACTIVITY 2
Aim: To investigate how plants transport water and minerals.
Requirements: A fresh green plant, a glass of water, natural food colour, a razor, slide and a
microscope.
Procedure:
Take a fresh green plant.
Give a cut at the basal end.
Put the cut segment in water with natural food colours.
Cut a transverse section of the stem and observe it under the microscope.
Discussion: What do you think the stem will look like?Could you see that some part of the stem appears coloured? Explain why.
Xylem is involved in uptake of water and mineral elements and phloem is involved in transport
of food material from source to the sink. The stem appeared coloured in the activity because
the water is rising through the specialized conducting tissues called the xylem.
Xylem: forms a continuous system running from the tips of the roots to the above ground
parts and also to the branches and leaves. It forms the long distance transport systems within
the plants. It is a complex tissue forming a part of vascular tissue. Xylem tissue is composed
of four types of cells: Tracheids, Vessels, Xylem fibres and Xylem parenchyma.
(a) Tracheids: Tracheids are elongated cells with blunt ends, present along the long axis of
the plant system. Tracheids are imperforate cells with bordered pits on their end walls.
They are arranged one above the other. These have broader lumen and lignified walls that
offer mechanical support to the plants. Sometimes an intermediate type of cell element
is also found in a vascular system known as fibre-tracheids.
(b) Vessels: Vessels are main transporting elements in xylem. These are long cylindrical tube
like structures made up of many cells called vessel members. These vessels are joined
end to end forming a continuous column. Sides of xylem vessels are lignified. These do
not have protoplasm and have perforations in their end walls.
(c) Xylem Parenchyma: These are thin walled cells that act as storage cells and their walls
are made up of cellulose. Radial conduction of water takes place by ray parenchymacells in thick tall trees.
(d) Xylem Fibres: These are cells with thick obliterated walls. These have narrow lumen
and their function is to attribute mechanical strength to the plants.
Xylem elements can be observed and studied well by using maceration technique. The slivers
of stem are cut and put into a maceration mixture. These are separated from the mixture,
washed, stained and mounted in glycerine and observed under microscope. Xylem elementsin macerated plant material as seen under microscope (Figures 8.1 and 8.2).
Figure 8.1: Diagram showing xylem elements fibres, parenchyma
cells, tracheids and vessels (macerated material)
8.3.2 Absorption of Water Through Roots
ACTIVITY 3
Aim: To demonstrate the water moves through xylem vessels.
Requirements: A small potted plant of tomato, eosine solution, beaker, stand, microscope,
slide, razor and water.
Method:
Dig out a small tomato plant.
Cut the stem at the base 1 to 2 cm above roots under water.
Immerse the cut end in eosine solution contained in a beaker.
Fix the shoot erect with the help of a stand.
Keep it for a day or two without disturbing.
Cut the transverse sections of stem.
Observe it under the microscope.
• Could you see that xylem and tracheids in the section looks red?
• Can you say why?
Observations and results: Xylem and tracheids in the section turn red indicating water moves
up in the xylem.
Observing plants in different situations allows learners to make inferences about watermovement through the plant material.
Soil is the main source of water for the plants. The principal source of soil water is the water
that is stored in the spaces between the soil particles after precipitation or rainfall. From the
root hair cells the water enters into the epidermis, cortex and finally endodermis (Figure 8.3).
Endodermis is impregnated with fatty substances along the wall called casparian strips. These
strips form networks and these seal off the spaces between the endodermal cells. From theendodermis water enters into the vascular tissue.
Figure 8.3: Diagram showing entry of water in the root system
The movement of water into the roots can take place by various pathways.
The first pathway is referred to as apoplast. It means that water is moving along the interconnecting
cell walls and spaces between the walls. Water moves through apoplast because of capillary action
or free diffusion along the gradient. It is also called the non-living continuum (Figure 8.4). The other
pathway is the symplast, in which water moves across the root hair membrane and through thecells themselves. Plasmodesmata act as channels to transport water between the cells (Figure 8.4).
Figure 8.5: Diagram showing the movement of water through
various pathways and through various cell layers in the roots
In natural conditions, the apoplastic and symplastic pathway do not separate and are operating
simultaneously within the system (Figure 8.5). The absorption of water through roots is affected
by various abiotic and biotic factors. When the soil temperature is high, movement is fast. Low
temperature reduces water uptake in plants. The branching pattern of the roots also affects
the uptake of water.
Water is a polar molecule. When two water molecules approach one another, the slightly
negative charged oxygen atom of one forms a hydrogen bond with a slightly positively charged
hydrogen atom in the other. This attractive force, along with other intermolecular forces, is one
of the principle factors responsible for the occurrence of surface tension in liquid water. It alsoallows plants to draw water from the root through the xylem to the leaf.
8.3.3 Anatomy of Root
In most of the herbaceous plants, the roots show various layers of cells through which the water
travels inside the root systems. The outer layer that is protective is termed as epidermis and is
followed by ground tissue consisting of multiple layers of cortex, endodermis and pericycle.
Two to six exarch xylem bundles are found. Phloem tissue is alternate with the xylem tissue(Figure 8.6).
Figure 8.6: Anatomy of ranunculus root
8.3.4 Rise of Water/Ascent of Sap
Cohesive and adhesive forces are important for the transport of water from the roots to the
leaves in plants. Various processes are operating inside the plant system and cells to facilitate
the movement of water from soil to roots and from one cell to another. Absorption of water by
root hairs from the soil and movement of water from one living cell to another within the plant
is brought about by osmosis. The most important factor that affects the uptake is the mineralconcentration of salts in soil.
Before we discuss other things, we should understand that solutions are classified on the basis of
mineral concentrations present in them. On the basis of mineral concentration in these, various
types of solutions are Hypotonic solutions, Hypertonic solutions and Isotonic solutions. Youhave already studied about them in Unit 2.
Water absorption in plants takes place with the expenditure or energy or without use of energy.
The two processes involved are: passive and active absorption.
Passive absorption: Root does not play an active role in water uptake. The root system merely
acts as a physical absorbing system. Energy is not spent for absorption. Passive absorptionaccounts for most of the water absorbed by plants.
Active absorption: Water is absorbed as a result of activity of the root. Root hairs take in
minerals by expenditure of energy. Then water moves from low solute concentration to highsolute concentration across the membrane.
APPLICATION 8.1
1. Complete sentences with appropriate terms:
(a) Xylem tissue is composed of four types of cells:
...................., ...................., .................... and .................... .
(b) Plants .................... water from soil and .................... it to aerial parts.
(c) Two pathways regulating uptake of water from roots are ................. and .................. .2. The diagram below represents a transversal section of a young stem of a dicotyledonous:
a) Name the parts A, B, C.
b) Give any one reason which proves that it is:
(i) A structure of a stem but not that of a root(ii) A primary structure but not a secondary structure.
8.3.5 Mechanism of Uptake of Water and Mineral Salts
It is quite clear that various mineral elements are present in the water and these are carried along
with water to the aerial parts of the plants. This is possible because water is a polar solvent and
many mineral ions are highly soluble in it. Many viewpoints have been put forward to help inunderstanding of water and mineral ions in the plants.
Root Pressure
As various ions from the soil are actively transported into
the vascular tissues of the roots, water follows (its potential
gradient) and increases the pressure inside the xylem. This
positive pressure is called root pressure, and can be responsible
for pushing up water to small heights in the stem. How can we
see that root pressure exists? Choose a small soft-stemmed plant
and on a day, when there is plenty of atmospheric moisture,
cut the stem horizontally near the base with a sharp blade,
early in the morning. You will soon see the drops of solutionooze out of the cut stem; this comes out due to the positive
Figure 8.7: Root pressure
root pressure. If you fix a rubber tube to the cut stem as a sleeve you can actually collect and
measure the rate of exudation, and also determine the composition of the exudates. Effects
of root pressure is also observable at night and early morning when evaporation is low, and
excess water collects in the form of droplets around special openings of veins near the tip
of grass blades, and leaves of many herbaceous parts. Such water loss in its liquid phase is
known as guttation (Figure 8.7).
Root pressure can, at best, only provide a modest push in the overall process of water
transport. It obviously does not play a major role in water movement up tall trees. The greatest
contribution of root pressure may be to re-establish the continuous chains of water molecules
in the xylem which often break under the enormous tensions created by transpiration. Root
pressure does not account for the majority of water transport; most plants meet their needby transpiration pull.
Transpiration Pull
Despite the absence of a heart or a circulatory system in plants, the flow of water upward
through the xylem in plants can achieve fairly high rates, up to 15 metres per hour. How is this
movement accomplished? A longstanding question is, whether water is ‘pushed’ or ‘pulled’
through the plant. Most researchers agree that water is mainly ‘pulled’ through the plant, and
that the driving force for this process is transpiration from the leaves. This is referred to as the
cohesion-tension-transpiration pull model of water transport (Figure 8.8). But, what generatesthis transpirational pull?
Figure 8.8: Transpiration pull
Water is transient in plants. Less than 1 per cent of the water reaching the leaves is used in
photosynthesis and plant growth. Most of it is lost through the stomata in the leaves. This water
loss is known as transpiration.
You have studied transpiration in an earlier class by enclosing a healthy plant in polythene bag
and observing the droplets of water formed inside the bag. You could also study water lossfrom a leaf using cobalt chloride paper, which turns colour on absorbing water.
1 MPa = 10 atmospheres or 14.5 pounds/inch2.
The root hairs are not developed in some of conifer plants; thus, water is absorbed with
the help of mycorrhizal associations. Mycorrhizal fungi also called vesicular arbuscular
mycorrhizae (VAM) also play an important role in absorption of water. Mycellia absorb
water and minerals and transfers to the roots. These fungal mycelia obtain their food from
the roots.
Velamen is a specialized tissue present on the outer side of the cortex found in epiphytes suchas orchid that absorb water through the hanging roots.
ACTIVITY 4
Aim: To observe various parts of a leaf.
Requirements: A fresh leaf (Bryophyllum), foreceps, needles, watch glass, slides, a razor blade
and compound microscope.
Methods: Cut a vertical section of the leaf.
Stain with a fast green and safranin.
Mount in glycerine.
Observe it under a microscope. What do you observe?
Discussion: Do you see some small openings in the lower layer of the leaf ? What are these
openings called?
Do you see any green coloured pigments in the leaf ?What are these pigments called?
Figure 8.9: Verticle Section of the leaf showing various parts
The outer layer of the leaf is epidermis surrounded by palisade parenchyma cells. The epidermis
has stomata composed of guard cells. Stomata can be present on both the leaf surfaces. In
many plants, stomata are present only on the lower leaf surface. The epidermis is protective in
function. The palisade layer is made up of columnar epithelial cells joined end-to-end having
many chloroplasts. Below the palisade layer are round spongy parenchyma cells that have
conspicuous intercellular spaces. The conducting tissue system consists of tissues present near or
at the centre of midrib region. The xylem is composed of vessels and trachieds and phloem has
fibres and parenchyma. Larger vascular bundles are surrounded by bundle sheaths (Figure 8.7).
Plants do not have systems for transporting oxygen and carbon dioxide. Instead, these gasesdiffuse through air space within stems, roots and leaves.
8.4 TRANSPIRATION IN PLANTS
ACTIVITY 5
Aim: To demonstrate the phenomenon of transpiration by bell jar method.
Requirement: A potted plant, glass plate, bell jar, oilcloth, grease and thread.
Methodology: Take a watered healthy plant. Cover the soil
by cloth to avoid evaporation.
Place the pot on a glass plate and cover with a bell jar.
Leave the apparatus for some time and observe.
What do you see at the inner side of the bell jar? Where do
these come from?
Results: Small drops of water start appearing on the inner side
of bell jar due to condensation of water vapour transpiredfrom the plant.
Transpiration is a physical process responsible for uptake of water in the form of water vapours
from the plants. Most of the transpiration takes place from the leaves through stomata, cuticles
and lenticels. Transpiration through stomata is called stomatal transpiration. It accounts to
90-95% of the total transpiration. Small quantity of water is also lost through the cuticle. Stomatal
opening and closing affects the rate of transpiration in plants. Changes in turgor pressure of
the guard cells cause stomata to open or close. Both the upper and lower leaf surfaces have a
flattened layer of cells called epidermis. Epidermis is covered by a waxy layer called cuticle. In
many plants, the lower epidermis has a pair of bean shaped cells called guard cells which along
with the subsidiary cells and other guard cells form stomatal complex. Guard cells in dicots are
kidney shaped and in monocots are dumbbell shaped. Guard cells have thickenings in inner
walls. The guard cells together form a stomatal pore or aperture. Of the total water taken up
by the plant most of it is lost in the form of water vapour. This type is cuticular transpiration.
Stomata are also meant for gaseous exchange of oxygen and carbon-dioxide, but transpiration
also occurs when they are open (Figure 8.10). There is a trade off during the gas-exchange that
is important for respiration and photosynthesis in the plant systems.
About less than 0.5% is lost through the lenticels, tissues on stem and fruits. This is calledlenticular transpiration.
Figure 8.10: Transpiration occurs through stomatal aperture
8.4.1 Factors Affecting Transpiration
The absorption of water from the roots is affected by many physico-chemical properties of
soil such as soil temperature, soil air, amount of water available in the soil and concentration
of mineral salts in the soil (Figure 8.11). Atmospheric humidity, temperature, wind velocity,
light and water availability in the soil affect the rate of transpiration in the plants. Study of
various temperature treatments on plants can be studied by using simple potometers.
In increased light intensity stomata open wider to allow more carbon dioxide into the leaf forphotosynthesis.
Figure 8.11: Diagram showing process of transpiration on a sunny day
With the increase in wind velocity, there is also increase in the rate of transpiration as water
evaporates fast.
Temperature affects transpiration indirectly through its effect on water vapour present in the air.
An increase in temperature brings about decrease in relative humidity of the air thus increasingrate of transpiration.
ACTIVITY 6
Aim: To study the effect of different light intensities on the rate of transpiration using a
potometer.
Requirements: Twig of Dracaena. Potometer, Luxmeter, Table lamp.
Procedure: Place a twig of plant in one end of the potometer and seal it air-tight. Fill the
entire apparatus with water so that there are no air spaces in between. The plant is exposedto different light intensities (Figure 8.12).
Do you see any changes in the level of water at the other end of the tube? Explain why.
Results: With the increase in light intensity, the level of the water drops indicating increase inrate of transpiration.
Figure 8.12: Study the effect of light intensities on rate of transpiration using potometer
8.4.2 Significance of Transpiration
Transpiration helps the plants in many ways. It has been considered as a necessary evil. This
is because plants lose lot of water due to the process but it is vital for many other physiological
processes. The reasons why this process is advantageous to plants are:
1. It maintains and regulates temperature by evaporative cooling.
2. It helps in absorption and transportation of mineral ions in plants.
3. It provides water to keep cells turgid in order to support the plant.4. It makes water available to leaf cells for photosynthesis.
8.4.3 Adaptations of Xerophytes to Reduce Water Loss
Many plants show various morphological features that help them survive in regions with low
water availability. The morphological variations are observed in root, stem, branching pattern,types of leaves and other parameters.
These variations are manifestations of changes taking place in the plants at various other levels
such as anatomical and biochemical level. These variations are termed as adaptations and help
plants to survive in a particular environment. Plants that grow in environments that have plenty of
water have stomata on both upper and lower epidermal cells of the leaves. These have isobilateral
leaves, aerenchyma in stems and undeveloped vascular bundles.
However, the plants growing in low water availability show various xeromorphic and xerophytic
characters. Xerophytic plants exhibit a variety of specialized adaptations to survive in such
conditions. Xerophytes may use water from their own storage, allocate water specifically to sites
of new tissue growth, or lose less water to the atmosphere and so convert a greater proportion
of water in the soil to growth.
Xerophytic adaptation of reducing water loss by transpiration:
1. Xerophytes have thick waxy cuticle which reduces evaporation as it acts as a barrier.
The shiny surface also reflects heat and so lowers temperatures reducing water loss.
2. They have rolled leaves or leaves reduced to spines to reduce water loss.
3. Stomata are present in pits. They are sunken. They open at night to reduce the
amount of water lost by transpiration.
4. Roots are deep and/or spreading to maximize the absorption of underground water.
5. They exhibit crassulacean acid metabolism, i.e., CAM Physiology.
6. They have fleshy stems or leaves—some cells in stems or leaves have very largevacuoles that act as water storage areas. These stems are also called succulent stems.
APPLICATION 8.2
Complete with appropriate terms:
(i) The association of Mycorrhizal fungi is ……………….. .
(ii) ……………….. is the loss of water from plants.
(iii) ……………….. is used to study the rate of transpiration.
(iv) Xerophytes have ……………….. stomata.(v) Xerophytes exhibit ……………….. metabolism.
8.5 TRANSPORT OF FOOD
Process of the movement of food synthesized during photosynthesis from the leaves to the
roots and other parts of a plant through the phloem is called translocation. This is carried outby another conducting tissue phloem.
8.5.1 Structure of Phloem
An analysis of the phloem exudate obtained by making an incision into the phloem tissue
provides evidence that photoassimilates are transported through phloem.
The phloem collects photoassimilates in green leaves, distributes them in the plant and supplies
it to the other heterotrophic plant organs. Phloem is composed of various specialized cells called
sieve tubes, companion cells, phloem fibres, and phloem parenchyma (Figure 8.13).
1. Sieve Tubes: Sieve tubes are series of cells joined end to end. The cross walls between
successive sieve elements become perforated forming sieve plates. The cell walls are
thin. Although the cell contents are living, the nucleus disintegrates and disappears. The
lumen is filled with a slimy sap which is composed mainly of protein. The function of
sieve tubes is transport of organic compounds.
2. Companion Cells: These are specialized parenchyma cells which always appear with the
sieve tube elements. They are also elongated, thin-walled and have distinct nucleus in the
cytoplasm of the companion cell. Their function is to regulate the metabolic activities
of the sieve tube elements.
3. Phloem Fibres: These cells are elongated and tapering. They have thickened walls.
Phloem fibres give mechanical strength to the plants.
4. Phloem Parenchyma: These are living cells with thin walls. Phloem parenchyma storescompounds such as starch.
Figure 8.13: (a) and (b) A Structure of phloem: RLS of the stem
showing various parts of phloem
8.5.2 Movement of Sugar in Plants
As sugar is synthesized in the leaves by the process of
photosynthesis its high concentration inside the phloem cells
of the leaf creates a diffusion gradient by which more water
is transported into the cells. Translocation takes place in the
sieve tubes, with the help of adjacent companion cells. Food
is translocated in the form of sucrose. The movement of water
and dissolved minerals in xylem is always upward from soil to
leaves against the gravitational pull. However, the movement
of food can be upward as well as downward depending upon
the needs of the plants (Figure 8.14).
Figure 8.14: Phloem Transport can be bidirectional
represented by red arrows
8.5.3 Mechanism of Uptake of Food in Plants
As explained earlier, leaves manufacture food by the process of photosynthesis and transport
it to other parts of the plants. Part of plants where food is formed more than requirement is
known as source. Leaves act as a source and where these are stored and sent is the sink. Roots
act as sinks for food. Movement of food takes place from source to the sink. Leaves prepare
food in the form of glucose that is converted into sucrose. Sucrose enters into the phloem
at the expense of energy in the form of ATP. It is noteworthy that in plants, roots, fruits
and other organs also act as storage organs for food and from here the food is translocated
to other parts. So, the direction of movement of the phloem can be both upwards and
downwards and hence the movement is bidirectional. Sugars, hormones and amino-acids are
also transported or translocated through phloem. Transport of food involves 3 steps: Phloem
loading, translocation and phloem unloading. Sucrose and other carbohydrates are actively
loaded into the sieve tubes at the source by a chemiosmotic mechanism. It requires ATP.
• ATP supplies energy to pump protons out of the sieve tube members into the apoplast.
• Creates proton gradient.
• The gradient drives the uptake of sucrose into the symplast through channels by the
cotransport of protons back into the sieve tube members.
Osmotic concentration of phloem increases due to presence of sucrose. Therefore, water enters
into sieve tubes by osmosis, due to which the hydrostatic pressure is created in phloem. High
pressure in the phloem allows the movement of food to all parts of the plants having low
pressure in their tissues. The pressure-flow hypothesis proposed by Munch is the simplest model and
continues to earn widespread support among plant physiologists. The pressure-flow mechanism
is based on the mass transfer of solute from source to sink along a hydrostatic (turgor) pressure
gradient. Translocation of solute in the phloem is closely linked to the flow of water in the
transpiration stream and a continuous recirculation of water in the plant.
Theory proposes that food molecules flow under pressure through the phloem. The food is
mixed with water. The pressure is created by the difference in water concentration of the
solution in the phloem and the relatively pure water in the nearby xylem. Sugars manufactured
in mesophyll cells are driven by energy into the companion cells and sieve tube elements
of the phloem. After accumulation into the phloem, water enters in cells by osmosis. A
pressure is built up in sieve tubes called turgor pressure. Due to this pressure, sugars are
removed by the cortex of both stem and root and consumed or converted into storage
products such as starch. Starch does not exert any osmotic effect. Hence, osmotic pressure
of phloem cells decreases. Thus, the pressure gradient created between leaves and shoot androot drives the contents of the phloem up and down through the sieve tubes (Figure 8.15).
Figure 8.15: Illustrated account of the phloem transport in plants
Assimilates including sucrose, amino acids and nutrients are transferred into sieve elements
of fully expanded leaves against significant concentration and electrochemical gradients. This
process is referred to as phloem loading. The cellular pathways of phloem loading, and hence
transport mechanisms and controls, vary between plant species. Longitudinal transport of
assimilates through sieve elements is achieved by mass flow and is termed phloem translocation.
Mass flow is driven by a pressure gradient generated osmotically at either end of the phloem
pathway, with a high concentration of solutes at the source end and a lower concentration at
the sink end. At the sink, assimilates exit the sieve elements and move into recipient sink cells
where they are used in growth or storage processes. Movement from sieve elements to recipient
sink cells is called phloem unloading. The cellular pathway of phloem unloading, and hencetransport mechanisms and controls, vary depending upon sink function.
Figure 8.16: (a) Girdling in trees
Figure 8.16: (b) Picture showing the role of phloem in translocation of food
A simple experiment, called girdling, was used to identify the tissues through which food is
transported (Figure 8.16(a)). On the trunk of a tree a ring of bark up to a depth of the phloem
layer, can be carefully removed. In the absence of downstream movement of food, the portion
of the bark above the ring on the stem becomes swollen after a few weeks (Figure 8.16(b)). This
simple experiment shows that phloem is the tissue responsible for translocation of food : andthat transport takes place in one direction, i.e., towards the roots.
To Investigate Mass Flow Hypothesis
• In mass flow, Munch’s model demonstrates that fluid flows from the region of high
hydrostatic pressure to the region of low hydrostatic pressure.
• As fluid flows, it carries the whole mass of different substances.
• In osmometer A, concentrated sucrose solution (leaf) has lower water potential. Water
flows into it from a high water potential region (xylem vessel) to a low water potential
region (leaf cells) by osmosis.
• This create high hydrostatic pressure in A and forces sucrose solution to enter into the
connecting tube (sieve tube) and pass to B (root cell).
• As the flow of mass from osmometer A to osmometer B continues, the sucrose solution
is pushed along and finally appears in B.
• In B, contain water/dilute sugar solution, water moves out from a higher water potential
region by the hydrostatic pressure gradient produced and redistributed through connecting
tube (xylem vessels) between the two containers.
• Mass flow continues until the concentration of sugar solution in A and B are equal (balanced).
• In nature, equilibrium is not reached because solutes are constantly synthesized at sourceA and utilized at the sink B.
Figure 8.17: Demonstration of mass flowhypathesis
APPLICATION 8.3
Complete with the appropriate terms:
(i) …………… is the process to show food flows from leaves to roots.
(ii) Phloem transports phloem sap from …………. to ………………… .
(iii) Phloem …………….. gives mechanical strength to cells.(iv) Movement from sileve elements to recipient sink cells is called ……….
8.6 SUMMARY
• Water is an important solvent and acts as a reagent in various chemical reactions in the plants.
• It helps to maintain turgidity of cells and is important for growth of plants as it serves
as a raw material for photosynthesis.
• Transport of water is an important process in plants and has been well understood.
• Several physical phenomena such as imbibition, diffusion, osmosis, turgor and water
potential facilitate uptake of water in plants.
• Forces of cohesion and adhesion also play an important role in transport of the water upstream.
• Water enters the plants through active or passive absorption process. The upward
movement of water through stem is called the ascent of sap.
• Practically most of the water absorbed by plants is lost into the atmosphere through the
process of transpiration.
• A variety of internal and external stimuli govern the rate of transpiration in plants.
• Atmospheric humidity, temperature, light, wind velocity, leaf area, leaf structure and
availability of water affect the process.
• Plants also take up inorganic nutrients from the soil with water. The sugars synthesized in
leaves are translocated downwards, upwards and to lateral organs mostly through phloem.
• Experiments have been conducted to demonstrate the movement of food through phloem.
Besides sugars that are end products of photosynthesis, amino acids are also transportedthrough phloem.
8.7 GLOSSARY
• Active transport: Transport of ions or molecules across a cell membrane against a
concentration gradient.
• Adhesion: Attraction of water molecules to polar surfaces is called adhesion.
• Apoplast: Intercellular spaces, excluding the protoplasts is called apoplast.
• Aquaporins: Protein channels for transport across membranes.
• Casparian strips: A band like structure in endodermis of root cells that contain suberin
and lignin.
• Cohesion: Mutual attraction between water molecules is called cohesion.
• Cuticle: A three layered structure present on the epidermis that prevents movement of
gases and water to move into or out of the plants.
• Diffusion: Movement of substance from high concentration to low concentration.
• Lignin: An aromatic polymer that rigidifies may secondary cell walls.
• Lenticels: Pores on woody stems and roots for gaseous exchange.
• Osmosis: Movement of water from area of low to area of high solute concentration.
• Phloem: the photosynthate-conducting tissue of plants.
• Plasmodesmata: Connection between protoplasts of adjacent cells through cell walls.
• Plasmolysis: Shrinkage of cytoplasm under the influence of hypotonic solution.
• Root pressure: Hydrostatic pressure created inside the roots due to absorption of water.
• Root: The portion of a plant axis produced by the root apical meristem.
• Stem: The portion of a plant axis produced by the shoot apical meristem.
• Leaf: A lateral appendage of the stem produced by the shoot apical meristem.
• Sieve element: a conducting cell in the phloem.
• Surface tension: Any liquid has a tendency to occupy the least possible surface area.
This property is called surface tension.
• Symplast: The continuous system of protoplasts connected by plasmodesmata.
• Tensile strength: It is a measure of maximum force per unit area that would be needed
to break a continuous column of water.
• Tracheid: A conducting cell of the xylem characterized by an elongated shape and
lignified secondary cell wall.
• Turgor Pressure: Hydrostatic pressure developed inside cell vacuole that presses cytoplasm
against the cell wall.
• Vascular tissue system: Tissues derived from the procambium or vascular cambium that
transports water and photosynthates.
• Vessels: Tracheary element with perforation plates.
• Water potential: Chemical potential of water in relation to pure water.
• Xylem: The water-conducting tissue of plants.
END UNIT ASSESSMENT 8
I. Multiple Choice Questions
1. Much of the transpiration takes place through
(a) stomata (b) lenticels (c) cuticle (d) epidermis
2. The roots absorb water through
(a) epidermal hairs (b) root hairs (c) root xylem (d) root phloem
3. The ascent of sap in plants takes place due to
(a) root pressure (b) transpiration pull
(c) osmosis (d) both (a) and (b)
4. Stomata open and close due to
(a) presence of valves (b) hormonal control
(c) turgor pressure of guard cells (d) concentration gradient of the gases
5. The food is transported in the phloem in the form of
(a) glucose (b) sucrose (c) amino acids (d) fats
6. The movement of particles from the region of their higher concentration to the region
of their lower concentration is called
(a) osmosis (b) diffusion (c) active transport (d) ascent of sap
7. Plant transport system does not transport
(a) CO2 (b) organic salts (c) water (d) plant hormones
8. The strongest force to pull water up the xylem and into the leaf is
(a) capillary action (b) root pressure(c) transpiration pull (d) active transport
9. The loss of water in the form of vapours by the leaves and stem of a plant is called
(a) translocation (b) osmosis
(c) active transport (d) transpiration
10. The transport of sugar from the leaf to the rest of the plant is called
(a) translocation (b) osmosis(c) active transport (d) transpiration
II. Long Answer Type Questions
1. Name the two transport tissues present in the plant.
2. What are the factors affecting the rate of diffusion?
3. Explain why pure water has the maximum water potential.
4. Differentiate between the following:
(i) Diffusion and Osmosis
(ii) Active and Passive Transport
(iii) Osmosis and Diffusion
(iv) Transpiration and Evaporation
5. Discuss the factors responsible for ascent of xylem sap in plants.
6. How is turgor pressure created in the sieve elements?
7. What is the difference between apoplast and symplast?
8. Explain in detail the absorption of water through root hairs up to the xylem.
9. Discuss the structure of phloem and its components in plants.
10. Explain how food prepared in the leaves reaches the other parts of the plant.
11. Explain the hypothesis proposed by Munch regarding translocation of food in plants.
12. Appreciate the importance of transport systems in plants.
13. Draw and label, from prepared slides, the cells in roots, stems and leaves using
transverse and longitudinal sections.
14. There are a million processes that account for life on Earth. All these processes play
a major role in balancing the climate of Earth. Investigate in the same regard the role
of transport in plants in regulating the environment of surroundings. Also necessitate
the presence of plants or a wholesome regulation of atmosphere.
15. Draw this picture in your exercise book. It shows various internal parts of a leaf.These are marked us A, B, C, D, E and F. Identify and name these parts.
