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Transportation in Plants
Transportation in plants is the process in which a substance is absorbed or synthesized in one part of the
plant and is transported to the other part of the plant.
Transport of Water and Minerals
Need for Water Water is required by the plant for various purposes.
Photosynthesis Water is one of the raw materials required to carry out
photosynthesis.
Transportation Transportation of minerals and sugar from roots to other
parts of the plant body.
Cooling Water is used to cool the plant by evaporation through
leaves when it is hot outside.
Need for Minerals Minerals are required as constituents of the cell and are also required for the synthesis of different
compounds and enzymes.
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Characteristics of Roots for Absorbing Water
The root system of plants consists of the main root, which gives out lateral roots.
The lateral roots bear a large number of fine outgrowths called root-hairs.
A root hair is a long protuberance of a cell.
The ability of roots to draw water from the soil is dependent on the following factors:
The root hairs cover a large surface
area. More the surface area, the greater
is the absorption.
The cell membrane is semi-permeable.
The vacuoles present in the root hair
cells contain cell sap. The water
concentration in cell sap is lower than the
concentration of water in soil.
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Experiment to show only Roots absorb Water
Take four test tubes and mark them A, B, C and D.
Fill water in test tubes A, B and C up to about three-quarters. Add only a little amount of water in test
tube D.
Fix a cork on the mouth of test tube A. Take three small-sized plants with their roots intact.
Wash their roots under tap water and insert them in test tubes B, C and D in a manner that the roots get
dipped fully in water in test tube B and C but remains above the water level in test tube D.
In test tube C, add pink dye to water.
Pour a few drops of oil in B and C to prevent the loss of water by evaporation.
Mark the levels of water in the four test tubes and leave the set up. Observe the change in the water
levels after 24 hours.
Observation:
Test Tube Observation Conclusion
A
Water level remains
unchanged.
No loss of water
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B
Water level falls. Water was absorbed by
the roots of the plant
dipped in it.
C
Water level falls.
The veins of the leaves
have taken up the pink
colour of the dye.
Water was absorbed by
the roots of the plant
dipped in it and it is
transported to leaves.
D
The leaves wilt. Leaves of the plant
experience a loss of water
due to transpiration and
this loss is not replaced by
the absorption of water by
roots.
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Osmosis
Absorption of water occurs through root hairs by the process of osmosis.
Osmosis is the movement of water molecules from a region of high concentration of water molecules to a
region of low concentration of water molecules across a semi-permeable membrane.
Semi-permeable Membrane
A semi-permeable membrane is a membrane that allows only selective materials to pass through it.
Raisins swell up in water and grapes shrink in thick syrup.
Put some raisins in bowl containing water and some grapes in another bowl
containing thick syrup of sugar. Observe the changes after 10-12 hours.
After 10-12 hours raisins swell up and the grapes shrink. The raisins absorb water
because they are placed in water while grapes lose water because they are kept in a
strong sugar solution. In both the cases, water movement takes place by osmosis.
The gain or loss of water is through the peel which acts as a semi-permeable
membrane.
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Absorption of Mineral Salts by Roots
Roots not only absorb water from the soil but also mineral salts.
Depending on the concentration of minerals, two mechanisms are involved in their absorption:
Diffusion Diffusion is the free movement of molecules of a substance from the region of its higher concentration to
the region of its lower concentration, until their concentration in both the regions is equal.
If the concentration of mineral outside the cell is higher than the cytoplasm of root hair cell, then the
mineral salts enter by the process of diffusion.
Diffusion Osmosis
Movement of a substance from its
higher concentration to lower
concentration when the two are in
direct contact.
No membrane is involved.
Direct contact between the two
substances is necessary.
It occurs in gases as well as in
liquids.
Diffusion of water molecules
across a semi-permeable
membrane from a more dilute
solution to a less dilute solution.
Osmosis takes place across a
semi-permeable membrane.
Two solutions are separated by a
semi permeable membrane.
Only water, a liquid can move
from its high concentration to low
concentration region across a
semi-permeable membrane.
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Active Transport Active transport involves the transportation of particles against a concentration gradient i.e. from a region
of low concentration to a region of high concentration.
Nutrients which cannot pass through the cell membrane of the roots cells easily are transported by
active transport.
Such substances are ions such as nitrates, sulphates, potassium, zinc, manganese, etc.
Active transport involves expenditure of energy. The energy utilized here is in the form of ATP.
Vascular Bundles
Vascular bundles constitute of xylem and phloem.
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1. Xylem It conducts water upwards in a plant.
It is located in the centre of the plant body.
Xylem chiefly consists of tracheids and vessels.
2. Phloem It conducts manufactured food from leaves to different parts of the plant.
The food in phloem can move in upwards as well as in downward direction.
Phloem chiefly consists of sieve tube cells and companion cells.
Together, xylem and phloem constitute of conducting tissues and are known as vascular tissues.
Experiment to show that xylem carries water and phloem carries manufactured food in the plant.
Procedure:
Take a pot containing a small, woody plant.
Of the main stem, remove the outer bark for about 4 to 6 inches vertically.
Make sure the central core of the stem is not removed.
Water the pot well and leave it in bright sunlight.
Observation:
After a few days, leaves on the top appear very fresh and healthy. This indicates that water from
the soil has reached the leaves through xylem located in the core of the stem.
Just above the cut region, a small bulge can be seen on the stem. This is because the bark which
was removed earlier is the location of phloem tissue. Due to its removal, the food prepared in the
leaves cannot go all down, but accumulates above the cut.
Conclusion:
The experiment proves that xylem located in the core of the stem conducts water while the phloem
located in the outer region of the stem conducts food.
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Root Pressure
It is the pressure developed in the roots due to continuous inflow of water.
Root pressure helps in pushing the plant sap upwards.
Due to root pressure water enters the xylem vessels.
Root Pressure
Process of Conduction of Water and Minerals
Water enters the root hairs through osmosis and mineral ions enter the root cells by active transport.
Both, water and minerals move upward from cell to cell through the cortex of the root by osmosis.
From the cortex, water and minerals are brought to the xylem.
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Ascent of Sap
The sap which contains water and dissolved minerals move upwards from the root cells to xylem. The
upwards movement of sap is called the ascent of sap.
Transpiration
Transpiration is the loss of water in the form of water vapours from the aerial parts of a plant.
Transpiration occurs through openings called stomata.
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During transpiration, water evaporates from the mesophyll cells of the leaves and escapes through the
open stomata.
Water loss through evaporation lowers the concentration of water inside the mesophyll cells. Due to
this, water enters the mesophyll cells from the neighbouring xylem vessels through osmosis.
More water is pulled upwards through the xylem vessels in stem, from the roots to replace the
water loss.
Therefore the pull is created by transpiration i.e. transpiration pull causes water and dissolved minerals
to move up through the stem.
Factors Affecting the Rate of Transpiration
Sunlight In day time, stomata remain open. Therefore, the rate of transpiration is faster.
During dark, the stomata are closed. Hence transpiration hardly occurs at night.
Temperature Increase in temperature increases the rate of transpiration.
Transpiration is faster on hot summer days than in cold winters.
Velocity of wind Transpiration increases with the blowing wind.
When the wind is blowing faster the water vapours from the leaves are
removed faster.
Humidity Transpiration is reduced if the air is humid.
Air cannot hold any water molecules when it is already laden with moisture.
Significance of Transpiration
Cooling Effect Maintaining Concentration of Cell Sap
During transpiration, water gets evaporated from
the plants.
The heat required for this evaporation is obtained
from the plant itself (latent heat).
Therefore, the plant is able to cool itself when it is
hot outside.
Transpiration ensures the maintenance of the
concentration of cell sap.
If there is no transpiration, then the
absorption of water by roots will make the cell
sap more dilute.
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Experiments to Demonstrate Transpiration:
Experiment 1: To demonstrate transpiration.
Procedure:
Take a small sized, well-watered plant.
Cover the plant with a transparent polythene bag completely and tie its mouth around the base of
the stem.
Keep the plant in sunlight and note the change after a few hours.
Observation:
Drops of water appear on the inner side of the polythene bag.
Inference:
The plant gives off water vapour which saturates and condenses on the inner surface of the
polythene bag.
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Experiment 2: To demonstrate that the plants lose water through its leaves.
Procedure:
Take a small sized, well-watered plant having a few branches.
Place a polythene bag over its one branch (consider this branch as A) as shown in the figure. Tie the
bag at the base of the stem with a rubber band.
From its other branch, remove all the leaves and cover it with another polythene bag as shown
in figure. (Consider this branch as B).
Place the plant in sunlight for 4-6 hours.
Observation:
Drops of water appear on the inner side of the polythene bag used to cover branch A.
No water drops appear on the polythene bag used to cover branch B.
Inference:
Most of the water gets evaporated from the plant through its leaves.
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Experiment 3: To demonstrate that leaves lose more water from their lower surface.
Procedure:
Fold strips of dry cobalt chloride paper on both the surfaces of a leaf of a well-watered potted plant.
Cover the two strips by glass slides tied together by elastic bands. This is to hold the strip in position.
Remove the paper after 1-2 hours and observe the changes in the cobalt chloride paper.
Observation:
The cobalt chloride paper attached to the lower surface of the leaf turns pinker than the one attached
to the upper surface of the leaf.
Inference:
Leaves lose more water from the lower surface. This is because the lower surface of the leaves have
more number of stomata.