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Thursday, 15 October 2015


                                            TRANSPORT IN PLANTS

Translocation : Transport of substances in plants over longer distances
through the vascular tissue (Xylem and Phloem) is called translocation.
Means of transport : The transport of material into and out of the cells is
carried out by a number of methods. These are diffusion, faciliated diffusion and
active transport.
Diffusion : Diffusion occurs from region of higher concentration to region
of lower concentration across the permeable membrane. It is passive and slow
process. No energy expenditure takes place.
Facilitated diffusion : The diffusion of hydrophilic substances along the
concentration gradient through fixed membrane transport protein without involving
energy expenditure is called facilitated diffusion. For this the membrane
possess aquarporins and ion channels. No energy is utilized in this process.
Active transport : Active transport is carried by the movable carrier proteins
(pumps) of membrane. Active transport uses energy to pump molecules
against a concentration gradient from a low concentration to high concentration
(uphill-transport). It is faster than passive transport.
Water potential : The chemical potential of water is called water potential.
It is denoted by ΨW (Psi) and measured in pascals (Pa). The water potential of a
cell is affected by solute potential (Ψs) and pressure potential (Ψp).
ΨW = Ψs + Ψp
Water potential of pure water at standard temperature which is not under
any pressure is taken to be zero (by convention).
Osomosis : Osmosis is movement of solvent or water molecules from the
region of their higher diffusion pressure or free energy to the region of their
lower diffusion pressure or free energy across a semi-permeable membrane.
Water molecules move from higher water potential to lower water potential
until equilibrium is reached.
Plasmolysis : Process of shrinkage of protoplasm in a cell due to exosmosis
in hypertonic solution.
Casparian strip : It is the tangential as well as radial walls of endodermal
cells having the deposition of water impermeable suberin.
Imbibition : Imbibition is the phenonmenon of adsorption of water or any
other liquid by the solid particles of a substance without forming a solution.
Some examples of Imbibition :
(i) If a dry piece of wood is placed in water, it swells and increases in its volume.
(ii) If dry gum or pieces of agar-agar are placed in water, they swell and their volume increases.
(iii) When seeds are placed in water they swell up.
Mass flow : Mass flow is the movement of substances (water, minerals and
food) in bulk from one point to another as a result of pressure differences between
two points.
Transport of water in plants : Water is absorbed by root hairs, then water
moves upto xylem by two pathways − apoplast and symplast pathway.
The transport of water to the tops of trees occurs through xylem vessels.
The forces of adhesion and cohesion maintain a thin and unbroken columns of
water in the capillaries of xylem vessels through which it travesl upward. Water
is mainly pulled by transpiration from leaves.
(Cohesion-tension-transpiration pull Model)
Root pressure : A hydrostatic pressure existing in roots which pushes the
water up in xylem vessels.
Guttation : The water loss in its liquid phase at night and early morning
through special openings of vein near the tip of leaves.
Transpiration : The loss of water through stomata of leaves and other aerial
parts of plants in form of water vapour.Factors affecting transpiration : Temperature, light, humidity, wind speed,
number and distribution of stomata, water status of plant.
Uptake and transport of mineral nutrients : Ions are absorbed by the
roots by passive and active transport. The active uptake of ions require ATP
energy. Specific proteins in membranes of root hair cells actively pump ions
from the soil into the cytoplasm of epidermal cells and then xylem. The further
transport of ions to all parts of the plant is carried through the transpiration
The Pressure or Mass Flow Hypothesis: The glucose is prepared at the
 source by the process of photosynthesis and is converted to sucrose (sugar). This
sugar is then moved into sieve tube cells by active transport. It produces hypertonic
condition in phloem. Water in the adjacent xylem moves into phloem by
osmosis. Due to osmotic (turgor) pressure, the phloem sap moves to the areas of
lower pressure.
At the sink, osmostic pressure is decreased. The incoming sugar is actively
transported out of the phloem and removed as complex carbohydrates (sucrose).
As the sugar is removed, the osmotic pressure decreases, the water moves out of
the phloem and returns to the xylem
Very Short Answer Questions (1 mark each)
1. Which part of the root is related with the absorption of water ?
2. What makes the raisins to swell up when kept in water ?
3. Define water potential.
4. What will happen to water potential when a solute is added to water ?
5. A plant cell when kept in a solution got plasmolysed. What was the nature of
the solution ?
6. Mention two ways of absorption of water in plants.
7. Which form of sugar is transported through phloem ?
8. Give one example of imbibition.
9. A flowering plant is planted in an earthen pot and irrigated. Urea is added to
make the plant grow faster, but after some time the plant dies. Give its
possible reason.
10. Why is energy required to develop root pressure ?
Short Answer Questions-II (2 marks each)
11. A well watered potted herbaceous plant shows wilting in the afternoon of a
dry sunny day. Give reason.
12. Do different species of plants growing in the same soil show the same rate
of transpiration of a particular time ? Justify your answer.
13. What is casparian strip ? Write its significance in plants.
14. Xylem transport is unidirectional and phloem transport bi-directional. Why ?
15. How is transpiration different from guttation ? Give two points.
Short Answer Questions-I (3 marks each)
16. When any dry plant material or seeds are kept in water, they swell up.
(a) Name the phenomenon involved in this change.
(b) Define this phenomenon.
(c) Give two conditions essential for the phenomenon to occur.
17. Plants show temporary and permanent wilting. Differentiate between the
two. Do any of them indicate the water status of the soil ?  18. What is mycorrhiza ? How is the mycorrhizal association helpful in absorption
of water and minerals in plants ?20. Give the scientific term for the following statements/processes :
(a) Movement of water in roots through the cell wall exclusively.
(b) The positive hydrostatic pressure developed inside the cell or cell wall.
(c) A solution having relatively less concentration.
(d) Loss of water vapour from the aerial parts of the plants in the form of
water vapour.
(e) Movement of a molecule across a membrane independent of other
(f) Water loss in its liquid phase through the special openings of veins
near the tip of leaves of many herbaceous plants.
Long Answer Questions (5 marks each)
21. Minerals are present in the soil in sufficient amount. Do plants need to adjust
the types of solutes that reach the xylem ? Which molecules help to
adjust this ? How do plants regulate the type and quantity of solutes that
reach xylem.22. How do plants absorb water ? Explain transpiration pull model in this regard.
23. (a) Describe the pressure flow hypothesis of translocation of sugar in plants.
(b) Explain the mechanism of closing and opening of stomata.
Very Short Answers (1 mark)
1. Root hairs.
2. Endosmosis.
3. Water potential is the potential energy of water.
4. Water potential will decrease.
5. Hypertonic.
6. Apoplast and symplast pathway.
7. Sucrose.
8. Swelling of seed when put in water/moist soil.
9. Due to exosmosis.
10. Every activity requires energy. Root pressure develops due to activity to
living cell.

Read more topics......
vitamins fat-soluble-vitamins water

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