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Senior Inter Botany
Transport in Plants
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Short Answer Questions
1.
Define and explain Water Potential.
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A: Water possesses kinetic energy or chemical energy called water potential. It is the
energy with which water shows net movement of water occurs from system with
higher energy to the one with lower energy.
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★ The term Water potential was coined by Slatyr and Taylor. Its symbol is ΨW
(Psi - Greek). Its units are pressure units (Pascals = Pa)
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★ The value of water potential of pure water is greatest. It is orbitrarily taken as
Zero. It is a constant. Water potential of any system (water + solutes) is
compared to that of pure water and it is always negative. The value of water
potential is a relative one. It is a fundamental concept about water movement.
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★ Water potential of a cell is affected by 2 other components.
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★ They are solute potential (Ψs) and pressure potential (ΨP).
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★ Their relation can be shown as ΨW = ΨS + ΨP
2.
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What is meant by plasmolysis? How is it practically useful to us?
A: If a plant cell is kept in hypertonic solution water moves out of the cell. It is lost
first from the cytoplasm and then from the vacuole. The size of the vacuole
decreases. Protoplast (Protoplasm + Plasma membrane) shrinks. Plasma
membrane separates from cell wall. This shrinking of plasma membrane along
with protoplasm due to loss of water is called plasmolysis or exosmosis.
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★ It is first observed in the corners of the cell. It is called as incipient
plasmolysis.
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Water
Incipient Plasmolysis
Cytoplasm
Nucleus
Plasma membrane
Cell Wall
Corner
R-8-1-15
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★ A completely plasmolysed cell is called Flaccid cell.
e.g.: Wilting leaf cells solute potential in a flaccid cell reaches maximum and
pressure potential is zero.
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ΨW = ΨS + 0
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∴ ΨW = ΨS
★ This phenomenon is useful in:
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(1) Salting of pickles
(2) Preservation of meat and fish in salt
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(3) Preparation of Jelly and Jam
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(4) Preparation of dry grapes (Raisins or kismis) and dry fruits.
3.
Explain pressure flow hypothesis of translocation of sugars in plants?
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A: Phloem transports food (sucrose) from the source to the sink in the form of sap.
Source (leaf) and sink (other parts and storage organs) relationship is variable.
Phloem sap consists water, sucrose, other sugars, hormones and amino acids. The
translocation in phloem is bidirectional as source-sink relationship is variable.
The transport in phloem is explained by pressure flow hypothesis. It was
proposed by Munch. It is also called Mass flow hypothesis. It occurs in 2 steps.
(1) Loading (2) Unloading.
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Loading of Phloem:
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1. Glucose in the mesophyll cells (source) is converted to sucrose.
2. Sucrose moves to companion cells.
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3. It moves from there to sieve tube cells by active transport.
4. Concentration of sap in these cells become hypertonic.
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5. Water from Xylem cells moves by osmosis into sieve tube cells.
6. Mass and Pressure increase.
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Unloading:
1. As mass increases in sieve tube cells, pressure flow begins.
2. Sugars in the sink are used up.
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3. Sap in the cells become hypotonic.
4. Water moves out of the phloem.
5. Water potential gradient is established between sink and source.
6. Sugars are unloaded. They move into the cells which use sugar.
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4.
Explain the mechanism of opening and closing of stomata.
A: Stomata open during day time and close in the night time. These movements are
called scoto active movements.
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Opening of the Stomata:
1. Glucose formed in chloroplasts of guard cells becomes Malic acid that
dissociates into Malate− and H+.
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2. H+ accumulated in the guard cells move into subsidiary cells actively. pH in
guard cells increases. Equal number of K+ from subsidiary cells move
passively into guard cells.
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3. Due to consumption Malate−, Cl− move passively from subsidiary cells into
guard cells.
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4. Guard cells become hypertonic (due to Malate−, K+ & Cl−). Water from
subsidiary cells enters guard cells due to endosmosis.
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5. Guard cells become turgid. As the outer walls are thin and elastic, cells expand
outwardly and stoma opens. Opening is helped by radial micro fibrils in cell
walls.
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Closure of Stomata:
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1. K+ and Cl− move out of guard cells passively.
2. H+ move from subsidiary cells into guard cells passively.
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3. pH in the guard cells decreases.
4. Water potential value in the guard cells increases.
5. Exosmosis occurs. It leads to the closure of stoma.
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Stoma Opened
Outer thin elastic
cell wall
Active eflux of H+
Stoma
Passive influx of K+
Passive influx of Cl-
Inner, thick non elastic
cell wall
Stoma Opened
K+
ClH+
Writer: Dr. M. Chandraiah
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