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PLANT PHYSIOLOGY

The Galaxy Education System – Department of Biology

Chapter -1 : Absorption by roots

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Contents

1. Importance of water and minerals …….……………...……………………………. 03

2. Absorption by roots…………………………………………………………………... 04

3. Imbibition and diffusion……………………………………………………………… 05

4. Diffusion in liquids and gases ……………………………………………………….. 06

5. Osmosis…………………………….………………………………………………….. 07

6. Types of Osmosis - Exosmosis and Endosmosis…………………………………….. 08

7. Types of Solutions (Tonicity)......……………………………………………………...09

8. Demonstration of Osmosis using thistle funnel …………………………………….. 09

9. Demonstration of osmosis using a Potato Osmoscope……………………………… 10

10. Turgor pressure and wall pressure………………………………………………….. 11

11. Plasmolysis and Deplasmolysis………………………………………………………. 11

12. Root Pressure ………………………………………………………………………….12

13. Active and passive transport ………………………………………………………… 13

14. Mechanism of Ascent of sap………………………………………….………………. 14

15. Transpirational pull theory……………………………..……………………………. 14

16. Experiment: To show that water is conducted through xylem…………………….. 16

17. Ringing or girdling experiment ……………………………………………………... 16

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IMPORTANCE OF WATER All living things need water to stay alive. However, plants need much more water than animals do, because they use much more water than do animals. Plants also contain more water than animals do; they are 90 percent water, while the percentage of water in animals goes as low as 75 percent. Water is the main component of protoplasm and makes up approximately 80-85 per cent of protoplasm. Protoplasm is the physical basis of life. So, in absence of water, protoplasm becomes inactive.

Water is most essential for the following reasons… …

1. Transportation: Water transports mineral elements to the different parts of plant by ascent of sap. Transport of minerals occurs in solution from the aerial parts of the plants

2. Translocation : The transport of prepared food(Photosynthates) through the phloem from the leaves to all the parts of the plants is known as translocation

3. Turgidity : Water makes the cells/tissues/organs turgid 4. Photosynthesis : Water is an essential raw material for photosynthesis 5. Opening and closing of stomata : Water makes the guard cells turgid and thereby helps

in the opening of stoma 6. Germination of seeds : Seeds swell up due to imbibition of water and germinate into

seedlings 7. Transpiration: Due to water, transpiration rate accelerates 8. Plant Growth: Scarcity of water affects plant growth.

Importance of minerals in the life of plants

All minerals required by plants are obtained from the soil itself by root absorption. Various

mineral nutrients are absorbed as ions (elements like Potassium. calcium, magnesium etc.) or

combined form (phosphates, nitrates, sulphates etc.)

Mineral nutrients may be needed by plants as raw materials for growth of cells, for making

enzymes and pigments as well as other metabolic activities.

Potassium is an important constituent of protoplasm. Phosphorus is necessary for cell division, growth and metabolism in plants.

Calcium influences water movement in cells and is necessary for maintaining the

permeability of cell membrane.

Magnesium is necessary for the formation of chlorophyll molecule.

Sulfur is a structural component of amino acids.

Nitrogen is a mineral required for protein synthesis and growth.

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ABSORPTION BY ROOTS All terrestrial plants except epiphytes

(plant that grows upon another plant) and

parasites absorb water and minerals from the

soil with the help of their roots. The absorbed

water and mineral salts are then transported to

all the plant parts. Major parts of water

absorbed by the plants are lost by their aerial

parts. This maintains a continuous suction

process in the plant body which makes the roots absorb water continuously according

to its need.

A root has three primary functions:

1. To fix the plant to the soil

2. To absorb water and minerals from the soil.

3. To conduct the absorbed substances through its tissues to the upper parts of the plant.

Root Hair

Root hairs are delicate prolongations

of Epiblema (epidermis). The cell

wall of the root hair is made up of

cellulose. Next to cell wall is plasma

membrane. It encloses cytoplasm,

nucleus and vacuole. Vacuole is

quite large and contains cell sap. Cell

sap is of higher osmotic potential as

some salts are dissolved in it. Plasma

membrane acts as semi permeable

membrane.

Diagram of a Root Hair Cell

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HOW ARE ROOTS ADAPTED FOR ABSORPTION OF WATER

Plants absorb water from the soil by osmosis. Root hair cells are adapted for this by ……

Larger Surface area: Roots contain root hairs that provide a larger surface area (figure alongside) for absorption of water.

Permeability: Epidermis of root hair is permeable to water: Root hairs have very thin cellulose walls that are permeable to water.

Concentration: Most of the absorption of water occurs near the root tips. Root hairs grow from the outer layer of cortex. The concentration of cell sap in the root hair is more than that of the

soil water. Water enters the plant body form the soil through the root system. Before studying the intake of water by plants, it is necessary for us to understand imbibition, diffusion, osmosis, and active transport IMBIBITION The absorption of water by hydrophilic surfaces is called imbibition. Materials capable of imbibition such as proteins and cellulose in living beings cause seeds to swell when immersed in water. Sticking of doors and the desk drawers in the rainy season are the common examples of imbibition. This is caused by an increase in size due to imbibition of moisture by the cells of dry wood. DIFFUSION Diffusion is the movement of molecules of substances from higher concentration towards the lower concentration. The rate of diffusion is affected by the concentration, density of medium, temperature and pressure. All substances are made up of minute particles called molecules. In solid form the molecules are packed relatively closely together with little freedom to move, in liquids the molecules are spaced further apart and are free to move, the molecules of a gas are most loosely packed and move about freely. The entire molecules move due to their own kinetic energy, the direction of movement of these particles is from the region of their higher concentration to the region of their lower concentration to equalize the concentration of two regions. This process is called diffusion.

Seeds swell when placed in water, doors become tight and jammed due to imbibition of water by wood during rainy season.

Representation of Diffusion

Diagram showing Root hairs

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Diffusion in liquids Experiment to show the process of diffusion Take a beaker full of water and put a crystal of

potassium permanganate into it. Leave the beaker undisturbed for some time. It will be observed that the molecules of the

crystal from its surface spread out and get uniformly distributed in all directions.

Diffusion of Gases When we open a bottle of perfume in a corner of a room we can smell it everywhere in the room because the molecules of perfume diffuse through the air. This movement continues until the molecules are uniformly distributed. The rate of diffusion increases with the rise of temperature. It is fastest in gases, less rapid in liquids and slowest in solids. IMPORTANCE OF DIFFUSION

Diffusion in plants:

CO2, and 02 are exchanged by diffusion and water enters inside the root by a special type

of diffusion.

Diffusion in Animals: The lungs contain millions of air sacs called alveoli which maximize the diffusion of carbon

dioxide and oxygen.

Most of the digested food is absorbed by diffusion in the small intestine.

Before After

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OSMOSIS Osmosis is defined as the movement of solvent or water, across a semipermeable membrane, from the region of higher concentration (dilute solution) to a region of lower concentration (concentrated solution) Osmosis is a particular type of diffusion which occurs across a semi-permeable membrane. Such a membrane is a thin, soft, pliable sheet, usually of plant or animal origin. When such a membrane has pores large enough to allow molecules of a fluid to pass through them, it is called a permeable membrane. If a membrane has pores large enough to allow molecules of the solvent to pass, but too small to let molecules of the solute to pass easily through them, it is called a semi-permeable or differentially permeable or selectively permeable membrane.

Membranes

Permeable If a membrane allows the passage of both solvent and solute molecules, it is called permeable E.g. Cell wall. Semi-permeable If a membrane allows the passage of solvent molecules. It is called semi-permeable E.g. plasma membrane It is also called differently permeable membrane. Impermeable If a membrane does not allow the passage of any substance called Impermeable E.g. rubber sheet

Diffusion Osmosis It need not involve any membrane It takes place through the semi permeable

membrane Diffusion takes place in solid, liquids and gases

Osmosis takes place in liquid (solvents) only.

It is a slow process It is a rapid process

Significance of osmosis 1. Maximum amount of water (70 per cent) moves across the tonoplast (vacuolar membrane) by

osmosis and plays a major role in water absorption than any process of water entry in the plant 2. The phenomenon of plasmolysis is dependent on osmosis. 3. The intact plant cell when placed in distilled water will expand due to absorption of water by

osmosis 4. The shape of the organelles in a cell is maintained due to osmosis 5. During unfavorable condition, the resistance increases the osmotic pressure of their cells

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TYPES OF OSMOSIS: ENDOSMOSIS AND EXOSMOSIS

Endosmosis If a cell is placed in a solution of lower concentration of solute, water enters into the cell. This phenomenon is known as endosmosis. Because of endosmosis, cell becomes turgid. Exosmosis Inversely, when a cell is placed in a solution of higher concentration than that inside its vacuole, water diffuses out of the cell. This flow of water is called exosmosis. Because of exosmosis, cell becomes flaccid. Experiment

To demonstrate exosmosis and endosmosis

Put some raisins in a dish full of water. Observe after a few hours. They swell due to the entry of water. In raisins, there is a stronger solution of sugar and the outer membrane of raisins serves as a semi-permeable membrane. Thus water enters into the raisins and makes them turgid. This shows endosmosis.

Figure: above demonstrates osmosis in raisins.

A - Raisins swell due to endosmosis and B - Shrink due to exosmosis when place in hypotonic solution and hypertonic solutions respectively

DO YOU KNOW?

If a fresh water plant is transferred to marine water, it dies due exosmosis.

If a marine plant is transferred to fresh water, it bursts due to endosmosis,

A

B

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TYPES OF SOLUTIONS (TONICITY) In relation to cell sap, solutions can be of following three types;

1. Hypertonic Solution (hyper = more than)

A solution whose concentration is more than that of the cell sap is known as hypertonic. If a cell

is placed in such a solution, water will diffuse out of it and the protoplasm would contract or

shrink… (Exosmosis occurs)

2. Hypotonic Solution (hypo= less than)

When the concentration of a solution is less than that of the cell sap, it is known as hypotonic. A

cell is immersed in hypotonic solution, water will diffuse into the cell and it will Increase In size.

(Endosmosis occurs)

3. Isotonic Solution (so = the same)

A solution with concentration equal to that of the cell sap is known as Isotonic. If a cell is placed

in isotonic solution there would be no diffusion of water. As a result there is no change in the

volume and weight of the cell. (No osmosis)

Demonstration of Osmosis Osmosis is demonstrated by a simple

experiment in laboratory. Goat's

bladder, or parchment paper is tied to

the wide mouth of a thistle funnel

Concentrated sugar solution, or syrup is

filled in the tube of the thistle funnel.

Now the wide mouth of thistle funnel is

immersed in a beaker containing water.

The level of the solution in the tube of

the funnel is marked. After some time,

the level in the tube increases. This is due to the entry of water molecules from beaker into the

thistle funnel. The concentration of water molecules in the beaker is more than their

concentration inside the thistle funnel. Therefore, water molecules move from region of higher

concentration (from the beaker) to the region of lower concentration (inside the funnel).

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Experiment Objective: To demonstrate osmosis with the help of a potato osmoscope Procedure Peel a large-sized potato. Cut it into two halves; Remove the central tissue so as to form two cups. Boil one cup for about 2-3 minutes so that the cells are killed. Suspend both the cups (one living) and the other (dead) in separate beakers containing water. Half-fill the cavities of the cups with sugar solution. Add water in the beaker so that the level of water in the beaker and the level of sugar solution in the potato cup are the same, Mark the level of solution in potato cups by pins. After 24 hours, again note the level of solution in both the cups. The level of solution in the unboiled cup rises. Observation and Inference: The increase in the level of sugar solution in the cavity of potato is due to the entry of water in called endosmosis. OSMOTIC PRESSURE Osmotic pressure is defined as the pressure needed to prevent the entry of pure water into an aqueous solution through a semi-permeable membrane, thereby preventing art increase in the volume of the solution. Osmotic pressure of a solution is directly proportional to the number of solute molecules per unit volume of solvent molecules. OSMOTIC POTENTIAL The potential of water molecules to move from a hypotonic solution (more water, less solutes) to a hypertonic solution (less water, more solutes) across a semi permeable membrane Turgidity When the cell wall is fully stretched i.e. when the pressure exerted by the cell contents outwards equals the resistance of the cell wall inwards, the cell is said to be turgid and the condition as turgidity. A cell is said to be fully turgid. When it cannot absorb any more water, or. In other words when its turgor pressure is at its maximum i.e. its absorbing capacity is at its minimum, the pressure that causes water to enter a plant cell is known as the suction pressure.

SIGNIFICANCE OF TURGIDITY 1. Opening and closing of stomata are regulated by the turgidity of the guard cells 2. Turgidity also is the force that pushes roots through the soil. 3. It provides mechanical support to leaves and stem 4. Turgidity helps in maintaining the shape and form of the plants 5. Leaf movements of plants like Mimosa pudica are controlled by loss and gain of cell turgor

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TURGOR PRESSURE AND WALL PRESSURE Turgor pressure When water enters the vacuole of a cell the volume of the cell sap increases. The pressure exerted by the contents of the cell against the rigid cell wall also increases. This pressure is known as turgor pressure (TP).

Figure: above shows turgor pressure and wall pressure in a plant cell

Wall Pressure The cell wall resists the expansion of the cell contents by pushing inwards with a pressure equal in magnitude but opposite in direction to the turgor pressure. This pressure is referred as wall pressure (WP). Flaccidity When a living cell is placed in a hypertonic solution, fluid (water) is drawn out of the cell sap by Exosmosis. The turgidity is lost and the cell becomes soft and limp. Such a cell is said to have become flaccid. As the cells become flaccid, the organs like leaves, twigs etc. Especially in herbaceous plants, droop down. This phenomenon is called wilting. PLASMOLYSIS AND DEPLASMOLYSIS Plasmolysis In hypertonic solution water comes out of the cell, due to exosmosis. As a result of continued exosmosis the protoplasm shrinks and pulls away from the cell wall, the phenomenon is called plasmolysis and the cell is said to be plasmolysed. In a plasmolysed plant cell, the space between the contracted protoplasm and the cell wall remains filled with external solution. The initial stage of plasmolysis is called incipient plasmolysis. If a cell showing incipient plasmolysis is immersed in water or in a solution whose concentration is less than that of the cell sap (hypotonic solution), the cell regains its turgidity as endosmosis takes place. However, if the exosmosis continues it causes permanent plasmolysis. Such a cell cannot regain turgidity even after it is transferred to a hypotonic solution. It eventually dies.

Fig. showing plasmolysis in plant cells

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Practical application of plasmolysis Salting of meat or addition of salt to the pickle is a

method of killing bacteria or germs by plasmolysis i.e. water is drawn out of the bacterial cells.

Weeds may be killed in a playground by sprinkling excessive salt around their base.

Excessive use of fertilizers may damage the roots and diminish the crop yield.

Deplasmolysis

If a plasmolysed cell is kept in water, or a hypotonic solution, water molecules diffuse into the cell by endosmosis, and the cell gradually regains its normal turgid condition. It is called Deplasmolysis.

ROOT PRESSURE It is a pressure exerted on the liquid contents of the cortical cells of the root, under fully turgid condition, forcing a quantity of water into the xylem vessels and through them upwards into the stem up to a certain height. Root pressure is a sort of hydrostatic pressure developed in the mots due to accumulation of absorbed water. Root pressure develops largely due to osmotic phenomenon.

Osmosis Plasmolysis 1. Osmosis is a physical process 1. Plasmolysis is a vital process, concerned

with living cells. 2. In this process water molecules move

through semi-permeable membrane from region of higher diffusion pressure to lower diffusion pressure towards concentrated solution

2. In this process water diffuses out of the cell as a result of exosmosis. This causes protoplasm to contract away from the cell wall

SIGNIFICANCE OF PLASMOLYSIS 1. Plasmolysis is a vital phenomenon as it explains the process of osmosis. 2. It also proves the permeability of the cell wall and semi- permeability of the

protoplasm. 3. Secondly, this is helpful in determining whether a cell is living or dead as the

plasmolysis does not take place in a dead or nonliving cell. 4. It is also used to determine the osmotic pressure of a cell.

IMPORTANT TO NOTE

The term osmosis was coined by Nollet.

Bacteria do not survive in salted

pickles because they get plasmolysed.

Common salt kills weeds by

plasmolysis.

Due to salting meat and fish we can

prevent mould Infestation.

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Experiment

Objective: To demonstrate root pressure in plants.

Procedure: Take a healthy herbaceous plant the stem

or the plant whose stem is cut a few

centimeters above the base with a sharp

knife and immediately attached to a

manometer through a rubber tube. The

manometer is supported by a stand. After

a few hours we observe that the level of

mercury rises in the manometer. This is

due to the pressure created by water

exuded from the cut end of the plant on

account of root pressure generated due to

entry of water in the root cells.

ACTIVE TRANSPORT

Active transport is just the reverse of diffusion. In diffusion, the molecules of a substance are transferred from a region of their higher concentration to the region of lower concentration, whereas in active transport the molecules are transferred from a region of lower concentration of high concentration. Such a movement is accomplished only by using up the energy (In the form of ATP) liberated during cellular respiration. Most of the living cells can carry out active transport. The active transport, as a matter of fact, is a physiological process which involves the movement of materials across the cell membranes. By this process cells of plants and animals can absorb and concentration of certain materials from very weak solutions. For example, sea-weeds absorb iodine from sea water and accumulate it in their cells in very high concentrations. Likewise, cells of the thyroid gland absorb and accumulate iodine from the blood, where its concentration is very low. Sugars and amino acids move into the cells by active transport. PASSIVE TRANSPORT Passive transport is the diffusion of substances across a membrane. As we stated above, this is a spontaneous process and cellular energy is not expended. Molecules will move from higher concentration to lower concentration.

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ASCENT OF SAP

Once the water is absorbed by the root hairs, it is translocated to various parts of the plant. The fluid that moves upward in the stem is not pure water; it is a dilute solution of mineral ions absorbed from the soil and is referred to as sap.

The process of translocation of sap from the roots to the top of the plant is called ascent of sap.

Mechanism of Ascent of Sap

To explain the mechanism of ascent of sap following views have been given by scientists.

Root Pressure Theory

The followers of this theory believe that the root pressure developed in xylem vessels pushes up water to leaves and branches In herbs. This theory is not applicable in case of tall plants.

Capillary Theory

According to capillary theory xylem vessels and tracheids occur in the form of thin capillaries, thus the xylem capillaries apply a physical force which helps in ascent of sap. This theory may be possible up to some extent in small plants. The ascent of sap in tall plaints is not possible by this force.

Transpirational pull theory

This theory was developed by Dixon andly 1894 lt is also called the Cohesion Tension Theory

because the main forces behind the ascent of sap are cohesion and tension of water molecules,

according to this theory.

The main characters of the theory are:

1. Continuous Water Column

A continuous column of water exists from root to leaves through the stem. This column exists

in tracheary elements. Even if a few tracheary elements get blocked, the other maintain the

continuity of the column.

Figure: Pathway of conduction of water

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2. Cohesion

Water molecules stick together by a strong force of attraction called cohesion or cohesive

form This mutual attraction is due to the presence of hydrogen bonds fa type of chemical

bond} between adjacent water molecules.

3. Adhesion

Water molecules also stick to wails of xylem vessels due to attraction. This is called adhesion

or adhesive force. Cohesion and adhesion ensure that the water column is not broken.

4. Development of Transpirational pull

Mesophyllic tissue is present in leaves, The Intercellular spaces amongst mesophyll cells of

leaves are always saturated with water vapour. These spaces come in contact with outside air

through stomatal openings. Since the outside air is rarely saturated with water vapour. The

vapor from the intercellular spaces diffuses out by Transpiration. The mesophyll cells then

absorb moisture from deeper cells to overcome this loss of moisture. This creates a pull

(tension) in the water column through the stem to the roots.

Since the column is continuous It Is pulled up like a rope due to this tension.

Thus, when water transpires from leaves it pulls the water molecules of the column upwards

and water is able to rise up inside the plant. This pull is referred to as Transpirational pull.

Path way for movement of water from soil to xylem

Soil particles -> Epidermis -> Cortex -> Endodermis -> Pericycle -> Xylem

Diagram above showing Pathway of water from soil to the xylem vessel

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Experiment

Objective: To show that water is conducted through the tracheary elements of xylem

Experiment showing path of ascent of sap using eosin

Procedure:

A dilute solution of eosin (a red dye that stains lignified tissues such as xylem.) is taken in a beaker in which a balsam twig is placed; after some time the veins of the leaves appear red and some parts of the stem become pink. A section of the stem or a petiole shows that only the xylem elements have taken the stain indicating that the solution (water) moves through the xylem. It is thus established that the water is conducted through the tracheary elements of xylem i.e. tracheids and vessels. These are dead cells with lignified secondary wall. The lumen of these elements allows an easy upward flow of water.

Girdling or Ringing experiment

Ringing was first introduced by Malpighi in 1675. It consists of removing a ring of bark i.e. all the tissues outside vascular cambium including the vital vascular tissue of phloem. Girdling or ringing experiment is done as follows.

Two small twigs or branches are taken. Girdle or a ring of bark is removed from one of these branches by a sharp knife. In the second twig, xylem is removed by careful manipulation without causing much injury to the bark, or the xylem blocked with wax or grease. Thus, the girdled part of the first twig contains only xylem and that of the second twig has all tissues except xylem. Both the twigs are placed in separate beakers containing water. After some time leaves on the first twig appear turgid while those on the second twig with no xylem, the leaves droop. The leaves appear turgid in the first case because these continue to receive water in the presence of xylem. In second twig, leaves wilt because there is no conduction of water due to absence or blocked xylem elements. Thus, this experiment shows that water is conducted through xylem (tracheids)

Source: Certificate biology, Google books Compiled by – Nishant Kapdi

Diagram showing ascent of sap using eosin

The Galaxy Education System

Department of Biology

Compiled by Nishant Kapdi