Maharashtra Board Class 12 Biology Chapter 6 Plant Water Relation PDF Download

Plant Water Relation

Plant obtains variety of substances like water, minerals, nutrients, food and gases like O2 and CO2, from its surroundings. Productivity in plants is mainly affected by the non-availibility of water. Water is considered as 'elixir of life'. Water constitutes almost 90 to 95% of most plant cells and tissues. Water helps the cells to maintain turgidity and shape. It shows following properties due to which it has great biological importance.

6.1 Properties Of Water

It is in the liquid form at room temperature and is the best solvent for most of the solutes.

It is inert inorganic compound with neutral pH when in pure form. Due to this, water is best transporting medium for dissolved minerals and food molecules.

It is best aqueous medium for all biochemical reactions occurring in the cells.

It is an essential raw material for photosynthesis.

Water has high specific heat, high heat of vaporization and high heat of fusion. Due to this, it acts as thermal buffer.

These various properties are due to hydrogen bonds between the water molecules.

Water molecules have good adhesive and cohesive forces of attraction. Due to high surface tension and high adhesive and cohesive force, it can easily rise in the capillaries. It is therefore, a significant molecule that connects physical world with biological processes.

Teacher's Note

Water is very important for all living things. In India, farmers need water for their crops. Without water, plants cannot grow.

Exam Trick

Remember: Water makes up 90-95% of plant cells. Just like our body needs water to stay healthy, plants also need water in the same way.

Points to Remember

Water is the best solvent for most substances.
Water helps cells stay firm and healthy.
Water has high heat capacity which keeps plants cool.
Water molecules stick together because of hydrogen bonds.
Water moves up in small tubes because of adhesion and cohesion.

6.2 Water Absorbing Organ: Root

Root is the main organ of water and mineral absorption. In terrestrial plants, plants absorb water in the form of liquid from the soil however, epiphytic plants like orchids absorb water vapours from air with the help of epiphytic roots having special tissue called velamen.

Typical root is divisible into four different regions. In the zone of absorption, epidermal cells (epiblema cells) form unicellular hair like extensions called root hairs.

Structure Of Root Hair

Root hair is cytoplasmic extension (prolongation) of epiblema cell. Each root hair may be approximately 1 to 10mm long and tube like structure. It is colourless, unbranched, short-lived (ephemeral) and very delicate.

It has a large central vacuole surrounded by thin film of cytoplasm, plasma membrane and thin cell wall, which is two layered. Outer layer is composed of pectin and inner layer is made up of cellulose.

Cell wall is freely permeable but plasma membrane is selectively permeable.

Teacher's Note

Root hairs are very small and help the plant drink water from soil. Think of them like little straws that the plant uses to drink water.

Exam Trick

Remember: Root hair is short-lived and very thin. It is like a tiny tube that sucks water from soil.

Points to Remember

Root hair is a small tube-like growth from the root.
It is 1 to 10mm long and colourless.
The outer layer of cell wall is made of pectin.
The inner layer of cell wall is made of cellulose.
Cell wall lets water pass freely but membrane does not.

6.3 Water Available To Roots For Absorption

Plants absorb water from the rhizosphere (the microenvironment surrounding the root). Water present in the soil occurs as gravitational (free) water, hygroscopic water, combined water and capillary water.

Water percolates deep, due to the gravity, in the soil, is called 'gravitational water'. This is not available to plants for absorption.

Fine soil particles imbibe/ adsorb water and hold it. This is called 'hygroscopic water'. Roots cannot absorb it.

Water present in the form of hydrated oxides of silicon, aluminum, etc., is called 'combined water'. It is also not available to plants for absorption.

Some amount of water is held in pores present between the neighbouring soil particles, due to capillarity. This is called capillary water that is avilable for absorption.

Teacher's Note

Not all water in soil can be used by plants. Only capillary water can be taken by roots. Think of it like this - in a field, only some water can be drunk by the plant.

Exam Trick

Remember: Capillary water = water between soil particles = only water plants can use. Other water types are trapped or too deep.

Points to Remember

Gravitational water goes deep into soil and is not useful.
Hygroscopic water sticks to soil particles and roots cannot take it.
Combined water is in oxides and cannot be used.
Capillary water is the only water that roots can absorb.
Capillary water sits between soil particles.

6.4 Absorption Of Water By Roots From Soil

Root hair absorbs water by employing three physical processes that occur sequentially- viz. imbibition, diffusion and osmosis.

a. Imbibition

Imbibition is swelling up of hydrophillic colloids due to adsorption of water. Substance that adsorbs water / liquid, is called as imbibant and water/ liquid, that gets imbibed is called as imbibate.

The root hair cell wall is made up of pectic compounds and cellulose which are hydrophillic colloids. During Imbibition, water molecules get tightly adsorbed without the formation of solution. Imbibition continues till the equilibrium is reached. In other words, water moves along the concentration gradient.

Imbibition is significant in soaking of seeds, swelling up of dried raisins, kneading of flour etc.

Teacher's Note

Imbibition is when dry things soak up water. When you soak rice or dal in water at home, that is imbibition happening. The grains swell up.

Exam Trick

Remember: Imbibition = soaking and swelling. Like when raisins swell in water or seeds get bigger when wet.

Points to Remember

Imbibition means water sticks to cell walls.
Root hair cell walls are made of pectin and cellulose.
These are hydrophillic which means they love water.
Water molecules join cell walls but do not make a solution.
Imbibition stops when water balance is reached.

b. Diffusion

Diffusion means to disperse. Diffusion can be defined as the movement of ions/ atoms/ molecules of a substance from the region of their higher concentration to the region of their lower concentration. The movement is due to the kinetic energy of the molecules. Diffusion continues till an equilibrium is reached. Thus, water passes into the cell by diffusion through a freely permeable cell wall. Water is now at the interface of cell wall and plasma membrane.

Diffusion results in the diffusion pressure (D. P.) which is directly proportional to the number of diffusing particles. Diffusion pressure of pure solvent (pure water) is always more than the diffusion pressure of solvent in a solution.

The difference in the diffusion pressures of pure solvent and the solvent in a solution is called Diffusion Pressure Deficit (DPD) or Suction Pressure (SP). The term was coined by B.S. Meyer (1938). Now a days, term water potential is used for DPD.

In colloquial language, the term DPD is actually the thirst of a cell with which it absorbs water from the surroundings. Water arround cell wall has more diffusion pressure than cell sap. Due to this, water moves in the cell by diffusion.

Diffusion is significant in plants in the absorption of water, minerals, conduction of water against the gravity, exchange of gases and transport and distribution of food.

Teacher's Note

Diffusion is when things spread from where there is more to where there is less. When you open a perfume bottle in one corner of a room, the smell spreads everywhere. This is diffusion.

Exam Trick

Remember: Diffusion = spreading from high to low concentration. DPD = thirst of cell = how much the cell wants water.

Points to Remember

Diffusion is movement from high to low concentration.
Water moves by diffusion through cell wall freely.
Diffusion pressure is based on number of particles.
DPD means the difference in diffusion pressure.
DPD is like the thirst a cell has for water.

c. Osmosis

It is a process by which water enters into the cytoplasm of the root hair cell. Osmosis is a special type of diffusion of solvent through a semipermeable membrane. The cytoplasm of root hair cell contains minerals, sugars, etc. In other words, solution inside the cell is more concentrated (stronger) than outside the cell (weaker). Therefore, solvent from weaker solution enters into cytoplasm (i.e. to stronger solution) of cell through a semipermiable plasma membrane. This migration of solvent is called Osmosis.

Thus, water at the interface of cell wall and plasma membrane, enters into the cytoplasm of the root hair cell due to osmosis.

With respect to the concentration and osmotic migration, three types of solutions are recognized viz,

i. Hypotonic (weak solution or strong solvent) having low osmotic concentration.

ii. Hypertonic (strong solution or weak solvent) having high osmotic concentration.

iii. Isotonic having such a concentration of solution where there is neither gain nor loss of water in an osmotic system. In other words, concentration outside and inside the cell is same.

Osmosis is of two types viz, Exo-osmosis and Endo-osmosis.

Exo-osmosis : It is the migration of solvent from the cell outside. It causes flaccidity of cell.

Endo-osmosis : It is the migration of the solvent into the cell. It causes turgidity of cell i.e. cytoplasm becomes turgid. Turgidity increases the turgor pressure (T. P.) of the cell. T. P. is the pressure exerted by turgid cell sap on to the cell membrane and cell wall. In a fully turgid cell, DPD is zero. Cell wall being thick and rigid, exerts a counter pressure on the cell sap. This is called Wall pressure (W. P.). In a fully turgid cell, T. P. = W. P. but operating in opposite direction.

Osmotic pressure (O. P.) : The pressure exerted due to osmosis is osmotic pressure.

Teacher's Note

Osmosis is when water moves into a cell to balance things out. If salt water is outside and pure water inside, water moves in to make them equal. This happens in all living cells.

Exam Trick

Remember: Osmosis = water moves through membrane. Hypertonic = more salt outside = water leaves cell. Hypotonic = less salt outside = water enters cell.

Points to Remember

Osmosis is water moving through a special membrane.
Hypotonic solution has less salt than the cell.
Hypertonic solution has more salt than the cell.
Isotonic solution has same salt as the cell.
Turgor pressure keeps cells firm and healthy.

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