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WATER

Introduction

Water is one of the most common and most important substances on the earth's surface. It is

essential for the existence of life, and most abundant substance in living systems, making up

70% or more of the weight of most organisms. Water pervades all portions of every cell and is

the medium in which the transport of nutrients, the enzyme-catalyzed reactions of metabolism,

and the transfer of chemical energy occur. It is essential for the existence of life, and the kinds

and amounts of vegetation occurring on various parts of the earth's surface depend more on

the quantity of water available than on any other single environmental factor. The importance of

water was recognized by early civilizations, and it occupied a prominent place in ancient

cosmologies and mythologies. The early Greek philosopher Thales asserted that water was the

origin of all things, and it was one of the four basic elements (earth, air, fire, water) recognized

by later Greek philosophers such as Aristotle. It was also one of the five elemental principles

(water, earth, fire, wood, metal) of early Chinese philosophers. Today it is realized that the

availability of water not only limits the growth of plants but can also limit the growth of cities

and industries.The first living organisms probably arose in the primeval oceans; evolution was

shaped by the properties of the medium in which it occurred. All aspects of cell structure and

function are adapted to the physical and chemical properties of water. The strong attractive

forces between water molecules result in water's solvent properties. The slight tendency of water

to ionize is also of crucial importance to the structure and function of biomolecules. The water

molecule and its ionization products, H+ and OH -, profoundly influence the structure, self

assembly, and properties of all cellular components, including enzymes and other proteins,

nucleic acids, and lipids. The noncovalent interactions responsible for the specificity of

"recognition" among biomolecules are decisively influenced by the solvent properties of water.

Structure of water molecule:

The water molecule consists of two atoms of hydrogen and an Oxygen atom.

Each hydrogen atom shares an electron pair with the central oxygen atom.

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The geometry of water molecule is dictated by the shapes of the outer electron orbitals of the

oxygen atom, which are similar to the sp3 bonding orbitals of carbon.

These orbitals describe a rough tetrahydron, with a hydrogen atom at each of two corners and

unshared electron pairs at the other two corners.

The H-O-H bond angle is 104.5o, slightly less than the 109.5 o of a perfect tetrahedron

because of crowding by the nonbonding orbitals of the oxygen atom.

There is electrostatic attraction between oxygen atom of one water molecule and hydrogen

atom of other, called hydrogen bond.

Hydrogen bonds are relatively week. The bond dissociation energy of liquid water is a 23

kJ/mol, compared with 470 kJ /mol for the covalent O-H bond in wateror 384 kJ/molfor a

covalent C-C bond.

Fig. : The dipolar nature of the H20 molecule, shown (a) by ball-and-stick and (b) by spacefilling models. The

dashed lines in (a) represent the nonbonding orbitals. There is a nearly tetrahedral arrangement of the outer shell

electron pairs around the oxygen atom; the two hydrogen atoms have localized partial positive charges and the

oxygen atom has two localized partial negative charges. (c) Two H2O molecules joined by a hydrogen bond

(designated by three blue lines) between the oxygen atom of the upper molecule and a hydrogen atom of the lower

one. Hydrogen bonds are longer and weaker than covalent O-H bonds.

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Properties of water:

Universal solvent: Water is a solvent for a great number of molecules which form ionized

solutions in water. So, it is also called as universal solvent which facilitates chemical

reactions both outside of and within biological systems.

High heat capacity: When a given amount of heat is applied, there occurs a smaller

temperature rise in water as compared to most other substances. Thus it acts as a temperature

buffer. It helps in keeping body temperature constant.

Surface tension: Water has the highest surface tension (of 72.8) of any known liquid. This is

the reason why water rises to unusually high levels in narrow capillary tubes.

Expansion on freezing: Most of the substances decrease in volume as their temperature

increase but in case of water when the temperature goes below 4oC it expands. So, organisms

living at the bottom of fresh water lakes are thus protected from freezing.

Polarity of water: When electrons are not shared equally in a covalent bond, the molecule is

described as polar. Water molecules are ploar. This means that while water molecules are

neutral as a whole, one end of the water molecule tends to have a positive charge while the

other has a negative charge. The oxygen end has a slight negative charge while the hydrogen

end has a slight positive charge.

Density: Another property of water is density during phase changes. The density of most

substances increases when a liquid becomes a solid. Solid water is actually less dense than

liquid water. It is for this reason that ice floats. This fact is essential for the survival of many

aquatic ecosystems and ultimately life on earth.

Capillary Action: Cohesion of water causes capillary attraction, which is the ability of water

to move upward in small spaces. Cohesion makes it possible for water to move up the fibers

of a plant. This is how plants get the water they need to survive. In addition, it moves water

upwards in soil. Cohesion of water also causes surface tension, water's invisible skin which

allows water striders to walk on water.

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Weak interactions in aqueous solutions-

A) Hydrogen bonding between water molecules:

The nearky tetrahedral arrangement of the oxygen electrons (Bond angle 104.5o) allows

each water molecule to form hydrogen bond with 4 neighbouring water molecules.

At any moment in liquid water at room temperature, each water molecule forms hydrogen

bonds with an average of 3.4 other water molecules.

The water molecules are in continuous motion in the liquid state; hence hydrogen bonds

are constantly and swiftly being broken and formed.

B) Hydrogen bonding between water and solute molecules:

Hydrogen bonds can be readily formed between any electronegative atom and a hydrogen

atom covalently bonded to another electronegative atom in the same or another molecule.

The hydrogen atoms covalently bonded to carbon atoms (non electronegative), do not

participate in hydrogen bonding.

C) Interaction between water and charged solutes:

Water is a polar solvent. It readily dissolves most biomolecules, which are generally

charged or polar compounds.

Compounds that readily dissolve in water are known as hydrophilic compounds.

D) Interaction between water and non polar gases:

The biologically important gases CO2, N2 and O2 are non polar. Hence these gases are

poorly soluble in water.

Some organisms have water soluble carrier proteins (Haemoglobin and myoglobin) that

facilitate the transport of oxygen.

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E) Interaction between water and non polar molecules:

When non polar compounds like benzene mixed with water they form two phases. These

phases are insoluble with each other. These compounds are also known as hydrophobic

compounds.

Dissolving hydrophobic solutes in water results in a measurable decrease in entropy.

F) Van Der Waals interaction:

These interactions are week, non atomic, interatomic attractions and come in to play

when any two uncharged atoms are 3 to 4 oA apart.

When two atoms are far apart, there is a very week interaction which becomes stronger as

the atoms move closer. But if the atoms move very close to each other, then the force of

repulsion occurs.

At a certain distance (Called Van Der Waals radius) there is the balance between the

forces of attraction and repulsion.

The basis for this bonding is that the distribution of electronic charge around an atom

changes in time.

These bonds are weaker than the hydrogen or electrostatic bond.

Imbibition

Imbibitions may be defined as a physical process in which living or dead plant materials

take up water or liquid mainly by adsorption due to the presence of hydrophilic or

lyophilic colloids inside them through the submicroscopic capillaries present on their

general surface of the body.

Due to imbibitions, sometimes a considerable pressure is developed which is known as

Imbibition pressure, e.g., a wooden piece driven into a hole in the rock on imbibition

swells so much that the pressure developed by it breaks the rock.

First step in imbibitions is adsorption, i.e., attachment of liquid on the surface.

Adsorption is the property of colloids and hence the material which have high

proportions of colloids, are good imbibants.

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Colloidal material and large molecules usually develop electrical charges when they are

wet, and thus attract water molecules.

Ex. Wooden doors in rainy season swell due to imbibitions phenomenon.

Characteristics of Imbibition:

1. Volume is increased, in the imbibant (Solid).

2. Heat is produced and is known as heat of wetting.

3. Pressure is developed.

Significance of Imbibition:

1. It is the first step of water ansorption.

2. It is also the first phenomenon in germination of seeds.

Diffusion

It is a spontaneous movement of molecules from a region of higher concentration to a

region of lower concentration.

Move along a concentration gradient until equilibrium reached.

Diffusion of particles of one substance is independent of another substance.

Factors affecting Diffusion:

1. Temperature: Directly proportional factor, because temperature increases kinetic

energy of particles.

2. Density: Hydrogen diffused 4 times faster than O2, according to Graham’s law of

diffusion, i.e., rate of diffusion of substance is inversely proportional to square root of

their densities.

3. Concentration gradient: More is the concentration gradient (Difference in

concentration on two sides), more will be the rate of diffusion.

4. Concentration of medium through which diffusion occurs: More dense is the

medium, lesser will be the rate of diffusion.

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Fig. Diffusion of dye molecules from lower concentration to higher concentration

Osmosis

Diffusion through semipermiable membrane is known as osmosis. or Osmosis is

diffusion of water through a differentially permeable membrane from a region where the

water is more concentrated to a region where it is less concentrated.

Osmosis is discovered by Pfeffer.

Water enters a cell by osmosis until the osmotic potential is balanced by the resistance to

expansion of the cell wall (Turgor Pressure)

Water Potential of a plant is essentially its osmotic potential and pressure potential

combined.

Water flows from the xylem to the leaves, evaporates within the leaf air spaces, and

transpires through the stomata into the atmosphere.

There are two types of Osmosis- Exosmosis and Endosmosis

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Exosmosis- When a cell is placed in hypertonic solution comes out from the cell into

outer solution, this is known as Exosmosis.

Endosmosis- If the flaccid cell is placed in in hypotonic solution water enters into cell,

this phenomenon is known as Endodmosis.

Plasmolysis

The shrinking of protoplasm away from the cell wall of a plant or bacterium due to water

loss from osmosis, thereby resulting in gaps between the cell wall and cell membrane.

When a plant cell is placed in a highly concentrated solution, water diffuses out of the

cell, and turgor pressure is lost causing the cell to become flaccid. Further loss of water

will result in plasmolysis.

Loss of water through osmosis is accompanied by shrinkage of protoplasm away from the

cell wall.

Fig. Process of Plasmolysis

Turgor pressure (TP):

Pressure exerted by the protoplast against the cell wall is known as turgor

pressure. When a living plant cell is placed in water, it swells up due to the absorption of

water by osmosis. As a result of this entry of water in to the cell sap, a pressure developed in

the protoplast which is exerted it against the cell wall. This actual pressure is Turgor

pressure. The cell wall being rigid and elastic exerts, an opposite pressure equal in magnitude

to the turgor pressure. This pressure is known as Wall pressure (W.P.)

W.P.= T.P.

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Diffusion Pressure Deficit (D.P.D)

This term is given by Meyer.

Every liquid is having a definite diffusion pressure and diffusion pressure of pure

solvent is always more than diffusion pressure of its solution. E.g. If a sugar solution

is made in water, then the D.P. of solution is lower than water (Solvent).

The amount by which D.P. of solution is lower than that of its pure solvent, is known

as Diffusion Pressure Deficit (D.P.D)

In case of D.P.D., the solution will try to wipe off this difference in D.P of solution by

sucking more water (solvent) molecules and hence D.P.D. is the measure of index of

sucking power and hence it is also called sucking pressure (S.P.)

So, D.P.D. = S.P.

Relationship between D.P.D., (S.P.), Osmotic pressure and turgor pressure:

D.P.D. (S.P.) = O.P. – T.P. (W.P)

Initially (or in flaccid cell), T.P. = 0. So, D.P.D. (S.P) = O.P. Therefore cell absorbs or

sucks water with a force equal to O.P.

In a fully turgid cell D.P.D. = 0. i.e. Cell has no further capacity to absorb any water.

Concept of Water potential, Solute potential and Pressure potential:

Concept of wate potential is given by Slater and Taylor.

It is the difference in the free energy (Chemical potential) of water molecules in the

solution and that of pure water at the same temperature and pressure.

It is indicated by greek letter psi, the symbol for which is Ψw. Ψw of pure water is 0. Ψw

of a solution is always negative.

Solute potential (Ψs): It is defined as the amount by which the water potential is reduced

as a result of the presence of solute. It is also known as osmotic potential which is always

expressed in negatine values and represented in bars. Osmotic potential and osmotic

pressure are numericaly equal but osmotic potential has a negative sign.

Pressure potential (Ψp): The pressure potential operates in plant cells as wall pressure

and turgor pressure and usually has a positive value.

Ψw = Ψs + Ψp + Ψg

Ψw = Ψs + Ψp + Ψm

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Where, Ψw is water potential, Ψs is Solute potential or osmotic potential, Ψm is Matric

potential and Ψm is gravitational potential.

Ψm and Ψg are negligible

Therefore, Ψw = Ψs + Ψp, Where, Ψw and Ψs are negative values.

Active Transport:

Plants absorb and retain solutes against a diffusion, or electrical, gradient through the

expenditure of energy. It involves proton pump. Various scientists proposed theories for active

transport of water.

A) Movement of water in the xylem "Cohesion Tension Theory":

When the negatively charged end of one water molecule comes close to the positively

charged end of another water molecule, weak hydrogen bonds hold the molecules

together.

Water molecules adhering to capillary walls, and each other, create a certain amount of

tension.

When water transpires, the cells involved develop a lower water potential than the

adjacent cells. Creates tension on water columns, drawing water from one molecule to

another, throughout the entire span of xylem cells.

B) Regulation of Transpiration through the stomata

Changes in turgor pressure occur when osmosis and active transport between the guard cells

and other epidermal cells cause shifts in solute concentrations.

When photosynthesis is not occurring in the guard cells, potassium ions leave, and the

stomata close.

An increase in potassium ions causes a lowering of the water potential and osmosis

leading to turgid guard cells

Stomata of most plants are open during the day and closed at night. Stomata of many

desert plants open only at night. Thus conserves water, but makes carbon dioxide

inaccessible during the day.

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Humidity plays an inverse role in transpiration rates. High humidity reduces

transpiration, while low humidity accelerates it.

If a cool night follows a warm, humid day, water droplets may be produced through

hydathodes at the tips of veins of some plants (Guttation).

Transport of Organic Solutes in Solution: one of most important functions of water in the

plant involves the translocation of food substances in solution by the phloem.

C) Pressure-Flow Hypothesis:

Organic solutes flow from a source where water enters by osmosis. Organic solutes

are moved along concentration gradients between sources and sinks

Soil water enters the root through its epidermis. It appears that water then travels in

both

The cytoplasm of root cells — called the symplast — that is, it crosses the plasma

membrane and then passes from cell to cell through plasmodesmata.

In the nonliving parts of the root — called the apoplast — that is, in the spaces

between the cells and in the cells walls themselves. This water has not crossed a

plasma membrane.

However, the inner boundary of the cortex, the endodermis, is impervious to water

because of a band of lignified matrix called the casparian strip. Therefore, to enter

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the stele, apoplastic water must enter the symplasm of the endodermal cells. From

here it can pass by plasmodesmata into the cells of the stele.

Once inside the stele, water is again free to move between cells as well as through

them. In young roots, water enters directly into the xylem vessels and/or tracheids

These are nonliving conduits so are part of the apoplast.

Once in the xylem, water with the minerals that have been deposited in it (as well as

occasional organic molecules supplied by the root tissue) move up in the vessels and

tracheids.

At any level, the water can leave the xylem and pass laterally to supply the needs of

other tissues.

At the leaves, the xylem passes into the petiole and then into the veins of the leaf.

Water leaves the finest veins and enters the cells of the spongy and palisade layers.

Here some of the water may be used in metabolism, but most is lost in transpiration.

D) Transpiration-Pull

In 1895, the Irish plant physiologists H. H. Dixon and J. Joly proposed that water is

pulled up the plant by tension (negative pressure) from above.

Water is continually being lost from leaves by transpiration. Dixon and Joly believed

that the loss of water in the leaves exerts a pull on the water in the xylem ducts and

draws more water into the leaf.

But even the best vacuum pump can pull water up to a height of only 34 ft (10.4 m) or

so. This is because a column of water that high exerts a pressure of ~15 lb/in2 (103

kilopascals, kPa) just counterbalanced by the pressure of the atmosphere. How can

water be drawn to the top of a sequoia (the tallest is 370 feet [113meters] high)?

Taking all factors into account, a pull of at least 270 lb/in2 (~1.9 x 103 kPa) is

probably needed.

The answer to the dilemma lies the cohesion of water molecules; that is the property

of water molecules to cling to each through the hydrogen bonds they form.

When ultrapure water is confined to tubes of very small bore, the force of cohesion

between water molecules imparts great strength to the column of water. It has been

reported that tensions as great as 3000 lb/in2 (21 x 103 kPa) are needed to break the

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column, about the value needed to break steel wires of the same diameter. In a sense,

the cohesion of water molecules gives them the physical properties of solid wires.

Because of the critical role of cohesion, the transpiration-pull theory is also called the

cohesion theory.

Metabolic Role of water:

In living systems water is pervasive and ubiquitous and cannot be considered as a

simple dilutent. It performs many functions: it transports, structures, stabilizes,

lubricates, reacts and partitions. Proof for the finely tuned involvement of water in

life processes is provided by the fact that heavy water is toxic to these processes. It is

widely appreciated that water molecules play an invaluable role in governing the

structure, stability, dynamic, and function of many biomolecules. Indeed, they lack

activity in the absence of water. It is however only in recent years that water has been

quantitatively treated as an integral component of biomolecular systems.

I. Role of water in protein folding, structure and stability

The hydrophobic effect and hydrogen bonds are of prime importance and water is an

actor in these contributions to protein structure and stability.

The hydrophobic effect is generally considered to be the major driving force for the

folding of globular proteins. It results in the burial of the hydrophobic amino acid side-

chains in the core of the protein.

Water tends to form ordered cages around non-polar groups (hydrophobic hydration)

which leads to a decrease in entropy of the system. These water molecules gain entropy

when they are released after hydrophobic surfaces are put in contact with each other.

This contributes in a very favorable way to the free energy of stabilisation of the protein.

Water is therefore fundamental in protein folding because of its role in defining

hydrophobic attractions.

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The hydrophobic “collapse” of the protein is necessarily accompanied, and guided, by

hydrogen-bond formation between favorable functional groups

A hydrogen bond (H-bond) occurs when two electronegative atoms, such as nitrogen

and oxygen, interact with the same hydrogen.

The strength of a hydrogen bond is around 2 to 20 kJ/mol. This is however not

necessarily the amount of energy that the hydrogen bond contributes towards the

stabilization of a folded protein.

Indeed, in the unfolded state, potential hydrogen bonding partners in the polypeptide

chain are satisfied by hydrogen bonds to water.

When the protein folds these protein-to-water H bonds are broken, some are replaced by

intra-protein H-bonds and the entropy of the solvent increases.

The balance between the entropy and enthalpy terms is close but H-bonds make a

positive contribution to protein stabilisation.

Despite the small contribution made to protein stability by hydrogen bonds, if an

intramolecular hydrogen bond in a protein is broken or deleted without the possibility of

forming a compensating H-bond to solvent, the protein will be destabilized and can loose

its structure.

Figure 1: Schematic representation of the various ways that water molecules are implicated in

protein structure and stability.

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II. Role of water on the structure and stability of nucleic acids

Like in proteins, water is an integral part of nucleic acid structures. A shell of hydration,

impermeable to cations, is located around DNA double helices.

It consists of about 18-19 water molecules per nucleotide in B-DNA and of about 13-14

in A-DNA.

These water molecules are specifically bound to the phosphate groups and to the bases.

X-ray studies show that there are six hydration sites per phosphate and that the positions

and occupancies of these sites are dependent on the conformation and type of nucleotide.

Studies on A-DNA crystal structures have shown that water molecules bridge

neighbouring phosphate groups and thus stabilize the A-form at lower water activities.

The water molecules around the phosphate groups are not permanently situated however,

due to the rather diffuse electron distribution of the phosphate groups.

The paired bases are capable of hydrogen-bonding to water within the grooves (Figure 2).

Figure 2: Watson-Crick base-pairing and possible sites for water H-bonding to the paired bases.

.

A spine of hydration is for example observed by x-ray crystallography in the minor

groove of B-DNA.

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Energy calculations suggest that the presence of this spine of hydration is the prime

reason for the narrowing of the minor groove.

Hydration more persistent around the bases than around the phosphate groups but the

residence times that are reported for these water molecules are still relatively short

compared to those of bound water molecules in proteins.

A second hydration shell, indistinguishable from bulk water as far as permeability to ions

is concerned, surrounds the DNA double helix.

In this shell the properties of the water molecules are different from those of bulk water

even if residence times are in the sub-nanosecond range.

RNA has a greater extent of hydration than DNA due to its extra oxygen atoms (the

ribose O2’) and unpaired base sites. Stable hydration patterns are observed, like in DNA

around double stranded regions [20 ;25].

III. Role of water in molecular recognition involving bio-molecules

Water is a highly versatile component at the interface of biomolecular complexes.

It can act both as a hydrogen bond donor and acceptor, imposing few steric constraints on

bond formation and can take part in multiple hydrogen bonds.

Water can thus confer a high level of adaptability to a surface allowing promiscuous

binding yet it can also provide specificity and increased affinity to an interaction.

Water in protein-ligand interactions can for example function as an extension of protein

structure allowing varied ligands to be accommodated in a given binding site or

increasing affinity for specific ligand (Figure 3).

For these water molecules, the energetic gain from water-mediated contacts is greater

than the entropic cost resulting from their immobilization.

Inclusion of water molecules in structure based ligand design is however currently largely

overlooked because the structural and thermodynamic effects of water’s inclusion are

hard to determine and model.

(Bio)molecular associations, especially protein-protein and protein-DNA complexes, will

of course also be stabilized as a result of the hydrophobic effect and of the screening, by

water, of charge repulsion.

.

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Figure 3: Schematic representation of various models explaining protein-ligand interactions.

Significance of water in biological system:

1. Transport:

Uptake of minerals by plants from soil across root hairs occurs in solution. Transpiration

stream and water-based movement of sugars and amino acids, hormones etc. in phloem

occurs in solution.

All transport fluids used in animals (e.g. cytoplasm, blood, plasma and tissue fluid) are

water-based.

Many essential metabolites dissolve completely e.g. glucose, amino acids,vitamins and

minerals.

Larger molecules e.g. proteins are transported as colloids.

Transpiration stream is held together by cohesion (water molecules hydrogen bond to

other water molecules) and adhesion (water molecules bind to side of xylem vessel).

Such forces alsogive rise to capillarity in tubes of very small diameter. Low viscosity of

water enables it to flow easily through tubes e.g. xylem vessels.

2. Chemical reactions (Metabolism):

Combination of thermal stability and excellent solvent properties makes water an ideal

environment for chemical reactions. All enzyme reactions of photosynthesis, respiration,

excretion etc. occur in solution.

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Water also acts as a reactant for example, in:

Light dependent stage of photosynthesis when photolysis splits water to release electrons

which move to photosystem II (PSII) and then through electron carriers in non-cylic

photophosphorylation pathway (NCP).

Hydrolytic reactions (e.g. digestive enzymes). 3. Temperature control:

High specific heat capacity allows water to act as a buffer; essential in endothermic

organisms that need to maintain a constant body temperature in order to optimise enzyme

activity and thereby regulate metabolism.

High incidence of hydrogen bonding also makes at it difficult for water molecules to

evaporate. When they do so, much energy is released and this is involved in cooling

mechanisms.

Water remains a liquid over a huge temperature range - essential for metabolism and

useful for aquatic organisms which avoid freezing. 4. Support:

In plant cells water confers turgidity. This is essential for example, in:

Maintaining maximum leaf surface area, hence light absorption, hence photosynthesis.

Maintaining aerial parts of the plant to maximise seed dispersal or pollination. Loss of

water in very hot conditions may lead to leaves wilting. This decreases their surface area,

hence light absorption, temperature and water loss.

In animals, water-filled tissues also contribute to skeletal support. In organisms which

possess a hydrostatic skeleton (e.g. annelids), water is the major component of the fluid

in the coelom against which muscles can act.

For aquatic organisms, water provides support through buoyancy.

5. Movement:

Nastic movements, i.e. those which do not involve growth in a particular direction as a

response to a directional stimulus, depend upon the osmotic inflow of water into tissues,

e.g. the opening and closing of flowers or ‘snapping’ of the carnivorous.

6. Reproduction:

Organisms which employ sexual reproduction use water to bring the male and female

gametes together in the process of fertilisation.

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In mammals the foetus develops in a water filled sac which provides physical and thermal

stability.

Bryophytes release antherozoids in moist conditions which use flagella to swim to

oospheres by chemotaxis.