PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

4. WATER RELATIONS

Structure and Properties of Water

Water is the only substance on Earth that simultaneously exists in large quantities in all three states of matter. Its Structure differs in each of these states. The solid state occurs in at least two forms: crystalline (ice) and non-crystalline (vitreous). The latter is formed during rapid freezing and does not damage living Cells.

Class="center">MOLECULAR Structure and properties

Physical properties

The physiological function of water is primarily determined by its physical properties. It is the most anomalous substance, despite being used as the standard for density and volume measurements for other liquids. While most substances expand upon heating, thereby decreasing in density, water reaches its maximum density at 4 °С; upon freezing, its volume increases sharply (by 11%), and its density decreases.

Due to this anomaly, as water bodies cool, the density of the surface water increases, causing the upper layers to sink until the Temperature throughout the entire depth reaches the point of maximum density (4 °С). Upon further cooling, the water becomes lighter and remains On the surface, protecting the deeper layers from cooling to lower temperatures. At 0 °С, the water body becomes covered with ice.

Volumetric changes in water during freezing and thawing are a critical factor influencing parent material during soil formation. Under such conditions, pressure can reach up to 2400 atm, which is sufficient to break down rock.

At a pressure of 1 atm, the freezing and boiling points are 0 °С and 100 °С, respectively. As pressure increases, the boiling point rises, while the freezing point decreases.

Water has a relatively high latent heat of fusion (≈ 335 J/g). Its latent heat of vaporization is 7 times higher. The specific heat capacity of water (The amount of heat required to raise its temperature by 1 °С) is 5–30 times greater than that of other substances.

The high heat capacity of water protects plants from sudden temperature fluctuations. The high heat of vaporization plays a key role in thermoregulation. High melting and boiling points, along with high heat capacity, indicate significant intermolecular interactions.

Water is characterized by high surface tension due to strong cohesive forces between its molecules (only mercury has a higher surface tension), as well as The phenomenon of adhesion, which manifests in its ability to rise against gravitational forces, for instance, in tree Tissues.

Water also possesses high thermal conductivity (The transfer of energy from warmer to cooler areas of a body), which allows it to evaporate even at 0 °С. Another essential property of water is its well-known ability to dissolve gases.

Thanks to its unique physicochemical properties, water has become the most suitable internal medium for All living organisms.

Molecular structure of water

In a water molecule, two pairs of electrons are shared between the hydrogen and oxygen nuclei. They have elongated orbits, and one part of the molecule carries a partial positive charge (near the hydrogen atoms), while the other carries a negative charge (near the oxygen atom), thus forming a dipole.

A dipole is a pair of point electric charges that are equal in magnitude and opposite in sign.

Spatially, the H₂O molecule forms a tetrahedral structure with four poles of electric charge (2⁺ and 2^-). As a dipole with a tetrahedral distribution of electrons around the oxygen atom, each water molecule can interact with four other water molecules through electrostatic forces. This results in a pentagonal structure, and this interaction, which has a small covalent component, is called a Hydrogen bond. Hydrogen bonding is caused by electrostatic attraction arising from the uneven distribution of electrons between atoms participating in a covalent bond.

The interaction between molecules due to electrostatic forces with a small covalent component is called a hydrogen bond.

Hydrogen Bonds are continuously forming and breaking. Furthermore, there is constant proton dissociation, i.e., the dissociation of water into H⁺ and OH^- (hydrogen and hydroxyl ions). At 25 °С, the concentration of hydrogen or hydroxyl ions in pure water is 1x10^-7 mol/L, which corresponds to a pH of 7.

According to modern concepts, The structure of water is based on an ordered framework consisting of a crystal lattice and a fraction of molecules disrupted by thermal motion. Ordered regions (clusters) constantly appear and disappear (their lifetime is 10^-10 s), which is why they are called "flickering clusters." At any given moment, up to 2/3 of water molecules are involved in The formation of "flickering clusters." Some researchers believe that liquid water has a homogeneous ice-like (open) structure, in the voids of which monomeric water molecules reside without forming hydrogen bonds.

The quasi-crystalline (from Latin quasi - as if, seemingly) structure of water is the main feature that distinguishes it from other liquids. In other words, both in liquid and solid states, water molecules form a specific structure. Ice has a hexagonal crystalline structure. Liquid water contains ordered regions (clusters with an ice-like structure) and disordered regions with a small number of hydrogen bonds.

The universality of water as a solvent is due to the polarity of its molecules and its ability to form hydrogen bonds. Crystals of inorganic salts dissolve through the Hydration of their ions. Organic substances containing carboxyl, hydroxyl, and other groups with which water can form hydrogen bonds also dissolve well in water. The polarity of water molecules makes it a better solvent for many substances than other liquids.

These properties of water determine the unique role it plays in the biophysical and biochemical processes that sustain all living systems.

THE STATE OF WATER IN SOLUTIONS

Electrolyte solutions

In solutions containing ions, the structure of water is significantly altered. Small ions with high charge density exert a stronger influence on the structure of pure water compared to large ions with low charge density. Both types disrupt the water structure: the former attract water molecules, while the latter disrupt the ice-like framework due to their large size. This process also alters the viscosity of the aqueous solution.

In the electric field of a cation, surrounding water molecules are oriented with their negative poles inward, whereas around an anion, the positive poles are directed inward. This gives rise to primary hydration. During Electrophoresis, this layer moves along with the ion as a single unit. As a result of ion-dipole interactions, more distant water molecules also undergo orientation, which is known as secondary hydration.

Hydration is the orientation of water molecules within the electrostatic field of an ion.

Water bound to ions is also referred to as osmotically bound water. It is a critical component of the osmotic pressure in plant cells.

As the concentration of the solution increases, the system transitions from the structure of pure water to that of a crystalline hydrate.

The Effect of hydrophobic radicals on water structure

Substances with large hydrophobic radicals increase the heat capacity of a solution. This is explained by an increase in the degree of Structural Organization of the water. Crystalline hydrates with pentagonal cavities form around non-polar molecules. Such crystals do not melt at 0 °С, but rather at 5-3 °С; this phenomenon accounts for the freezing of grain and crop damage due to ice formation in tissues at 4-5 °С.

Protein solutions

In Proteins, hydration is driven by the interaction of water molecules with hydrophilic (ionic) and hydrophobic (non-polar) groups, as well as by the immobilization of water within enclosed spaces inside macromolecules during conformational changes. A protein is least hydrated at its isoelectric point, which is also where its solubility is observed to be at its lowest.

Water bound to both internal and surface groups is referred to as colloidally bound water.

Immobilization is the mechanical entrapment of water during conformational changes of biopolymer molecules, which results in water molecules becoming trapped within closed spaces.