Biomolecules: Basic Molecules and Enzymes - Water Properties

Water (H₂O) is essential for life, serving as a vital chemical constituent and habitat.
A water molecule consists of an oxygen atom covalently bonded to two hydrogen atoms.
Oxygen is more electronegative than hydrogen, resulting in polar covalent bonds.
Partial negative charges (δ-) are located on the oxygen atom, while partial positive charges (δ+) are on the hydrogen atoms.
Hydrogen bonds form between the δ- oxygen of one water molecule and the δ+ hydrogen of another.
These hydrogen bonds give water its unique properties.
The most stable arrangement of charges is a tetrahedron.
The H-O-H angle is $104.5^{\circ}$ , slightly less than a perfect tetrahedron due to the greater space occupied by partial negative charges.

Water is an excellent solvent due to its polarity.
It effectively dissolves polar molecules and ionic compounds.
When a salt crystal (NaCl) is placed in water:
- The δ+ hydrogen ends of water molecules surround chloride ions (Cl⁻).
- The δ- oxygen ends of water molecules surround sodium ions (Na⁺).
This process forms dispersive hydration shells around the ions.
Water dissolves polar molecules because its positive and negative poles are attracted to oppositely charged regions of the dissolving molecules.
Charged and polar molecules are hydrophilic and dissolve readily in water. Examples: sugars and amino acids.
For instance, glucose contains polar hydroxyl (OH) groups that form hydrogen bonds with water, creating a hydration shell.
Water also dissolves gases, such as oxygen and carbon dioxide.

Uncharged, non-polar molecules (e.g., fats and oils) do not dissolve in water and are termed hydrophobic.
Nonpolar molecules aggregate in water due to hydrophobic exclusion, leading to specific shapes and structures.
Lipids, being immiscible with water, can separate aqueous solutions into compartments, such as membranes.
Hydrophobic exclusion influences the structure of DNA and proteins.

Specific heat capacity: the amount of heat (in joules) needed to raise the temperature of 1 kg of water by $1^{\circ}C$ .
Water has a high heat capacity, requiring a large amount of heat energy to raise its temperature.
Much of the energy is used to break hydrogen bonds, restricting molecular mobility.
High heat capacity minimizes temperature changes within water, which is vital for living organisms.

A relatively large amount of energy is needed to vaporize water, resulting in a cooling effect.
This is utilized in sweating and panting for thermoregulation in mammals.
The high heat of vaporization allows a large amount of heat loss with minimal water loss from the body.

Latent heat of fusion: the heat energy required to melt a solid (ice).
Water requires a relatively large amount of heat energy to thaw and releases a large amount of heat energy to freeze.
This property reduces the likelihood of cell contents and environments freezing.

Ice is less dense than liquid water, causing it to float.
When a pond or lake freezes, the ice forms an insulating layer at the top, delaying further freezing of the water below.
If ice sank, ponds and lakes in cold regions would freeze solid during winter, harming aquatic life and limiting access to liquid water.

Water exhibits high cohesion – water molecules tend to stick together.
(Adhesion is the sticking of water molecules to other types of molecules).
Cohesion among water molecules at the surface creates surface tension.
High cohesion is important in cells and for water translocation through xylem in plants.
Many small organisms rely on surface tension to live on the water's surface.

All Organisms
- High water content in protoplasm provides structure.
- Acts as a solvent and medium for diffusion.
- Functions as a reagent in hydrolysis.
- Offers support for aquatic organisms.
- Facilitates fertilization via swimming gametes.
- Aids dispersal of seeds, gametes, and larval stages of aquatic organisms, as well as seeds of some terrestrial species.
Plants
- Important for osmosis and turgidity, supporting the guard cell mechanism.
- Functions as a reagent in photosynthesis.
- Crucial for transpiration and translocation of inorganic ions and organic compounds.
- Essential for seed germination as water causes swelling and breaking open of the testa which stimulates further seed development
Animals
- Necessary for transport processes.
- Important for osmoregulation.
- Enables cooling through evaporation (sweating, panting).
- Acts as a lubricant, e.g., in joints.
- Provides support for hydrostatic skeletons.
- Offers protection, e.g., lachrymal fluid, mucus.
- Facilitates migration in ocean currents.