Unique Properties of Water and Their Importance in Biology

Water: Essential Properties for Life

Polarity and Hydrogen Bonds in Water Molecules

• A water molecule comprises an oxygen atom covalently bonded to two hydrogen atoms.
• Oxygen is more electronegative than hydrogen, leading to an unequal sharing of electrons.
• This creates a polar covalent bond, with hydrogen having a partial positive charge and oxygen having a partial negative charge.
• The bent shape of water molecules results in a positive charge concentration on one side and a negative charge on the opposite side.
• The partial positive charge is attracted to the partial negative charge, creating an intermolecular attraction between water molecules called a hydrogen bond.
• Hydrogen bonds are the strongest intermolecular forces, but they are still considered weak bonds. However, the sheer number of H2O molecules gives water its unique properties, making it essential to life.

Hydrogen Bonding and Dipolarity: Key to Water's Properties

Thermal Properties

Water has a high specific heat capacity, meaning it can absorb or release a significant amount of heat without a drastic temperature change. This is due to the extensive hydrogen bonding between water molecules.

Water also has a high latent heat of vaporization, requiring substantial heat to evaporate from liquid to vapor. This property is crucial for cooling mechanisms, such as sweating in humans. As sweat evaporates, it absorbs heat from the skin, thus cooling the body.

Cohesive Properties

• Water is a polar molecule with a negative oxygen end and a positive hydrogen end.
• Hydrogen bonds between water molecules create a high level of attraction, linking them together. This attraction between like molecules is called cohesion.
• Cohesive forces enable water to move up plant vascular tissue against gravity. They also create surface tension, allowing some organisms to walk on water.

Adhesive Properties

• Water not only binds strongly to itself but also forms hydrogen bonds with other polar molecules. This is called adhesion.
• Adhesion is vital in transpiration, as water adheres to cellulose in xylem vessel walls.
• As water evaporates from stomata, adhesion helps it move up through the xylem.

Solvent Properties

• Water is known as the "universal solvent" due to its ability to dissolve many substances because of its polarity.
• Water can dissolve other polar molecules, such as carbohydrates, proteins, and DNA, as well as positively and negatively charged ions like Na+.
• This property is essential, allowing water to act as a transport medium (e.g., blood and cytoplasm) for important molecules in biological organisms.

Hydrophilic and Hydrophobic Substances

• Hydrophilic means "water-loving."
• Substances that dissolve in water, including charged ions (e.g., Na+) or polar molecules (e.g., glucose, fructose), are hydrophilic. Molecules attracted to water, like phospholipid heads, are also hydrophilic.
• Hydrophobic molecules are "water-fearing." These are non-polar substances insoluble in water, such as lipids.