Water

Electrons are not always shared equally in a covalent bond. 

• Some atoms, such as oxygen, tend to be highly electronegative and so attract electrons towards themselves, resulting in a partial negative charge (δ- ).

• Other atoms, such as hydrogen, are less capable of attracting electrons, leading to a partial positive charge (δ+). 

This imbalance creates a dipole, making the molecule polar. 

When one dipole encounters another, the positive end of one molecule is weakly attracted to the negative end of another molecule. 

This attraction forms a hydrogen bond. Hydrogen bonds are weaker compared to covalent or ionic bonds and are relatively easy to break. However, the cumulative strength of many hydrogen bonds between two molecules can be significantly higher.

Hydrogen bonds between water molecules.

Function

Water, composed of two hydrogen atoms and one oxygen atom, is a polar molecule. Hydrogen bonds between water molecules bestow water with several important and unique properties:

• The attraction between water molecules is known as cohesion. Cohesion gives water its surface tension, allowing some organisms to walk on water without breaking the surface. It also enables water to flow in continuous columns, such as in blood vessels and xylem vessels.

• Water molecules are also attracted to other polar or charged particles, a property known as adhesion. This characteristic is crucial for water's movement in columns up xylem vessels.

• Specific heat capacity refers to the amount of heat needed to raise the temperature of 1 gram of a substance by 1 degree Celsius. Water has a high specific heat capacity due to the numerous hydrogen bonds holding its molecules together. Since living cells are mostly composed of water, this property stabilizes and prevents extreme temperature fluctuations within cells, even when external temperatures change.

• Latent heat of vaporization is the amount of heat energy required to change a substance from a liquid to a gas. Water has a high latent heat of vaporization because breaking its hydrogen bonds requires a significant amount of energy. This trait benefits organisms that sweat, as it takes a considerable amount of heat energy from the organism to evaporate the water.

• Water's attraction to any charged or polar molecule makes it an excellent solvent, often referred to as the "universal solvent." This allows reactants, nutrients, and products to dissolve in the cytoplasm of cells and various substances to dissolve in blood plasma and tissue fluid.

• Many biological reactions require the breakdown of macromolecules, such as proteins, carbohydrates, and nucleic acids. These hydrolysis reactions necessitate water to break the covalent bonds between molecules. Conversely, water is also produced during condensation reactions when large biological molecules are synthesized.

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