Water

Water is an indispensable biological molecule, forming a major component of cells and providing the environment for life itself. Typically, 70% to 95% of a cell's mass is water, and about 60% of a human body is water. Most of the chemical reactions fundamental to life occur in an aqueous solution.

Molecular Structure and Polarity

A water molecule (H2O) consists of one oxygen atom covalently bonded to two hydrogen atoms. The key to water's unique properties lies in its polarity.

• The shared negative hydrogen electrons are pulled towards the oxygen atom because oxygen attracts electrons more strongly.

• This leaves the other side of each hydrogen atom with a slight positive charge (δ+).

• The unshared negative electrons on the oxygen atom give it a slight negative charge (δ-).

• This unequal distribution of charge makes water a polar molecule (also called a dipole or dipolar molecule).

Hydrogen Bonding

Due to its polarity, hydrogen bonds form between water molecules.

• These are weak bonds formed between a slightly positively charged hydrogen atom in one molecule and a slightly negatively charged oxygen atom in another water molecule.

• While individual hydrogen bonds are weak and constantly breaking and reforming, their large number contributes to a strong structure and accounts for many of water's unique and useful properties.

Properties and their Biological Importance

The properties of water, largely influenced by hydrogen bonding, make it essential for life:

• Good Solvent:

  • Water is an excellent solvent for polar substances (like sugars, amino acids, glycerol, and glucose) and ionic substances (like salts, e.g., sodium and chloride ions).

  • Its polarity allows it to surround ions and polar molecules, causing them to dissolve.

  • This solvent action is crucial because most metabolic reactions take place in solution (e.g., in cytoplasm), and allows for the transport of dissolved substances throughout organisms (e.g., glucose, oxygen, ions, hormones, urea in blood plasma and tissue fluid; water and mineral ions, dissolved sugars, and amino acids in plants via xylem and phloem).

  • Non-polar substances, such as lipids, are repelled by water and are described as hydrophobic. This property is important in the formation of cell membranes.

• High Specific Heat Capacity:

  • Water has a relatively high specific heat capacity, meaning it takes a lot of energy (heat) to raise the temperature of 1 gram of the substance by 1 °C.

  • This is because much of the heat energy is used to break the hydrogen bonds between water molecules, rather than increasing their kinetic energy and thus temperature.

  • This property helps buffer (resist) changes in temperature, providing stable habitats in aquatic environments and helping organisms maintain a constant internal body temperature for optimal enzyme activity.

• High Latent Heat of Vaporization:

  • Water has a high latent heat of vaporization, meaning a lot of energy (heat) is required to turn liquid water into water vapor.

  • This energy is used to break the hydrogen bonds holding water molecules together so they can escape as a gas.

  • This property is vital for cooling living organisms through evaporation, such as sweating in mammals or transpiration from plant leaves. It allows for significant heat loss with minimal water loss, reducing dehydration risk.

• Cohesion and Adhesion:

  • Cohesion is the strong attraction between molecules of the same type (water molecules sticking together). This is due to their polarity and hydrogen bonding.

  • Adhesion is the force by which water molecules cling to surrounding materials or surfaces that are hydrophilic.

  • Strong cohesion helps water to flow, making it excellent for transporting substances. In plants, cohesion and adhesion allow water to travel in continuous columns up the xylem vessels (cohesion-tension theory) from roots to leaves, resisting breakage under tension.

  • High cohesion also results in high surface tension where water meets air, which allows some small organisms (e.g., pond skaters) to move on its surface.

• Density Anomaly (Ice floats):

  • Water is unique in that its solid state (ice) is less dense than its liquid state, causing ice to float.

  • This property is advantageous for aquatic organisms as it allows large bodies of water to freeze from the surface downwards, insulating the liquid water below and preserving aquatic life.

Role as a Metabolite / Reagent

Water is a metabolite and an important reagent in many metabolic reactions.

• It is directly involved in condensation reactions (where water is released as bonds form) and hydrolysis reactions (where water is added to break bonds). For instance, amino acids join to form proteins via condensation reactions. ATP releases energy through a hydrolysis reaction.

• In photosynthesis, water is a reactant; light energy splits water into protons, electrons, and oxygen (O2), which is released as a waste product. The hydrogen from water is used to make glucose.

Water in Biological Systems

• Cellular Component: Water makes up a large percentage of a cell's contents and cytoplasm.

• Osmotic Balance: Water potential describes the tendency of water to move into or out of a solution. Organisms maintain osmoregulation to control the water potential of their blood and tissue fluid, preventing cells from swelling or shrinking due to osmosis.

• Transport in animals: Water is the main component of blood plasma (95%) and tissue fluid, enabling transport of nutrients, waste, hormones, and heat. Plasma proteins like albumin regulate blood water potential.

• Transport in plants: Water is transported in xylem vessels and carries dissolved mineral ions. Transpiration, the loss of water vapor from leaves, creates a tension that pulls water up the plant.

Water's various properties, arising primarily from its polar nature and hydrogen bonding, make it fundamental to the existence and functioning of all living organisms.