2.1 Properties of water and its importance in biological systems

Water, the foundation of life, plays a crucial role in biological systems. Its unique properties, stemming from its polar nature and hydrogen bonding abilities, make it an ideal medium for cellular processes and chemical reactions.

From cohesion and adhesion to its high specific heat capacity, water's characteristics shape the world around us. Understanding these properties is key to grasping how life thrives in aqueous environments and how water influences biological functions.

Chemical Structure and Bonding

Polarity and Hydrogen Bonding

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• Water is a polar molecule due to the unequal sharing of electrons between oxygen and hydrogen atoms
• Oxygen atom has a higher electronegativity than hydrogen atoms, resulting in a partial negative charge on oxygen and partial positive charges on hydrogens
• Hydrogen bonding occurs when the partially positive hydrogen of one water molecule is attracted to the partially negative oxygen of another water molecule
• Hydrogen bonds are relatively weak compared to covalent bonds but collectively contribute to the unique properties of water
• Hydrogen bonding gives water a higher boiling point than expected for a molecule of its size (100°C) compared to other small molecules like methane (-161°C)

Hydrophilicity and Hydrophobicity

• Hydrophilic molecules or regions of molecules are attracted to water and readily dissolve in water (sugar, salt)
• Hydrophilic substances typically contain polar or charged groups that can form hydrogen bonds with water molecules
• Hydrophobic molecules or regions of molecules are repelled by water and do not readily dissolve in water (oils, fats)
• Hydrophobic substances are typically nonpolar and cannot form hydrogen bonds with water molecules
• Amphipathic molecules contain both hydrophilic and hydrophobic regions (phospholipids, detergents)
• Amphipathic molecules can self-assemble in water to form structures like micelles and lipid bilayers with hydrophobic regions shielded from water

Unique Properties of Water

Cohesion, Adhesion, and Surface Tension

• Cohesion is the attractive force between water molecules due to hydrogen bonding
• Cohesion allows water to maintain its liquid state and resist separation
• Adhesion is the attractive force between water molecules and other surfaces
• Adhesion allows water to "stick" to surfaces like cell walls in plants, enabling capillary action and water transport
• Surface tension is the tendency of water molecules at the surface to be pulled together, creating a "skin" on the surface
• Surface tension allows small objects to float on water (water striders) and enables the formation of droplets and bubbles

High Specific Heat Capacity and Solvent Properties

• Specific heat capacity is the amount of heat required to raise the temperature of a substance by 1°C
• Water has a high specific heat capacity (4.18 J/g°C) compared to many other substances
• High specific heat capacity allows water to absorb and release large amounts of heat energy without significant temperature changes
• This property enables water to regulate temperature in living organisms and Earth's climate system (oceans as heat reservoirs)
• Water is an excellent solvent for polar and ionic compounds due to its polarity and ability to form hydrogen bonds
• Water can dissolve a wide range of substances (nutrients, waste products) and facilitate chemical reactions in biological systems
• The solvent properties of water are crucial for the transport of substances within organisms (blood, sap) and the environment (rivers, oceans)