Summary
Highlights
Water is composed of two hydrogen atoms covalently bonded to an oxygen atom. Oxygen's higher electronegativity creates partial negative charges on the oxygen and partial positive charges on the hydrogens, making water a polar molecule with a bent shape. This polarity leads to hydrogen bonding between water molecules, which is crucial for its unique properties.
Water's polarity makes it an excellent solvent for hydrophilic substances, as it forms hydrogen bonds with them. This 'universal solvent' property was essential for the formation of the first cells and is still vital for biological reactions occurring in the cytoplasm of cells. Conversely, hydrophobic substances like lipids do not dissolve in water but repel it, necessitating special transport mechanisms like lipoproteins in blood.
Water's physical properties, such as high specific heat capacity, thermal conductivity, buoyancy, and viscosity, make it an ideal aquatic habitat. Its high specific heat capacity allows oceans to absorb and release heat slowly, buffering temperatures. High thermal conductivity means heat transfers quickly in water, requiring insulation for aquatic animals. Water's density provides buoyancy, aiding movement for marine organisms, and its viscosity provides resistance for propulsion.
Due to hydrogen bonding, water molecules exhibit cohesion (attraction to each other), leading to surface tension, which supports small organisms on water surfaces. Water also displays adhesion (attraction to other substances), visible when water sticks to surfaces. Both cohesion and adhesion are crucial for biological phenomena.
Cohesion and adhesion together cause capillary action, where water moves upward in thin tubes against gravity. In plants, this is seen in the xylem, where water adheres to the hydrophilic cellulose walls and cohesive forces pull subsequent water molecules up. Capillary action also occurs in soil, where water adheres to soil particles and cohesively moves through pore spaces, ensuring water reaches plant roots.
Water's polarity, ability to form hydrogen bonds, solvent properties, and physical characteristics (specific heat capacity, buoyancy, viscosity, thermal conductivity) make it a fundamental component of biological systems and the natural environment. Cohesion and adhesion are essential for processes like surface tension and capillary action in plants and soil.