Summary
Highlights
Water is fundamental to life, with its first cell forming in water. Water molecules consist of one oxygen and two hydrogen atoms. Their asymmetrical structure and polar covalent bonds lead to an unequal distribution of charges, making water molecules polar. This polarity is crucial because opposite charges attract, leading to the formation of hydrogen bonds between water molecules.
Cohesion is the property of water molecules attracting and sticking to each other, primarily due to hydrogen bonds. This is vital for biological processes like water transport in plants through xylem, where transpiration pulls water up against gravity. Surface tension, enabled by cohesion, allows insects like water striders to walk on water. Adhesion is the ability of water molecules to stick to other surfaces, also due to polarity. This is seen in capillary action in soil, allowing water to move through porous spaces, and in plant xylem, where water adheres to the tube walls, aiding upward movement.
Water's solvent property stems from its polarity, allowing it to dissolve hydrophilic (polar) substances by forming hydrogen bonds. When salt (sodium chloride) dissolves, water molecules surround and separate the charged ions. This makes water a 'universal solvent,' crucial for metabolic reactions and transport, like blood plasma carrying dissolved substances. However, hydrophobic (nonpolar) molecules, like phospholipids in cell membranes, do not dissolve in water, which is essential for maintaining cellular structures.
Water has a high specific heat, meaning it requires significant energy to raise its temperature. This property helps organisms maintain stable internal temperatures. Water also exhibits high thermal conductivity, effectively transferring heat. This is exemplified by the fennec fox using its large, blood-vessel-rich ears to dissipate heat, with the water-based blood facilitating this transfer to the environment.
Buoyancy refers to an object's ability to float, determined by its density relative to water. Organisms, being largely composed of water, have densities close to water, which aids buoyancy. For example, green algae use oxygen produced during photosynthesis and gas vacuoles to increase buoyancy and position themselves for optimal sunlight. Viscosity describes a substance's resistance to flow. Water has a relatively low viscosity, but it increases with added solutes. This is important for aquatic organisms and the flow of bodily fluids like blood plasma.
The physical properties of water have varying impacts on different organisms. Comparing a black-throated loon (bird) and a ringed seal (mammal), the seal, spending more time in water, is more affected by water's viscosity and thermal conductivity. Moving through water requires more energy than flying through air, and the seal loses more heat to the water due to its higher thermal conductivity. Adaptations like a thick fur coat help the seal retain body heat in cold water.