Water is liquid at most Earth temperatures, which is crucial because most cellular metabolic reactions occur with dissolved substances in water-based solutions.
Molecular formula: H2O. Water consists of one oxygen atom covalently bonded to two hydrogen atoms.
Covalent bonds and polarity:
The O–H covalent bond is formed by sharing electrons.
Electrons are shared unequally, pulled slightly toward the oxygen atom, creating partial charges: δ− on oxygen and δ+ on hydrogens.
This unequal sharing and the resulting dipole make water a polar molecule.
Molecular drawing conventions:
A solid line denotes a covalent bond within a molecule.
A dashed or dotted line denotes a hydrogen bond between two different water molecules.
Hydrogen bonds:
A hydrogen bond is an attraction between the partially negative end (on oxygen) of one molecule and the partially positive end (on hydrogen) of a neighboring molecule.
These bonds are not as strong as covalent bonds, but collectively they give water many unique properties.
Hydrogen Bonds and Cohesion
Cohesion:
Definition: an attraction between molecules of the same substance, i.e., water–water interactions.
Origin: hydrogen bonding between adjacent water molecules.
Significance of cohesion:
Enables transpiration in plants via a continuous water column in the xylem.
Evaporation from stomata in leaves creates a pulling force that transmits up the water column through cohesive forces.
Helps create surface phenomena where water can support organisms or form a stable surface film.
Transpiration and the water column:
Water moves up through the plant xylem due to cohesive tension created by evaporation at stomata.
A continuous chain of hydrogen-bonded water molecules is required; any break in the column stops tension transmission.
Surface interactions:
Cohesion allows certain organisms to rest on the water surface without breaking into the water, by maintaining a strong surface tension.
Example scenario: an insect standing on the water surface relies on cohesive forces to keep the surface intact under its weight.
Adhesion and Capillary Action
Adhesion:
Definition: the attraction between water molecules and a solid surface.
In capillary tubes, water adheres to the glass walls, helping to pull water upward against gravity.
Capillary action:
Result of adhesion between water and solid surfaces and the cohesive forces between water molecules.
Visual cue: the meniscus, a curved surface in a narrow tube, caused by the balance of cohesive and adhesive forces.
Relevance to plants:
Adhesion to the sides of xylem vessels aids in lifting water, complementing cohesion to move water from soil to aerial parts.
Water as a Solvent
Key vocabulary:
Solute: the substance being dissolved.
Solvent: the liquid that dissolves the solute.
Solution: the homogeneous mixture formed.
Water’s solvent quality:
Water is a very good solvent, but not universal.
It dissolves hydrophilic substances (water-loving) and ions, which are typically polar or charged.
Hydrophilic vs hydrophobic:
Hydrophilic (water-loving): polar molecules or ions; examples include glucose or dissolved ions.
Hydrophobic (water-hating): nonpolar substances with low solubility in water (e.g., lipids, oxygen molecules).
Why water dissolves many substances:
Water’s polarity allows it to stabilize charged or polar solutes via hydration shells.
Metabolism and solubility:
Metabolic reactions rely on reactants, nutrients, and waste products being in solution.
Water’s polar nature makes it a suitable transport medium in biology.
Transport implications:
Plants: water transports dissolved minerals in the xylem; carbohydrates in the phloem must be in solution.
Animals: blood plasma is mainly water and serves as the primary solvent for ions, glucose, and amino acids (hydrophilic solutes).
Lipids and cholesterol are not highly soluble in water and require transport by proteins or carriers.
Hydrophilic vs solubility in blood:
Hydrophilic solutes are soluble and found in plasma.
Hydrophobic molecules require association with proteins or cell components for transport.
Buoyancy, Density, and Adaptations
Buoyancy basics:
Buoyancy is the upward force exerted by a fluid opposing gravity.
Objects less dense than water tend to float; more dense objects sink.
Water’s buoyancy helps maintain an object’s position in the water with minimal energy expenditure.
Density balance in aquatic life:
Most aquatic organisms have densities similar to water to maintain stable positions in the water column.
Buoyancy adaptations:
Swim bladder: a gas-filled organ in fish that can be filled to reduce density (float) or deflated to increase density (sink).
This adjustment allows control over vertical position without expending energy through swimming.
Viscosity and Its Biological Implications
Viscosity:
Definition: a liquid’s resistance to flow.
In water, viscosity is a factor in how easily organisms can move and how solutes move during circulation.
Effects of dissolved solutes:
Higher solute concentration increases viscosity, potentially impeding flow in fluids like blood.
There are practical limits to solubility before flow becomes too resistant.
Thermal Conductivity, Heat Capacity, and Temperature Regulation
Thermal conductivity:
Water is a good conductor of heat, allowing heat to transfer efficiently within water and between water and surroundings.
Pros and cons depend on context (e.g., metabolic heat distribution vs. rapid heat loss in cold water).
Real-world implications:
In aquatic environments, high heat transfer can protect or threaten organisms depending on circumstances.
Organisms need adaptations to utilize or mitigate heat exchange with their environment.
Specific heat capacity:
Water has a high specific heat capacity, meaning it can absorb a lot of heat before a large change in temperature occurs.
This yields a moderating effect, stabilizing temperatures of bodies of water and the internal environment of organisms.
Practical consequence: water buffers temperature fluctuations in air and within organisms, aiding homeostasis.
Moderating effect in nature:
Large bodies of water maintain relatively stable temperatures despite air temperature swings, benefiting aquatic ecosystems.
Within organisms, water helps moderate cellular and tissue temperatures during metabolic activity.
Practical and ecological implications:
Aquatic environments experience slower temperature changes than air, supporting stable habitats and biological processes.
Real-world examples:
African elephant example: large ears reduce overheating by exposing blood to cooler outside air, leveraging heat exchange with air rather than relying solely on water’s properties.
Real-World Examples Illustrating Water Properties
Ring seal (sea mammal):
Lives mostly in water; uses buoyancy to float and conserve energy.
Additionally, heavy blubber minimizes heat loss to the surrounding water, leveraging water’s high thermal conductivity to exchange heat efficiently while keeping internal temperatures stable.
Arctic loon (illustrative, live in air):
Arctic loon evolves to live in air; air is less buoyant than water, requiring more energy to stay aloft.
Air is less viscous than water, reducing energy needed to move through it, and is a poorer thermal conductor, resulting in fewer heat losses via surrounding medium.
Implications of adaptations:
Organisms tailor their body structures (e.g., blubber, ears, lungs, or air sacs) to balance buoyancy, viscosity, and heat exchange depending on whether they inhabit air or water.
Connecting to broader themes:
Physical properties of water (cohesion, adhesion, solvent capabilities, buoyancy, viscosity, thermal properties) drive a wide range of biological structures and ecological strategies.
Foundational connections:
The described properties relate to central chemistry concepts (polarity, hydrogen bonding) and physics (density, buoyancy, viscosity, heat transfer).
Practical implications:
Understanding water’s properties informs agriculture (transpiration, capillary action in xylem), medicine (blood plasma as solvent), and environmental science (aquatic climate buffering).
Key Terminology Recap
Solute, solvent, solution: components of a mixture; water as solvent.
Hydrophilic vs hydrophobic: affinity for water vs aversion to water.
Polarity: separation of electric charge within a molecule, enabling dipole interactions.
Cohesion: water–water attraction due to hydrogen bonding.
Adhesion: water–surface attraction (to walls of xylem, capillary tubes).
Capillary action: rise of liquid in narrow tubes due to cohesion and adhesion.
Buoyancy: upward force exerted by a fluid on an immersed object.
Density: mass per unit volume; determines whether an object sinks or floats.
Viscosity: resistance to flow; affects movement of organisms and transport fluids.
Thermal conductivity: rate at which heat is transferred through a material.
Specific heat capacity: amount of heat required to raise the temperature of a substance by one degree.
Moderating effect: stabilizing influence of large bodies of water on temperature.
Connections to Foundational Principles and Real-World Relevance
Chemistry foundations:
Polarity and hydrogen bonding underpin water’s solvent properties and many biological interactions.
The structure of water explains macroscopic phenomena like cohesion, adhesion, and surface tension.
Biology and physiology:
Water as the primary solvent enables solutes to participate in metabolic reactions.
Transport systems (blood plasma in animals, xylem and phloem in plants) rely on water’s solvent properties.
Buoyancy and density influence habitat choice and animal adaptations (swim bladders, energy-efficient floatation).
Temperature regulation through high specific heat capacity supports homeostasis across organisms.
Ecology and environment:
Water’s moderating effect stabilizes aquatic ecosystems and climate by buffering temperature changes.
Practical implications:
Agricultural practices depend on understanding transpiration and capillary action to manage water delivery to crops.
Biomedical and veterinary sciences rely on the solvent properties of water for nutrient transport and metabolic reactions.
Ethical and practical considerations:
Ensuring clean, accessible water supports health, agriculture, and biodiversity.
Understanding water properties informs environmental stewardship and climate resilience strategies.