Water’s polarity and its ability to form hydrogen bonds give it unique characteristics that are essential for life and support key biological functions.
Water as a Polar Molecule
Molecular Composition of Water
Water, with the chemical formula H2O, is a simple yet vital molecule composed of two hydrogen atoms covalently bonded to a single oxygen atom. Despite its simplicity, the geometry of water makes it a profoundly important compound in biological systems. Due to the bent shape of the water molecule, it exhibits a significant unequal distribution of electrons, which leads to the formation of partial charges.
Electronegativity and Polarity
The oxygen atom in water has a higher electronegativity than hydrogen, meaning it pulls the shared electrons in the covalent bonds closer to itself. As a result:
The oxygen atom becomes partially negative (δ⁻)
Each hydrogen atom becomes partially positive (δ⁺)
This separation of charge makes water a polar molecule, where one end is more negative, and the other more positive. This polarity is essential for many of the molecule's interactions with other substances and is responsible for its ability to form hydrogen bonds.
Dipole Moment
A dipole moment refers to the separation of charge in a molecule, leading to positive and negative poles. In water, the dipole moment enhances its interactions with other polar substances and its role as a solvent in biological systems.
Hydrogen Bonding
Nature of Hydrogen Bonds
Hydrogen bonds are weak interactions that occur between a hydrogen atom bonded to an electronegative atom (such as oxygen or nitrogen) and another nearby electronegative atom. These bonds are intermolecular forces, not true chemical bonds like covalent or ionic bonds, but they are strong enough to have significant effects on molecular behavior and structure.
Though weaker than covalent bonds, hydrogen bonds are crucial in biological systems:
They help maintain the three-dimensional structure of proteins
They stabilize the double helix structure of DNA
They give water its unique physical properties
Hydrogen Bonding Between Water Molecules
Each water molecule can form up to four hydrogen bonds with neighboring water molecules:
Two from the hydrogen atoms acting as donors
Two from lone pairs on the oxygen atom acting as acceptors
These transient bonds are constantly forming and breaking, giving water its liquid nature, but also resulting in strong cohesive properties.
Hydrophilic and Hydrophobic Interactions
Hydrophilic Substances
Hydrophilic means “water-loving.” These substances interact easily with water due to their charge or polarity. Examples include:
Salts like NaCl
Sugars like glucose
Proteins with polar side chains
Water surrounds these molecules and dissolves them, making water an excellent medium for biological reactions.
Hydrophobic Substances
Hydrophobic means “water-fearing.” These are nonpolar substances that do not dissolve in or interact favorably with water. Examples include:
Fats and oils
Lipid membranes
In water, hydrophobic substances tend to aggregate together, minimizing their surface area exposed to water. This phenomenon is critical in membrane formation and protein folding.
Oil and Water: A Common Analogy
Trying to mix oil and water demonstrates how polarity affects solubility. Water molecules are polar and attract each other via hydrogen bonds, whereas oil molecules are nonpolar and cannot form hydrogen bonds. As a result, the two separate into distinct layers. This principle is vital in understanding cellular environments and membrane dynamics.
Image courtesy of ABCteach.
Cohesion and Adhesion
Cohesion: Molecules Sticking Together
Cohesion refers to the attraction between molecules of the same kind, such as water molecules. This is due to the hydrogen bonds that link them together. Cohesion leads to:
Droplet formation
High surface tension
Water movement in plants
Adhesion: Attraction to Different Substances
Adhesion is the tendency of water to stick to other substances, particularly those that are polar or charged. It allows water to:
Cling to cell walls in plants
Form thin films on surfaces
Both cohesion and adhesion are essential in capillary action, which is the ability of water to move upward through narrow tubes, such as xylem vessels in plants.
Capillary Action in Plants
Through the combination of cohesion (molecules pulling each other up) and adhesion (water sticking to the vessel walls), water can move against gravity from the roots to the leaves in tall plants. This is vital for transporting nutrients and maintaining turgor pressure in plant cells.
Surface Tension
Surface Tension Explained
Surface tension is the resistance of a liquid’s surface to external force. In water, it arises because molecules at the surface experience stronger cohesive forces with neighboring molecules. Since there's no water above the surface, molecules are pulled inward, creating a tense film on the surface.
Examples in Nature
Water striders, insects that can walk on water, exploit surface tension to remain afloat
Raindrops form spherical shapes due to surface tension minimizing surface area
Surface tension also contributes to the formation of bubbles, capillary action, and droplet behavior.
High Specific Heat
Definition of Specific Heat
Specific heat is defined as the amount of energy required to raise the temperature of 1 gram of a substance by 1 degree Celsius. Water has a very high specific heat due to the energy needed to disrupt hydrogen bonds between molecules.
Biological Implications
Regulates climate by stabilizing temperatures in oceans and lakes
Helps organisms maintain homeostasis by resisting rapid temperature changes
Stabilizes internal body temperature
Example
The ocean absorbs heat during the day and slowly releases it at night, maintaining a relatively stable environment for marine life.
Heat of Vaporization and Evaporative Cooling
Heat of Vaporization
The heat of vaporization is the amount of energy required to convert 1 gram of liquid to vapor without a temperature change. Water’s high heat of vaporization means it takes a large amount of energy to evaporate.
Evaporative Cooling
When water evaporates, it removes heat energy from the surface it leaves. This is known as evaporative cooling, which helps:
Cool organisms through sweating or panting
Regulate temperature in plants through transpiration
Examples
Humans sweat to cool down during exercise or hot weather
Dogs pant to evaporate water from their tongue and respiratory tract
Plants lose water vapor through stomata, which cools the leaf surface
Water as a Universal Solvent
What Makes Water a Good Solvent?
Water is often referred to as the "universal solvent" because it can dissolve a wide variety of substances. This is due to:
Its polarity, which allows water to interact with ions and polar molecules
The ability to form hydration shells around dissolved particles
How Solvation Works
When a salt like NaCl is added to water:
The Na+ ions are surrounded by the partially negative oxygen atoms of water
The Cl− ions are surrounded by the partially positive hydrogen atoms
This process keeps the ions in solution, allowing for transport and reactions in cells.
Biological Importance
Enables transport of nutrients and waste in blood and cytoplasm
Supports enzymatic reactions in aqueous environments
Allows cells to absorb and distribute dissolved substances
Dissociation of Water and pH
Autoionization of Water
Water molecules can undergo a process called autoionization:
H2O + H2O ⇌ H3O+ + OH−
This means that water can simultaneously act as both an acid and a base, a process that maintains the balance of H+ and OH− ions in solution.
Acids and Bases
An acid increases H+ (or H3O+) concentration in a solution
Example: Hydrochloric acid → HCl → H+ + Cl−
A base decreases H+ concentration, often by releasing OH−
Example: Sodium hydroxide → NaOH → Na+ + OH−
The pH Scale
pH stands for “power of hydrogen”
pH is calculated as:
pH = −log[H+]
The pH scale ranges from 0 to 14:
pH < 7: Acidic
pH = 7: Neutral
pH > 7: Basic (alkaline)
Biological pH Range
Most biological fluids have a pH between 6 and 8, with some exceptions (e.g., stomach acid). Enzyme function and cellular processes are sensitive to pH changes, making this balance crucial for life.
Molecular Behavior in Aqueous Solutions
Solvation Shells and Ion Stability
When polar or charged molecules enter water, water forms solvation shells (or hydration shells) around them:
Prevents re-association of ions
Keeps molecules evenly dispersed in solution
This supports the efficiency of reactions in living organisms, as reactants must be in the same phase and accessible to enzymes.
Dynamic Equilibrium
In any given sample of pure water, a small proportion of molecules dissociate into H+ and OH−. Despite this, the overall concentration remains very low, and the product of [H+] and [OH−] is always:
[H+][OH−] = 1 x 10⁻¹⁴ at 25°C
This equilibrium supports the neutral pH of 7.
Biological Relevance of Water's Properties
Transport in Plants
Water’s cohesive and adhesive forces enable the upward transport of water from the roots to the leaves via capillary action and transpiration. This system allows for:
Delivery of nutrients
Cooling of leaf surfaces
Maintenance of cell turgor
Thermoregulation in Organisms
Water buffers temperature changes, helping organisms stay in balance. For example:
Blood distributes heat throughout the body
Sweat evaporates to remove excess heat
Cellular water absorbs and releases heat to prevent internal damage
Water in Cellular Structure
Water forms the majority of the cytoplasm, facilitates diffusion, supports organelle function, and enables enzyme-substrate interactions. Dehydration disrupts these processes, underlining water’s central role in life.
FAQ
Hydrogen bonds in water are relatively weak compared to covalent bonds, with each bond lasting only a few picoseconds in liquid form. This constant breaking and reforming allows water molecules to slide past one another, giving water its fluidity. These dynamic interactions are critical for maintaining the liquid state at a wide range of temperatures.
In liquid water, hydrogen bonds are transient, allowing mobility of molecules.
In ice, bonds are more stable and organized, forming a crystalline lattice.
As temperature increases, more bonds break, increasing molecular motion and leading to evaporation.
This dynamic behavior also contributes to water’s high surface tension and viscosity.
Water is one of the few substances where the solid form (ice) is less dense than the liquid. This is due to the unique arrangement of hydrogen bonds when water freezes. As temperature drops, water molecules slow down and form a hexagonal lattice structure that spaces the molecules farther apart than in liquid form.
Hydrogen bonds in ice hold molecules in fixed positions with maximum separation.
In liquid water, molecules are closer together due to constant motion and bond flexibility.
This structural arrangement allows ice to float, providing insulation in aquatic environments.
The lower density of ice is critical for aquatic life during freezing temperatures.
Water’s high heat of fusion means it requires a significant amount of energy to change from solid to liquid without a change in temperature. This property helps stabilize environmental and cellular temperatures when ice melts, acting as a buffer against sudden thermal changes.
Absorbs energy without a rapid rise in temperature during melting.
Protects cells and tissues from thermal damage during seasonal transitions.
Helps regulate climate and delay ice melting in polar ecosystems.
Prevents large-scale freezing of aquatic habitats, allowing life to persist under ice layers.
Although water transport in animals differs from plants, cohesion and adhesion still play vital roles in blood and lymph circulation. Water’s cohesive forces help maintain the continuous flow of fluid, while adhesion supports interaction with vessel walls, reducing turbulence and supporting capillary action in microvessels.
Cohesion maintains integrity of blood as it flows through vessels.
Adhesion assists in capillary refill and blood interaction with vessel linings.
These forces reduce cavitation and ensure smooth blood movement.
Important in microcirculation, especially in tissues with narrow capillaries like the lungs and kidneys.
Water is amphoteric, meaning it can act as both an acid (proton donor) and a base (proton acceptor). This dual role enables water to participate in a wide range of biochemical reactions, particularly those involving acid-base chemistry, which are common in metabolism and enzyme activity.
Water self-ionizes to form H3O+ and OH− ions: H2O + H2O ⇌ H3O+ + OH−
Acts as a buffer by neutralizing excess acids or bases in biological fluids.
Supports enzymatic function by maintaining proper pH conditions.
Crucial in hydrolysis reactions, where water breaks down large molecules like proteins and nucleic acids.
Practice Questions
Explain how the structure and polarity of water molecules contribute to two of its emergent properties that are essential for life.
Water’s structure is bent, with oxygen being more electronegative than hydrogen, creating a polar molecule with partial charges. This polarity enables hydrogen bonding between water molecules. Cohesion, one emergent property, results from hydrogen bonds holding water molecules together, allowing for transport in plant xylem. Another is water’s high specific heat, which stabilizes temperature by absorbing heat with minimal temperature change. This property helps organisms maintain homeostasis and supports stable aquatic environments. These features arise directly from water’s ability to form hydrogen bonds due to its polar structure and are essential for biological function and environmental stability.
Describe how water’s role as a solvent supports molecular interactions in biological systems. Include a specific example.
Water is a polar molecule, with partially charged regions that allow it to surround and separate other polar or charged substances. As a result, it acts as an excellent solvent by forming hydration shells around ions and polar molecules, keeping them suspended in solution. This enables crucial biochemical reactions by facilitating molecular collisions and transport. For example, in blood plasma, water dissolves electrolytes like sodium and chloride ions, allowing nerve signaling and muscle contraction. Without water’s solvent properties, essential molecules could not move freely, interact properly, or participate in metabolism within cells and bodily fluids.
