Biology: Bonding, Water, Hydrophilicity, Solutions, and pH (Study Notes)
Covalent vs Ionic bonds and bond representations
In diagrams, a solid line between atoms typically represents a covalent bond; dotted lines between atoms indicate a different type of interaction or bond (not covalent).
Water is a covalent compound, specifically a polar covalent bond. Polar means unequal sharing of electrons.
Oxygen has atomic number 8. For oxygen:
Total electrons: 8
First shell (orbital) holds 2 electrons (filled).
Second shell starts with 6 electrons remaining.
Oxygen tends to want 2 electrons in its outer shell to be stable.
Hydrogen atoms each have 1 electron; in water, they share electrons with oxygen, forming covalent bonds.
Is the sharing equal? In water, sharing is unequal due to oxygen’s higher electronegativity; this causes polarity. The electrons spend more time around oxygen than hydrogen.
This unequal sharing creates electronegativity differences and partial charges on atoms within the molecule.
Water’s polarity leads to a partial negative charge on the oxygen side and partial positive charges on the hydrogen sides; this uneven distribution is central to water’s behavior in biology.
Hydrogen atoms in water: each H has 1 proton and 1 electron; they form covalent bonds by sharing with oxygen.
The electrons bound in the first shell around the oxygen are limited to that region; the remaining electrons occupy higher-energy regions; oxygen carries four nonbonding electrons (two lone pairs).
Because of electronegativity and electron distribution, water is polar and tends to pull electron density toward the oxygen, reinforcing polar character.
Hydrogen bonds and water’s unique properties
Polar covalent bonds within water molecules allow them to hydrogen-bond with other water molecules (dotted lines between molecules).
Hydrogen bonds are weaker than covalent bonds but numerous; collectively they confer important properties to water.
Surface tension arises from hydrogen bonding at the air–water interface, making water droplets cling and form beads; this also explains why water can stick and form a surface film.
When water is heated, hydrogen bonds break, allowing water to boil and produce steam; energy input is required to break these bonds.
Hydrogen bonds are essential for biological structure and function; they help maintain macromolecular structure and enable interactions in cellular environments.
Water’s role in biology and terminology
Water acts as the solvent in most biological reactions; life’s chemistry occurs in aqueous environments.
Hydrophilic (water-loving) substances generally are polar and interact well with water; hydrophilic can be described as polar and water-loving.
Lipophilic (lipid-loving) and lipophobic (lipid-hating) describe substances that prefer nonpolar environments; lipophilic compounds are nonpolar and interact with nonpolar surroundings (lipids) via nonpolar interactions.
Lipophilic = nonpolar; lipophobic = dislikes polar environments (water).
Solutions: solvent and solute
A solution consists of a solvent and solute(s). The solvent for biological systems is often water.
In a solution: solvent dissolves solute(s). Example: caffeine and sugar dissolved in a beverage represent solutes in water (the solvent).
Nonpolar molecules and interactions
O₂ is nonpolar because its electrons are shared equally between the two oxygen atoms.
For O₂:
First shell: 2 electrons shown.
After filling, there are 6 electrons in the second shell; because the molecule is symmetric, sharing is even -> nonpolar.
Nonpolar molecules interact with each other via Van der Waals forces.
Polar molecules interact with other polar molecules via hydrogen bonds or dipole–dipole interactions.
Ionic bonds and dissociation in water
Ionic bonds involve the attraction between oppositely charged ions; it is a donation or acceptance of electrons rather than sharing.
Example: NaCl (sodium chloride) in water dissociates into Na⁺ and Cl⁻ ions.
Dissociation: the crystal lattice breaks apart, and ions become solvated by water molecules, keeping them apart in solution.
In NaCl dissolution:
Sodium becomes Na⁺ (cation).
Chlorine becomes Cl⁻ (anion).
In water, the Na⁺ ions are typically surrounded by oxygen atoms of water molecules, while Cl⁻ ions are typically surrounded by hydrogen atoms of water molecules (hydration).
Ionic bonds exist in the solid form; once dissolved, the ionic lattice is broken, and ions are dispersed in solution.
Acids, bases, and pH (simplified)
Acid (in biology): may donate a proton (H⁺).
Base (in biology): may receive (accept) a proton (H⁺).
pH is a measure of proton (H⁺) concentration; a lower pH means higher [H⁺], a higher pH means lower [H⁺].
pH is defined as:
where brackets denote concentration.A pH scale from 0 to 14 is a simplified reference: lower values indicate acidity, higher values indicate basicity; pH 7 is neutral (in pure water at standard conditions).
The acidity or basicity of a solution reflects proton availability; more protons means more acidic; fewer protons means more basic (as a base has accepted protons).
The exact pH is a logarithmic measure; each unit change represents a tenfold change in proton concentration.
Practical note: when discussing acidity, avoid vague terms like “move up the scale” without specifying direction; simply say "more acidic" or "more basic" and specify the direction in terms of proton concentration.
Real-world example: stomach acid can be very strong with a low pH (around pH = 1).
Extremes: the transcript mentions very strong acids and bases beyond the typical 0–14 range (e.g., references to pH values like -33.1 on some theoretical scales). These are not typical classroom values but illustrate that extremely strong acids/bases exist in theory; biological systems operate within the practical 0–14 range.
Hydration and pH interplay with macromolecular structure: acidity/basicity can affect the structure and behavior of biomolecules.
Oxidation-reduction (redox) basics
Oxidation-reduction concepts are introduced via: "oil rig" and the memory aid LEO the lion says GER (loss of electrons is oxidation; gain of electrons is reduction).
Oxidation: loss of electrons (or loss of hydrogen).
Reduction: gain of electrons (or gain of hydrogen).
In biology, these concepts help describe how electrons and protons are transferred during metabolic processes; for the course, the simplified statements are:
Oxidation = loss of electrons/H; reduction = gain of electrons/H.
Note: H⁺ is a proton and plays a central role in acid-base chemistry and redox-related processes in biology.
Reactions and arrows in chemistry notation
A chemical reaction is often shown with an arrow indicating the direction of the reaction.
A single-headed arrow (→) implies the reaction proceeds in one direction (though some reactions can be effectively one-way under certain conditions).
A double-headed arrow (⇌) indicates a reversible reaction at equilibrium.
Reactants on the left may become products on the right, and products can revert to reactants under appropriate conditions, maintaining dynamic balance.
Quick cross-links and practical implications
Polarity and hydrogen bonding influence molecular interactions in cells and tissues; water’s properties support the chemistry of life, including solvent effects, macromolecule stabilization, and reaction environments.
The concept of hydrophilic vs lipophilic helps predict solubility and localization of molecules in aqueous and lipid environments, guiding understanding of membrane biology and transport.
Ionic compounds and their hydration in water explain how salts influence osmolarity, ion gradients, and enzyme activities in biological systems.
Acid-base chemistry underpins pH regulation, enzyme activity, and metabolism; understanding proton transfer helps explain many biological processes.
The presence of hydrogen bonds and water networks is essential for the structure and function of biomolecules (e.g., DNA, proteins) and for processes like protein folding, enzyme catalysis, and molecular recognition.
Summary of key terms to remember
Covalent bond: sharing of electrons between atoms (often drawn as lines).
Polar covalent bond: unequal sharing of electrons due to electronegativity differences; creates partial charges.
Nonpolar covalent bond: equal sharing of electrons; typically leads to nonpolar molecules.
Electronegativity: tendency of an atom to attract electrons in a bond; drives polarity.
Hydrogen bond: a weak interaction between a hydrogen atom bonded to a highly electronegative atom (like O) and another electronegative atom in a nearby molecule; important for water’s properties and biomolecular structure.
Hydrophilic: water-loving; typically polar.
Lipophilic: lipid-loving; typically nonpolar.
Lipophobic: lipid-hating; typically polar.
Solvent vs solute: the solvent (often water in biology) dissolves solutes.
Ionic bond: electrical attraction between oppositely charged ions; dissociates in water.
Cation: positively charged ion (e.g., Na⁺).
Anion: negatively charged ion (e.g., Cl⁻).
pH: negative logarithm of hydrogen ion concentration; measures acidity/basicity.
[H⁺]: concentration of hydrogen ions; bracket notation denotes molar concentration.
Oxidation: loss of electrons (or H).
Reduction: gain of electrons (or H).
Reversible reaction: indicated by a two-headed arrow; equilibrium can be established.
Notable illustrative demonstrations mentioned
Water’s surface tension demonstrated with two glass slides kept together by hydrogen bonding; heating or introducing air disrupts surface tension and allows separation.
Syrup example illustrating how water can help remove sticky substances via hydrogen bonding networks.
Conceptual demonstrations show how hydrogen bonds, although individually weak, collectively create strong, life-sustaining properties.
Connections to foundational biology and real-world relevance
Water’s unique properties are foundational to cellular chemistry, reaction environments, and macromolecular structures.
The polarity of water influences solvent behavior, molecular interactions, and the location and movement of ions in biological systems.
Understanding polar vs nonpolar interactions informs expectations about solubility, membrane behavior, and biochemical pathways.
Acid-base balance and pH regulation are central to metabolic processes, enzyme function, and homeostasis in organisms.
Redox chemistry underlies energy transduction in metabolism, signaling, and maintenance of cellular redox states.
Covalent vs Ionic bonds and bond representations
A solid line represents a covalent bond; dotted lines indicate other interactions.
Water () is a polar covalent compound; oxygen unequally shares electrons due to higher electronegativity, creating partial charges.
Oxygen's outer shell needs 2 electrons to be stable; hydrogen atoms share 1 electron each.
Water's polarity results in a partial negative charge on oxygen and partial positive charges on hydrogens.
Hydrogen bonds and water’s unique properties
Polar water molecules form hydrogen bonds with each other (weaker than covalent bonds).
Numerous hydrogen bonds give water unique properties like surface tension and high boiling point.
They are crucial for biological structures and functions.
Water’s role in biology and terminology
Water is the primary solvent in biological reactions.
Hydrophilic substances are polar and