Source: Practice Exam Midterm #1, Codon Learning
Tags: covalent bonds, hydrogen bonds, ionic bonds, hydrophobic, hydrophilic, amphipathic, polar, nonpolar, partial charges, ions, acids, bases, protons, pH, water chemistry
Difficulty: Foundational Prerequisites: Basic atomic structure (protons, neutrons, electrons, electronegativity).
This material underpins everything else in the course. Every macromolecule you will study, from proteins to DNA, behaves the way it does because of the bonds and interactions covered here. If you do not understand why some bonds break easily and others do not, or why certain molecules dissolve in water while others cluster away from it, the later units on proteins, nucleic acids, and lipids will feel arbitrary. These concepts also appear repeatedly in application questions, so expect the exam to test whether you can reason with them, not just recite definitions.
Covalent bonds are strong and stable; hydrogen bonds are weak and constantly breaking and reforming. Whether a molecule is polar or nonpolar determines how it interacts with water. Acids release protons, bases accept them, and ions are atoms or molecules carrying a charge.
Covalent bond
A strong chemical bond formed when two atoms share one or more pairs of electrons. These bonds hold atoms together within molecules and do not break and reform under normal biological conditions.
In simple terms, this is the "permanent scaffolding" inside a molecule.
Hydrogen bond
A weak interaction that forms between a hydrogen atom bonded to an electronegative atom (like O or N) and another electronegative atom on a nearby molecule. These bonds constantly break and reform as molecules move in solution.
Think of it as a temporary handshake between molecules, not a permanent attachment.
Polar covalent bond
A covalent bond in which electrons are shared unequally because one atom is more electronegative than the other, creating regions of partial positive and partial negative charge.
In simple terms, one end of the bond is slightly negative and the other slightly positive.
Nonpolar covalent bond
A covalent bond in which electrons are shared roughly equally, producing no significant partial charges. C–C and C–H bonds are the classic examples.
Think of it as an even tug-of-war where neither side wins.
Hydrophobic
Describes a molecule (or region of a molecule) that does not interact favourably with water, typically because it contains mostly nonpolar covalent bonds such as C–C and C–H.
In simple terms, "water-fearing." These molecules are pushed away from water rather than dissolving in it.
Hydrophilic
Describes a molecule (or region of a molecule) that interacts favourably with water, usually because it carries partial charges or full charges that can form hydrogen bonds with water.
Think of it as "water-loving." These molecules dissolve readily.
Amphipathic
A molecule that contains both a hydrophilic region and a hydrophobic region. Phospholipids are the textbook example.
In simple terms, one end likes water and the other avoids it, which is why these molecules form membranes.
Ion
An atom or molecule that carries a net electrical charge because it has gained or lost one or more electrons.
Think of it as an atom that is no longer electrically neutral.
Acid
An ion or molecule that releases (donates) a proton (H⁺) into solution, increasing the proton concentration.
In simple terms, acids are proton donors.
Base
An ion or molecule that accepts (removes) a proton from solution, decreasing the proton concentration.
In simple terms, bases are proton acceptors.
Covalent bonds are strong and stable under normal cellular conditions. They hold atoms together within a single molecule.
Hydrogen bonds are much weaker. In solution, they are constantly being broken and reformed as molecules move around.
When a researcher observes some interactions breaking and reforming while others remain fixed, the explanation is that the transient ones are hydrogen bonds and the stable ones are covalent bonds.
A hydrogen bond forms between the hydrogen atom of one water molecule and the oxygen atom of another.
This happens because oxygen is highly electronegative, making the O–H bond polar: oxygen carries a partial negative charge and hydrogen carries a partial positive charge.
The partial positive hydrogen is attracted to the partial negative oxygen on a neighbouring molecule.
To dissolve in water, a molecule needs regions with partial charges (polar bonds, charged groups). These allow it to form hydrogen bonds or electrostatic interactions with water.
Molecules dominated by nonpolar bonds (C–C, C–H) are hydrophobic and do not dissolve.
Molecules with O–H, N–H, or full charges are hydrophilic.
Lipid molecules are dominated by C–C and C–H bonds, which are nonpolar. This is what makes lipids hydrophobic.
A molecule with both hydrophilic and hydrophobic regions is called amphipathic.
An acid releases a proton into solution, raising the H⁺ concentration.
A base accepts a proton from solution, lowering the H⁺ concentration.
If a substance accepts a proton from the surrounding environment, it is acting as a base, and the proton concentration in solution decreases.
Students often think hydrogen bonds are a type of covalent bond. They are not. They are weak, non-covalent interactions.
Students confuse "hydrophobic" with "repelled by water." Hydrophobic molecules are not actively repelled; they simply lack the partial charges needed to interact with water, so water molecules preferentially interact with each other instead.
Students sometimes think acids have a high pH. Acids release protons and lower pH; bases accept protons and raise pH.
Students mix up ions and isotopes. Ions differ in electron count (and therefore charge). Isotopes differ in neutron count (and therefore mass).
⚠️ The distinction between covalent and hydrogen bonds (strong/stable vs. weak/transient) is tested directly and through application scenarios.
⚠️ Knowing which bond types make a molecule hydrophobic (C–C, C–H) vs. hydrophilic (O–H, N–H, charged groups) is essential for lipid and membrane questions.
⚠️ Acid = proton donor, base = proton acceptor. Expect a scenario question where you must predict what happens to proton concentration after a substance acts on the solution.
⚠️ The hydrogen bond between two water molecules forms specifically between H on one molecule and O on the other, not O–O or H–H.
True or false: Hydrogen bonds between water molecules are permanent once formed.
Fill in the blank: Molecules that contain mostly ______ covalent bonds are hydrophobic.
True or false: A base increases the proton concentration in solution.
Fill in the blank: An atom or molecule carrying a net charge is called an ______.
True or false: C–C and C–H bonds are polar.
Answers: 1. False (they constantly break and reform). 2. Nonpolar. 3. False (a base decreases it). 4. Ion. 5. False (they are nonpolar).
Q: A researcher observes that some molecular interactions in solution are constantly breaking and reforming while others remain stable. What explains this?
A: Hydrogen bonds are weak and constantly break and reform under normal conditions, while covalent bonds are strong and remain stable.
Q: Between which atoms does a hydrogen bond form when two water molecules interact?
A: Between a hydrogen atom on one molecule and the oxygen atom on another molecule.
Q: You need to design a molecule that dissolves easily in the watery environment of a cell. What structural feature should you include?
A: Regions with partial charges (polar bonds or charged groups), which allow the molecule to interact with water.
Q: What type of bonds dominate lipid molecules and make them hydrophobic?
A: C–C and C–H bonds (nonpolar covalent bonds).
Q: A substance in solution accepts a proton from the surrounding environment. What happens to the proton concentration?
A: It decreases, because a base removed a proton from solution.
Q: What is the difference between an ion and an isotope?
A: An ion has a net electrical charge (gained or lost electrons). An isotope has a different number of neutrons (different mass, same charge).
This material connects directly to protein folding: hydrogen bonds stabilise secondary structures (alpha-helices and beta-sheets), while hydrophobic interactions drive tertiary folding. It also connects to membrane biology, where the amphipathic nature of phospholipids depends on having both polar and nonpolar regions. Understanding acids and bases is essential for buffer systems and enzyme activity, which appear in later units.
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