2.1 (R) - Weak interactions in Aqueous Systems

Weak interactions

Noncovalent forces that are individually weak but collectively stabilize biological structures.

What are the four major weak interactions in biology?

Hydrogen bonds, ionic interactions, hydrophobic interactions, and van der Waals forces.

Why are weak interactions important in biology?

They determine protein folding, DNA structure, membrane formation, and enzyme-substrate binding.

Why is water considered a polar molecule?

Oxygen is more electronegative than hydrogen, creating partial negative charge on oxygen and partial positive charges on hydrogens.

What are the partial charges in water?

Oxygen carries δ− and hydrogens carry δ+.

What is the bond angle of water?

Approximately 104.5 degrees.

How many hydrogen bonds can one water molecule form?

Up to four hydrogen bonds.

What type of intermolecular force primarily holds water molecules together?

Hydrogen bonds.

How strong is a typical hydrogen bond compared to a covalent bond?

Hydrogen bonds are much weaker (~23 kJ/mol) than covalent bonds (~350–470 kJ/mol).

Why does water have a high boiling point compared to similar molecules?

Strong hydrogen bonding between water molecules requires more energy to break.

Why does ice float on water?

Ice forms a rigid hydrogen-bonded lattice that is less dense than liquid water.

What happens to entropy when ice melts into liquid water?

Entropy increases because molecules become more disordered.

What is a hydrogen bond donor?

A hydrogen atom covalently bonded to an electronegative atom such as oxygen or nitrogen.

What is a hydrogen bond acceptor?

An electronegative atom such as oxygen or nitrogen with a lone pair of electrons.

Why do C–H bonds rarely form hydrogen bonds?

Carbon is not electronegative enough to create strong partial charges.

Which biological molecules commonly form hydrogen bonds with water?

Alcohols, sugars, proteins, aldehydes, ketones, and DNA bases.

What makes hydrogen bonds highly specific in biological systems?

Their directionality and optimal strength when atoms align linearly.

Why is water considered a good solvent for ionic compounds?

Its polarity stabilizes charged ions through hydration.

What happens during hydration of ions?

Water molecules surround ions and weaken ionic attractions.

How does water orient around Na⁺ ions?

Oxygen atoms face Na⁺ ions.

How does water orient around Cl⁻ ions?

Hydrogen atoms face Cl⁻ ions.

What is the dielectric constant of water?

Approximately 78.5.

Why does water weaken ionic interactions?

Its high dielectric constant reduces electrostatic attraction between ions.

What happens to entropy when salts dissolve in water?

Entropy increases because ions become more dispersed.

What are hydrophilic molecules?

Polar or charged molecules that dissolve easily in water.

What are hydrophobic molecules?

Nonpolar molecules that do not dissolve well in water.

Why do nonpolar molecules not dissolve in water?

They cannot form hydrogen bonds and disrupt water structure.

What structure does water form around nonpolar molecules?

Ordered cages called clathrate-like structures.

What happens to entropy when water forms cages around hydrophobic molecules?

Entropy decreases.

What is the hydrophobic effect?

Nonpolar molecules cluster together in water to minimize disruption of hydrogen bonding.

Why does the hydrophobic effect occur?

Clustering increases entropy of surrounding water molecules.

What biological structures rely on the hydrophobic effect?

Cell membranes and protein folding.

What are amphipathic molecules?

Molecules containing both hydrophilic and hydrophobic regions.

What is an example of an amphipathic molecule?

Phospholipids.

What structure forms when amphipathic molecules interact with water?

Micelles or lipid bilayers.

What is a micelle?

A spherical structure with hydrophobic tails inside and hydrophilic heads outside.

What drives lipid bilayer formation?

Hydrophobic interactions among lipid tails.

What are van der Waals forces?

Weak attractions caused by temporary dipoles formed from electron movement.

How strong are van der Waals forces compared to hydrogen bonds?

Much weaker (~4 kJ/mol).

When do van der Waals forces become significant?

When many atoms are packed closely together.

What determines van der Waals radius?

The minimum distance atoms can approach without repelling each other.

Why are many weak interactions stronger together than individually?

Their cumulative effect stabilizes large biological structures.

What types of interactions stabilize enzyme-substrate binding?

Hydrogen bonds, ionic interactions, hydrophobic interactions, and van der Waals forces.

What are colligative properties?

Properties that depend on the number of dissolved particles, not their identity.

What are the four colligative properties?

Vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure.

What is osmosis?

Movement of water across a semipermeable membrane from high water concentration to low water concentration.

What is osmotic pressure?

The pressure required to stop water movement across a membrane.

What is the van’t Hoff factor?

The number of particles formed when a solute dissolves.

What is the van’t Hoff factor for NaCl?

2 (Na⁺ and Cl⁻).

What does isotonic mean?

Equal solute concentrations inside and outside a cell.

What does hypertonic mean?

Higher solute concentration outside the cell.

What happens to cells in hypertonic solutions?

Cells lose water and shrink.

What does hypotonic mean?

Lower solute concentration outside the cell.

What happens to cells in hypotonic solutions?

Cells gain water and swell or burst.

What is osmotic lysis?

Bursting of a cell due to excessive water intake.

What creates rigidity in plant cells?

Turgor pressure generated by osmotic water entry.

Why do cells store glycogen instead of free glucose?

To reduce osmotic pressure inside the cell.

What role do tightly bound water molecules play in proteins?

They stabilize structure and assist in catalysis.

Why is the hydrophobic effect considered one of the most important forces in biology?

It drives formation of membranes and folding of proteins.