Chemical Bonds, Water Properties, and Nucleic Acids in Biology

Chemical Bonds in Biological Molecules

Electronegativity and Its Biological Importance

Electronegativity is a fundamental concept in chemistry and biology, describing how strongly an atom attracts electrons in a chemical bond. The relative electronegativities of the most common elements in living organisms—oxygen, nitrogen, carbon, and hydrogen—determine the types of bonds they form and the properties of biological molecules.

• Electronegativity: The tendency of an atom to attract electrons toward itself in a chemical bond.

• Order of Electronegativity (strongest to weakest): O > N > C ≈ H

• Implication: Oxygen is the most electronegative, followed by nitrogen. Carbon and hydrogen have similar, lower electronegativities.

Example: In water (H2O), oxygen attracts electrons more strongly than hydrogen, resulting in a polar molecule.

Types of Chemical Bonds

Atoms in biological molecules are held together by different types of chemical bonds, each with distinct properties and biological implications.

• Nonpolar Covalent Bonds: Electrons are shared equally between atoms of similar electronegativity (e.g., C-H in methane).

• Polar Covalent Bonds: Electrons are shared unequally between atoms of different electronegativity, resulting in partial charges (e.g., O-H in water, N-H in ammonia).

• Hydrogen Bonds: Weak electrical attractions between a partially positive hydrogen atom (bonded to O or N) and a partially negative atom (usually O or N) in another molecule.

Bond Type	Mechanism	Example
Nonpolar Covalent	Equal sharing of electrons	Methane (CH4)
Polar Covalent	Unequal sharing of electrons	Water (H2O), Ammonia (NH3)
Hydrogen Bond	Attraction between partial charges (Hδ+ and O/Nδ−)	Between water molecules

Comparison: Covalent bonds (both polar and nonpolar) are much stronger than hydrogen bonds. However, hydrogen bonds are crucial for the structure and function of biological molecules, such as DNA and proteins.

Properties of Water and Their Biological Implications

Polarity and Hydrogen Bonding

Water is a polar molecule due to the difference in electronegativity between oxygen and hydrogen. This polarity allows water molecules to form hydrogen bonds with each other and with other polar substances.

• Cohesion: Water molecules stick to each other via hydrogen bonds, leading to surface tension.

• Adhesion: Water molecules can also stick to other polar or charged surfaces.

• Solvent Properties: Water dissolves many ionic and polar substances, making it an excellent solvent for biological reactions.

Example: Salt (NaCl) dissolves in water because the partial charges on water molecules surround and separate the ions. Oil does not dissolve because it is nonpolar and cannot form hydrogen bonds with water.

Thermal Properties and Biological Effects

Hydrogen bonding gives water unique thermal properties that are essential for life.

• High Specific Heat: Water absorbs a lot of heat before its temperature rises, moderating Earth's climate and stabilizing temperatures in organisms.

• High Heat of Vaporization: It takes significant energy to convert water from liquid to gas, which is why sweating cools the body.

• Surface Tension: Water's surface tension allows small objects, like leaves or insects, to rest on its surface without sinking.

Example: Coastal climates are more moderate than inland climates because large bodies of water absorb and release heat slowly.

Acids, Bases, and the pH Scale

Definitions and the pH Scale

Acids and bases are substances that affect the concentration of hydrogen ions (H+) in a solution, which is measured by the pH scale.

• Acid: A compound that donates a proton (H+).

• Base: A compound that accepts a proton (H+).

• Amphoteric: A substance that can act as both an acid and a base (e.g., water).

• pH Scale: A logarithmic scale measuring H+ concentration: $\text{pH} = -\log [\text{H}^+]$

• Neutral pH: 7 (pure water)

• Acidic: pH < 7 (higher [H+])

• Basic: pH > 7 (lower [H+])

• Each unit change in pH represents a tenfold change in [H+].

Example: A solution with pH 6 is ten times more acidic than a solution with pH 7.

Nucleic Acids: DNA and RNA

Structure and Function

Nucleic acids are large biological molecules essential for storing and expressing genetic information. The two main types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• Monomer: Nucleotide

• Polymer: Polynucleotide chain (DNA or RNA)

• Bonds: Nucleotides are joined by phosphodiester bonds.

• Function: DNA stores genetic information; RNA helps express genetic information.

• Structure: DNA is typically a double helix; RNA is usually single-stranded but can form complex structures.

Nucleotide Structure

Each nucleotide consists of three components:

• Phosphate group

• Pentose sugar (deoxyribose in DNA, ribose in RNA)

• Nitrogenous base (adenine, guanine, cytosine, thymine in DNA; uracil replaces thymine in RNA)

The phosphate group is attached to the 5' carbon of the sugar, and the nitrogenous base is attached to the 1' carbon.

Component	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strands	Double (usually)	Single (usually)
Function	Genetic information storage	Gene expression, protein synthesis

Example: The sequence of nucleotides in DNA encodes the instructions for building proteins.

Summary Table: Key Properties of Water

Property	Biological Importance
Cohesion	Enables transport of water in plants; surface tension supports small organisms
High Specific Heat	Stabilizes temperature in organisms and environments
Solvent Ability	Facilitates biochemical reactions in cells
High Heat of Vaporization	Allows cooling by evaporation (sweating, panting)

Additional info: These notes expand on brief points from the original materials to provide a self-contained study guide suitable for exam preparation in General Biology.