Water, Acidity, and Introduction to Biological Macromolecules

Water Molecules and Their Properties

Structure and Polarity of Water

Water (H2O) is a polar molecule, meaning it has regions of partial positive and negative charge due to the unequal sharing of electrons between oxygen and hydrogen atoms. This property is fundamental to many of water's unique behaviors in biological systems.

• Polar Covalent Bond: The bond between oxygen and hydrogen in water is a polar covalent bond, where electrons are shared unequally. Oxygen is more electronegative, pulling electrons closer and acquiring a partial negative charge (δ-), while hydrogen becomes partially positive (δ+).

• Molecular Geometry: Water has a bent molecular shape with an angle of approximately 104.5°, contributing to its polarity.

• Hydration Shells: When ionic compounds like NaCl dissolve in water, water molecules surround the ions. For Na+, the oxygen atoms (δ-) of water point inward toward the cation, stabilizing it in solution.

Example: In a solution of NaCl, Na+ ions are surrounded by water molecules with their oxygen atoms facing the ion, while Cl- ions are surrounded by water molecules with their hydrogen atoms facing the ion.

Acids, Bases, and the pH Scale

Understanding Acidity and pH

The pH scale measures the concentration of hydrogen ions (H+) in a solution, indicating its acidity or basicity. The scale ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

• Acidic Solutions: Have a higher concentration of H+ ions than OH- ions (pH < 7).

• Basic Solutions: Have a higher concentration of OH- ions than H+ ions (pH > 7).

• Neutral Solution: Pure water has equal concentrations of H+ and OH- ions (pH = 7).

• pH Calculation: $pH = -\log_{10}[H^+]$

• Relative Acidity: Each unit change in pH represents a tenfold change in H+ concentration. For example, a solution with pH 3 is 1,000 times more acidic than one with pH 6.

Example: Wine (pH ~3) is about 1,000 times more acidic than urine (pH ~6).

Acid-Base Reactions

• Adding a Base: Adding a base (such as NaOH) to a solution increases the concentration of OH- ions, which combine with H+ ions to form water, thus reducing acidity.

• Buffer Systems: Biological systems use buffers to maintain stable pH by absorbing excess H+ or OH- ions.

Carbon: The Foundation of Biological Macromolecules

Properties of Carbon

Carbon is a versatile element that forms the backbone of all biological macromolecules due to its ability to form four covalent bonds with other atoms, including itself.

• Tetravalency: Carbon has four valence electrons, allowing it to form up to four covalent bonds, resulting in a variety of stable structures (chains, rings, branches).

• Double Bonds: Carbon can also form double bonds, which affect the shape and reactivity of molecules.

• Hydrocarbons: Compounds consisting only of carbon and hydrogen. Hydrocarbons are hydrophobic and store significant energy.

Example: Methane (CH4), ethene (C2H4), and benzene (C6H6) are all hydrocarbons.

Macromolecules in Biology

Biological macromolecules are large, complex molecules essential for life. They are typically polymers made from smaller subunits called monomers.

• Major Classes:

  • Carbohydrates

  • Lipids

  • Proteins

  • Nucleic Acids

• Monomers and Polymers: Monomers are the building blocks (e.g., amino acids, nucleotides), and polymers are long chains of monomers (e.g., proteins, DNA).

Chemical/Functional Groups in Biological Molecules

Common Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic properties and reactivities. They play a crucial role in the structure and function of biological molecules.

Functional Group	Structure	Properties/Examples
Hydroxyl	—OH	Polar, forms hydrogen bonds; found in alcohols
Carbonyl	>C=O	Polar; found in aldehydes and ketones
Carboxyl	—COOH	Acts as an acid; found in amino acids and fatty acids
Amino	—NH2	Acts as a base; found in amino acids
Sulfhydryl	—SH	Forms disulfide bonds; found in some amino acids (e.g., cysteine)
Phosphate	—OPO32-	Contributes negative charge; involved in energy transfer (e.g., ATP)
Methyl	—CH3	Nonpolar; affects gene expression and molecular properties

Example: Estradiol and testosterone differ only by a few functional groups, yet have very different biological effects.

Functional Groups and Molecular Properties

• Hydrophilic Groups: Most functional groups (except methyl) are hydrophilic due to their polarity, increasing solubility in water.

• Disulfide Bridges: Sulfhydryl groups can form covalent disulfide bonds, stabilizing protein structure (e.g., in hair shape).

• Energy Transfer: Phosphate groups are key in molecules like ATP, which store and transfer energy in cells.

Additional info: Functional groups are often used to identify and classify organic molecules in biochemistry. Their presence determines the chemical reactivity and interactions of biomolecules.