Chemistry of Cell Biology Part II

Objectives

• Explain what makes water polar and the properties that result from its polarity, making it essential for cellular life.

• Define salt, acid, base, anion, and cation.

• Understand the pH scale and its importance for maintaining stable cellular environments.

• Describe the structure and function of carbohydrates, lipids, proteins, and nucleic acids.

Important Biological Molecules

Types of Molecules

• Inorganic molecules: Typically do not contain carbon; examples include water, salts, acids, and bases.

• Organic molecules: Contain carbon; examples include carbohydrates, proteins, nucleic acids, and lipids.

Water (H2O) and Its Properties

Polarity of Water

Water is a polar molecule, meaning it has an unequal distribution of electrical charge. This property is crucial for its role in biological systems.

• Polar molecule: A molecule with regions of partial positive and negative charge due to unequal sharing of electrons.

• In water, oxygen is more electronegative than hydrogen, attracting electrons more strongly and resulting in a partial negative charge on oxygen and partial positive charges on hydrogens.

• This polarity allows water molecules to form hydrogen bonds with each other and with other substances.

Key Properties of Water Due to Polarity

• Universal solvent: Water can dissolve many substances, facilitating chemical reactions in cells.

• Hydrogen bonding: The attraction between the slightly positive hydrogen atom of one water molecule and the slightly negative oxygen atom of another.

Example: Sodium chloride (NaCl) dissolves in water, dissociating into sodium (Na+) and chloride (Cl-) ions.

Cohesion and Adhesion

Water molecules exhibit both cohesion and adhesion, which are essential for many biological processes.

• Cohesion: Attraction between molecules of the same kind (e.g., water molecules sticking together).

• Adhesion: Attraction between molecules of different kinds (e.g., water molecules sticking to cell walls).

• These properties contribute to the structure of cell membranes and the behavior of DNA and proteins in cells.

High Specific Heat Capacity

Water has a high specific heat capacity due to hydrogen bonding, which allows it to absorb and retain heat, helping cells resist temperature fluctuations.

• Specific heat capacity: The amount of heat required to raise the temperature of a substance by one degree Celsius.

• This property stabilizes cellular and environmental temperatures.

Acids, Bases, Salts, Anions, and Cations

Definitions

• Salt: An ionic compound formed from the neutralization of an acid and a base.

• Acid: A substance that releases hydrogen ions (H+) in solution.

• Base: A substance that releases hydroxide ions (OH-) or accepts hydrogen ions in solution.

• Anion: A negatively charged ion (e.g., Cl-).

• Cation: A positively charged ion (e.g., Na+).

The pH Scale

The pH scale measures the concentration of hydrogen ions in a solution, indicating its acidity or basicity.

• pH: Potential of hydrogen; scale ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

• Each unit change in pH represents a tenfold change in hydrogen ion concentration.

• Cells must maintain pH between 6.5 and 8.5 for optimal function, often using buffers to stabilize pH.

Equation:

Example: A change from pH 7 to pH 9 means a 100-fold decrease in hydrogen ion concentration.

Organic Molecules in Cells

Carbon and Molecular Diversity

Organic molecules are based on carbon, which can form four covalent bonds, allowing for a variety of structures such as chains and rings. This versatility enables the formation of complex biological molecules.

• Covalent bond: A chemical bond formed by the sharing of electron pairs between atoms.

• Carbon's bonding properties allow for the creation of macromolecules essential for life.

Four Major Classes of Organic Molecules

• Carbohydrates

• Proteins

• Lipids

• Nucleic acids

These macromolecules are formed by linking smaller building blocks through dehydration synthesis and broken down by hydrolysis.

Carbohydrates

Structure and Function

Carbohydrates are the primary energy source for cells and serve structural roles in cell walls.

• Monosaccharides: Simple sugars with 3-7 carbon atoms (e.g., glucose).

• Disaccharides: Two monosaccharides joined together (e.g., sucrose).

• Polysaccharides: Long chains of monosaccharides (e.g., cellulose in plant cell walls, chitin in fungi).

Lipids

Structure and Function

Lipids are hydrophobic molecules that store energy and form cell membranes.

• Triglycerides: Composed of glycerol and three fatty acids; stored in fat cells.

• Phospholipids: Contain two fatty acids and a phosphate group; major component of cell membranes. Hydrophobic tails face inward, hydrophilic heads face outward toward water.

• Steroids: Lipids with carbon ring structures (e.g., cholesterol), which help maintain membrane fluidity.

Proteins

Structure and Function

Proteins perform diverse functions, including catalyzing biochemical reactions (enzymes), transporting molecules, providing structural support, and enabling movement.

• Composed of amino acids (20 types), linked by peptide bonds formed through dehydration synthesis.

• Levels of protein structure:

  • Primary: Sequence of amino acids.

  • Secondary: Folding into alpha helices and beta sheets via hydrogen bonding.

  • Tertiary: Three-dimensional folding due to interactions among side chains.

  • Quaternary: Association of multiple polypeptide chains.

Nucleic Acids

Structure and Function

Nucleic acids store and transmit genetic information and direct protein synthesis.

• Made of repeating subunits called nucleotides, each consisting of a phosphate group, a pentose sugar, and a nitrogenous base.

• DNA: Contains deoxyribose sugar, bases A, T, C, G; double-stranded; stores genetic material.

• RNA: Contains ribose sugar, bases A, U, C, G; single-stranded; involved in protein synthesis.

Additional info: Some explanations and examples have been expanded for clarity and completeness, including definitions, equations, and table entries.