Carbon and the Molecular Diversity of Life & The Structure and Function of Large Biological Molecules

Chapter 4: Carbon and the Molecular Diversity of Life

Section 4.1: Organic Chemistry is the Study of Carbon Compounds

Organic chemistry focuses on the study of carbon-containing compounds, which form the basis of all living organisms. The unique properties of carbon allow it to form a vast array of molecules essential for life.

• Organic Compounds: Molecules that contain carbon, often bonded to hydrogen, oxygen, nitrogen, or other elements.

• Vitalism: The historical idea that living organisms are governed by a life force outside the realm of physical and chemical laws. This concept has been replaced by mechanism.

• Mechanism: The view that all natural phenomena, including life processes, are governed by physical and chemical laws.

Section 4.2: Carbon Atoms Can Form Diverse Molecules by Bonding to Four Other Atoms

Carbon's ability to form four covalent bonds allows for a diversity of stable and complex molecules, making it the backbone of biological macromolecules.

• Tetravalence: Carbon has four valence electrons, enabling it to form four covalent bonds with other atoms.

• Valence Shell: The outermost electron shell of an atom, where bonding electrons are found.

• Isomers: Molecules with the same molecular formula but different structures and properties.

  • Structural Isomers: Differ in the covalent arrangements of their atoms.

  • Geometric Isomers (cis-trans isomers): Have the same covalent bonds but differ in spatial arrangement around a double bond.

  • Enantiomers: Mirror-image isomers that differ in spatial arrangement, often referred to as "handedness."

Section 4.3: Functional Groups are the Parts of Molecules Involved in Chemical Reactions

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.

• Hydroxyl Group (–OH): Hydrogen bonded to oxygen, which is bonded to the carbon skeleton; makes molecules water-soluble (alcohols).

• Carbonyl Group (C=O): Carbon double bonded to oxygen; found in aldehydes and ketones.

• Aldehyde: Carbonyl group at the end of a carbon skeleton.

• Ketone: Carbonyl group within a carbon skeleton.

• Carboxyl Group (–COOH): Carbon double bonded to oxygen and also bonded to a hydroxyl group; acts as an acid.

• Amino Group (–NH2): Nitrogen bonded to two hydrogens and the carbon skeleton; acts as a base.

• Sulfhydryl Group (–SH): Sulfur bonded to hydrogen; helps stabilize protein structure via disulfide bonds.

• Phosphate Group (–PO4): Phosphorus atom bonded to four oxygens; important in energy transfer (e.g., ATP) and nucleic acids.

• Other Functional Groups: Alcohols, amines, thiols, organic phosphates, and carboxylic acids are classes of organic compounds named after their functional groups.

• Adenosine Triphosphate (ATP): A molecule with high-energy bonds; serves as the primary energy carrier in cells.

Chapter 5: The Structure and Function of Large Biological Molecules

Section 5.1: Macromolecules are Polymers Built from Monomers

Macromolecules are large, complex molecules essential for life, constructed from smaller subunits called monomers. The diversity of macromolecules arises from the arrangement of these monomers.

• Macromolecule: A very large molecule, often composed of thousands of atoms, built from repeated smaller units (monomers).

• Monomer: The smaller subunit that makes up a polymer.

• Polymer: A long molecule consisting of many similar or identical monomers linked by covalent bonds.

• Dehydration Reaction (Condensation Reaction): A chemical reaction that joins monomers by removing a water molecule.

• Hydrolysis: A chemical reaction that breaks bonds between monomers by adding water.

Section 5.2: Carbohydrates, Lipids, Proteins, and Nucleic Acids (Overview / Structure-Function)

There are four major classes of biological macromolecules, each with distinct structures and functions essential for life.

• Carbohydrates: Molecules ranging from small sugars to large polysaccharides; serve as energy sources and structural materials.

• Monosaccharide: The simplest carbohydrate (single sugar unit), e.g., glucose.

• Disaccharide: Two monosaccharides joined by a glycosidic linkage, e.g., sucrose.

• Polysaccharide: A polymer made of many monosaccharide units, e.g., starch, cellulose.

• Glycosidic Linkage: A covalent bond formed between two monosaccharides by a dehydration reaction.

• Lipids: Diverse class of hydrophobic molecules, not true polymers; include fats, phospholipids, and steroids.

• Fatty Acid: A long hydrocarbon chain with a carboxyl group at one end.

• Unsaturated vs. Saturated Fatty Acids: Saturated fatty acids have no double bonds (more hydrogen), while unsaturated fatty acids have one or more double bonds.

• Phospholipid: A lipid with a hydrophilic (polar) head and two hydrophobic tails; main component of cell membranes.

• Steroid: Lipid characterized by a carbon skeleton of four fused rings; includes cholesterol and hormones.

Section 5.3: How Amino Acids, Peptides, and Proteins are Built / Primary to Tertiary Structure

Proteins are polymers of amino acids, and their structure determines their function. Protein structure is organized into four levels: primary, secondary, tertiary, and quaternary.

• Amino Acid: The monomer of proteins; consists of a central carbon, an amino group, a carboxyl group, a hydrogen atom, and an R group (side chain).

• R Group (Side Chain): The variable part of an amino acid that determines its chemical nature (e.g., polar, nonpolar, charged).

• Peptide Bond: The covalent bond between amino acids formed by dehydration reaction.

• Polypeptide: A chain of amino acids linked by peptide bonds.

• Primary Structure: The specific linear sequence of amino acids in a protein.

• Secondary Structure: Local folding patterns within a protein, such as α-helix and β-pleated sheet, stabilized by hydrogen bonds.

• Tertiary Structure: The overall 3D shape of a polypeptide, determined by interactions among R groups (hydrophobic interactions, ionic bonds, hydrogen bonds, disulfide bridges).

Section 5.4: Proteins Include a Diversity of Structures, Resulting in a Wide Range of Functions

Proteins perform a vast array of functions in living organisms, and their function is directly related to their structure.

• Quaternary Structure: When a protein is made of two or more polypeptide subunits, how they assemble.

• Denaturation: The process in which a protein loses its native structure (and function) due to environmental changes (temperature, pH, salt concentration).

• Chaperonins: Protein molecules that assist in the proper folding of other proteins.

• Hydrophobic Interaction: Tendency for nonpolar side-chains to cluster away from water, driving folding.

• Disulfide Bridge (Bond): Covalent bond between two cysteine side chains that stabilizes the tertiary or quaternary structure.

Table: Types of Isomers in Organic Molecules

Type of Isomer	Description	Example
Structural Isomer	Different covalent arrangements of atoms	Butane vs. isobutane
Geometric Isomer	Same covalent bonds, different spatial arrangement (cis/trans)	Cis-2-butene vs. trans-2-butene
Enantiomer	Mirror-image isomers, differ in handedness	L-alanine vs. D-alanine

Key Equations

• Dehydration Reaction: $\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + \text{H}_2\text{O}$

• Hydrolysis Reaction: $\text{Polymer} + \text{H}_2\text{O} \rightarrow \text{Monomer}_1 + \text{Monomer}_2$