Carbon and the Molecular Diversity of Life

Overview: Carbon Compounds and Life

Living organisms are primarily composed of carbon-based compounds, which form the foundation of biological molecules. Carbon's unique chemical properties allow it to create large, complex, and diverse molecules essential for life.

• Carbon-based compounds are the main constituents of living matter, aside from water.

• Organic compounds are molecules containing carbon atoms bonded to other elements, typically hydrogen, oxygen, and nitrogen.

• Carbon's ability to form four covalent bonds makes it highly versatile, enabling the construction of complex molecular structures.

Major Classes of Biological Molecules

All living things are composed of four main classes of large biological molecules, each with distinct functions and properties.

• Carbohydrates: Serve as fuel and building material.

• Lipids: Diverse group of hydrophobic molecules, important for energy storage and membrane structure.

• Proteins: Perform a wide range of functions, including catalysis, defense, transport, and structural support.

• Nucleic acids: Store, transmit, and express hereditary information.

• The first three classes (carbohydrates, lipids, proteins) can form large molecules called macromolecules.

Carbon: Structure and Bonding

Electron Configuration and Bonding

Carbon's electron configuration allows it to form four covalent bonds with other atoms, resulting in a variety of molecular shapes and sizes.

• Carbon atoms have four valence electrons, enabling them to bond with up to four other atoms.

• Molecules with multiple carbons can have tetrahedral shapes or planar arrangements, depending on the type of bonds (single or double).

• Carbon can bond with hydrogen, oxygen, nitrogen, and other elements, forming diverse organic molecules.

Carbon Skeletons and Molecular Diversity

Carbon chains form the skeletons of most organic molecules, varying in length, branching, and ring structures.

• Carbon skeletons can be straight, branched, or arranged in rings.

• Hydrocarbons are organic molecules consisting only of carbon and hydrogen; they are hydrophobic and can store energy.

• Functional groups attached to carbon skeletons determine the chemical properties and reactivity of organic molecules.

Functional Groups Important to Life

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

• Seven key functional groups: hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, phosphate, methyl.

• The number and arrangement of functional groups give each molecule its unique properties.

Macromolecules: Polymers and Monomers

Polymers and Monomers

Macromolecules are large molecules formed by joining smaller units called monomers. The process of polymerization involves specific chemical reactions.

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

• Monomer: The repeating unit that serves as the building block of a polymer.

• Dehydration reactions join monomers by removing a water molecule; hydrolysis breaks polymers into monomers by adding water.

• Enzymes facilitate the synthesis and breakdown of polymers.

Carbohydrates

Monosaccharides and Disaccharides

Carbohydrates include simple sugars (monosaccharides) and their polymers. They serve as energy sources and structural materials.

• Monosaccharides: Simple sugars with molecular formulas usually multiples of CH2O (e.g., glucose: C6H12O6).

• Classified by the number of carbons (triose, pentose, hexose) and the position of the carbonyl group.

• Disaccharide: Formed by a dehydration reaction joining two monosaccharides via a glycosidic linkage.

Polysaccharides

Polysaccharides are polymers of sugars with storage and structural roles.

• Starch: Storage polysaccharide in plants, composed of glucose monomers (amylose is the simplest form).

• Glycogen: Storage polysaccharide in animals, mainly in liver and muscle cells.

• Cellulose: Structural polysaccharide in plant cell walls; differs from starch in glycosidic linkage (β vs. α).

• Chitin: Structural polysaccharide in arthropod exoskeletons and fungal cell walls.

Comparison of Starch and Cellulose

Lipids

Fats and Fatty Acids

Lipids are hydrophobic molecules that do not form true polymers. Fats are important for energy storage and insulation.

• Fat: Constructed from glycerol and fatty acids.

• Glycerol: Three-carbon alcohol with a hydroxyl group attached to each carbon.

• Fatty acid: Carboxyl group attached to a long carbon skeleton.

• Fats are formed by joining three fatty acids to glycerol via ester linkages, creating a triacylglycerol.

• Saturated fatty acids: No double bonds; solid at room temperature (animal fats).

• Unsaturated fatty acids: One or more double bonds; liquid at room temperature (plant and fish oils).

Phospholipids

Phospholipids are major components of cell membranes, forming bilayers due to their hydrophilic heads and hydrophobic tails.

• Composed of two fatty acids and a phosphate group attached to glycerol.

• Self-assemble into bilayers in aqueous environments, forming the basis of biological membranes.

Steroids

Steroids are lipids with a carbon skeleton consisting of four fused rings. Cholesterol is a key steroid in animal cell membranes.

• Cholesterol: Essential for membrane structure but high levels can contribute to cardiovascular disease.

Proteins

Structure and Function

Proteins are the most diverse macromolecules, performing a wide range of functions in cells.

• Functions include catalysis (enzymes), defense (antibodies), transport (hemoglobin), storage, communication, movement, and structural support.

• Proteins are polymers of amino acids, linked by peptide bonds to form polypeptides.

Amino Acids

Amino acids are organic molecules with carboxyl and amino groups, differing in their side chains (R groups).

• 20 different amino acids, classified by properties of their side chains (nonpolar, polar, charged).

• Sequence of amino acids determines protein structure and function.

Levels of Protein Structure

Proteins have four levels of structure, each contributing to their function.

• Primary structure: Unique sequence of amino acids.

• Secondary structure: Coils and folds (α-helix, β-pleated sheet) due to hydrogen bonding.

• Tertiary structure: Overall 3D shape due to interactions among side chains.

• Quaternary structure: Association of multiple polypeptide chains.

Protein Structure Table

Protein Denaturation

Physical and chemical conditions (pH, temperature, salt concentration) can cause proteins to lose their native structure, becoming biologically inactive.

• Denaturation: Loss of protein structure and function.

• Protein folding is complex and often assisted by cellular machinery.

Nucleic Acids

DNA and RNA

Nucleic acids store and transmit hereditary information. DNA and RNA are polymers of nucleotides.

• DNA (deoxyribonucleic acid): Stores genetic information; double helix structure.

• RNA (ribonucleic acid): Involved in protein synthesis; usually single-stranded.

• Genes are segments of DNA that encode the sequence of amino acids in proteins.

Nucleotide Structure

Nucleotides consist of a nitrogenous base, a pentose sugar, and one or more phosphate groups.

• Nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), Thymine (T, DNA only), Uracil (U, RNA only).

• Pentose sugar: Deoxyribose in DNA, ribose in RNA.

• Nucleotides are joined by phosphodiester bonds between the 3' hydroxyl and 5' phosphate groups.

DNA Structure and Base Pairing

DNA consists of two antiparallel strands forming a double helix, with complementary base pairing.

• Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C).

• Base pairs are joined by hydrogen bonds.

• RNA uses Uracil (U) instead of Thymine (T).

Genetic Information and Evolution

The sequence of nucleotides in DNA is inherited and can be used to assess evolutionary relationships among species.

• Closely related species have more similar DNA sequences.

• Molecular biology provides tools for studying evolutionary kinship.

Key Equations and Formulas

• General formula for monosaccharides:

• Dehydration reaction (joining monomers):

• Hydrolysis (breaking polymers):

• ATP hydrolysis:

Additional info: Some details and examples have been expanded for clarity and completeness based on standard biology textbooks.