Carbon and the Molecular Diversity of Life: Structure and Function of Biomolecules

Carbon and the Molecular Diversity of Life

Introduction to Biomolecules

Living organisms are composed of a vast array of organic molecules, all of which are based on the element carbon. The four main classes of biomolecules—carbohydrates, lipids, proteins, and nucleic acids—are essential for life’s structure and function. These molecules are large, complex, and diverse, enabling the complexity of biological systems.

3.1 Carbon Atoms Can Form Diverse Molecules by Bonding to Four Other Atoms

Properties of Carbon

Carbon is unique in its ability to form four covalent bonds due to its four valence electrons. This property allows carbon to act as an intersection point from which a molecule can branch off in up to four directions, resulting in a diversity of molecular shapes and sizes.

• Valence: The number of covalent bonds an atom can form, determined by the number of unpaired electrons in its outer shell.

• Tetrahedral Geometry: When carbon forms four single bonds, the molecule adopts a tetrahedral shape. Double bonds create planar (flat) regions in molecules.

Formation of Bonds with Carbon

Carbon can bond with many elements, most commonly hydrogen, oxygen, and nitrogen, as well as with other carbon atoms. This versatility allows for the formation of long carbon chains, branched molecules, and rings, which serve as the skeletons of organic molecules.

• Hydrocarbons: Molecules consisting only of carbon and hydrogen. They are major components of cell membranes and energy storage molecules.

• Carbon Dioxide (CO2): An example of a simple carbon compound important in cellular respiration and photosynthesis.

Carbon Skeletons

Carbon chains form the backbone of most organic molecules. These skeletons can vary in:

• Length (number of carbons)

• Branching (linear or branched chains)

• Double bond position (location and number of double bonds)

• Presence of rings (cyclic structures)

Isomers

Isomers are compounds with the same molecular formula but different structures and properties. There are three main types:

• Structural Isomers: Differ in the covalent arrangement of atoms.

• Cis-trans (Geometric) Isomers: Differ in spatial arrangement around inflexible double bonds.

• Enantiomers: Mirror images of each other, often with only one form biologically active.

Functional Groups

Functional groups are specific groups of atoms attached to carbon skeletons that participate in chemical reactions and confer specific properties to molecules. Seven functional groups are most important in the chemistry of life: hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, phosphate, and methyl.

ATP: The Energy Currency of the Cell

Adenosine triphosphate (ATP) is an organic molecule that stores and releases energy for cellular processes. Hydrolysis of ATP releases energy by removing a phosphate group, forming ADP (adenosine diphosphate).

3.2 Macromolecules Are Polymers, Built from Monomers

Polymers and Monomers

Most macromolecules are polymers, long molecules built from repeating units called monomers. The four classes of macromolecules are carbohydrates, proteins, nucleic acids, and (to a lesser extent) lipids.

• Dehydration (Condensation) Reaction: Joins two monomers by removing a water molecule, forming a covalent bond.

• Hydrolysis Reaction: Breaks a covalent bond by adding water, splitting a polymer into monomers.

• Enzymes: Biological catalysts that speed up both dehydration and hydrolysis reactions.

Carbohydrates

Monosaccharides

Carbohydrates are sugars and their polymers. The simplest carbohydrates are monosaccharides, which generally have the formula (CH2O)n. Glucose (C6H12O6) is the most common monosaccharide and a primary energy source for cells.

• Classification: By number of carbons and position of the carbonyl group (aldose or ketose).

• Ring Formation: In aqueous solutions, most five- and six-carbon sugars form rings.

Disaccharides and Glycosidic Linkages

Disaccharides are formed when two monosaccharides are joined by a dehydration reaction, creating a glycosidic linkage. Sucrose (table sugar) is a common disaccharide composed of glucose and fructose.

Polysaccharides

Polysaccharides are large carbohydrate polymers with storage or structural roles:

• Starch: Storage polysaccharide in plants, composed of glucose monomers.

• Glycogen: Storage polysaccharide in animals, highly branched and stored in liver and muscle cells.

• Cellulose: Structural polysaccharide in plant cell walls; differs from starch in glycosidic linkage, making it indigestible to most animals.

• Chitin: Structural polysaccharide in arthropod exoskeletons and fungal cell walls; similar to cellulose but with nitrogen-containing groups.

Lipids

General Properties

Lipids are hydrophobic molecules that do not form true polymers. They are mainly composed of hydrocarbons and serve as energy storage, structural components of membranes, and signaling molecules.

Fats (Triglycerides)

Fats are constructed from glycerol and three fatty acids via dehydration reactions, forming ester linkages. They are efficient energy storage molecules.

Saturated vs. Unsaturated Fatty Acids

• Saturated Fatty Acids: No double bonds; solid at room temperature; mostly animal fats.

• Unsaturated Fatty Acids: One or more double bonds; liquid at room temperature; mostly plant and fish fats.

• Trans Fats: Unsaturated fats with trans double bonds, often produced industrially.

Phospholipids

Phospholipids consist of two fatty acids, a phosphate group, and glycerol. They are amphipathic, with hydrophobic tails and a hydrophilic head, and form the bilayer structure of cell membranes.

Steroids

Steroids are lipids with a carbon skeleton consisting of four fused rings. Cholesterol is a key steroid, maintaining membrane fluidity and serving as a precursor for other steroids such as hormones.

Proteins

Structure and Function

Proteins are polymers of amino acids and account for more than 50% of the dry mass of most cells. They perform a wide variety of functions, including catalysis, defense, transport, support, movement, and regulation.

Amino Acids and Peptide Bonds

Amino acids have a central carbon, an amino group, a carboxyl group, a hydrogen atom, and a variable R group. Peptide bonds link amino acids into polypeptides via dehydration reactions.

Levels of Protein Structure

• Primary Structure: Unique sequence of amino acids.

• Secondary Structure: Coils (α-helix) and folds (β-pleated sheet) stabilized by hydrogen bonds.

• Tertiary Structure: Overall 3D shape due to interactions among R groups (hydrophobic interactions, disulfide bridges, ionic bonds, hydrogen bonds).

• Quaternary Structure: Association of two or more polypeptides (e.g., hemoglobin).

Protein Structure and Disease

A single amino acid substitution can drastically affect protein function, as seen in sickle-cell disease, where a change in hemoglobin leads to abnormal red blood cell shape and function.

Denaturation

Protein structure can be affected by environmental factors such as temperature, pH, and salt concentration. Denaturation is the loss of a protein’s native structure, rendering it biologically inactive. Sometimes, denaturation is reversible.

Nucleic Acids

DNA and RNA

Nucleic acids store, transmit, and help express hereditary information. DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are polymers of nucleotides. Genes are segments of DNA that encode the amino acid sequence of polypeptides.

Nucleotide Structure

Each nucleotide consists of a nitrogenous base, a pentose sugar (deoxyribose in DNA, ribose in RNA), and one or more phosphate groups. Nitrogenous bases are classified as pyrimidines (C, T, U) or purines (A, G).

Nucleotide Polymers

Nucleotides are joined by phosphodiester linkages, forming a sugar-phosphate backbone. DNA is typically double-stranded, forming a double helix with complementary base pairing (A-T, G-C). RNA is usually single-stranded, with uracil replacing thymine.

Genomics and Proteomics

Genomics is the study of whole sets of genes and their interactions, while proteomics is the study of entire sets of proteins. Advances in bioinformatics have enabled rapid sequencing and analysis of genomes, revolutionizing biology and medicine.

DNA and Proteins as Tape Measures of Evolution

Comparing DNA and protein sequences among species provides insights into evolutionary relationships. Closely related species have more similar DNA sequences than distantly related species, allowing molecular biology to assess evolutionary kinship.