Overview: The Molecules of Life

All living organisms are composed of four main classes of large biological molecules, or macromolecules: carbohydrates, lipids, proteins, and nucleic acids. These macromolecules are essential for structure, function, and regulation of the body's cells, tissues, and organs. Their unique properties arise from the specific arrangement of their atoms.

Concept 5.1: Macromolecules are Polymers, Built from Monomers

Polymers and Monomers

• Polymers are long molecules made of repeating units called monomers, linked by covalent bonds.

• Three of the four classes of macromolecules—carbohydrates, proteins, and nucleic acids—are polymers.

• Enzymes catalyze the synthesis and breakdown of polymers.

Dehydration and Hydrolysis Reactions

• Dehydration reaction: Monomers are joined by covalent bonds with the loss of a water molecule.

• Hydrolysis: Polymers are broken down by the addition of water, splitting the covalent bond.

• Example: Digestion involves hydrolysis of polymers into monomers, which are then absorbed and reassembled into new polymers by cells.

Diversity of Polymers

• Cells use about 40–50 common monomers to build a vast array of polymers.

• The diversity of macromolecules arises from the arrangement and combination of these monomers.

Concept 5.2: Carbohydrates Serve as Fuel and Building Material

Classification of Carbohydrates

• Monosaccharides: Simple sugars (e.g., glucose, C6H12O6), serve as fuel and raw material for other molecules.

• Disaccharides: Two monosaccharides joined by a glycosidic linkage (e.g., maltose, sucrose, lactose).

• Polysaccharides: Polymers of many monosaccharides, with storage or structural roles (e.g., starch, glycogen, cellulose, chitin).

Monosaccharide Structure and Function

• General formula: (CH2O)n

• Classified by carbonyl group position (aldose or ketose) and carbon skeleton length (triose, pentose, hexose).

• Most sugars form rings in aqueous solution.

• Major nutrient for cellular respiration and building blocks for other molecules.

Polysaccharides: Storage and Structure

• Starch: Storage polysaccharide in plants, composed of α-glucose monomers; forms helical structures (amylose: unbranched, amylopectin: branched).

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

• Cellulose: Structural polysaccharide in plant cell walls, composed of β-glucose monomers; forms straight, rigid structures (microfibrils).

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

Comparison of Major Polysaccharides

Concept 5.3: Lipids are a Diverse Group of Hydrophobic Molecules

General Properties of Lipids

• Lipids are not true polymers and are hydrophobic due to nonpolar hydrocarbon chains.

• Main types: fats, phospholipids, steroids.

Fats (Triacylglycerols)

• Composed of glycerol and three fatty acids joined by ester linkages.

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

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

• Hydrogenation can convert unsaturated fats to saturated and produce trans fats, which are linked to cardiovascular disease.

• Fats are efficient energy storage molecules, storing more than twice the energy of carbohydrates.

• Adipose tissue stores fat, cushions organs, and provides insulation.

Phospholipids

• Composed of glycerol, two fatty acids, and a phosphate group (often with additional polar groups).

• Amphipathic: hydrophilic head and hydrophobic tails.

• Form bilayers in aqueous environments, making up the fundamental structure of cell membranes.

Steroids

• Lipids with four fused carbon rings.

• Cholesterol: Essential component of animal cell membranes and precursor for steroid hormones.

• High cholesterol and trans fats are associated with cardiovascular disease.

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

Functions of Proteins

• Structural support, storage, transport, cellular communication, movement, defense, and catalysis (enzymes).

• Proteins are the most structurally complex biological molecules.

Amino Acids and Polypeptides

• Amino acids: Monomers with a central (α) carbon, amino group, carboxyl group, hydrogen atom, and variable R group.

• 20 different amino acids, classified by properties of R groups (nonpolar, polar, acidic, basic).

• Amino acids are linked by peptide bonds (dehydration reaction) to form polypeptides.

• Polypeptides have directionality: N-terminus (amino end) and C-terminus (carboxyl end).

Levels of Protein Structure

• Primary structure: Unique sequence of amino acids.

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

• Tertiary structure: Overall 3D shape stabilized by interactions among R groups (hydrogen bonds, ionic bonds, hydrophobic interactions, van der Waals forces, disulfide bridges).

• Quaternary structure: Association of two or more polypeptide subunits (e.g., hemoglobin, collagen).

Protein Folding and Denaturation

• Protein function depends on correct folding; misfolding can cause diseases (e.g., sickle-cell disease, Alzheimer's).

• Denaturation: Loss of native structure due to changes in pH, temperature, or chemicals; usually results in loss of function.

• Chaperonins assist in proper folding of some proteins.

Determining Protein Structure

• Methods: X-ray crystallography, NMR spectroscopy, and bioinformatics.

• Primary structure is determined by DNA sequence.

Concept 5.5: Nucleic Acids Store, Transmit, and Help Express Hereditary Information

Genes and Nucleic Acids

• Genes are units of inheritance made of DNA, a nucleic acid polymer.

• Two types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• DNA directs its own replication and controls protein synthesis via RNA (gene expression).

Nucleotide Structure

• Each nucleotide consists of a nitrogenous base, a pentose sugar (ribose or deoxyribose), and one or more phosphate groups.

• Nitrogenous bases: Pyrimidines (cytosine, thymine [DNA], uracil [RNA]) and purines (adenine, guanine).

• Nucleotides are joined by phosphodiester linkages between the 3' OH and 5' phosphate.

DNA and RNA Structure

• DNA: Double helix of two antiparallel strands; sugar-phosphate backbone outside, bases inside.

• Base pairing: Adenine (A) pairs with thymine (T) in DNA, and with uracil (U) in RNA; guanine (G) pairs with cytosine (C).

• RNA: Usually single-stranded, but can form complex shapes via internal base pairing.

• Genetic information is encoded in the sequence of bases.

Flow of Genetic Information

• Central dogma:

• Messenger RNA (mRNA) carries genetic instructions from DNA to ribosomes for protein synthesis.

• Transfer RNA (tRNA) brings amino acids to ribosomes during translation.

Concept 5.6: Genomics and Proteomics Have Transformed Biological Inquiry and Applications

Genomics and Proteomics

• Genomics: Study of whole sets of genes and their interactions.

• Proteomics: Study of large sets of proteins, their structures, and functions.

• Bioinformatics uses computational tools to analyze large biological data sets.

• Genome sequencing projects (e.g., Human Genome Project) have revolutionized biology.

DNA and Proteins as Evolutionary Tape Measures

• Comparing DNA and protein sequences reveals evolutionary relationships among species.

• Closely related species have more similar DNA/protein sequences than distantly related species.

• Example: Human and gorilla hemoglobin differ by only 1 amino acid; human and frog hemoglobin differ by 67 amino acids.