Chapter 5: The Structure and Function of Large Biological Molecules

Macromolecules: Polymers Built from Monomers

Macromolecules are large, complex molecules essential for life, constructed from smaller units called monomers. These include carbohydrates, proteins, and nucleic acids, all of which are polymers.

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

• Monomer: The repeating unit that serves as a building block for polymers.

• Examples: Carbohydrates, proteins, and nucleic acids are all polymers.

The Synthesis and Breakdown of Polymers

• Enzymes: Specialized macromolecules that speed up chemical reactions, including those that build or break down polymers.

• Dehydration Reaction: Joins two monomers by removing a water molecule.

• Hydrolysis: Breaks polymers into monomers by adding water, essentially the reverse of dehydration.

Diversity of Polymers

• Cells contain thousands of different macromolecules.

• Macromolecules vary among cells, species, and even more between species.

• A huge variety of polymers can be built from a small set of monomers.

Carbohydrates: Fuel and Building Material

Carbohydrates are sugars and their polymers, serving as energy sources and structural components.

• Monosaccharides: Simple sugars with molecular formulas usually multiples of CH2O. Glucose is the most common.

• Classification: Based on the location of the carbonyl group (aldose or ketose) and the number of carbons.

• Polysaccharides: Polymers composed of many sugar monomers.

Sugars

• Monosaccharides often form rings in aqueous solutions.

• Serve as major fuel for cells and raw material for building molecules.

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

Polysaccharides

• Have storage and structural roles.

• Function and architecture determined by sugar monomers and glycosidic linkages.

Storage Polysaccharides

• Starch: Storage polysaccharide in plants, made of glucose monomers. Stored as granules in chloroplasts and plastids. Amylose is the simplest form.

• Glycogen: Storage polysaccharide in animals, mainly in liver and muscle cells. Hydrolysis releases glucose when needed.

Structural Polysaccharides

• Cellulose: Major component of plant cell walls. Polymer of glucose, but with different glycosidic linkages than starch.

• Cellulose molecules are straight and unbranched, allowing hydrogen bonding between parallel molecules.

• Enzymes that digest starch cannot digest cellulose; cellulose acts as "insoluble fiber" in human food.

• Chitin: Found in arthropod exoskeletons and fungal cell walls.

Lipids: Diverse Hydrophobic Molecules

Lipids are hydrophobic molecules, not true polymers, and include fats, phospholipids, and steroids.

• Fats: Constructed from glycerol and fatty acids. Glycerol is a three-carbon alcohol; fatty acids have a carboxyl group and a long carbon skeleton.

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

• Saturated Fatty Acids: No double bonds, maximum hydrogen atoms, solid at room temperature (animal fats).

• Unsaturated Fatty Acids: One or more double bonds, liquid at room temperature (plant and fish fats).

• Trans Fats: Created by hydrogenation, may contribute to cardiovascular disease.

• Functions: Energy storage, cushioning organs, insulation.

Phospholipids

• Two fatty acids and a phosphate group attached to glycerol.

• Hydrophobic tails and hydrophilic head.

• Form bilayers in water, making up cell membranes.

Steroids

• Characterized by four fused rings.

• Cholesterol: Component of animal cell membranes and precursor for other steroids.

Proteins: Diversity of Structure and Function

Proteins are polymers of amino acids, accounting for more than 50% of cell dry mass. They perform a wide range of functions, including catalysis, defense, storage, transport, communication, movement, and structural support.

• Enzymes: Proteins that act as catalysts.

• Constructed from 20 amino acids.

• Polypeptides: Unbranched polymers of amino acids, linked by peptide bonds.

• Each polypeptide has a unique sequence, with a carboxyl (C-terminus) and amino (N-terminus) end.

Amino Acids

• Organic molecules with amino and carboxyl groups.

• Differ in properties due to side chains (R groups).

Four Levels of Protein Structure

• Primary Structure: Sequence of amino acids, determined by genetic information.

• Secondary Structure: Coils (α helix) and folds (β pleated sheet) from hydrogen bonds in the backbone.

• Tertiary Structure: Overall shape from interactions between R groups (hydrogen bonds, ionic bonds, hydrophobic interactions, van der Waals forces, disulfide bridges).

• Quaternary Structure: Two or more polypeptide chains forming one macromolecule (e.g., collagen, hemoglobin).

Sickle-Cell Disease

• Caused by a single amino acid substitution in hemoglobin.

• Leads to abnormal hemoglobin, causing red blood cells to deform and aggregate.

Protein Structure Determinants

• Physical and chemical conditions (pH, salt, temperature) can affect structure.

• Denaturation: Loss of native structure, rendering protein inactive.

• Denaturation can sometimes be reversed.

Protein Folding in the Cell

• Protein structure prediction is complex.

• Misfolded proteins are associated with diseases (Alzheimer’s, Parkinson’s, mad cow disease).

• Structure determination methods: X-ray crystallography, NMR spectroscopy, bioinformatics.

Nucleic Acids: Storage and Expression of Hereditary Information

Nucleic acids are polymers of nucleotides, storing and transmitting genetic information. DNA and RNA are the two types.

• Gene: Unit of inheritance, made of DNA.

• DNA directs its own replication and synthesis of messenger RNA (mRNA), which controls protein synthesis (gene expression).

Components of Nucleic Acids

• Nucleotide: Nitrogenous base, pentose sugar, and phosphate group.

• Nucleoside: Nitrogenous base + sugar (no phosphate).

• Pyrimidines: Cytosine, thymine, uracil (single ring).

• Purines: Adenine, guanine (double ring).

• DNA sugar: deoxyribose; RNA sugar: ribose.

Nucleotide Polymers

• Nucleotides linked by phosphodiester bonds, forming a sugar-phosphate backbone.

• Sequence of bases is unique for each gene.

Structures of DNA and RNA

• DNA: Double helix, antiparallel strands, complementary base pairing (A-T, G-C).

• RNA: Single-stranded, complementary pairing (A-U, G-C), variable in form.

Summary Table: Comparison of Biological Macromolecules

Additional info: The above notes expand on brief points from the original slides, providing definitions, examples, and context for each macromolecule type. The summary table is inferred for clarity and comparison.