Macromolecules: Polymers and Monomers

Introduction to Macromolecules

Large biological molecules, or macromolecules, are essential for life and include carbohydrates, proteins, and nucleic acids. These molecules are polymers, meaning they are composed of repeating subunits called monomers.

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

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

• Enzymes are specialized macromolecules that catalyze the synthesis and breakdown of polymers.

Polymer Assembly and Disassembly

Polymers are assembled and disassembled through two main types of reactions:

• Dehydration Reaction: Two monomers are covalently bonded with the loss of a water molecule, forming a polymer.

• Hydrolysis: Polymers are broken down into monomers by the addition of water, reversing the dehydration reaction.

Carbohydrates: Structure and Function

Monosaccharides

Carbohydrates are sugars and their polymers. The simplest carbohydrates are monosaccharides (simple sugars), which generally have molecular formulas that are multiples of CH2O. Glucose (C6H12O6) is the most common monosaccharide.

• Monosaccharides are classified by the location of their carbonyl group (as aldose or ketose) and the number of carbons in the skeleton.

Ring Structures of Sugars

In aqueous solutions, many monosaccharides form ring structures, which are more stable than linear forms.

Disaccharides

Disaccharides are formed when two monosaccharides are joined by a dehydration reaction, creating a glycosidic linkage.

• Examples: Sucrose (glucose + fructose), Lactose (glucose + galactose), Maltose (glucose + glucose).

Polysaccharides

Polysaccharides are polymers of sugars and serve storage or structural roles. Their properties depend on the types of sugar monomers and the positions of glycosidic linkages.

• Starch: Storage polysaccharide in plants, composed of glucose monomers. Stored as granules in chloroplasts.

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

• Cellulose: Structural polysaccharide in plant cell walls. Also a glucose polymer, but with different glycosidic linkages (β instead of α), making it indigestible to most animals.

Lipids: Structure and Function

Overview of Lipids

Lipids are a diverse group of hydrophobic molecules that do not form true polymers. They are mainly composed of hydrocarbons and include fats, phospholipids, and steroids.

Fats

Fats are constructed from glycerol and fatty acids. A fat molecule (triglyceride) consists of three fatty acids joined to glycerol by ester linkages.

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

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

• Trans fats: Produced by hydrogenating unsaturated fats; associated with health risks.

• Main function: Long-term energy storage, insulation, and cushioning of organs.

Phospholipids

Phospholipids consist of two fatty acids and a phosphate group attached to glycerol. The fatty acid tails are hydrophobic, while the phosphate head is hydrophilic. In water, phospholipids self-assemble into bilayers, forming the basis of cell membranes.

Steroids

Steroids are lipids with a carbon skeleton consisting of four fused rings. Cholesterol is an important steroid in animal cell membranes and a precursor for other steroids.

Proteins: Structure and Function

Functions of Proteins

Proteins are the most diverse macromolecules, accounting for more than 50% of the dry mass of most cells. They function as enzymes, in defense, storage, transport, cellular communication, movement, and structural support.

Amino Acids and Polypeptides

Proteins are polymers of amino acids, which have a central carbon, an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group). Amino acids are linked by peptide bonds to form polypeptides.

Levels of Protein Structure

The function of a protein is determined by its structure, which has four levels:

• Primary structure: Unique sequence of amino acids.

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

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

• Quaternary structure: Association of multiple polypeptide chains.

Protein Structure and Disease

A single amino acid change can drastically affect protein function, as seen in sickle-cell disease. Environmental factors such as pH, salt concentration, and temperature can denature proteins, causing loss of function.

Nucleic Acids: DNA and RNA

Structure and Function of Nucleic Acids

Nucleic acids store, transmit, and help express hereditary information. The two types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). DNA provides instructions for its own replication and for protein synthesis via RNA, a process known as gene expression.

Nucleotide Structure

Nucleic acids are polymers called polynucleotides, made of monomers called nucleotides. Each nucleotide consists of a nitrogenous base, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a phosphate group. Nitrogenous bases are classified as pyrimidines (C, T, U) or purines (A, G).

DNA and RNA Structure

DNA consists of two antiparallel polynucleotide strands forming a double helix, with complementary base pairing (A with T, G with C). RNA is usually single-stranded and uses uracil (U) instead of thymine (T).

Summary Table: Comparison of Major Macromolecules