The Structure and Function of Large Biological Molecules

The Structure and Function of Large Biological Molecules

Introduction to Biological Macromolecules

All living organisms are composed of four major classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids. These macromolecules are essential for life, serving as structural components, energy sources, and carriers of genetic information.

Macromolecules: Polymers and Monomers

Polymer Structure

Most macromolecules (except lipids) are polymers, long molecules made by linking together smaller units called monomers. The diversity of life arises from the arrangement of these monomers into unique polymers.

• Carbohydrates, proteins, and nucleic acids are true polymers.

• Lipids are not true polymers but are large and hydrophobic.

Synthesis and Breakdown of Polymers

Polymers are synthesized and broken down by specific chemical reactions:

• Dehydration reaction: Monomers are joined by removing a water molecule, forming a covalent bond.

• Hydrolysis: Polymers are broken down into monomers by adding a water molecule, breaking the covalent bond.

Carbohydrates: Fuel and Building Material

Monosaccharides

Monosaccharides are the simplest carbohydrates, often called simple sugars. They have molecular formulas that are multiples of (CH2O)n. The most common monosaccharide is glucose (C6H12O6).

• Classified by the location of the carbonyl group (aldose or ketose) and the number of carbons in the skeleton (triose, pentose, hexose).

Ring Structures of Sugars

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

Disaccharides and Glycosidic Linkages

Disaccharides are formed when two monosaccharides are joined by a dehydration reaction, creating a glycosidic linkage. Examples include maltose (glucose + glucose) and sucrose (glucose + fructose).

Polysaccharides: Storage and Structure

Polysaccharides are polymers of sugars with storage or structural roles:

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

• Glycogen: Storage polysaccharide in animals, highly branched.

• Cellulose: Structural polysaccharide in plant cell walls, composed of β-glucose monomers.

Chitin

Chitin is a structural polysaccharide found in the exoskeleton of arthropods and the cell walls of fungi. It is similar to cellulose but contains nitrogen-containing appendages.

Lipids: Hydrophobic Molecules

Fats

Fats are constructed from glycerol and fatty acids. Their main function is energy storage. Fats can be:

• Saturated fats: No double bonds, solid at room temperature, mostly animal fats.

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

Phospholipids

Phospholipids consist of two fatty acids and a phosphate group attached to glycerol. They are amphipathic, with hydrophobic tails and a hydrophilic head. In water, they form bilayers, which are the foundation of cell membranes.

Steroids

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

Proteins: Structure and Function

Protein Functions

Proteins are the most diverse macromolecules, accounting for more than 50% of the dry mass of most cells. Their functions include catalysis (enzymes), defense, storage, transport, communication, movement, and structural support.

Amino Acids and Polypeptides

Proteins are polymers of amino acids, which have a central carbon (α carbon) bonded to 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 depends on its specific structure, which is organized into four levels:

• 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 stabilized by interactions among R groups (hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bridges).

• Quaternary structure: Association of multiple polypeptide chains.

Protein Denaturation

Proteins can lose their structure (and function) when exposed to changes in pH, salt concentration, temperature, or other environmental factors. This process is called denaturation. Some proteins can refold (renature) if the denaturing agent is removed.

Nucleic Acids: Information Molecules

Types and Functions

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 intermediates.

Structure of Nucleic Acids

Nucleic acids are polymers called polynucleotides, made of monomers called nucleotides. Each nucleotide consists of a nitrogenous base, a pentose sugar (deoxyribose or ribose), and a phosphate group.

DNA and RNA Molecules

DNA consists of two polynucleotide strands forming a double helix, with complementary base pairing (A with T, G with C). RNA is usually single-stranded and can fold into complex shapes.