The Structure and Function of Large Biological Molecules

Overview: The Molecules of Life

Large biological molecules, or macromolecules, are essential for the structure and function of living cells. These molecules are typically formed by the joining of smaller organic molecules and play diverse roles in cellular processes.

• Macromolecules include carbohydrates, lipids, proteins, and nucleic acids.

• Macromolecules are polymers composed of thousands of covalently bonded atoms, often exceeding 100,000 daltons in mass.

• Biochemists have determined that macromolecules have complex, ordered structures that enable their specific functions.

Concept 5.1: Macromolecules are Polymers, Built from Monomers

Most macromolecules are polymers, which are long chains of repeating units called monomers. The diversity and complexity of life arise from the arrangement and combination of these monomers.

• Carbohydrates, proteins, and nucleic acids are polymers; lipids are not true polymers.

• A polymer is a long molecule consisting of many similar or identical building blocks linked by covalent bonds.

• The repeating units are called monomers.

• Monomers are connected by dehydration reactions, which remove a water molecule to form a covalent bond.

• Polymers can be broken down into monomers by hydrolysis, which adds a water molecule to break the bond.

• Enzymes catalyze both dehydration and hydrolysis reactions.

Equation for Dehydration Reaction: Equation for Hydrolysis:

• An immense variety of polymers can be built from a small number of monomers, similar to how words are formed from a limited alphabet.

Concept 5.2: Carbohydrates Serve as Fuel and Building Material

Carbohydrates are organic molecules consisting of sugars and their polymers. They are a primary source of energy and structural material in cells.

• Monosaccharides are simple sugars (e.g., glucose, fructose).

• Disaccharides are composed of two monosaccharides joined by a glycosidic linkage (e.g., sucrose, lactose).

• Polysaccharides are polymers of many monosaccharides (e.g., starch, glycogen, cellulose).

Sugars: Monosaccharides and Disaccharides

• Monosaccharides generally have the molecular formula .

• They are classified by the location of the carbonyl group (aldose or ketose) and the length of the carbon skeleton.

• Disaccharides are formed by dehydration reactions between two monosaccharides.

Polysaccharides: Storage and Structural Roles

• Starch is a storage polysaccharide in plants, composed of glucose monomers.

• Glycogen is a storage polysaccharide in animals, stored mainly in liver and muscle cells.

• Cellulose is a structural polysaccharide in plant cell walls, composed of beta-glucose monomers.

• Enzymes that digest starch by hydrolyzing alpha linkages cannot hydrolyze beta linkages in cellulose.

• Some microbes and fungi can digest cellulose, aiding in the breakdown of plant material.

• Chitin is a structural polysaccharide found in the exoskeletons of arthropods and cell walls of fungi.

Concept 5.3: Lipids are a Diverse Group of Hydrophobic Molecules

Lipids are not true polymers but are large biological molecules that are hydrophobic due to their molecular structure. They serve as energy storage, structural components of membranes, and signaling molecules.

• Fats are constructed from glycerol and fatty acids.

• A fat molecule consists of three fatty acids joined to glycerol by an ester linkage, forming a triacylglycerol.

• Saturated fatty acids have no double bonds; they are solid at room temperature.

• Unsaturated fatty acids have one or more double bonds; they are liquid at room temperature.

• Phospholipids have two fatty acids and a phosphate group attached to glycerol; they form cell membranes.

• Steroids are lipids with a carbon skeleton consisting of four fused rings (e.g., cholesterol).

Equation for Fat Formation:

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

Proteins are the most structurally complex molecules known, accounting for more than 50% of the dry mass of most cells. They perform a vast array of functions, including catalysis, structural support, transport, communication, and defense.

• Proteins are polymers of amino acids, linked by peptide bonds.

• There are 20 different amino acids, each with a specific side chain (R group).

• The sequence of amino acids determines a protein's structure and function.

• Proteins can have four levels of structure: primary, secondary, tertiary, and quaternary.

Equation for Peptide Bond Formation:

Additional info:

• Protein folding is critical for function; misfolded proteins can lead to diseases.

• Proteins may require cofactors or coenzymes for activity.