Macromolecules in Biology

Definition and Classes of Macromolecules

Macromolecules are large, complex molecules essential for life, formed by joining smaller molecules. They make up the majority of the structure and function of living cells. The four main classes of biological macromolecules are carbohydrates, lipids, proteins, and nucleic acids.

• Macromolecule: A large molecule composed of thousands of atoms, typically formed by polymerization of smaller subunits.

• Monomer: A small, repeating unit that serves as the building block of a polymer. Example: glucose is a monomer for starch.

• Polymer: A long molecule consisting of many similar or identical monomers linked together. Example: cellulose is a polymer of glucose.

• Examples: Proteins (polymers of amino acids), DNA (polymer of nucleotides), starch (polymer of glucose).

Additional info: Nucleic acids and proteins are also major biological macromolecules, but this lecture focuses on carbohydrates and lipids.

Formation and Breakdown of Polymers

Dehydration Synthesis and Hydrolysis

Polymers are formed and broken down by specific chemical reactions involving water.

• Dehydration Synthesis: A reaction in which two monomers are joined by removing a water molecule, forming a covalent bond. This process builds polymers such as proteins and carbohydrates.

• Hydrolysis: A reaction in which a polymer is broken down into monomers by adding water, breaking the covalent bond. This process is important in digestion.

Equation for Dehydration Synthesis:

Equation for Hydrolysis:

Carbohydrates

Structure and Types of Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with the formula . They serve as energy sources and structural components in cells.

• Monosaccharides: Simple sugars with one sugar unit (e.g., glucose, fructose, galactose). They vary in the number of carbons (triose, pentose, hexose) and arrangement of functional groups.

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

• Oligosaccharides: Short chains of 3-10 monosaccharides, often involved in cell recognition.

• Polysaccharides: Long chains of monosaccharides (more than 10), such as starch, glycogen, and cellulose.

Glycosidic Bond Formation: Monosaccharides are joined by dehydration synthesis to form glycosidic bonds.

Functions of Carbohydrates

Carbohydrates play key roles in energy storage and structural support.

• Energy Storage: Starch (in plants) and glycogen (in animals) are polysaccharides that store glucose for energy.

• Structural Support: Cellulose provides rigidity to plant cell walls.

• Cell Recognition: Oligosaccharides on cell surfaces are involved in cell-cell recognition.

Example: Glycogen is highly branched, allowing rapid release of glucose in animal muscle and liver.

Lipids

Structure and Types of Lipids

Lipids are hydrophobic molecules, primarily composed of long hydrocarbon chains or rings. They are not true polymers but are formed by dehydration reactions. Major types include triglycerides, phospholipids, and steroids.

• Triglycerides: Composed of one glycerol and three fatty acids. Serve as long-term energy storage and insulation.

• Phospholipids: Contain a glycerol backbone, two fatty acids, and a phosphate group. Major component of cell membranes.

• Steroids: Lipids with four fused rings, such as cholesterol and hormones (e.g., testosterone).

Saturated, Unsaturated, and Trans Fats

The structure of fatty acids determines their properties and health effects.

• Saturated Fatty Acids: Have no double bonds between carbon atoms; saturated with hydrogen. Pack closely, solid at room temperature.

• Unsaturated Fatty Acids: Have one or more double bonds, causing kinks that prevent tight packing. Liquid at room temperature.

• Trans Fats: Unsaturated fats with trans double bonds, produced by hydrogenation. Associated with increased risk of coronary heart disease.

Comparison Table:

Additional info: Trans fats are artificially produced and have been linked to significant health risks, including coronary artery disease.

Phospholipids and Cell Membranes

Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions. This property allows them to form bilayers, which are the foundation of cell membranes.

• Structure: Glycerol backbone, two fatty acid tails (hydrophobic), and a phosphate group (hydrophilic).

• Function: Form the lipid bilayer of cell membranes, creating a barrier that separates the cell from its environment.

Example: The hydrophilic phosphate heads face outward toward water, while hydrophobic tails face inward, away from water.

Steroids

Steroids are lipids characterized by a structure of four fused rings. They serve various functions in the body.

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

• Steroid Hormones: Include testosterone and estrogen, which regulate physiological processes.

Additional info: The function of steroids is determined by the chemical groups attached to the rings.

Key Terms and Concepts to Understand

• Macromolecule, monomer, polymer

• Dehydration synthesis, hydrolysis

• Monosaccharide, disaccharide, oligosaccharide, polysaccharide

• Glycosidic bond

• Saturated, unsaturated, trans fats

• Phospholipid, steroid