Chapter 5: The Structure and Function of Large Biological Molecules

Overview of Organic Molecules in Living Organisms

Large biological molecules, also known as macromolecules, are essential for the structure and function of living cells. They include carbohydrates, lipids, proteins, and nucleic acids. Each class has unique properties and roles in biological systems.

• Polymers and Macromolecules: Polymers are long chains of repeating units called monomers. Macromolecules are large molecules formed by polymerization. Enzymatic reactions are crucial for their synthesis and breakdown.

• Enzymatic Reactions: Enzymes catalyze the formation and hydrolysis of macromolecules, allowing cells to build and degrade complex structures efficiently.

Carbohydrates

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

• Monosaccharides: Simple sugars such as glucose, fructose, and galactose. Classified by the number of carbon atoms (e.g., triose, pentose, hexose).

• Disaccharides: Formed by joining two monosaccharides (e.g., sucrose, lactose, maltose).

• Polysaccharides: Long chains of monosaccharides. Examples include starch (energy storage in plants), glycogen (energy storage in animals), and cellulose (structural component in plants).

• Major Types: Starch, glycogen, cellulose, chitin.

• Functions: Energy storage, structural support, cell recognition.

Example: Starch is a polysaccharide used by plants to store energy, while cellulose provides structural support in plant cell walls.

Lipids

Lipids are hydrophobic molecules that include fats, oils, phospholipids, and steroids. They play roles in energy storage, membrane structure, and signaling.

• Fats: Composed of triglycerides (glycerol + three fatty acids). Can be saturated (no double bonds) or unsaturated (one or more double bonds).

• Properties: Saturated fats are solid at room temperature (e.g., butter), while unsaturated fats are liquid (e.g., olive oil).

• Phospholipids: Consist of two fatty acids, a phosphate group, and glycerol. They form the bilayer of cell membranes due to their amphipathic nature (hydrophilic head, hydrophobic tails).

• Steroids: Lipids with a characteristic four-ring structure. Cholesterol is a common steroid in animal cell membranes.

Example: Phospholipids are essential for forming biological membranes, creating a barrier that separates the cell from its environment.

Proteins

Proteins are polymers of amino acids and perform a wide variety of functions, including catalysis, transport, and structural support.

• Amino Acids: There are 20 different amino acids, each with a variable side chain (R group). Amino acids are linked by peptide bonds to form polypeptides.

• Protein Structure: Proteins have four levels of structure:

  • Primary: Sequence of amino acids.

  • Secondary: Local folding (e.g., alpha helices, beta sheets).

  • Tertiary: Overall 3D shape of a polypeptide.

  • Quaternary: Association of multiple polypeptide chains.

• Protein Folding: Proper folding is essential for function. Misfolded proteins can lead to diseases.

Example: Hemoglobin is a protein with quaternary structure that transports oxygen in the blood.

Equation:

Nucleic Acids

Nucleic acids store and transmit genetic information. The two main types are DNA and RNA.

• Nucleotides: Building blocks of nucleic acids, each consisting of a phosphate group, a five-carbon sugar (deoxyribose or ribose), and a nitrogenous base.

• DNA: Double-stranded molecule with bases adenine (A), thymine (T), cytosine (C), and guanine (G). Stores genetic information.

• RNA: Single-stranded molecule with bases adenine (A), uracil (U), cytosine (C), and guanine (G). Involved in protein synthesis and gene regulation.

• Flow of Genetic Information: DNA is transcribed into RNA, which is then translated into protein.

Equation:

Example: Messenger RNA (mRNA) carries genetic instructions from DNA to ribosomes for protein synthesis.

Comparison Table: Major Classes of Biological Macromolecules

Additional info: Academic context and examples have been expanded for clarity and completeness.