LEC 4 Macromolecules: Structure and Function in Biology

Macromolecules

Introduction to Macromolecules

Macromolecules are large, complex molecules essential for life. They are typically polymers, formed by joining smaller units called monomers. The four main classes of macromolecules in biology are carbohydrates, lipids, proteins, and nucleic acids.

• Polymer: A large molecule made by linking together many similar or identical monomers.

• Dehydration (Condensation) Reaction: The process by which monomers are joined, releasing a molecule of water.

• Hydrolysis: The process by which polymers are broken down into monomers by the addition of water.

Carbohydrates

Structure and Types

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with the formula (CH2O)n. They serve as energy sources and structural components in cells.

• Monosaccharides: Simple sugars (e.g., glucose, galactose, fructose) that are the basic building blocks of carbohydrates.

• Disaccharides: Consist of two monosaccharides joined by a glycosidic bond (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 plant cell walls).

Example: Glucose is a monosaccharide with the formula C6H12O6. Starch is a polysaccharide made of glucose units.

Lipids

Structure and Types

Lipids are hydrophobic molecules, mainly composed of long hydrocarbon chains. They are not true polymers but are essential for energy storage, membrane structure, and signaling.

• Fats (Triglycerides): Composed of one glycerol and three fatty acids. Used for long-term energy storage.

• Saturated Fatty Acids: Have no double bonds between carbon atoms; solid at room temperature (e.g., animal fats).

• Unsaturated Fatty Acids: Have one or more double bonds; liquid at room temperature (e.g., plant oils).

• Phospholipids: Major component of cell membranes; consist of two fatty acids, a glycerol, and a phosphate group. They form bilayers in aqueous environments.

• Steroids: Lipids with a structure of four fused rings; include cholesterol and hormones.

Example: Phospholipids form the basic structure of biological membranes, with hydrophilic heads and hydrophobic tails.

Proteins

Structure and Function

Proteins are polymers of amino acids, which fold into specific three-dimensional shapes to perform a wide variety of functions in cells.

• Amino Acids: The building blocks of proteins; 20 different types exist.

• Peptide Bond: The covalent bond linking amino acids in a protein chain.

• Levels of Protein Structure:

  • Primary Structure: The sequence of amino acids in a polypeptide chain.

  • Secondary Structure: Local folding into alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds.

  • Tertiary Structure: The overall three-dimensional shape of a polypeptide, stabilized by interactions among side chains (R groups).

  • Quaternary Structure: The arrangement of multiple polypeptide chains in a functional protein (e.g., hemoglobin).

• Protein Function: Determined by its specific conformation; even small changes can affect function (e.g., sickle cell anemia caused by a single amino acid change in hemoglobin).

Example: Enzymes are proteins that catalyze biochemical reactions.

Nucleic Acids

Structure and Function

Nucleic acids store and transmit genetic information. The two main types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• Nucleotide: The monomer of nucleic acids, consisting of a five-carbon sugar, a phosphate group, and a nitrogenous base.

• DNA: Stores genetic information; double-stranded helix; bases are adenine (A), thymine (T), cytosine (C), and guanine (G).

• RNA: Involved in protein synthesis; usually single-stranded; bases are adenine (A), uracil (U), cytosine (C), and guanine (G).

• Base Pairing: In DNA, A pairs with T, and G pairs with C via hydrogen bonds. The two strands are complementary.

• Phosphodiester Bond: The linkage between nucleotides in a nucleic acid strand.

Example: If one DNA strand has the sequence 5'-AGCTTCCGAAG-3', the complementary strand is 3'-TCGAAGGCTTC-5'.

Summary Table: Major Classes of Macromolecules

Macromolecule	Monomer	Bond Type	Function	Examples
Carbohydrates	Monosaccharide	Glycosidic bond	Energy, structure	Glucose, starch, cellulose
Lipids	Fatty acids, glycerol	Ester bond	Energy storage, membranes	Fats, oils, phospholipids
Proteins	Amino acid	Peptide bond	Catalysis, structure, transport	Enzymes, hemoglobin
Nucleic Acids	Nucleotide	Phosphodiester bond	Genetic information	DNA, RNA

Key Equations and Concepts

• General Formula for Monosaccharides: $C_nH_{2n}O_n$

• Formation of a Peptide Bond: $\text{Amino acid}_1 + \text{Amino acid}_2 \rightarrow \text{Dipeptide} + H_2O$

• Formation of a Triglyceride: $\text{Glycerol} + 3 \times \text{Fatty acid} \rightarrow \text{Triglyceride} + 3H_2O$

• Base Pairing in DNA: $A = T,\ G = C$

Applications and Importance

• Molecular components are essential for the continuity of life and evolutionary relationships among species.

• Understanding macromolecules helps explain cellular structure, function, and heredity.