General biology unit 5

Biological Molecules: The Building Blocks of Life

Biological molecules are essential for the structure and function of all living organisms. The four major classes—polymers, carbohydrates, lipids, and proteins—form the foundation of cellular structures and organelles.

Concept 5.1 and Figure 5.2: Polymers

Polymers are large molecules composed of repeating subunits called monomers. They are formed by dehydration reactions and broken down by hydrolysis.

• Monomer: A small molecule that can join with other similar molecules to form a polymer.

• Polymer: A large molecule made up of repeating monomer units.

• Dehydration Reaction: A chemical reaction in which two molecules are covalently bonded to each other with the removal of a water molecule.

• Hydrolysis: A chemical reaction that breaks bonds between two molecules by the addition of water; functions in disassembly of polymers to monomers.

• Examples of Polymers: Polysaccharides (carbohydrates), proteins, and nucleic acids.

• Not all biological molecules are polymers: Lipids are not considered true polymers because they are not composed of repeating monomer units.

Example: Starch is a polymer of glucose monomers, while triglycerides (a type of lipid) are not polymers.

Concept 5.2: Carbohydrates

Carbohydrates are sugars and their polymers. They serve as energy sources and structural materials in cells.

• Monosaccharides: Simple sugars (e.g., glucose, fructose) that are the monomers of carbohydrates.

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

• Polysaccharides: Large polymers of monosaccharides (e.g., starch, glycogen, cellulose, chitin).

• Glycosidic Bond: A covalent bond formed between two monosaccharides by a dehydration reaction.

• Dehydration Reaction: Forms glycosidic bonds in carbohydrates by removing water.

• Functions: Energy storage (starch in plants, glycogen in animals), structural support (cellulose in plants, chitin in arthropods and fungi).

Example: Cellulose is a polysaccharide that provides structural support in plant cell walls.

Concept 5.3: Lipids

Lipids are a diverse group of hydrophobic molecules, including fats, phospholipids, and steroids. They are not true polymers.

• Lipids: Hydrophobic molecules that include fats, oils, phospholipids, and steroids.

• Fats: Composed of glycerol and three fatty acids; formed by dehydration reactions.

• Glycerol: A three-carbon alcohol that forms the backbone of fat molecules.

• Fatty Acids: Long hydrocarbon chains with a carboxyl group at one end.

• Dehydration Reaction: Joins fatty acids to glycerol, forming ester bonds and producing triglycerides.

• Saturated vs. Unsaturated Fats: Saturated fats have no double bonds between carbon atoms (solid at room temperature); unsaturated fats have one or more double bonds (liquid at room temperature).

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

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

Example: Phospholipids form the bilayer structure of cell membranes.

Concept 5.4 and Figure 5.18: Proteins

Proteins are polymers of amino acids and perform a vast array of functions in cells, including catalysis, structure, transport, and signaling.

• Amino Acids: The monomers of proteins, each with a central carbon, amino group, carboxyl group, hydrogen atom, and a unique side chain (R group).

• Polypeptide: A polymer of amino acids linked by peptide bonds.

• Peptide Bond: Covalent bond formed between the amino group of one amino acid and the carboxyl group of another via dehydration reaction.

• Side Chain (R Group): Determines the chemical properties and function of each amino acid.

• Protein Structure:

  • Primary Structure: Sequence of amino acids in a polypeptide.

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

  • Tertiary Structure: Overall 3D shape of a polypeptide, determined by interactions among R groups.

  • Quaternary Structure: Association of multiple polypeptide chains to form a functional protein.

• Functions: Enzymes, structural proteins, transport proteins, signaling molecules, and more.

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

Concept 5.5 and Figure 5.23: Nucleic Acids

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

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

• Purines: Nitrogenous bases with a double-ring structure (adenine and guanine).

• Pyrimidines: Nitrogenous bases with a single-ring structure (cytosine, thymine, and uracil).

• Phosphodiester Bond: Covalent bond that links nucleotides together in a nucleic acid chain, formed by dehydration reaction.

• DNA Structure: Double helix formed by two complementary strands held together by hydrogen bonds between nitrogenous bases.

Example: DNA stores genetic information in the sequence of its bases, which is used to direct protein synthesis.

Supplemental Materials

• Short talks and videos on large biological molecules are recommended for further study.

Summary Table: Major Biological Molecules

Molecule Type	Monomer	Polymer	Bond Type	Main Functions
Carbohydrates	Monosaccharide	Polysaccharide	Glycosidic bond	Energy storage, structure
Lipids	Glycerol & Fatty Acids	Not true polymers	Ester bond	Energy storage, membranes, signaling
Proteins	Amino acid	Polypeptide	Peptide bond	Catalysis, structure, transport
Nucleic Acids	Nucleotide	DNA/RNA	Phosphodiester bond	Genetic information storage and transfer