The Structure and Function of Large Biological Molecules

Chapter 5: The Structure and Function of Large Biological Molecules

Introduction to Biomolecules

Large biological molecules, or macromolecules, are essential for life and include carbohydrates, lipids, proteins, and nucleic acids. These molecules are polymers built from monomers, and their structure determines their function in living organisms.

Monomers, Polymers, and Macromolecules

Definitions and Relationships

• Monomer: A simple organic molecule that serves as a building block for polymers.

• Polymer: A chain of monomers linked together by covalent bonds.

• Macromolecule: A large molecule formed by the joining of many polymers; examples include carbohydrates, proteins, and nucleic acids.

Examples of monomer to macromolecule relationships:

• Monosaccharide → Carbohydrate → Bread

• Glycerol/Fatty acids → Lipids → Bee’s wax

• Amino acid → Protein → Hemoglobin

• Nucleotide → Nucleic acid → DNA

Synthesis and Breakdown of Polymers

Polymers are synthesized and broken down by specific chemical reactions:

• Dehydration Reaction: Joins monomers by removing a water molecule, forming a new bond.

• Hydrolysis: Breaks bonds between monomers by adding a water molecule.

Carbohydrates: Fuel and Building Material

Structure and Classification

Carbohydrates are sugars and their polymers, serving as energy sources and structural materials.

• General formula: H-C-OH; ratio of O to H is 1:2.

• Aldose: Aldehyde sugar (C=O at the end).

• Ketose: Ketone sugar (C=O within the skeleton).

• Classified by carbon number: triose (3C), pentose (5C), hexose (6C).

• Monosaccharide: Simple sugar (e.g., glucose, 6C).

Glucose

Glucose is a common monosaccharide, often forming a ring structure for stability. It is central to cellular respiration and serves as a precursor for amino acids and fatty acids.

Disaccharides

Disaccharides are formed by joining two monosaccharides via a dehydration reaction, creating a glycosidic linkage.

• Maltose: Glucose + Glucose

• Sucrose: Glucose + Fructose

• Lactose: Glucose + Galactose

Polysaccharides

Polysaccharides are large polymers of monosaccharides, serving storage and structural roles.

• Starch: Storage in plants (in plastids such as chloroplasts).

• Glycogen: Storage in animals (liver and muscle cells).

• Cellulose: Structural component in plant cell walls; straight chains provide strength. Most animals cannot digest cellulose, but some (e.g., cows) have gut microbes that can.

• Chitin: Structural polysaccharide in exoskeletons of arthropods.

Lipids: Energy Storage and Membranes

Types and Properties

Lipids are hydrophobic molecules, including fats, oils, phospholipids, and steroids.

• Fats: Glycerol + 3 fatty acids; typically animal origin.

• Oils: Plant origin; usually unsaturated.

• Saturated fatty acids: No double bonds (e.g., lard, butter).

• Unsaturated fatty acids: One or more double bonds (e.g., vegetable oils, fish oils).

• Trans fats: Produced by hydrogenation; found in processed foods.

Phospholipids and Steroids

Phospholipids are major components of cell membranes, forming a bilayer with hydrophobic tails and hydrophilic heads. Steroids, such as cholesterol, have four fused rings and contribute to membrane integrity and hormone synthesis.

Proteins: Structure, Enzymes, and Functions

Amino Acids and Peptide Bonds

Proteins are polymers of amino acids, linked by peptide bonds. Each amino acid contains an amino group (NH2), a carboxyl group (COOH), a hydrogen atom, and a variable R group attached to a central carbon.

Protein Structure and Function

The function of a protein is determined by its structure, which is organized into four levels:

• Primary structure: Sequence of amino acids.

• Secondary structure: Alpha helix or beta sheet formed by hydrogen bonding.

• Tertiary structure: Overall 3D folding driven by interactions among R groups.

• Quaternary structure: Association of two or more polypeptide chains.

Protein shape is crucial for function, such as enzyme activity or antibody-antigen binding. Denaturation (loss of structure) can occur due to heat, pH changes, or salt concentration, leading to loss of function. Misfolding is implicated in diseases like cystic fibrosis, Alzheimer's, Parkinson's, and mad cow disease.

Nucleic Acids: Storage and Transmission of Genetic Information

Structure and Types

Nucleic acids (DNA and RNA) are polymers of nucleotides, each composed of a phosphate group, a pentose sugar, and a nitrogenous base. Nitrogenous bases are classified as purines (adenine, guanine) or pyrimidines (cytosine, thymine, uracil).

DNA vs. RNA

	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	Adenine, Guanine, Thymine, Cytosine	Adenine, Guanine, Uracil, Cytosine
Strands	Double	Single
Helix	Yes	No

Summary Table: Organic Compounds

Macromolecule	Monomer	Function
Proteins	Amino Acids	Enzymes, structural components (e.g., muscle proteins)
Carbohydrates	Glucose	Energy storage (starch in plants, glycogen in animals), plant cell walls (cellulose)
Lipids	Glycerol, fatty acids	Long-term energy storage, cell membrane structure
Nucleic Acids	Nucleotides	Genetic material (DNA), protein synthesis (RNA)