Macromolecules in Biology

Introduction to Biological Macromolecules

All living organisms are composed of four major classes of large biological molecules, known as macromolecules: carbohydrates, lipids, proteins, and nucleic acids. These macromolecules are essential for structure, function, and regulation of the body's tissues and organs.

• Carbohydrates: Serve as fuel and building material.

• Lipids: Important for energy storage, membrane structure, and signaling.

• Proteins: Perform a vast array of functions including catalysis, structure, transport, and defense.

• Nucleic Acids: Store and transmit genetic information.

Carbohydrates

Structure and Storage Forms

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with a hydrogen:oxygen atom ratio of 2:1. They are classified as monosaccharides, disaccharides, and polysaccharides.

• Monosaccharides: Simple sugars (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides joined (e.g., sucrose, lactose).

• Polysaccharides: Long chains of monosaccharides; important for energy storage and structural support.

Storage Polysaccharides

• Starch: The main storage polysaccharide in plants. Exists in two forms: amylose (unbranched) and amylopectin (branched).

• Glycogen: The main storage polysaccharide in animals. Highly branched, allowing rapid release of glucose when needed.

• Cellulose: A structural polysaccharide in plant cell walls. Composed of unbranched chains of glucose with different glycosidic linkages than starch and glycogen.

Alpha and Beta Glucose Linkages

• In starch and glycogen, all glucose monomers are in the alpha configuration.

• In cellulose, all glucose monomers are in the beta configuration.

Advantages of Branched Polysaccharides

• Branching in glycogen and amylopectin allows for more rapid mobilization of glucose, as enzymes can act on multiple ends simultaneously.

Example: Identifying Polysaccharides

• Given a molecular structure, the type of glycosidic linkage (alpha or beta) and the degree of branching can help identify whether the molecule is starch, glycogen, or cellulose.

Lipids

Types and Properties of Lipids

Lipids are a diverse group of hydrophobic molecules, including fats, phospholipids, and steroids. They are not true polymers but are assembled from smaller molecules by dehydration reactions.

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

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

• Steroids: Characterized by a carbon skeleton with four fused rings (e.g., cholesterol).

Saturated vs. Unsaturated Fatty Acids

• Saturated fatty acids: Contain only single bonds between carbon atoms. The hydrocarbon chain is straight, allowing tight packing and resulting in solid fats at room temperature (e.g., butter, lard).

• Unsaturated fatty acids: Contain one or more double bonds, causing kinks in the chain. This prevents tight packing, making these fats liquid at room temperature (e.g., olive oil, fish oil).

Effect of Saturation on Membrane Structure

• High saturation leads to rigid membrane structures, which are better suited for hot environments to maintain integrity.

• High unsaturation leads to more fluid membranes, which are advantageous in cold environments to prevent solidification.

Energy Storage

• Fats store more than twice the energy per gram compared to polysaccharides.

Examples from Daily Life

• Butter (high in saturated fats) is solid at room temperature and becomes firmer when refrigerated.

• Vegetable oils (high in unsaturated fats) remain liquid even at low temperatures.

Special Types of Lipids

• Hydrogenated oils: Unsaturated fats that have been artificially saturated by adding hydrogen, often resulting in trans fats, which are associated with negative health effects.

Proteins

Structure and Function

Proteins are polymers of amino acids and account for more than 50% of the dry mass of most cells. They perform a wide variety of functions:

• Enzymes: Catalyze biochemical reactions.

• Defense: Antibodies protect against disease.

• Transport: Move substances across cell membranes.

• Hormones: Regulate physiological processes (e.g., insulin).

• Structure: Provide support (e.g., keratin in hair and nails).

• Movement: Enable muscle contraction (e.g., actin, myosin).

• Receptors: Detect signals and initiate cellular responses.

• Energy storage: Serve as a reserve of amino acids.

Amino Acids and Peptide Bonds

• Proteins are made from 20 different amino acids, each with a central carbon, an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group).

• Amino acids are linked by peptide bonds formed through dehydration synthesis.

Classification of Amino Acids

• Nonpolar (hydrophobic): e.g., glycine, alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, proline.

• Polar (hydrophilic): e.g., serine, threonine, cysteine, tyrosine, asparagine, glutamine.

• Acidic (negatively charged): aspartic acid, glutamic acid.

• Basic (positively charged): lysine, arginine, histidine.

Levels of Protein Structure

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

• Secondary structure: Local folding into alpha helices or beta pleated sheets, stabilized by hydrogen bonds.

• Tertiary structure: The overall three-dimensional shape, stabilized by interactions among side chains (hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions).

• Quaternary structure: Association of multiple polypeptide subunits (e.g., hemoglobin consists of four subunits).

Protein Denaturation

• Changes in temperature, pH, or other environmental factors can cause proteins to lose their structure and function, a process called denaturation.

Example: Sickle Cell Disease

• Caused by a single amino acid substitution (valine for glutamic acid) in hemoglobin, affecting its structure and function.

Nucleic Acids

Structure and Function

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

• DNA: Stores genetic information and directs its own replication and the synthesis of RNA.

• RNA: Functions in protein synthesis as a messenger (mRNA), transfer (tRNA), or ribosomal (rRNA) molecule.

Nucleotide Structure

• Each nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphate group.

• The sugar is deoxyribose in DNA and ribose in RNA.

Nitrogenous Bases

• Pyrimidines: Cytosine (C), Thymine (T, DNA only), Uracil (U, RNA only).

• Purines: Adenine (A), Guanine (G).

Polymerization and Directionality

• Nucleotides are joined by phosphodiester bonds between the 3' hydroxyl of one sugar and the 5' phosphate of the next.

• Nucleic acid strands have directionality, with a 5' end (phosphate group) and a 3' end (hydroxyl group).

DNA Structure

• DNA consists of two antiparallel strands forming a double helix.

• Base pairing occurs via hydrogen bonds: Adenine pairs with Thymine (A-T), Guanine pairs with Cytosine (G-C).

RNA Structure

• RNA is usually single-stranded, but can form complex secondary structures.

• Uracil (U) replaces Thymine (T) in RNA and pairs with Adenine (A).

Summary Table: Comparison of Macromolecules

Key Equations and Concepts

• Dehydration Synthesis: Formation of polymers by removal of water.

• Hydrolysis: Breakdown of polymers by addition of water.

• Number of possible polypeptides: For a chain of n amino acids, the number of possible sequences is .

Additional info: Some explanations and examples have been expanded for clarity and completeness, including the summary table and the explanation of hydrogenated oils and trans fats.