The Structure and Function of Large Biological Macromolecules

The Molecules of Life

Biological macromolecules are essential for life, serving as structural components, energy sources, and information carriers. Understanding their classes and properties is foundational in biology.

• Four Major Classes of Macromolecules: Carbohydrates, lipids, proteins, and nucleic acids.

• Monomers vs. Polymers: Monomers are small, repeating units that join to form polymers, which are large molecules composed of many monomers.

• Dehydration Synthesis and Hydrolysis: Dehydration synthesis joins monomers by removing water; hydrolysis breaks polymers into monomers by adding water.

Example: Glucose (monomer) forms starch (polymer) via dehydration synthesis.

Carbohydrates Serve as Fuel and Building Material

Carbohydrates are organic molecules consisting of carbon, hydrogen, and oxygen. They provide energy and structural support.

• Monosaccharides, Disaccharides, Polysaccharides: Monosaccharides are single sugar units (e.g., glucose), disaccharides are two sugars joined (e.g., sucrose), and polysaccharides are long chains (e.g., cellulose, starch).

• Starch vs. Cellulose: Both are polysaccharides of glucose, but starch is digestible by humans, while cellulose is not due to differences in glycosidic linkages.

• Synthesis in Animals: Animals synthesize glycogen for energy storage; cellulose is synthesized by plants.

Example: Cellulose forms plant cell walls; starch stores energy in plants.

Lipids are a Diverse Group of Hydrophobic Molecules

Lipids are nonpolar molecules, including fats, phospholipids, and steroids, important for energy storage and membrane structure.

• Building Blocks: Most lipids are formed from fatty acids and glycerol.

• Phospholipids: Major component of cell membranes, with hydrophilic heads and hydrophobic tails.

• Saturated vs. Unsaturated Fats: Saturated fats have no double bonds; unsaturated fats have one or more double bonds, affecting fluidity.

• Examples: Steroids (e.g., cholesterol), plant oils, animal fats.

Proteins have Many Structures, Resulting in a Wide Range of Functions

Proteins are polymers of amino acids, performing diverse functions such as catalysis, transport, and structural support.

• Levels of Protein Structure: Primary (amino acid sequence), secondary (α-helix, β-sheet), tertiary (3D folding), quaternary (multiple polypeptides).

• Amino Acids: 20 different amino acids, each with a unique side chain (R group).

• Protein Folding: Determined by chemical interactions; misfolding can cause diseases.

• Denaturation: Loss of structure and function due to environmental changes.

Example: Hemoglobin (quaternary structure) transports oxygen in blood.

Nucleic Acids Store and Transmit Hereditary Information

Nucleic acids (DNA and RNA) are polymers of nucleotides, encoding genetic information.

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

• Types of Nitrogenous Bases: Purines (adenine, guanine) and pyrimidines (cytosine, thymine, uracil).

• DNA Structure: Double helix with complementary base pairing (A-T, G-C).

Example: DNA stores genetic instructions; RNA is involved in protein synthesis.

A Tour of the Cell

How We Study Cells

Microscopy and cell fractionation are key techniques for studying cell structure and function.

• Types of Microscopes: Light microscope, transmission electron microscope (TEM), scanning electron microscope (SEM).

• Cell Fractionation: Technique to separate cell components for study.

Example: TEM reveals internal cell structures; SEM shows surface details.

A Panoramic View of the Cell

Cells are classified as prokaryotic or eukaryotic, with distinct structural features.

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles (e.g., bacteria).

• Eukaryotic Cells: Have a nucleus and organelles (e.g., plants, animals).

• Compartmentalization: Eukaryotic cells have specialized compartments for different functions.

The Nucleus and Ribosomes

The nucleus stores genetic material; ribosomes synthesize proteins.

• Nuclear Envelope: Double membrane with pores for transport.

• Nucleolus: Site of ribosome synthesis.

• Ribosomes: Free ribosomes (cytoplasm) and bound ribosomes (ER).

The Endomembrane System

The endomembrane system coordinates synthesis, transport, and processing of cellular materials.

• Components: Endoplasmic reticulum (rough and smooth), Golgi apparatus, lysosomes, vacuoles, plasma membrane.

• Functions: Protein and lipid synthesis, modification, sorting, and transport.

Mitochondria and Plastids

Mitochondria and chloroplasts are energy-converting organelles with unique features.

• Mitochondria: Site of cellular respiration; converts glucose to ATP.

• Chloroplasts: Site of photosynthesis in plants and algae.

• Endosymbiotic Theory: Suggests mitochondria and chloroplasts originated from free-living bacteria.

Cell Surfaces and Junctions

Cell Walls and Extracellular Matrix

Cell walls provide structural support; the extracellular matrix (ECM) mediates cell interactions.

• Plant Cell Wall: Composed of cellulose; primary, secondary, and middle lamella layers.

• Animal ECM: Contains proteins (collagen, elastin) and carbohydrates.

Cell Junctions

Cell junctions connect cells and facilitate communication.

• Tight Junctions: Prevent leakage between cells.

• Desmosomes: Anchor cells together.

• Gap Junctions: Allow passage of ions and small molecules.

Membrane Structure and Function

Membrane Structure

Biological membranes are composed of a phospholipid bilayer with embedded proteins, providing selective permeability.

• Fluid Mosaic Model: Membranes are dynamic, with proteins and lipids moving laterally.

• Amphipathic Molecules: Phospholipids have hydrophilic heads and hydrophobic tails.

• Membrane Proteins: Integral (span membrane) and peripheral (surface-associated).

• Factors Affecting Fluidity: Temperature, fatty acid composition, cholesterol.

Membrane Transport

Membranes regulate the movement of substances via passive and active transport mechanisms.

• Passive Transport: Diffusion, osmosis, facilitated diffusion (no energy required).

• Active Transport: Requires energy (ATP) to move substances against concentration gradients.

• Osmosis: Movement of water across membranes.

• Isotonic, Hypotonic, Hypertonic Solutions: Affect cell volume and water movement.

• Endocytosis and Exocytosis: Bulk transport of materials into and out of cells.

Example: Sodium-potassium pump ( ATPase) maintains ion gradients in animal cells.

Table: Comparison of Transport Mechanisms

Equations and Formulas

• Osmotic Pressure:

• Diffusion Rate (Fick's Law):

Summary

This study guide covers the major macromolecules, cell structure, and membrane function, providing foundational knowledge for understanding biological systems.