Cell Theory and Basic Cell Structure

Overview of Cell Theory

• Cells are the building blocks of all plants and animals.

• All cells arise from the division of preexisting cells.

• Cells are the smallest units that perform all vital physiological functions.

• Each cell maintains homeostasis at the cellular level.

Three Basic Components of Any Cell

• Plasma Membrane: Flexible outer boundary of the cell, acts as a selectively permeable barrier.

• Cytoplasm: Intracellular fluid packed with organelles, small structures that perform specific cell functions.

• Nucleus: The control center, an organelle that controls cellular activities; the nucleus lies near the cell's center.

Cell Components and Their Functions

Major Organelles and Structures

• Plasma Membrane: Flexible outer boundary, regulates entry and exit of substances.

• Cytoplasm: Fluid matrix containing organelles.

• Nucleus: Contains genetic material (DNA), controls cell activities.

• Mitochondria: Site of ATP (energy) production.

• Ribosomes: Sites of protein synthesis.

• Endoplasmic Reticulum (ER): Synthesizes proteins (rough ER) and lipids (smooth ER).

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Contain digestive enzymes for breakdown of waste.

• Peroxisomes: Detoxify harmful substances.

• Microvilli: Increase surface area for absorption.

Plasma Membrane: Structure and Function

Phospholipid Bilayer and Fluid Mosaic Model

• The plasma membrane is primarily composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• The Fluid Mosaic Model describes the membrane as a dynamic structure with various molecules moving within the lipid bilayer.

Lipid Components

• Phospholipids:

  • Hydrophilic heads: Attracted to water; form inner and outer surfaces of the membrane.

  • Hydrophobic tails: Avoid water; form the center of the membrane.

• Cholesterol: Stabilizes the membrane, making it less fluid and more rigid. Comprises about 20% of membrane lipids.

• Lipid Rafts: Specialized membrane microdomains rich in cholesterol and sphingolipids; function in cell signaling and membrane trafficking.

Protein Components

• Integral Proteins: Span the membrane; have both hydrophobic and hydrophilic regions.

  • Function as channels, carriers, receptors, enzymes, or cell-adhesion molecules.

• Peripheral Proteins: Loosely attached to integral proteins or membrane lipids; function in signaling or maintaining cell shape.

Glycocalyx and Cell Recognition

• Glycocalyx: Sugar coating on the cell surface formed by glycoproteins and glycolipids.

  • Each cell type has a unique pattern of sugars, serving as identification tags for cell recognition.

  • Allows immune system to distinguish "self" from "non-self".

  • Cancer cells may alter their glycocalyx to evade immune detection.

Summary Table: Major Plasma Membrane Components

Types and Functions of Integral Proteins

• Transport: Allow substances to cross the membrane via channels or carriers.

• Receptors for Signal Transduction: Bind chemical messengers (e.g., hormones), triggering cellular responses.

• Enzymatic Activity: Catalyze reactions at the membrane surface.

• Cell-Cell Recognition: Glycoproteins serve as identification tags recognized by other cells.

• Cell Joining: Membrane proteins of adjacent cells may hook together in various kinds of intercellular junctions.

• Attachment to Cytoskeleton and Extracellular Matrix: Maintain cell shape and stabilize membrane location.

Cell Junctions

Types of Cell Junctions and Their Functions

• Tight Junctions:

  • Form impermeable seals between adjacent cells.

  • Prevent passage of fluids and most molecules between cells.

  • Location: Epithelial cells of organs, kidney tubules.

• Desmosomes:

  • Anchor cells together at plaques (thickenings on plasma membrane).

  • Provide mechanical stability, especially in tissues subject to stress (e.g., cardiac muscle, skin).

• Gap Junctions:

  • Transmembrane proteins (connexons) form channels allowing small molecules and ions to pass between cells.

  • Location: Cardiac and smooth muscle, nervous system.

Membrane Transport Mechanisms

Passive Transport

• Definition: Movement of substances across the membrane without energy input (no ATP required).

• Substances move down their concentration gradient.

• Types:

  • Simple Diffusion: Direct movement of small, nonpolar molecules (e.g., O2, CO2) through the lipid bilayer.

  • Facilitated Diffusion: Movement of larger or polar molecules via membrane proteins (channels or carriers).

  • Osmosis: Diffusion of water through aquaporins.

  • Filtration: Movement of water and solutes across a membrane due to hydrostatic pressure (e.g., in capillaries).

Factors Affecting Diffusion Rate

• Molecule Size: Smaller molecules diffuse faster.

• Temperature: Higher temperature increases diffusion rate.

• Concentration Gradient: Greater difference increases rate.

Solute vs. Solvent

• Solute: Substance dissolved in a solution (e.g., salt).

• Solvent: Substance present in the greatest amount, dissolves the solute (e.g., water in biological systems).

Factors Determining Membrane Permeability

• Lipid Solubility: Lipid-soluble substances (e.g., O2, CO2, steroid hormones) diffuse easily.

• Size: Small molecules pass more readily than large ones.

Facilitated Diffusion: Carrier vs. Channel-Mediated

• Carrier-Mediated: Specific molecules bind to carrier proteins, which change shape to transport them across the membrane. Example: Glucose transport.

• Channel-Mediated: Ions or water move through protein channels. Example: Sodium and potassium ions.

• Osmosis: Water moves through aquaporins, specialized channels for water transport.

Summary Table: Types of Membrane Transport

Key Equations

• Fick's Law of Diffusion:

• Where J is the rate of diffusion, D is the diffusion coefficient, and is the concentration gradient.

Examples and Applications

• Osmosis in Kidney Tubules: Aquaporins facilitate water reabsorption, crucial for urine concentration.

• Tight Junctions in Intestinal Epithelium: Prevent leakage of digestive enzymes and pathogens.

• Desmosomes in Cardiac Muscle: Provide mechanical strength to withstand contraction forces.

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.