Cell-Structure-and-Organization--Understanding-Prokaryotic-and-Eukaryotic-Cells

Cell Structure and Organization

Understanding Prokaryotic and Eukaryotic Cells

  • Instructor: Ricardo V. Villanueva

Introduction to Cell Types

  • Traditional Classification: Prokaryotic vs Eukaryotic cells

  • Modern Classification: Archaea, Bacteria, and Eukaryotes

    • Key distinction: Presence or absence of nucleus

  • Terminology Shift: Movement away from "prokaryote" due to genetic differences

Prokaryotic Cells

  • Characteristics:

    • Lack a nucleus

    • Lack various internal structures bound by phospholipid membranes

    • Generally small in size (~1.0 µm in diameter)

    • Simple structure

    • Includes bacteria and archaea

Eukaryotic Cells

  • Characteristics:

    • Have a defined nucleus

    • Possess internal membrane-bound organelles

    • Larger in size (10-100 µm in diameter)

    • More complex structure

    • Includes algae, protozoa, fungi, animals, and plants

Cell Morphology Basics

  • Definition: Shape and structure of cells

  • Importance for Function:

    • Growth

    • Reproduction

    • Nutrient acquisition

    • Movement

  • Essential for maintaining cell shape and proper function

Common Bacterial Shapes

  • Coccus (plural: cocci): Spherical cells

  • Bacillus (plural: bacilli): Rod-shaped cells

  • Vibrio: Single curve

  • Spirilla/Spirochetes: Spiral shapes

  • Curved Rods: Pleomorphic - variable shapes

Cell Size and Surface-to-Volume Ratio

  • Average Sizes:

    • Bacterial/Archaeal cells: Few micrometers

    • Eukaryotic cells: ~10x larger

  • Surface-to-Volume Ratio Calculation:

    • Small cell (r=1 µm): Ratio = 3

    • Large cell (r=2 µm): Ratio = 1.5

Importance of Surface-to-Volume Ratio

  • Faster nutrient uptake

  • More efficient waste removal

  • Quicker growth and reproduction

  • Enhanced evolutionary speed due to higher ratio advantages

Exceptional Cases: Giant Bacteria

  • Size: 100-750 µm in length

  • Comparison: E. coli = 4 µm

  • Thiomargarita namibiensis:

    • Features large vacuoles, ruffled membranes for surface area maximization

  • Adaptations for survival in varying conditions

Universal Cell Components

Cytoplasm

  • Characteristics:

    • Gel-like fluid with aqueous environment

    • Composed of water, salts, and proteins

DNA

  • Role:

    • Genetic material organization

    • Found in nucleoid (prokaryotes) or nucleus (eukaryotes)

Ribosomes

  • Function: Protein synthesis factories

    • Bacterial/Archaeal: 70S

    • Eukaryotic: 80S

The Plasma Membrane

  • Primary Functions:

    • Selective barrier

    • Energy conservation

    • Environmental sensing

    • Cellular communication

  • Essential for cell survival

Plasma Membrane Functions

  • Semi-permeable Barrier for nutrient intake and waste excretion

  • Protection from toxins and energy production (PMF)

  • Signal reception mechanisms

Membrane Structure

Fluid-Mosaic Model

  • Composed of:

    • Phospholipid bilayer with hydrophilic heads and hydrophobic tails

    • Specific organization impacts function

Membrane Proteins

  • Types:

    • Integral Proteins (70-80%): Embedded within bilayer, some with lipid anchors

    • Peripheral Proteins (20-30%): Associated with membrane surface

Bacterial Cell Walls: Structure, Function, and Classification

Introduction to Cell Walls

  • Essential structural component in ~90% of bacteria

  • Two main types: Gram-positive and Gram-negative (identified using Gram stain)

    • Critical for bacterial survival and function

Functions of Bacterial Cell Walls

  • Provides structural strength and rigidity

  • Maintains cell shape and prevents osmotic lysis

  • Controls molecular transport

  • Contributes to pathogenicity and withstands internal pressure

Gram-Positive Bacterial Cell Walls

  • Characteristics:

    • Thick layer of peptidoglycan

    • Contains teichoic acids

    • Appears purple after Gram staining

    • Up to 60% mycolic acid in some bacteria for survival in desiccation

Gram-Negative Bacterial Cell Walls

  • Characteristics:

    • Thin layer of peptidoglycan

    • Bilayer membrane contains LPS, phospholipids, and proteins

    • Appears pink after Gram staining

    • Can impede treatment of diseases

Peptidoglycan Structure

  • Basic Components:

    • N-acetylglucosamine (NAG)

    • N-acetylmuramic acid (NAM)

  • Forms a lattice-like structure essential for maintaining cell integrity

Bacterial Cytoplasmic Membranes

  • Functions:

    • Energy storage and selective permeability

    • Essential for maintaining concentration and electrical gradients

  • Active and Passive Transport Mechanisms

Specialized Structures in Bacteria

Bacterial Inclusions

  • Serve functions in storage, motility, and metabolic processes

Plasmids

  • Non-essential, circular DNA providing antibiotic resistance and survival advantages

Endospores

  • Specialized survival forms in some Gram-positive bacteria resistant to extreme conditions

  • Structure includes multiple protective layers

    • Enables survival through extreme temperature and desiccation

Bacterial Surface Structures and Motility

Surface Layers Overview

  • All bacteria possess a cell membrane, many have a cell wall, and some have additional layers for survival advantages

Capsule and Slime Layer

  • Capsules provide protection and pathogenic capabilities; slime layers for adhesion and protection

Pili and Flagella

  • Fimbriae: Attachment

  • Pili: Specialized functions including DNA transfer and motility

  • Flagellar Structure: Composed of filament, hook, and motor; provides motility and propulsion through rotation

Chemotaxis

  • Definition: Movement toward/away from chemicals

  • Mechanisms for detecting and responding to environmental stimuli using sensory and motor adjustments

Conclusion

  • Understanding the structure and function of bacterial cells, including their complexity and adaptability, is crucial for microbiology and related fields.