Prokaryotic and Eukaryotic Cells

Introduction

  • Overview of the lecture focusing on prokaryotic cell structures.

Prokaryotic Cell Structures

  • Cells are primarily classified into two categories: prokaryotic and eukaryotic.

Size and Scale

  • Size comparison: Human cells are larger than bacteria; yeast cells are also relatively small.

  • Scale: Bacteria are significantly smaller than human cells, often not visible without microscopy.

  • Significance: There are more bacterial cells in the human body than human cells, which play vital roles in gut health, immune system support, and nutrient absorption.

Theory of Cell Generation

  • Biogenesis: Cells arise only from preexisting cells.

  • Spontaneous Generation: Disproven theory, famously tested by Louis Pasteur using the Swan's Neck experiment, demonstrating that microbes do not spontaneously generate.

Cell Types

Prokaryotic vs. Eukaryotic Cells

  • Prokaryotic cells: lack internal organelles and membrane-bound nucleus (nucleoid region instead).

  • Eukaryotic cells: possess a membrane-bound nucleus, organelles like mitochondria and Golgi apparatus.

  • Comparison: Prokaryotic cells can perform functions such as respiration and movement, similar to eukaryotic cells, but are considered simpler in structure.

Cell Shapes of Prokaryotes

Cocci (spherical) and Bacilli (rod-shaped)

  • Cocci arrangements:

    • Singly: individual cocci.

    • Pairs: diplococci.

    • Chains: streptococci.

    • Clusters: staphylococci.

  • Bacilli arrangements:

    • Singly: individual bacilli.

    • Chains: streptobacilli (can grow and split).

    • Variations in shape like vibrios (curved) and spirochetes (spiral).

Pleomorphic Bacteria

  • Definition: Refers to bacteria capable of varying shapes and sizes in response to environmental conditions.

Cell Structures and Functions

Nucleoid and Chromosome Structure

  • Nucleoid: region containing a single circular chromosome (DNA) comprising all essential genetic information.

  • Plasmid: small, circular DNA that can confer advantageous traits (e.g., antibiotic resistance).

Cytoplasm Composition

  • Composition: Gelatinous medium containing proteins, DNA, ribosomes, and organelles.

  • Cytoplasm Functions: Enzymatic reactions, glycolysis, and TCA cycles take place within the cytoplasm.

Ribosomes

  • Function: Sites for protein synthesis, consisting of a small (30S) and large (50S) subunit, forming a 70S ribosome.

  • Structure: Ribosomes read mRNA to synthesize proteins.

Membrane Structure and Function

  • Cell Membrane: Critical for maintaining cellular integrity and selective permeability. Functions to control the internal environment by regulating what enters and exits the cell.

  • Fluid Mosaic Model: Describes the dynamic nature of cell membranes, allowing for flexibility and adaptability.

Transport Mechanisms

  • Diffusion: Movement of molecules along a concentration gradient; includes simple diffusion (e.g., O2, CO2) and facilitated diffusion (requires transport proteins).

  • Active Transport: Moves substances against their concentration gradient using energy (ATP).

    • Example: Glucose uptake can be via active transport, where glucose is phosphorylated to trap it within the cell.

Cell Wall and Gram Staining

Structure and Function

  • Cell Wall: Provides structural support and shape, preventing lysis from osmotic pressure.

  • Types of Bacterial Cell Walls:

    • Gram-positive: Thick peptidoglycan layer with teichoic acids.

    • Gram-negative: Thin peptidoglycan layer with an outer lipopolysaccharide (LPS) membrane, which can influence virulence.

Antibiotic Action

  • Penicillin: Works primarily on gram-positive bacteria by inhibiting peptidoglycan synthesis, leading to cell lysis.

  • Gram-negative resistance: Often more resistant to antibiotics due to the protective outer membrane and effective porins that limit access to harmful substances.

Glycocalyx and Biofilms

Glycocalyx Functions

  • Types: Can be a capsule (rigid) or slime layer (looser structure).

  • Functions: Provides protection against desiccation, helps resist phagocytosis, and promotes adherence to surfaces (biofilms).

Biofilms

  • Formed by communities of bacteria protected by extracellular polysaccharides, often found in health care-associated infections (HAIs).

Appendages of Prokaryotes

Flagella

  • Function: Provides motility through spinning motion; different arrangements (e.g., monotrichous, lophotrichous).

  • Chemotaxis: Ability to move toward or away from chemical signals; bacteria can detect gradients.

Fimbriæ and Pili

  • Fimbriæ: Hair-like structures that assist in attachment to surfaces.

  • Sex Pili: Facilitate genetic exchange between bacterial cells through conjugation.

Eukaryotic Overview

Complexity of Eukaryotes

  • Eukaryotic cells are more complex with a variety of organelles, providing specialized functions.

  • Examples include mitochondria (energy production) and chloroplasts (photosynthesis).

Endosymbiotic Theory

  • Explanation of the origin of mitochondria and chloroplasts as descended from prokaryotic cells engulfed by early eukaryotic ancestors.

Conclusion

  • Cells are foundational to biological functions, with prokaryotic cells demonstrating essential life functions with simpler organization. This understanding forms a basis for further study in microbiology, immunology, and cellular biology.

Introduction

  • This lecture provides an in-depth exploration of prokaryotic cell structures, spanning the domains Bacteria and Archaea.

  • Detailed analysis includes the chemical composition, functional mechanisms, and evolutionary significance of specialized cellular components.

Prokaryotic Cell Structures

  • Cells are primary classified into prokaryotic and eukaryotic categories based on the presence of membrane-bound compartments.

Size and Scale

  • Microscopic Dimensions: Most bacteria range from 0.2 to 2.0 \mu m in diameter and 2 to 8 \mu m in length.

  • Surface Area-to-Volume (SA:V) Ratio:

    • Being small allows prokaryotes to maintain a high SA:V ratio, facilitating rapid nutrient uptake and waste removal.

    • This efficiency supports faster metabolic rates and shorter generation times compared to larger eukaryotic cells.

  • Significance: The human microbiome contains approximately 10^{13} to 10^{14} bacterial cells, outweighing human cells in number and performing essential metabolic functions.

Theory of Cell Generation

  • Biogenesis: The principle that living cells arise only from preexisting living cells.

  • Spontaneous Generation: The historical belief that life could arise from non-living matter.

  • Pasteur’s Swan’s Neck Experiment:

    • Louis Pasteur utilized S-shaped flasks to allow air entry while trapping dust and microbes in the neck.

    • This demonstrated that boiling killed existing microbes and that new microbes only appeared if the broth was exposed to contaminated air, definitively disproving abiogenesis for microorganisms.

Cell Types Comparison

  • Prokaryotes: DNA is not enclosed in a membrane (nucleoid), lacks histones (except for some Archaea), and lacks membrane-bound organelles.

  • Eukaryotes: DNA is found in a nucleus, associated with histones, and contains complex organelles (mitochondria, chloroplasts, lysosomes).

  • Metabolic Parity: While structurally simpler, prokaryotes possess all metabolic pathways (e.g., glycolysis, Krebs cycle) found in eukaryotes, though these occur in the cytoplasm or at the plasma membrane.

Morhpology and Arrangements
Basic Shapes

  • Cocci (Spherical):

    • Diplococci: Pairs remaining attached after division.

    • Streptococci: Chain-like patterns.

    • Tetrads: Groups of four in two planes.

    • Sarcinae: Cube-like groups of eight in three planes.

    • Staphylococci: Grape-like clusters in multiple planes.

  • Bacilli (Rod-shaped):

    • Diplobacilli: Pairs after division.

    • Streptobacilli: Chains.

    • Coccobacilli: Oval-shaped rods that resemble cocci.

  • Spiral Bacteria:

    • Vibrios: Curved rods.

    • Spirilla: Rigid corkscrew shape, moving via flagella.

    • Spirochetes: Flexible spirals, moving via axial filaments (endoflagella).

Pleomorphic Bacteria

  • Definition: Bacteria like Mycoplasma and Corynebacterium lack a rigid cell wall or have variable shapes, allowing them to change morphology in response to chemical and physical stressors.

Internal Structures and Functions
Nucleoid and DNA

  • The Bacterial Chromosome: A single, continuous, circular thread of double-stranded DNA containing essential genes. It is supercoiled to fit within the nucleoid region.

  • Plasmids: Extra-chromosomal DNA elements (1-200 kb) that replicate independently. They carry non-essential but advantageous genes, such as those for antibiotic resistance (R-factors) or toxin production.

Ribosomes

  • Function: Protein synthesis via translation of mRNA.

  • Structure: Prokaryotic ribosomes are 70S (composed of a small 30S subunit and a large 50S subunit).

  • Significance: The structural difference between prokaryotic 70S and eukaryotic 80S ribosomes is a target for selective toxicity in antibiotics (e.g., tetracycline).

Membrane Structure and Function

  • Plasma Membrane: A phospholipid bilayer containing proteins but generally lacking sterols (unlike eukaryotes which use cholesterol; bacteria often use hopanoids instead).

  • Selective Permeability: Regulates the passage of ions and molecules. Small nonpolar molecules (like O_2) pass easily, while ions require transport proteins.

  • Transport Mechanisms:

    1. Passive Transport: Simple diffusion and facilitated diffusion (via permeases).

    2. Active Transport: Requires ATP or proton motive force to move solutes against a gradient.

    3. Group Translocation: A specialized form of active transport unique to prokaryotes (e.g., the PEP-phosphotransferase system) where the substance is chemically altered (phosphorylated) as it enters the cell, preventing its exit.

The Bacterial Cell Wall
Peptidoglycan (Murein)

  • A polymer of repeating disaccharides: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

  • These chains are cross-linked by tetrapeptide side chains and peptide cross-bridges to create a rigid lattice.

Gram-Positive vs. Gram-Negative

  • Gram-Positive:

    • Characterized by thick layers of peptidoglycan.

    • Contains Teichoic Acids (wall teichoic and lipoteichoic), which regulate cation movement and provide antigenic specificity.

  • Gram-Negative:

    • Contains a thin layer of peptidoglycan located in the periplasmic space.

    • Features an Outer Membrane containing Lipopolysaccharides (LPS), lipoproteins, and phospholipids.

    • LPS Structure: Consists of Lipid A (an endotoxin), Core polysaccharide, and O-polysaccharide (O-antigen).

    • Porins: Channels in the outer membrane that control the entry of specific molecules, contributing to antibiotic resistance.

Medical Impacts

  • Lysozyme: An enzyme found in tears/saliva that hydrolyzes the bonds between NAG and NAM, destroying the cell wall.

  • Penicillin: Interferes with the final stage of peptidoglycan synthesis (cross-linking), making the cell susceptible to osmotic lysis.

External Structures
Glycocalyx

  • Capsule: Well-organized and firmly attached; prevents phagocytosis (e.g., Streptococcus pneumoniae).

  • Slime Layer: Unorganized and loosely attached; facilitates adherence and formation of Biofilms.

Motility and Appendages

  • Flagella: Long filamentous appendages consisting of the filament (flagellin protein), hook, and basal body (motor).

    • Taxis: Movement toward (positive) or away (negative) from stimuli like chemicals (chemotaxis) or light (phototaxis) through "runs" and "tumbles."

  • Fimbriae: Short, hair-like appendages used for attachment to surfaces or host tissues.

  • Pili: Longer than fimbriae; used for motility (twitching/gliding) and DNA transfer via conjugation.

Evolutionary Perspective: Endosymbiotic Theory

  • Proposes that mitochondria and chloroplasts originated as symbiotic prokaryotes.

  • Evidence: These organelles have their own circular DNA, 70S ribosomes, binary fission reproduction, and double membranes mimicking the engulfment process.

Conclusion

  • The intricate structural adaptations of prokaryotes enable them to survive in diverse and extreme environments, making them fundamental to the Earth's ecosystem and human health.