Functional Anatomy of Prokaryotic and Eukaryotic Cells

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Functional Anatomy of Prokaryotic and Eukaryotic Cells

Overview

This chapter explores the structural and functional differences between prokaryotic and eukaryotic cells, focusing on bacterial morphology, anatomy, and the mechanisms by which substances move across cellular membranes. Understanding these differences is fundamental to microbiology, as it underpins microbial classification, physiology, and the basis for many antimicrobial strategies.

Prokaryotic vs. Eukaryotic Cells

Key Differences

Prokaryotic Cells: DNA is not enclosed in a membrane, lack membrane-bound organelles, contain peptidoglycan in their cell walls, and divide by binary fission. Their DNA is 'naked' (not associated with histones).
Eukaryotic Cells: Possess a true nucleus with DNA associated with histones, contain membrane-bound organelles (e.g., mitochondria, ER, Golgi apparatus), have simple cell walls when present, and divide by mitosis.

Labeled diagram of a eukaryotic cell with organelles

Size, Shape, and Arrangement of Bacterial Cells

Bacterial Morphology

Size: Typically 0.2–2.0 μm in diameter and 2–8 μm in length.
Shapes: Cocci (spherical), Bacilli (rod-shaped), Spirals (vibrio, spirillum, spirochete).
Arrangements: Determined by planes of division and whether cells remain attached after division (e.g., diplo-, strepto-, staphylo-).

Bacterial cocci arrangements Bacilli arrangements Spiral bacterial shapes

Bacterial Anatomy

Major Structures

Glycocalyx: A polysaccharide/polypeptide layer outside the cell wall; can be a capsule (firmly attached) or slime layer (loosely attached). Functions in protection from phagocytosis, attachment, and environmental resistance.
Flagella: Long, filamentous appendages for motility, composed of flagellin (filament), hook, and basal body. Enable taxis (movement toward/away from stimuli).
Fimbriae and Pili: Fimbriae are short, hair-like structures for attachment; pili are longer and involved in DNA transfer (conjugation) and motility.
Cell Wall: Provides shape and protection; composed mainly of peptidoglycan in bacteria.
Cytoplasm: Gel-like substance inside the plasma membrane containing water, proteins, DNA, ribosomes, and inclusions.
Nucleoid: Region containing the bacterial chromosome (circular, double-stranded DNA) and sometimes plasmids.
Ribosomes: Sites of protein synthesis; prokaryotic ribosomes are 70S (smaller than eukaryotic 80S).
Inclusions: Reserve deposits of nutrients and other substances.
Endospores: Highly resistant, dormant structures formed by some Gram-positive bacteria under stress.

Labeled diagram of a prokaryotic cell

Flagella and Motility

Structure and Function

Flagella: Enable motility; composed of filament, hook, and basal body. The arrangement of flagella can be monotrichous (single), lophotrichous (tuft at one end), amphitrichous (one at each end), or peritrichous (all over the cell).
Motility: Bacteria move by "runs" (straight movement) and "tumbles" (random changes in direction), often in response to chemical (chemotaxis) or light (phototaxis) stimuli.

Flagellum structure Flagella arrangements Run and tumble motility

Axial Filaments (Endoflagella)

Found in spirochetes; wrap around the cell, anchored at one end, causing a corkscrew motion that enables movement through viscous environments (e.g., body fluids).

Axial filament structure in spirochetes Axial filament in spirochete

Fimbriae and Pili

Structure and Function

Fimbriae: Short, numerous, used for attachment to surfaces (important in pathogenesis).
Pili: Longer, usually 1–2 per cell; involved in DNA transfer (conjugation) and sometimes motility.

Fimbriae and pili on bacterial cell Fimbriae on bacterial cell Pilus joining two bacterial cells

Bacterial Cell Wall

Composition and Function

Peptidoglycan: Main component; consists of repeating disaccharides (N-acetylglucosamine and N-acetylmuramic acid) cross-linked by peptides.
Function: Maintains shape, prevents osmotic lysis, and provides protection.
Antibiotic Target: Penicillin inhibits peptidoglycan synthesis, leading to cell lysis.

Gram-positive cell wall structure

Gram-Positive vs. Gram-Negative Cell Walls

Gram-Positive: Thick peptidoglycan layer, teichoic acids (regulate cation movement, cell growth).
Gram-Negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharides (LPS), lipoproteins, and phospholipids. The outer membrane acts as a barrier to certain antibiotics and contains lipid A (endotoxin).

Gram-negative cell wall structure

Atypical Cell Walls

Mycoplasma: Lack cell walls; smallest bacteria.
Archaea: Contain pseudopeptidoglycan or other variants.
Mycobacterium: Contain mycolic acid (waxy lipid) instead of peptidoglycan.

Atypical cell wall structure

Plasma Membrane

Structure and Function

Phospholipid Bilayer: Fluid mosaic model; selectively permeable barrier.
Proteins: Integral and peripheral proteins serve as transporters, enzymes, or receptors.
Antimicrobial Target: Some antibiotics (e.g., polymyxins) disrupt the plasma membrane.

Plasma membrane structure Lipid bilayer of plasma membrane

Movement of Materials Across Membranes

Passive Transport

Simple Diffusion: Movement of molecules from high to low concentration without energy input.
Facilitated Diffusion: Utilizes carrier proteins (permeases) to transport substances down their concentration gradient.
Osmosis: Movement of water across a selectively permeable membrane. Types of solutions:
- Isotonic: Equal solute concentration inside and outside the cell.
- Hypotonic: Lower solute concentration outside; water enters, cell may lyse.
- Hypertonic: Higher solute concentration outside; water leaves, cell shrinks (plasmolysis).

Simple diffusion across plasma membrane Facilitated diffusion across plasma membrane

Active Transport

Requires energy (ATP) to move substances against their concentration gradient, often using transporter proteins.
Group Translocation: Substance is chemically modified during transport (e.g., glucose uptake in bacteria).

Active transport across plasma membrane

Cytoplasm

Composition and Function

80% water, contains proteins, carbohydrates, lipids, ions, DNA, ribosomes, and inclusions.
Site of most cellular processes.

Nuclear Area (Nucleoid) and Plasmids

Genetic Material

Nucleoid: Contains the bacterial chromosome (circular, double-stranded DNA), not surrounded by a membrane.
Plasmids: Small, circular DNA molecules; carry genes for antibiotic resistance, toxin production, and can be transferred between cells.

Ribosomes

Protein Synthesis

Composed of rRNA and protein; prokaryotic ribosomes are 70S, eukaryotic are 80S.
Target for many antibiotics (e.g., streptomycin, tetracycline).

Inclusions

Storage Granules

Reserve deposits for nutrients such as phosphate (volutin), glycogen, starch, lipids, sulfur, and gas vacuoles.

Endospores

Survival Structures

Formed by some Gram-positive bacteria (e.g., Bacillus, Clostridium) in response to adverse conditions.
Highly resistant to heat, desiccation, chemicals; important in food safety and sterilization.
Endospore formation is called sporulation; return to active state is germination.

Summary Table: Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No true nucleus	True nucleus present
DNA	Circular, naked, no histones	Linear, associated with histones
Organelles	Absent	Present (mitochondria, ER, etc.)
Cell Wall	Peptidoglycan (most)	Cellulose, chitin, or none
Division	Binary fission	Mitosis/meiosis
Ribosomes	70S	80S (cytoplasm), 70S (organelles)