Prokaryotic and Eukaryotic Cell Structure and Function: Study Guide

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Overview of Cell Types

Cells are classified as either prokaryotic or eukaryotic based on structural and genetic differences. Understanding these differences is fundamental to microbiology.

• Prokaryotes: Include Bacteria and Archaea. Characterized by the absence of a nucleus and membrane-bound organelles.

• Eukaryotes: Include Protists, Algae, and Fungi. Possess a true nucleus and various organelles.

Unique Features of Prokaryotes

• Genetic Material: Single, circular chromosome located in the nucleoid region.

• Cell Wall Chemicals: Peptidoglycan (in bacteria), unique cell wall components in archaea.

• Cell Division: Binary fission (asexual reproduction).

Unique Features of Eukaryotes

• Genetic Material: Multiple, linear chromosomes within a membrane-bound nucleus.

• Cell Wall Chemicals: Cellulose (in algae), chitin (in fungi), or absent (in some protists).

• Cell Division: Mitosis (and meiosis for sexual reproduction).

• Organelles: Mitochondria, endoplasmic reticulum, Golgi apparatus, etc.

• Histones: Proteins associated with DNA packaging.

Structures Common to Both Prokaryotes and Eukaryotes

• Cell Membrane: Phospholipid bilayer with embedded proteins; semi-permeable and involved in transport (fluid mosaic model).

• Cytoplasm: Gel-like substance containing water, enzymes, nutrients, wastes, and gases.

• Ribosomes: Sites of protein synthesis (70S in prokaryotes, 80S in eukaryotes).

• Genetic Material: DNA as the hereditary material.

Bacterial Shapes and Arrangements

Basic Shapes

• Coccus (spherical)

• Bacillus (rod-shaped)

• Spirillum (spiral)

Arrangements (Example: Cocci)

• Diplococci: Pairs

• Streptococci: Chains

• Staphylococci: Clusters

• Tetrads: Groups of four

• Sarcinae: Cubes of eight

Cell Membranous Structures

Cell Membrane

• Structure: Phospholipid bilayer with proteins.

• Function: Selective permeability, transport, energy generation, and cell signaling.

• Fluid Mosaic Model: Describes the dynamic arrangement of lipids and proteins.

Cell Wall

• Function: Maintains cell shape, prevents osmotic lysis, and provides structural support.

• Peptidoglycan: Main component in bacterial cell walls; composed of N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) linked by peptide bridges.

• Gram-Positive Bacteria: Thick peptidoglycan layer, teichoic acids.

• Gram-Negative Bacteria: Thin peptidoglycan layer, outer membrane with lipopolysaccharide (LPS), periplasmic space.

• Acid-Fast Bacteria: Mycolic acid in cell wall (e.g., Mycobacterium).

• Atypical Cell Walls: Archaea (pseudopeptidoglycan or S-layer), Mycoplasma (no cell wall).

Gram-Negative Cell Wall Components

• Outer Membrane: Contains LPS, porins, and proteins.

• Lipopolysaccharide (LPS): Composed of O-polysaccharide (antigenic), core polysaccharide, and Lipid A (endotoxin).

• Periplasm: Gel-like space between inner and outer membranes; contains enzymes and transport proteins.

• Porins: Channels for small molecules.

Gram-Positive Cell Wall Components

• Peptidoglycan: Thick, multilayered.

• Teichoic Acids: Provide rigidity and regulate cation movement.

Capsule

• Structure: Polysaccharide or polypeptide layer outside the cell wall.

• Function: Protection from phagocytosis, aids in attachment, contributes to virulence.

Appendages

Flagella

• Role: Motility (movement).

• Patterns:

  • Monotrichous: Single flagellum at one end

  • Amphitrichous: Single flagellum at both ends

  • Lophotrichous: Tuft of flagella at one or both ends

  • Peritrichous: Flagella distributed over the entire cell

• Structure: Filament (flagellin protein), hook, basal body (anchors flagellum to cell wall and membrane).

Movement and Taxis

• Chemotaxis: Movement in response to chemical stimuli.

• Mechanism:

  • Counter-clockwise (CCW) rotation: "Run" (straight movement)

  • Clockwise (CW) rotation: "Tumble" (random reorientation)

  • Receptors detect attractants or repellents, biasing movement (random biased walk).

Axial Filaments (Endoflagella)

• Location: Found in spirochetes.

• Function: Enables movement in viscous environments (e.g., mucus, mud).

Pili and Fimbriae

• Fimbriae: Short, numerous; used for attachment to surfaces and host tissues.

• Pili: Longer, fewer; involved in attachment and conjugation (sex pilus for DNA transfer).

• Role in Virulence: Aid in colonization and infection.

Cytoplasmic Structures

Cytoplasm

• Contents: Water, enzymes, nutrients, ribosomes, DNA, inclusion bodies.

DNA and Nucleoid

• Structure: Single, circular DNA molecule in prokaryotes; multiple, linear chromosomes in eukaryotes.

• Location: Nucleoid region (prokaryotes); nucleus (eukaryotes).

Ribosomes

• Prokaryotic Ribosomes: 70S (50S + 30S subunits).

• Eukaryotic Ribosomes: 80S (60S + 40S subunits).

• Function: Protein synthesis.

Mitochondria

• Present in: Eukaryotes only.

• Function: ATP production via aerobic respiration.

Inclusion Bodies

• Types: Storage granules for nutrients (e.g., glycogen, polyphosphate, sulfur granules).

• Function: Reserve deposits; help prevent osmotic lysis by reducing solute concentration in cytoplasm.

Endospores

Structure and Function

• Produced by: Certain Gram-positive bacteria (e.g., Bacillus, Clostridium).

• Function: Survival under harsh conditions (heat, desiccation, chemicals, radiation).

• Not for Reproduction: Endospores are dormant, non-reproductive structures; one cell forms one spore and vice versa.

Life Cycle

• Sporulation: Formation of an endospore from a vegetative cell.

• Germination: Return of the endospore to a metabolically active vegetative cell.

Key Table: Comparison of Prokaryotic and Eukaryotic Cell Features

Feature	Prokaryotes	Eukaryotes
Nucleus	Absent	Present
Chromosomes	Single, circular	Multiple, linear
Cell Wall	Peptidoglycan (bacteria), varied (archaea)	Cellulose (algae), chitin (fungi), absent (animals)
Ribosomes	70S	80S (cytoplasm), 70S (mitochondria/chloroplasts)
Organelles	Absent	Present
Cell Division	Binary fission	Mitosis/meiosis

Sample Review Questions and Answers

1. What are the three basic shapes of microbes? What is meant by arrangement (cocci)? Answer: Cocci (spherical), bacilli (rod-shaped), and spirilla (spiral). Arrangement refers to the grouping pattern of cells after division, such as diplococci (pairs), streptococci (chains), and staphylococci (clusters).

2. Describe these parts of the prokaryotic cell:

   • Flagella: Composed of filament, hook, and basal body; responsible for motility.

   • Chemotaxis: Movement toward or away from chemicals; involves CCW (run) and CW (tumble) rotation, receptors, attractants, and repellents.

   • Pili: Hair-like structures for attachment and DNA transfer (conjugation).

   • Peptidoglycan: Polymer of NAM and NAG with peptide cross-links; thick in Gram-positive, thin in Gram-negative.

   • LPS Layer: Outer membrane of Gram-negative bacteria; contains O-polysaccharide, Lipid A (endotoxin).

   • Periplasm: Space between inner and outer membranes in Gram-negative bacteria.

   • Nucleoid: Region containing the bacterial chromosome.

   • Ribosomes: 70S, site of protein synthesis.

   • Inclusions: Storage granules for nutrients.

   • Endospores: Dormant, resistant structures for survival.

3. Why are endospores only considered as survival structures, not reproductive or multiplication structures? Answer: Because each vegetative cell forms only one endospore, and each endospore germinates into one cell; there is no increase in cell number.

4. What are the two processes that a) form a spore from the vegetative cell and b) form a cell from a spore called? Answer: a) Sporulation; b) Germination.

Additional info: This guide expands on the study points with definitions, examples, and a comparison table for clarity and exam preparation.