Chapter 3: Introduction to Prokaryotic Cells – Study Guide

Introduction to Prokaryotic Cells

Overview

This chapter explores the structure, function, and diversity of prokaryotic cells, focusing on bacteria and archaea. Key cellular components, their roles, and differences between prokaryotic and eukaryotic cells are discussed, along with mechanisms of cell division, transport, and survival strategies.

Classification and Environments of Prokaryotes

Bacteria and Archaea: Environmental Adaptations

• Bacteria are found in diverse environments, often moderate conditions.

• Archaea are extremophiles, thriving in harsh environments (e.g., high temperature, salinity, acidity).

• Some archaea are methanogens (produce methane), halophiles (salt-loving), or thermophiles (heat-loving).

Cellular Arrangements and Morphology

Plasmogamy and Cell Groupings

• Plasmogamy refers to the fusion of cytoplasm from two cells, but in bacteria, the term often refers to the sharing of genetic material.

• Bacteria exhibit various shapes (cocci, bacilli, spirilla) and arrangements (chains, clusters) due to patterns of cell division and attachment.

Prokaryotic Cell Structures

Cell Wall and Glycocalyx

• Cell Wall: Provides structural support and shape; composed of peptidoglycan in bacteria.

• Glycocalyx: A polysaccharide layer outside the cell wall; includes capsules (organized, firmly attached) and slime layers (loose, unorganized).

• Functions of Glycocalyx:

  • Prevents desiccation (drying out)

  • Protects against phagocytosis

  • Facilitates attachment to surfaces

  • Can contribute to pathogenicity

Flagella, Fimbriae, and Pili

• Flagella: Long, whip-like structures for motility; rotate to propel the cell.

• Fimbriae: Short, hair-like projections for attachment to surfaces.

• Pili: Longer than fimbriae; involved in conjugation (DNA transfer) and attachment.

Cell Envelope: Gram-Positive vs. Gram-Negative Bacteria

Structural Differences

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Outer Membrane	Absent	Present (contains LPS)
Teichoic Acids	Present	Absent
Lipopolysaccharide (LPS)	Absent	Present
Periplasmic Space	Small/Absent	Present

Peptidoglycan Structure: Composed of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by peptide bridges.

Lipid A and Endotoxin

• Lipid A is a component of LPS in Gram-negative bacteria; released upon cell death, can cause endotoxic shock.

Cell Membranes and Transport

Differences Between Bacteria and Archaea

• Bacterial Membranes: Phospholipid bilayer with ester-linked fatty acids.

• Archaeal Membranes: Ether-linked lipids; may form monolayers or bilayers; branched isoprenoid chains.

Transport Mechanisms

• Passive Transport (no energy required):

  • Diffusion: Movement of molecules from high to low concentration.

  • Facilitated Diffusion: Uses membrane proteins to transport substances down their gradient.

  • Osmosis: Diffusion of water across a selectively permeable membrane.

• Active Transport (requires energy):

  • Uses ATP or proton motive force to move substances against their concentration gradient.

Osmotic Effects on Cells

• Hypotonic Solution: Water enters the cell; risk of lysis if the cell wall is weak.

• Hypertonic Solution: Water leaves the cell; cell may shrink (plasmolysis).

Internal Structures

Nucleoid

• Region where the bacterial chromosome (DNA) is located; not membrane-bound.

Cytoskeleton

• Maintains cell shape, aids in division, and organizes cellular contents.

Ribosomes

• Prokaryotic Ribosomes: 70S (composed of 50S and 30S subunits).

• Eukaryotic Ribosomes: 80S (composed of 60S and 40S subunits).

• Difference allows selective targeting by antibiotics.

Cell Division and Survival Structures

Binary Fission

• Primary method of reproduction in prokaryotes.

• Steps:

  1. DNA replication

  2. Cell elongation

  3. Septum formation and cell division

Endospores

• Dormant, highly resistant structures formed by some bacteria (e.g., Bacillus, Clostridium).

• Enable survival in harsh conditions (heat, desiccation, chemicals).

• Formation involves DNA replication, membrane invagination, cortex and coat synthesis, and release.

• Endospores can be dangerous due to their resistance and potential to cause disease.

Summary Table: Key Prokaryotic Structures and Functions

Structure	Function
Cell Wall	Shape, protection, prevents lysis
Glycocalyx	Protection, attachment, evasion of immune system
Flagella	Motility
Fimbriae	Attachment
Pili	DNA transfer, attachment
Nucleoid	Genetic material
Ribosome	Protein synthesis
Endospore	Dormancy, survival

Additional info:

• Some explanations and definitions have been expanded for clarity and completeness.

• Tables and diagrams referenced in the original notes have been recreated in text and HTML table format for accessibility.