Ch 3. Cell Structure and Function: Characterizing and Classifying Prokaryotes

Cell Structure and Function in Microbiology

Introduction

Understanding the structure and function of microbial cells is fundamental in microbiology. This section focuses on the characteristics of life in microbes, the basic unit of life (the cell), and the differences between prokaryotic and eukaryotic cells, with emphasis on prokaryotes.

Characteristics of Life in Microbes

Defining Life in Microorganisms

Microbes exhibit several characteristics that define living organisms. However, not all microbes meet every criterion, especially viruses.

• Growth: Increase in size; occurs in all bacteria, archaea, and eukaryotes, but not in viruses.

• Reproduction: Increase in number; occurs in all cellular microbes, while viruses replicate only within host cells.

• Responsiveness: Ability to react to environmental stimuli; present in all cellular microbes, some viruses respond to host signals.

• Metabolism: Controlled chemical reactions; present in all cellular microbes, viruses use host metabolism.

• Cellular Structure: Membrane-bound structure capable of all above functions; present in all cellular microbes, viruses lack true cellular structure.

Viruses are not considered living by many scientists because they do not meet all these criteria.

Table: Characteristics of Life and Their Distribution in Microbes

Characteristic	Bacteria, Archaea, Eukaryotes	Viruses
Growth	Occurs in all	Growth does not occur
Reproduction	Occurs in all	Host cell replicates the virus
Responsiveness	Occurs in all	Reaction to host cell seen in some viruses
Metabolism	Occurs in all	Viruses use host cell's metabolism
Cellular structure	Present in all	Viruses lack cytoplasmic membrane or cellular structure

The Cell: Basic Unit of Life

Types of Microbial Cells

• Prokaryotic cells: Include Bacteria and Archaea. These are small organisms composed of single cells.

• Eukaryotic cells: Include Fungi, Protozoans, and Helminths. These may be single-celled or multicellular.

Key differences between prokaryotes and eukaryotes include cell structure, presence of organelles, and complexity.

Size and Morphology of Prokaryotes

Prokaryotic Cell Size

• Prokaryotes are among the smallest living things, with diameters typically between 0.5 and 2.0 μm.

• For comparison, a human red blood cell (RBC) has a diameter of about 7 μm.

• Prokaryotes include Bacteria and Archaea.

• Some prokaryotic cells are not spherical, so diameter may not fully represent their size.

Gram Negative

• Lipopolysaccharides (LPS) are only present in Gram Negative Cell Walls.

• LPS is important for treatment of disease.

• LPS also known as endotoxin.

• Lipid portion is known as Lipid A.

• Porins are on the outer layer of Gram Neg cell walls that allow molecules to pass through it.

Characteristics of LPS

1. Dead cells release Lipid A when cell wall disintegrates.

2. Can cause fever, vasodilation, inflammation, shock, and blood clotting.

3. Can be released when antimicrobial drugs kill bacteria.

Desiccation - To Dry

Glycocalyces

2 types: Capsule and Slime Layer

Flagella

Fimbriae

Pili

Hami (only in archaea)

Prokaryotic Cell Morphology

Prokaryotic cells are classified by their size, shape, and grouping.

• Cell Shape: Common shapes include:

  • Coccus: Spherical

  • Bacillus: Rod-shaped

  • Coccobacillus: Short rod, intermediate between coccus and bacillus

  • Vibrio: Curved rod

  • Spirillum: Spiral-shaped, rigid

  • Spirochete: Spiral-shaped, flexible

  • Pleomorphic: Variable shape

• Cell Grouping: Cells may exist singly or in characteristic groupings (e.g., chains, clusters).

Summary

• Microbial life is defined by growth, reproduction, responsiveness, metabolism, and cellular structure.

• Viruses do not meet all criteria for life and are generally not considered living organisms.

• Prokaryotic cells (bacteria and archaea) are small, simple, and classified by their size, shape, and arrangement.

• Understanding cell morphology is essential for identifying and classifying microbes in microbiology.

Example: Escherichia coli is a rod-shaped (bacillus) bacterium commonly found in the human gut.