Cell Structure and Function in Microbiology

Cell Structure and Function

Introduction

This section provides an overview of the fundamental differences between prokaryotic and eukaryotic cells, as well as the characteristics of life as they pertain to microbes. Understanding these differences is essential for the study of microbiology, as it forms the basis for classifying and understanding microbial life.

Types of Cells

Prokaryotic Cells

Prokaryotes are unicellular organisms that lack a membrane-bound nucleus and other internal organelles. They are structurally simpler than eukaryotes and include the domains Bacteria and Archaea.

• Lack nucleus: Genetic material is not enclosed within a membrane-bound nucleus.

• Simultaneous transcription and translation: Prokaryotes can read DNA and synthesize proteins at the same time.

• Lack internal membrane-bound organelles: No mitochondria, endoplasmic reticulum, or Golgi apparatus.

• Size: Typically 1.0 μm in diameter or smaller.

• Examples: Bacteria and Archaea.

Eukaryotic Cells

Eukaryotes are organisms whose cells contain a true nucleus and various membrane-bound organelles. They are generally larger and more complex than prokaryotes.

• Have nucleus: DNA is enclosed within a nuclear membrane.

• Internal membrane-bound organelles: Includes mitochondria, endoplasmic reticulum, Golgi apparatus, etc.

• Size: Typically 10–100 μm in diameter.

• Complex structure: Greater compartmentalization allows for specialized functions.

• Examples: Algae, protozoa, fungi, animals, and plants.

Examples of Cell Types

• Bacterial cell (SEM image): Shows rod-shaped prokaryotic cells.

• Protozoan cell (LM image): Shows a eukaryotic cell with visible internal structures.

Characteristics of Life and Their Distribution in Microbes

Microbes can be classified based on several characteristics of life. The table below summarizes these characteristics and their presence in different groups of microbes.

Characteristic	Bacteria, Archaea, Eukaryotes	Viruses
Growth (increase in size)	Occurs in all	Growth does not occur
Reproduction (increase in number)	Occurs in all	Host cell replicates the virus
Responsiveness (ability to react to environmental stimuli)	Occurs in all	Reaction to host cells seen in some viruses
Metabolism (controlled chemical reactions of organisms)	Occurs in all	Viruses use host cell's metabolism
Cellular structure (membrane-bound structure capable of all functions)	Present in all	Viruses lack cytoplasmic membrane or cellular structure

Key Terms

• Prokaryote: An organism whose cells lack a nucleus and membrane-bound organelles.

• Eukaryote: An organism whose cells contain a nucleus and membrane-bound organelles.

• Virus: An acellular infectious agent that requires a host cell for replication and lacks independent metabolism.

Example

• Bacteria: Escherichia coli is a common prokaryotic bacterium found in the intestines of humans and animals.

• Protozoa: Paramecium is a eukaryotic microorganism commonly found in freshwater environments.