Cell Structure and Function

Introduction

This chapter provides an overview of the fundamental structures and functions of microbial cells, focusing on prokaryotes, eukaryotes, and viruses. Understanding these differences is essential for microbiology students, as cell structure underpins microbial physiology, classification, and response to environmental factors.

Processes of Life

Characteristics of Life and Distribution in Microbes

Microorganisms exhibit various characteristics that define life. These characteristics are distributed differently among prokaryotes, eukaryotes, and viruses.

Key Points:

• Prokaryotes and eukaryotes possess all characteristics of life, including growth, reproduction, responsiveness, metabolism, and cellular structure.

• Viruses lack independent growth and structure, and rely on host cells for reproduction and metabolism.

Example: Bacteria (prokaryotes) can grow and reproduce independently, while viruses must infect a host cell to replicate.

Historical Perspective: Discovery of Cells

Robert Hooke and the Cell Theory

Robert Hooke (1635–1703) was the first to describe cells based on his microscopic observations of cork. His work, published in Micrographia, laid the foundation for the cell theory, which states that the cell is the fundamental unit of life.

• Cell Theory: All living organisms are composed of cells.

• Microscopy: Hooke's observations revealed the presence of cell walls in cork, leading to the term "cell".

Example: The illustration from Hooke's work shows the compartmentalized structure of cork cells.

Overview of Cell Types

Prokaryotic Cells

Prokaryotes include bacteria and archaea. They are characterized by the absence of a nucleus and membrane-bound organelles.

• Nucleoid: Region containing DNA, not enclosed by a membrane.

• Cell membrane: Phospholipid bilayer controlling entry and exit of substances.

• Ribosomes: Sites of protein synthesis (70S type).

• Cell wall: Most have a cell wall composed of peptidoglycan (bacteria) or other polymers (archaea).

• Size: Typically 0.5–2.0 μm in diameter.

Example: Escherichia coli is a common prokaryotic bacterium found in the human gut.

Eukaryotic Cells

Eukaryotes include protozoa, fungi, plants, and animals. They possess a true nucleus and various membrane-bound organelles.

• Nucleus: Contains most of the cell's DNA, surrounded by a nuclear envelope.

• Organelles: Includes mitochondria, endoplasmic reticulum, Golgi apparatus, and others.

• Ribosomes: Larger (80S type) than prokaryotic ribosomes.

• Cytoskeleton: Provides structural support and facilitates movement.

• Size: Typically 10–100 μm in diameter.

Example: Saccharomyces cerevisiae (yeast) is a eukaryotic microorganism used in baking and brewing.

Viruses

Viruses are acellular entities that require a host cell for replication. They lack cellular structure and independent metabolism.

• Genetic material: DNA or RNA enclosed in a protein coat (capsid).

• Replication: Occurs only within host cells.

• Metabolism: None; viruses use host cell machinery.

Example: Influenza virus infects respiratory tract cells in humans.

Comparative Table: Cell Types

Additional info: Viruses are not considered living organisms by most definitions, as they do not carry out metabolism or possess cellular structure.