Cell Structure and Function – Microbiology Study Notes (Chapter 3)

Processes of Life

Defining Characteristics of Living Things

All living organisms share four fundamental processes that distinguish them from non-living matter. These processes are essential for the maintenance and propagation of life.

• Growth: An increase in size of an organism over time.

• Reproduction: The production of new individuals, leading to an increase in number.

• Responsiveness: The ability to sense and respond to environmental stimuli.

• Metabolism: The sum of all controlled chemical reactions that occur within an organism.

Note: Viruses do not exhibit all these characteristics independently; for example, they do not grow or metabolize outside a host cell.

Comparison of Life Characteristics in Microbes

The table below summarizes the presence of life characteristics in different types of microbes.

Characteristic	Bacteria, Archaea, Eukaryotes	Viruses
Growth	Occurs in all	Does not occur
Reproduction	Occurs in all	Host cell replicates the virus
Responsiveness	Occurs in all	Reaction not 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

Prokaryotic and Eukaryotic Cells: An Overview

Major Types of Microbial Cells

Microorganisms can be classified based on their cellular organization into prokaryotes and eukaryotes. Understanding their differences is fundamental in microbiology.

• Prokaryotes: Include Bacteria and Archaea. They are typically 0.2–2.0 μm in diameter, lack a nucleus, and do not have membrane-bound organelles. Transcription and translation occur simultaneously in the cytoplasm.

• Eukaryotes: Include algae, protozoa, fungi, animals, and plants. They are generally 10–100 μm in diameter, possess a true nucleus, and contain various internal membrane-bound organelles, resulting in a more complex structure.

Examples: Escherichia coli (bacterium, prokaryote), Paramecium (protozoan, eukaryote).

Relative Size of Microbial Cells

• Viruses: Typically 0.02–0.3 μm in diameter (e.g., Orthopoxvirus: 0.3 μm).

• Bacteria: Usually 0.5–5 μm in diameter (e.g., Staphylococcus: 1 μm).

• Eukaryotic cells: 10–100 μm in diameter (e.g., typical protozoan or algal cell).

• Chicken egg: ~47 mm (47,000 μm) in diameter (for scale comparison).

Additional info: The size difference between viruses, prokaryotes, and eukaryotes is significant and impacts their biological functions and methods of study.

Viral Replication (Example: Coronavirus)

Viral Life Cycle

Viruses, such as coronaviruses, replicate by hijacking the host cell's machinery. The process involves several key steps:

1. Attachment: Virus binds to specific receptors on the host cell surface.

2. Entry: Viral genetic material enters the host cell, often via endocytosis or membrane fusion.

3. Replication and Transcription: The viral genome is replicated and transcribed using host enzymes.

4. Translation: Viral proteins are synthesized by the host ribosomes.

5. Assembly: New viral particles are assembled from replicated genomes and synthesized proteins.

6. Release: New virions exit the host cell, often destroying it in the process (lysis or budding).

Additional info: Unlike cellular organisms, viruses are obligate intracellular parasites and cannot carry out metabolism or reproduction independently.