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.

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.