What is the primary component of the bacterial cell wall, and what is its function?

The primary component is murein (peptidoglycan). Its function is to provide structural support and protection, preventing the cell from bursting due to osmotic pressure.

How does the genetic material in a bacterium differ from that in a virus?

Bacteria always have a single circular DNA chromosome (and potentially plasmids), while viruses can have either DNA or RNA, which may be single-stranded or double-stranded.

Why are viruses considered 'non-cellular'?

Viruses lack a cytoplasm, a plasma membrane, and ribosomes. They do not perform metabolic processes and cannot reproduce without a host cell's machinery.

What is the role of the bacterial capsule?

The capsule acts as a protective 'slime layer' that prevents the bacterium from drying out and helps it hide from the host's immune system cells.

What is the difference between bacterial ribosomes and eukaryotic ribosomes?

Bacteria have 70S ribosomes, which are smaller and have a different structure than the 80S ribosomes found in the cytoplasm of eukaryotic cells.

What are plasmids, and why are they significant in microbiology?

Plasmids are small, circular loops of accessory DNA. They are significant because they often contain genes for antibiotic resistance and can be shared between bacteria.

What error is made when assuming all viruses have a lipid envelope?

Not all viruses are enveloped; some consist only of a nucleocapsid (nucleic acid + capsid). The envelope is an optional outer layer derived from the host cell membrane.

How do pili and flagella differ in function?

Flagella are used primarily for locomotion (movement), whereas pili are used for attachment to surfaces or for the transfer of genetic material between cells.

What is a common misconception regarding the 'nucleus' in bacteria?

A common error is stating bacteria have a nucleus; they actually have a nucleoid region where circular DNA is free-floating and not bound by a membrane.

What is the function of viral attachment proteins?

They allow the virus to identify and bind to specific complementary receptors on the host cell membrane, which is the first step of infection.

Structure of Bacteria & Viruses | Pearson Edexcel A-Level Biology

6. Immunity, Infection and Forensics

Structure of Bacteria & Viruses

Summary

Microorganisms are classified into distinct structural categories, primarily single-celled prokaryotic bacteria and non-cellular infectious viruses. Understanding their architecture is fundamental to microbiology, as it explains how these entities survive, reproduce, and interact with host organisms.

1. Definition & Core Concepts

Bacteria are unicellular prokaryotes, characterized by their lack of a membrane-bound nucleus and specialized organelles. They are significantly smaller than eukaryotic cells but possess the necessary machinery for independent metabolism and reproduction.
Viruses are defined as non-cellular infectious particles that exist at the edge of life. Unlike bacteria, they cannot reproduce independently and must hijack the biochemical machinery of a host cell to replicate their genetic material.
The fundamental difference lies in biological complexity: bacteria are complete living organisms with metabolic pathways, while viruses are essentially genetic packages wrapped in protein.

Comparison of a bacterial cell showing circular DNA and a virus showing a protein capsid.

2. Bacterial Architecture

The bacterial cell wall is composed of murein (also known as peptidoglycan), a complex glycoprotein that provides structural integrity and prevents osmotic lysis. This wall is a primary target for many antibiotics which interfere with its synthesis.
Internally, bacteria contain 70S ribosomes, which are smaller than the 80S ribosomes found in eukaryotes but perform the same essential function of protein synthesis. The cytoplasm also contains a single circular chromosome of DNA that is not associated with histone proteins.
Many bacteria possess plasmids, which are small, independent loops of DNA. These often carry accessory genes, such as those for antibiotic resistance, and can be transferred between individuals via horizontal gene transfer.

3. Viral Architecture

Every virus possesses a nucleic acid core, which can consist of either DNA or RNA. This genetic material may be single-stranded or double-stranded, depending on the specific viral family.
The genetic core is protected by a protein coat called a capsid, which is composed of individual protein subunits. Some viruses also feature an outer lipid envelope derived from the host cell's plasma membrane during the budding process.
Attachment proteins (or glycoproteins) protrude from the viral surface. These are critical for the virus's ability to recognize and bind to specific receptor molecules on the surface of a target host cell.

4. Specialized Bacterial Structures

5. Key Distinctions

6. Exam Strategy & Tips