How does the reproduction of a virus differ from that of a bacterium?

Bacteria reproduce independently through binary fission using their own internal machinery. Viruses are obligate parasites that must hijack a host cell's ribosomes and enzymes to replicate their components.

What is the structural difference between a viral capsid and a viral envelope?

The capsid is a protein coat made of capsomeres that surrounds the genetic material. The envelope is an outer lipid bilayer derived from the host cell's membrane that surrounds the capsid in some virus types.

Why can a virus infect a human respiratory cell but not a human skin cell?

Infection depends on specific 'attachment proteins' on the virus matching 'receptor molecules' on the host cell. If the skin cell lacks the specific receptor the virus recognizes, the virus cannot attach or enter.

What is a common mistake when describing the biological status of a virus in an exam?

A common error is describing viruses as 'living organisms' or 'cells'. They should always be referred to as 'non-cellular particles' because they lack metabolism and independent reproduction.

Why is it incorrect to say that viruses 'eat' or 'breathe'?

Viruses lack any form of metabolism, including respiration or nutrient uptake. They do not require energy input until they are inside a host cell, where they use the host's energy (ATP) for replication.

What error occurs if a student identifies a virus as being $10$ $\mu m$ in diameter?

This is a scale error; $10$ $\mu m$ is the size of a small eukaryotic cell. Viruses are much smaller, typically between $20$ and $300$ nanometers ($nm$).

Define the term 'Capsid'.

A capsid is the protective protein shell that surrounds the nucleic acid core of a virus particle. It is composed of protein subunits called capsomeres.

What are 'Attachment Proteins'?

These are specific proteins located on the outer surface of a virus (on the capsid or envelope) that bind to complementary receptors on the host cell membrane to initiate infection.

What is the 'Nucleic Acid Core' of a virus?

It is the central part of the virus containing the genetic material, which can be either DNA or RNA. This material encodes the instructions for making new viral particles.

Why are viruses considered 'obligate intracellular parasites'?

They are 'obligate' because they have no choice but to use a host, 'intracellular' because they must be inside a cell to function, and 'parasites' because they benefit at the expense of the host cell.

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AS-Level

Cambridge International Examinations

Biology

1. Cell Structure

Viruses

Summary

Viruses are unique, non-cellular biological entities that exist at the edge of life. They lack the metabolic machinery to reproduce independently and must hijack the biochemical processes of a host cell to replicate, making them obligate intracellular parasites.

1. Definition & Biological Status

Non-cellular Nature: Viruses are classified as biological particles rather than living organisms because they do not possess a cellular structure, cytoplasm, or organelles. They are unable to carry out fundamental life processes such as respiration, growth, or independent reproduction.
Obligate Intracellular Parasites: A virus can only replicate after infecting a compatible living host cell. It utilizes the host's ribosomes, enzymes, and energy (ATP) to synthesize viral components and assemble new particles.
Size and Scale: Viruses are significantly smaller than even the simplest prokaryotic cells, typically ranging from $20$ to $300$ nm in diameter. This small size allows them to pass through filters that would trap bacteria.

Diagram of a basic virus structure showing the protein capsid, internal nucleic acid core, and external attachment proteins.

2. Structural Architecture

Nucleic Acid Core: Every virus contains a genome consisting of either DNA or RNA, which can be single-stranded or double-stranded. This genetic material provides the instructions for synthesizing viral proteins once inside a host.
Capsid: The genome is enclosed in a protective protein coat known as a capsid. The capsid is composed of individual protein subunits called capsomeres, which determine the specific geometric shape of the virus.
Envelope: Some viruses possess an outer lipid bilayer called an envelope, derived from the host cell's own plasma membrane during the release phase. This envelope often hides the virus from the host's immune system.
Attachment Proteins: Projecting from the capsid or envelope are specific proteins that allow the virus to identify and bind to complementary receptor molecules on the surface of a target host cell.

3. The Viral Replication Cycle

4. Key Distinctions: Viruses vs. Cellular Life

5. Exam Strategy & Common Pitfalls