1.5.1Inquiry Process
Virus Structure — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 12 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Parts of a Virus
Viruses are tiny, usually 20 to 300 nm, and can only be seen with an electron microscope.
Genetic material
- Genetic material Each virus carries DNA or RNA, which may be single or double stranded, linear or circular depending on type.
Capsid
- Capsid A protein coat built from repeating subunits that surrounds and protects the nucleic acid.
Envelope
- Envelope In some viruses a lipid layer taken from the host membrane covers the capsid and carries glycoproteins.
Attachment proteins
- Attachment proteins Surface proteins that bind to specific receptors on host cells, deciding which cells can be infected.
Enzymes
- Enzymes Some viruses carry their own enzymes, such as reverse transcriptase in retroviruses like HIV.
Part 2 of 5Why Viruses Are Not Cells
Why Viruses Are Not Cells
No cytoplasm
- No cytoplasm Viruses have no cytoplasm, membrane-bound organelles or ribosomes, so they cannot make their own proteins.
No metabolism
- No metabolism They do not respire or grow independently and show activity only when inside a host cell.
Obligate parasites
- Obligate parasites They can reproduce only by using the machinery of a living host cell, so they are strictly parasitic.
Outside the domains
- Outside the domains Because they are non-cellular, they do not fit into Archaea, Bacteria or Eukarya in the three-domain system.
Part 3 of 5Examples of Viral Structure
1
The bacteriophage has a head containing DNA, a tail and tail fibres that attach to a bacterial surface.
2
HIV is an enveloped virus whose genome is two copies of single-stranded RNA.
3
Influenza virus has an envelope with spike proteins and a segmented RNA genome.
4
The tobacco mosaic virus is a helical rod with RNA wound inside a protein tube.
5
Naked viruses lack the envelope and so depend on their capsid proteins alone for attachment.
| Virus | Genome | Envelope |
|---|---|---|
| Bacteriophage T4 | dsDNA | No |
| HIV | ssRNA | Yes |
| Influenza | ssRNA | Yes |
Part 4 of 5Worked Example
Question
- A virus particle is 100 nm across and a bacterium is 2 μm across.
- Calculate how many times larger the bacterium is, and state what instrument is needed to see the virus.
- Convert to the same unit2 μm = 2000 nm.
- Divide2000 ÷ 100 = 20.
- A virus is smaller than the resolving power of a light microscope (about 200 nm), so an electron microscope is needed.
Answer
- The bacterium is 20 times larger;
- an electron microscope is required to see the virus.
Part 5 of 5Exam Checklist & Tips
1.5.1 – Key Points for the Exam
- Capsid The protein coat of a virus.
- Envelope An outer lipid layer derived from the host cell membrane.
- Obligate parasite An organism that can only survive by living inside a host.
- Bacteriophage A virus that infects bacteria.
Common Mistakes
- Calling viruses prokaryotes; they are non-cellular and sit outside the three domains.
- Saying every virus has DNA; many have RNA.
- Believing viruses are larger than bacteria; they are far smaller.
- Describing a virus as made of a cell membrane; only enveloped types carry a lipid layer, taken from the host.
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