What you need to know
- 6.5(i)Compare bacteria and viruses: nucleic acid, capsidThe protein coat of a virus, made of capsomeres. structure and envelopeAn outer covering membrane. The nucleus and chloroplasts have a double-membrane envelope; some viruses, such as HIV, have a lipid envelope taken from the host cell., with EbolaAn enveloped RNA virus that causes Ebola virus disease, a severe haemorrhagic fever., TMVTobacco mosaic virus: a rod-shaped RNA virus that infects plants., HIVHuman immunodeficiency virus: an RNA retrovirus that infects T helper cells and can lead to AIDS. and lambda phageA bacteriophage that infects E. coli and can follow either a lytic or a lysogenic cycle..
Prokaryotic organelles stay in 3A. Viruses are not a fourth domain (4B). Lytic and latency are 6A 2. How HIV and TB infect named human cells is 6A 7–8.
By the end, you should be able to…
- Contrast a bacterial cell with an acellular virus.
- Use nucleic acid, capsid and envelope as the comparison headings.
- Place Ebola, TMV, HIV and λ phage on those headings.
Build your understanding
A cell, or a packet of genes
A bacterium is a prokaryoticDescribes cells with no nucleus and no membrane-bound organelles, such as bacteria. Their DNA is free in the cytoplasm. cell: cell wallA rigid layer outside the cell membrane. In plants it is made mainly of cellulose; it supports the cell and resists bursting., cell surface membraneThe partially permeable membrane around a cell that controls which substances enter and leave., cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles., ribosomeA small organelle made of rRNA and protein, where translation takes place. and DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code., usually as a circular chromosomeA long DNA molecule wound around histone proteins. After replication it is two sister chromatids joined at a centromere. and sometimes plasmids. It can metabolise and divide by binary fissionAsexual reproduction in prokaryotes: the DNA is copied and the cell divides into two.. That is 3A language used here as the left-hand column of a comparison, not a second organelleA structure inside a cell with a particular function, such as a mitochondrion or ribosome. list.
A virus is acellular. It has nucleic acid inside a protein capsidThe protein coat of a virus, made of capsomeres. built from repeating capsomereOne protein subunit of a virus capsid.. Some viruses also have an envelopeAn outer covering membrane. The nucleus and chloroplasts have a double-membrane envelope; some viruses, such as HIV, have a lipid envelope taken from the host cell.: a host-derived lipid membrane, often carrying viral glycoproteinA protein with carbohydrate chains attached. In membranes, glycoproteins act as receptors and as antigens for cell recognition.. A virus has no cytoplasm or ribosomes of its own. It can reproduce only inside a host cell.
Nucleic acid, capsid, envelope
Bacterial nucleic acid is DNA. A virus may have DNA or RNARibonucleic acid: a single-stranded polynucleotide with ribose and uracil. mRNA, tRNA and rRNA take part in protein synthesis., never both as its genome in the four examples here. The capsid may be helical, as in tobacco mosaic virus, or a more complex head-and-tail, as in lambda phageA bacteriophage that infects E. coli and can follow either a lytic or a lysogenic cycle.. An envelope is either present or absent; it is not a cell wall.
Nucleic acid, capsid, envelope (continued)
EbolaAn enveloped RNA virus that causes Ebola virus disease, a severe haemorrhagic fever.: RNA, enveloped. TMVTobacco mosaic virus: a rod-shaped RNA virus that infects plants.: RNA, no envelope, helical rod. HIVHuman immunodeficiency virus: an RNA retrovirus that infects T helper cells and can lead to AIDS.: RNA, enveloped. λ phage: DNA, no envelope, icosahedral head with a tail that injects DNA into a bacterium. Learn the four as a table, not as four disease stories.
What not to invent
Do not give bacteria a nucleusThe organelle that contains the cell’s chromosomes, surrounded by a nuclear envelope with pores. or mitochondrionThe organelle where the link reaction, Krebs cycle and oxidative phosphorylation of aerobic respiration take place.. Do not give viruses a cytoplasm “because they are small cells”. Do not add a fourth domainThe highest rank in classification. The three domains are Bacteria, Archaea and Eukarya. for viruses: 4B already closed that door. Size helps a microscope argument; it is not the specification comparison.
HIV’s reverse transcriptaseAn enzyme from retroviruses, such as HIV, that makes DNA using RNA as a template. and TB’s survival in macrophages belong with 6.6. Here you only need to know HIV is an enveloped RNA virus so you can compare it with the other three.
| Virus | Nucleic acid | Envelope | Capsid note |
|---|---|---|---|
| Ebola | RNA | Yes | Filamentous particle |
| TMV | RNA | No | Helical rod |
| HIV | RNA | Yes | Enveloped particle |
| λ phage | DNA | No | Head and tail |
Make your reasoning visible
Start with cell versus particle
Then nucleic acid, then capsidThe protein coat of a virus, made of capsomeres., then envelopeAn outer covering membrane. The nucleus and chloroplasts have a double-membrane envelope; some viruses, such as HIV, have a lipid envelope taken from the host cell.. Disease names without those headings score little.
Envelope is host membrane
It is not peptidoglycan and not a second capsid.
Avoid this misconception
The claim: Viruses are the smallest kind of bacterium.
The correction: Viruses are acellular. They lack cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles., ribosomeA small organelle made of rRNA and protein, where translation takes place. and a plasma membrane of their own.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Compare the nucleic acid of a typical bacterium with that of a virus.
Show suggested answer
A bacterium has DNA, usually a circular chromosome and sometimes plasmids, inside a cell. A virus has DNA or RNA inside a capsid and has no cytoplasm of its own.
Practice question
Retrieval and explanation · suggested answer
What is a capsid, and what is an envelope made from?
Show suggested answer
A capsid is the protein coat of a virus, built from capsomeres. An envelope is a host-derived lipid membrane around some capsids.
Practice question
Retrieval and explanation · suggested answer
For TMV and λ phage, state nucleic acid and whether each has an envelope.
Show suggested answer
TMV: RNA, no envelope. λ phage: DNA, no envelope.
Practice question
Retrieval and explanation · suggested answer
Why is “Ebola is a bacterial disease” the wrong comparison?
Show suggested answer
Ebola is an enveloped RNA virus, not a prokaryotic cell.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A student says HIV and Ebola are the same because both have RNA and an envelope. What extra 6.5(i) detail still distinguishes λ phage from both, and what later lesson is needed to distinguish HIV from Ebola in the host?
Show a hint
Structure here; infection later.
Explore the reasoning
λ phage has DNA and no envelope, so it is already different on 6.5(i). HIV versus Ebola as diseases needs 6.6: HIV infects T helper cells; Ebola is not the TB/HIV case study. Structure headings do not make two enveloped RNA viruses identical in the body.
Recall this lesson
Lesson recall questions
Question 1 of 5: What is a capsid, and what protein units make it?
Bacteria vs viruses
Practice question
Retrieval and explanation · suggested answer
What is a capsid, and what protein units make it?
Lesson: 6A 1: Bacteria and Viruses
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
TMV was a tool before it was a syllabus name
6A · Bacteria and VirusesThe American Phytopathological Society’s TMV teaching pages describe a rod-shaped particle of RNA plus protein, stable enough for school work, that helped show a disease agent could be…Specification 6.5(i)Read & use the scienceClose article
Why this matters
The American Phytopathological Society’s TMV teaching pages describe a rod-shaped particle of RNA plus protein, stable enough for school work, that helped show a disease agent could be smaller than a bacterium. That history is enrichment. You still compare TMV with Ebola, HIV and λ phage using nucleic acid, capsid and envelope. That comparison matters outside the classroom because an envelope is a lipid membrane derived from the host, whereas a capsid is protein. An image alone cannot establish whether a particle carries RNA or DNA.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.5(i) be able to compare the structure of bacteria and viruses (nucleic acid, capsid structure and envelope) with reference to Ebola virus, tobacco mosaic virus (TMV), human immunodeficiency virus (HIV) and lambda phage (λ phage)
Use the science
Why does TMV’s stability in sap not mean it is a living bacterium?
Explore a possible response
Stability is a physical property of the particle. TMV still has no cytoplasm or ribosomes and still needs a host cell to reproduce.
Explore the source
- APSNet: tobacco mosaic virus, purpose and backgroundAmerican Phytopathological Society
The Ebola image is a structure, not a monster
6A · Bacteria and VirusesFilm posters often turn a virus into a spiky creature with a face. A CDC electron micrograph instead shows a filamentous Ebola particle with an outer envelope and…Specification 6.5(i)Read & use the scienceClose article
Why this matters
Film posters often turn a virus into a spiky creature with a face. A CDC electron micrograph instead shows a filamentous Ebola particle with an outer envelope and internal nucleocapsid. The shape may help a researcher recognise a family, but the image cannot reveal the RNA sequence or prove how that particular particle will behave in a host. Ebola is one of four specified viruses to compare by nucleic acid, capsid and envelope. A bacterium has a cellular structure with cytoplasm and ribosomes; the virus does not. A dramatic image is a starting observation, not a complete biological classification.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.5(i) be able to compare the structure of bacteria and viruses (nucleic acid, capsid structure and envelope) with reference to Ebola virus, tobacco mosaic virus (TMV), human immunodeficiency virus (HIV) and lambda phage (λ phage)
Use the science
What can the electron image support about Ebola, and what would require a different test?
Explore a possible response
It can support observations about particle shape and envelope projections. Genome type or sequence needs molecular analysis, not shape alone.
Explore the source
- Ebola virus electron micrographCDC Public Health Image Library