Structure of prokaryotic cells and of viruses - AQA A-Level Biology (Paper 1) Exam Questions

Practise 16 exam-style questions on Structure of prokaryotic cells and of viruses for AQA A-Level Biology (Paper 1). Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with the Study Helper on hand if you get stuck.

16 questions
1–9 marks each
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16 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

Some human hormones are glycoproteins. Genetically modified bacteria can make the protein part of one of these hormones, but they cannot make the complete glycoprotein. Genetically modified yeast cells can make the complete glycoprotein.

Use your knowledge of cell structure to suggest why.

Model answer

Bacteria are prokaryotes, so they have no membrane-bound organelles such as the Golgi apparatus. Yeast cells are eukaryotic and have a Golgi apparatus, which modifies proteins by adding carbohydrate to them, forming glycoproteins.

Explanation

Prokaryotic cytoplasm lacks membrane-bound organelles, so bacteria have no rough endoplasmic reticulum or Golgi apparatus. In eukaryotic cells such as yeast, the Golgi apparatus modifies proteins, including adding carbohydrate to make glycoproteins. This is why eukaryotic cells are used to make some human medicines. Bacteria do have ribosomes, so they can make the protein part.

Mark scheme

1 mark
Bacteria/prokaryotes have no Golgi apparatus / no membrane-bound organelles
Accept 'bacteria have no Golgi apparatus or rough ER'. Reject 'bacteria have no ribosomes'.
1 mark
Yeast/eukaryotic cells have a Golgi apparatus, which adds carbohydrate to proteins / modifies proteins to form glycoproteins
Accept 'yeast is a eukaryote with a Golgi apparatus that makes glycoproteins'.

Question 2

3 marks

The cytoplasm of a prokaryotic cell lacks membrane-bound organelles.

Name three membrane-bound organelles that are found in animal cells but not in prokaryotic cells. Give one function of each organelle you name.

Model answer

Nucleus - contains the DNA, whose genes code for the cell's polypeptides.
Mitochondrion - the site of aerobic respiration, which produces ATP.
Golgi apparatus - modifies proteins (for example by adding carbohydrate) and packages them into vesicles.

Explanation

Prokaryotic cells have no nucleus and none of the membrane-bound organelles of eukaryotic cells; their DNA lies free in the cytoplasm. Ribosomes are found in both types of cell but are not surrounded by a membrane (and prokaryotic ribosomes are smaller), so they are a common wrong answer; chloroplasts are membrane-bound but are not found in animal cells. For the nucleus, give a precise function such as containing the genes that code for polypeptides, not just 'controls the cell'.

Mark scheme

1 mark
Nucleus AND contains the DNA/genes that code for polypeptides/proteins OR site of DNA replication OR site of transcription / production of mRNA OR production of rRNA/ribosomes (in the nucleolus)
Max 3. 1 mark for each correctly named organelle with a correct function; both the name and the function must be correct. List rule: if more than three organelles are given, each wrong extra answer (e.g. ribosome, cell wall, chloroplast, vacuole) cancels one correct answer. Reject ribosomes: they are not surrounded by a membrane. Insufficient for the nucleus: 'controls the cell' or 'holds genetic material' with no reference to coding for proteins or to a named process.
1 mark
Mitochondrion AND (site of) aerobic respiration / production of ATP
Reject 'produces energy'.
1 mark
Golgi apparatus AND modifies/processes/packages proteins OR adds carbohydrate to proteins OR forms lysosomes / vesicles
Accept Golgi body.
1 mark
Rough endoplasmic reticulum AND (site of) protein synthesis / folds or transports proteins (to the Golgi apparatus)
Accept RER.
1 mark
Smooth endoplasmic reticulum AND synthesises/processes lipids
Accept SER.
1 mark
Lysosome AND contains/releases hydrolytic enzymes OR digests worn-out organelles / engulfed pathogens
Reject 'lysosomes are enzymes'.
1 mark
(Golgi / secretory) vesicle AND transports substances in the cell OR releases substances by exocytosis
Credit vesicles only once.

Question 3

4 marks

Scientists mixed phagocytes with bacteria of one species. They used two strains: strain K, which has a capsule, and strain L, a mutant of the same species that cannot make a capsule. After 30 minutes, they counted the bacteria inside 100 phagocytes.

- Strain K: 15 bacteria inside 100 phagocytes
- Strain L: 228 bacteria inside 100 phagocytes

Calculate how many times more bacteria of strain L than of strain K were taken in by the phagocytes. Give your answer to 2 significant figures.

Suggest an explanation for the difference between the two strains.

Model answer

228 ÷ 15 = 15.2, so 15 times more bacteria of strain L were taken in (2 s.f.).

The capsule covers the antigens and other molecules on the surface of the bacterium, so phagocytes cannot recognise and bind to bacteria of strain K. Strain L has no capsule, so phagocytes can attach to and engulf it far more often.

Explanation

Divide the larger value by the smaller one: 228 ÷ 15 = 15.2, which is 15 to 2 significant figures. A capsule is a slimy layer outside the cell wall of many bacteria. It makes it harder for phagocytes to bind to the bacterium, so bacteria with capsules are engulfed less often and are more likely to survive in the body. Comparing a mutant of the same species means the capsule is the only difference between the strains.

Mark scheme

1 mark
Correct method: 228 ÷ 15; or a correct final answer of 15 with no working
Max 4. A correct answer of 15 (times) with no working scores 2 marks (points 1 and 2). Accept for 1 mark: 15.2 (not rounded to 2 significant figures) OR 0.066 (15 ÷ 228, inverted).
1 mark
15 (times)
Accept 15 only for this mark.
1 mark
The capsule covers the cell wall / antigens / surface molecules of the bacterium
Points 3 to 5: max 2. Accept 'the capsule hides the bacterium's antigens'.
1 mark
So phagocytes cannot bind to / recognise / attach to the bacteria (with a capsule)
Accept 'the capsule is slippery so phagocytes cannot hold on to/engulf the bacteria'.
1 mark
So bacteria with a capsule are less likely to be engulfed/destroyed (and more likely to survive and cause disease)
Ignore 'the capsule protects the bacterium' unqualified.

Question 4

6 marks

Compare the structure of a virus particle with the structure of a bacterial cell.

Model answer

Similarities: both contain genetic material made of nucleic acid, and in both the genetic material is enclosed in an outer layer - a protein capsid in a virus and a cell wall and membrane in a bacterium.

Differences: a virus particle is acellular, with no cell-surface membrane, cytoplasm or ribosomes, whereas a bacterium is a cell with a cell-surface membrane, cytoplasm and ribosomes. A virus has a protein capsid with attachment proteins, whereas a bacterium has a cell wall containing murein and may have a capsule and flagella. A virus contains either DNA or RNA, but a bacterium contains DNA as a single circular molecule and may also have plasmids. Virus particles are much smaller than bacteria.

Explanation

A virus particle is just genetic material (DNA or RNA) in a protein capsid with attachment proteins; it has none of the structures of a cell. A bacterium is a prokaryotic cell with a membrane, cytoplasm, ribosomes, a murein cell wall and a circular DNA molecule. 'Compare' needs similarities as well as differences. Common errors are confusing capsid with capsule and saying that all viruses contain RNA.

Mark scheme

1 mark
Both contain genetic material / nucleic acid
Max 4 if the answer gives only similarities or only differences. Points 1 to 11: 1 mark each, max 6. Each difference must describe both, or be a clear comparative.
1 mark
Both have an outer layer that encloses/protects the genetic material: a capsid in viruses and a cell wall (and membrane) in bacteria
Accept 'both contain protein'. Reject 'both have a cell wall'.
1 mark
Virus is acellular / has no cell-surface membrane or cytoplasm AND bacterium has a cell-surface membrane and cytoplasm
Accept 'bacterium is a cell but a virus is not'.
1 mark
Virus has a capsid (of protein) AND bacterium has a cell wall containing murein
Reject 'capsule' for capsid.
1 mark
Virus has attachment proteins AND bacterium does not
Accept 'proteins that bind to receptors on host cells' for attachment proteins.
1 mark
Bacterium may have a capsule AND virus does not
Reject 'capsule' for capsid in a virus.
1 mark
Bacterium may have flagella / a flagellum AND virus does not
1 mark
Virus genetic material may be DNA or RNA AND bacterium has DNA (only)
Reject 'viruses have RNA, bacteria have DNA' unless the answer makes clear that some viruses have DNA.
1 mark
Virus has no ribosomes AND bacterium has ribosomes
Ignore 70S unless contrasted with nothing in the virus.
1 mark
Bacterium has a single circular DNA molecule (and may have plasmids) AND virus has no plasmids
Accept 'bacterial DNA is circular and free in the cytoplasm'.
1 mark
Virus particles are much smaller than bacterial cells
Accept a correct size comparison, e.g. viruses about 20 to 300 nm and bacteria about 1 to 10 μm.

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