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AS · Practice questions · Prokaryotic cells and viruses

Simpler cells, and not cells.

Six original Cambridge-style questions on prokaryotic cells and viruses: structure, comparisons with eukaryotes and the cell as the basic unit of life.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Wall, DNA, ribosomes, membranes.

01
Recall
[4 marks]

Outline four key structural features of a typical prokaryotic cell.

  • Unicellular and generally 1 to 5 μm in diameter. ✓
  • A cell wall made of peptidoglycan. ✓
  • Circular DNA, not associated with histones, free in the cytoplasm. ✓
  • 70S ribosomes and no organelles surrounded by double membranes (no nucleus, mitochondria or chloroplasts). ✓
02
Analysis
[4 marks]

Compare the structure of a bacterium with that of a plant mesophyll cell. Give two similarities and two differences.

  • Similarities (any two): both have a cell wall, a cell surface membrane, cytoplasm, ribosomes and DNA. ✓
  • Difference: bacterial wall is peptidoglycan; plant wall is cellulose. ✓
  • Difference: bacterium has 70S ribosomes only; plant cytoplasm has 80S ribosomes. ✓
  • Difference (also accept): bacterium has circular DNA without a nucleus; plant cell has a nucleus and chloroplasts and mitochondria with double membranes. ✓
03
Recall
[3 marks]

Describe the structure of a virus such as influenza.

  • A core of nucleic acid (single-stranded RNA in influenza). ✓
  • Surrounded by a protein capsid made of repeating subunits. ✓
  • An outer envelope of phospholipid, taken from the host cell membrane, with glycoprotein spikes. ✓
04
Explain
[3 marks]

Explain why viruses are described as non-cellular.

  • They have no cell surface membrane of their own enclosing cytoplasm (any envelope comes from the host). ✓
  • They have no cytoplasm, ribosomes or other organelles, and no metabolism. ✓
  • They can only reproduce inside a host cell, using its ribosomes, enzymes and ATP. ✓
05
Calculation
[2 marks]

A bacterium 2.5 μm long and a virus 100 nm across are shown in the same electron micrograph at ×20 000. Calculate the image size of each in mm.

  • Bacterium: 2.5 × 20 000 = 50 000 μm = 50 mm. ✓
  • Virus: 100 nm = 0.1 μm; 0.1 × 20 000 = 2000 μm = 2 mm. ✓
06
Explain
[2 marks]

Mitochondria are about the same size as bacteria, contain circular DNA and have 70S ribosomes. Explain why a mitochondrion is nevertheless not a prokaryotic cell.

  • A mitochondrion is an organelle inside a eukaryotic cell and cannot live or reproduce independently outside it. ✓
  • It has a double membrane and no peptidoglycan cell wall, so it lacks key features of a prokaryotic cell. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.