Prokaryotic cells and virusesAQA A-Level Biology: Revision notes
Section 1
Prokaryotic cells compared with eukaryotic cells
Prokaryotic cells (bacteria) are much smaller than eukaryotic cells, typically 1–5 μm compared with 10–100 μm. They differ from eukaryotes in having:
- cytoplasm that lacks membrane-bound organelles (no mitochondria, Golgi apparatus or endoplasmic reticulum)
- smaller ribosomes
- no nucleus: the DNA is a single circular molecule, free in the cytoplasm and not associated with protein
- a cell wall containing murein, a glycoprotein
The details of these structural differences (such as ribosome sizes in Svedberg units) are not required.
Eukaryotic DNA is linear and bound to protein; prokaryotic DNA is circular and not bound to protein. Mixing these up is a very common error.
Section 2
Other features of some prokaryotes
Many prokaryotic cells also have:
- one or more plasmids: small circular molecules of DNA, separate from the main DNA, often carrying genes such as antibiotic resistance that can be passed to other bacteria
- a capsule surrounding the cell, a protective layer that helps the cell avoid white blood cells and drying out
- one or more flagella, tail-like protein structures that rotate to move the cell
These are not present in every bacterium. All prokaryotes also have a cell-surface membrane.
Section 3
Antibiotics and prokaryote structure
Differences between prokaryotes and eukaryotes explain how antibiotics work. Penicillin prevents bacteria forming murein, so the cell wall is weakened and the cell bursts as water enters. Human cells have no murein, so they are not harmed. Other antibiotics bind to the smaller bacterial ribosomes and stop protein synthesis without affecting human ribosomes.
Viruses have no cell wall and no ribosomes, so these antibiotics have no effect on them.
Section 4
Viruses
Viruses are acellular (not made of cells) and non-living: they have no cell-surface membrane, cytoplasm or organelles, and cannot carry out metabolism or reproduce on their own. They replicate only inside a host cell, using its machinery.
A virus particle consists of:
- genetic material: DNA or RNA
- a protein coat, the capsid, which surrounds the genetic material
- attachment proteins on the surface, which bind to specific receptors on a host cell
Viruses are much smaller than bacteria.
Say 'acellular and non-living' when asked why a virus is not a cell: no cell structure, and no independent metabolism or reproduction.
Section 5
Comparing cell types
Prokaryote: small, no nucleus, circular DNA with no protein, small ribosomes, murein cell wall, no organelles with membranes.
Eukaryote: larger, nucleus, linear DNA with protein, bigger ribosomes, membrane-bound organelles, cell wall only in plants, algae and fungi.
Virus: not a cell; genetic material, capsid and attachment proteins only.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Prokaryotic cells and viruses
- A student compares the bacterium Escherichia coli, which is about 2 μm long, with a human liver cell, which is about 25 μm across.State two differences between the DNA of E. coli and the DNA of the human liver cell.2 marks
- Some antibiotics work by binding to bacterial ribosomes and stopping protein synthesis, but they do not affect protein synthesis in the cells of the person taking them. A scientist is investigating why.Suggest why an antibiotic that binds to bacterial ribosomes is unlikely to harm human cells, and why it stops the bacteria from growing.2 marks
- Pseudomonas aeruginosa is a bacterium that infects the lungs of patients with cystic fibrosis. Each cell has a capsule, flagella and several plasmids, and one plasmid carries a gene for antibiotic resistance. The bacteria live alongside human lung epithelial cells.Suggest how the capsule, the flagella and the plasmids help Pseudomonas aeruginosa to survive and spread in the lungs.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).