Prokaryotic cellsEdexcel International A Level Biology: Revision notes
Section 1
What is a prokaryotic cell?
Prokaryotic cells, such as bacteria, are much smaller and simpler than eukaryotic cells. They have no nucleus and no membrane-bound organelles, such as mitochondria or a Golgi apparatus. Their DNA lies free in the cytoplasm, and their ribosomes are smaller than those of eukaryotes.
Like all cells they have a cell surface membrane, cytoplasm, DNA and ribosomes.
Section 2
Cell wall and capsule
The cell wall is made of peptidoglycan, a polymer of sugars and amino acids. It gives the cell its shape, supports it, and prevents the cell bursting by osmosis when water enters in dilute surroundings. Antibiotics such as penicillin stop the wall forming properly, so the bacteria take in water and burst. Human cells have no cell wall, so they are not harmed.
The capsule is a layer of slime outside the cell wall in many bacteria. It protects the bacterium from phagocytes and from drying out, and helps the cell to stick to surfaces. Capsulated strains are often more harmful.
Bacterial cell walls are made of peptidoglycan, not cellulose or chitin.
Section 3
Flagella and pili
A flagellum is a long, whip-like protein filament that rotates to propel the bacterium through liquid. Some bacteria have several. It allows the cell to move towards food or a host surface.
Pili (singular pilus) are shorter, thinner hair-like protein projections. They attach the bacterium to surfaces and to host cells. A special type of pilus joins two bacteria together so that a plasmid can be transferred from one to the other (conjugation).
Do not mix up flagella (movement) with pili (attachment and conjugation).
Section 4
Circular DNA and plasmids
The main genetic material is a single circular DNA molecule, which is not surrounded by a nuclear envelope and is not wrapped around proteins like eukaryotic chromosomes. It carries the genes essential for the cell.
Plasmids are small, circular DNA molecules, separate from the main DNA, that can replicate independently. They carry extra genes, such as genes for antibiotic resistance. They can be transferred to other bacteria by conjugation, so resistance can spread rapidly through a population.
Section 5
Ribosomes and comparison with eukaryotes
Bacterial ribosomes are the site of protein synthesis. They are free in the cytoplasm and are smaller than eukaryotic ribosomes.
Comparing a bacterium with a eukaryotic cell:
- DNA is circular and free in the cytoplasm, not linear in a nucleus
- plasmids and a cell wall are present
- there are no membrane-bound organelles
- ribosomes are smaller
Both have a cell surface membrane, cytoplasm and ribosomes.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Prokaryotic cells
- Penicillin-type antibiotics work by preventing the formation of cross-links in the cell wall of growing bacteria. A culture of bacteria growing in a very dilute nutrient solution is treated with such an antibiotic and the cells are seen to burst.Explain why an antibiotic that prevents cell wall formation does not damage human cells.2 marks
- Two strains of a bacterium were compared. Strain S has a capsule and pili; strain R has neither. Both strains have circular DNA in the cytoplasm. When mice were injected with strain S they developed pneumonia and died. When mice were injected with strain R they survived and no bacteria were found in their lungs after a few days. Strain S bacteria were found attached to the lung epithelium of the dead mice.Explain why the mice injected with strain R survived.2 marks
- A hospital laboratory isolates a bacterium that is resistant to the antibiotic ampicillin. The gene for resistance is on a small circular DNA molecule that is separate from the main bacterial chromosome. When this strain was mixed with a non-resistant strain, a thin tube formed between pairs of cells. After several hours some of the previously non-resistant cells were able to grow on agar containing ampicillin.Explain how the non-resistant bacteria became resistant to ampicillin.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).