Prokaryotic vs Eukaryotic Cells
The structural differences between prokaryotic and eukaryotic cells: size, DNA, organelles, ribosomes, murein cell wall, plasmids, capsule and flagella. Includes the 4-mark exam question, a worked model answer and the examiner trap on never saying bacteria have no DNA.
Chapters (9)
Exam question [4 marks]
Describe four ways in which the structure of a bacterial cell differs from the structure of a plant cell, other than size.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Bacterial DNA is a single circular molecule, free in the cytoplasm / not in a nucleus (plant DNA is linear, in a nucleus) | "loop of DNA", "no nucleus", "not associated with proteins / histones" | "bacteria have no DNA"; "no genetic material"; "DNA is in plasmids only" |
| 2 | Bacterial cell wall contains murein / peptidoglycan (plant cell wall is cellulose) | "murein", "glycoprotein cell wall" | "bacterial wall is cellulose"; "chitin"; "bacteria have a cell wall" alone |
| 3 | Bacterial ribosomes are smaller | "70S, not 80S", "smaller ribosomes" | "bacteria have no ribosomes"; "ribosomes are missing" |
| 4 | Bacteria have no membrane-bound organelles (e.g. no mitochondria or chloroplasts) | "cytoplasm lacks membrane-bound organelles", a named organelle that is absent | "no organelles" alone; "bacteria are smaller" |
Each mark needs a bacterial feature and a clear contrast with the plant cell, or a clearly implied one. The question says "other than size", so "much smaller" is not credited here (in questions without that wording it is a valid difference). Plasmids, capsule and flagella are optional features of many bacteria, not shared by all, and are not credited as differences in this question. Mark point 1 is any one of the listed ideas; "no nucleus" may not also be credited again as point 4 (a nucleus is itself a membrane-bound organelle, so point 4 needs a different absent organelle).
Model answer (4/4)
Bacterial DNA is a single circular molecule that is free in the cytoplasm, not associated with proteins, and there is no nucleus, whereas plant DNA is linear and held in a nucleus. The bacterial cell wall contains murein, whereas the plant cell wall is made of cellulose. Bacterial ribosomes are smaller than plant ribosomes. Bacteria have no membrane-bound organelles, such as mitochondria or chloroplasts, which plant cells have.
Examiner trap
- Saying prokaryotes have 'no DNA'
- Saying the cell wall is cellulose or chitin
- Saying plasmids are the main genome
Key terms
- prokaryote
- murein
- plasmid
- capsule
- flagellum
- circular DNA
- 70S ribosome
- membrane-bound organelle
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:28Prokaryotes: small and simple
Prokaryotic cells, like bacteria, are much smaller than eukaryotic cells. The word prokaryote comes from before nucleus, and that is the big clue. Eukaryotic cells, like plant, animal and fungal cells, have a true nucleus. In the exam, always say much smaller, never just small, and compare the two cells, rather than describing one on its own.
Size is a real difference, but in this question it is not credited, so let's look at the structures that are.
1:00Genetic material
First, the genetic material. A eukaryotic cell keeps its nuclear DNA inside a nucleus, wrapped around proteins. A prokaryotic cell has no nucleus. Instead it has a single, circular molecule of DNA, lying free in the cytoplasm. And that DNA is not associated with proteins.
So bacteria do have DNA. It is just circular, free, and without proteins.
1:28Cytoplasm and ribosomes
Second, the cytoplasm. Eukaryotic cytoplasm is full of membrane-bound organelles, such as mitochondria, the endoplasmic reticulum and the Golgi apparatus. Prokaryotic cytoplasm lacks membrane-bound organelles. Third, ribosomes. Both cells have them, but prokaryotic ribosomes are smaller than the eukaryotic cytoplasmic ones.
You may see them called seventy S and eighty S. You do not need to learn those, but the word smaller scores.
2:01The murein cell wall
Fourth, the cell wall. Plant cell walls are made of cellulose. Fungal cell walls contain chitin. A prokaryotic cell wall contains murein, which is a glycoprotein. That means a sugar joined to protein. Murein is the word the mark scheme wants, and peptidoglycan is credited too.
If you write cell wall alone, you have said something both cells share, so you must name what it is made of.
2:28Plasmids, capsule, flagella
Finally, many prokaryotic cells have extra features. One or more plasmids, which are small, extra loops of DNA. A capsule, a slimy layer around the outside of the cell. And one or more flagella, which spin to move the cell.
Notice the wording: many have these. Not every bacterium does, so do not use them as your main differences.
2:53Summary table
Put it together in a table. Size: prokaryotes much smaller. DNA: single and circular, free in the cytoplasm, not with proteins, against linear DNA in a nucleus. Organelles: no membrane-bound ones, against many.
Ribosomes: smaller, against larger. Cell wall: murein, against cellulose in plants or chitin in fungi.
4:05Examiner trap and recap
The traps. Never say bacteria have no DNA, and never say their cell wall is cellulose or chitin. Plasmids are extra DNA, not the main genome. The main genome is the one circular molecule. Recap. Prokaryotes are smaller, with circular DNA, smaller ribosomes, no membrane-bound organelles and a murein wall.
Next time, viruses. Are they even alive?
Watch first
This lesson builds on:
- Nucleus, Mitochondria and Chloroplasts · AQA 7402 · 3.2.1.1
Keep going
More from 3.2 Cells
- Nucleus, Mitochondria and Chloroplasts · AQA 7402 · 3.2.1.1
- Rough ER, Golgi and Lysosomes Explained · AQA 7402 · 3.2.1.1
- Cell Wall, Vacuole and Specialised Cells · AQA 7402 · 3.2.1.1
- Viruses: Structure and Non-Living Status · AQA 7402 · 3.2.1.2
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-025.