Practise Eukaryotes and prokaryotes. 11 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Every cell is either eukaryotic, with its genetic material enclosed in a nucleus (animal and plant cells), or prokaryotic, with no nucleus (bacteria). Expect recall and 'compare' questions on cell structure, and short calculations on cell sizes using µm, nm, standard form and orders of magnitude.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Name eukaryotic and prokaryotic cellsAnimal and plant cells are eukaryotic; bacterial cells are prokaryotic.
4
Describe the structure of a bacterial cellCytoplasm and a cell membrane inside a cell wall, a single loop of DNA not in a nucleus, and often plasmids.
5
Compare prokaryotic and eukaryotic cellsSay what both have (cell membrane, cytoplasm, ribosomes) and how they differ, above all whether the DNA is enclosed in a nucleus.
5
Convert between mm, µm and nmMultiply by 1000 for each step down (mm → µm → nm) and divide by 1000 for each step up.
6
Write cell sizes in standard formUse 1 µm = 10−6 m and 1 nm = 10−9 m, e.g. 4 µm = 4 × 10−6 m.
7
Compare sizes using orders of magnitudePut both sizes in the same unit, divide, and write the ratio as a power of ten: 1000 times bigger is 3 orders of magnitude.
Notes
Two kinds of cell
Eukaryotic cells have a cell membrane, cytoplasm and genetic material enclosed in a nucleus. Animal and plant cells are eukaryotic.
Prokaryotic cells are bacterial cells. Their genetic material is not enclosed in a nucleus.
Prokaryotic cells are much smaller. Most bacteria are a few micrometres long (about 1 to 10 µm), while most animal and plant cells are about 10 to 100 µm across.
Inside a bacterial cell
Cytoplasm and a cell membrane, surrounded by a cell wall. This wall is not made of cellulose (only plant and algal cell walls are).
The genetic material is a single loop of DNA, free in the cytoplasm.
There may also be one or more plasmids: small rings of DNA, separate from the main loop.
Bacteria have ribosomes, but no nucleus, no mitochondria and no chloroplasts.
Units, standard form and orders of magnitude
1 mm = 1000 µm (micrometres) and 1 µm = 1000 nm (nanometres).
In metres: 1 cm = 10−2 m, 1 mm = 10−3 m, 1 µm = 10−6 m, 1 nm = 10−9 m.
To compare two sizes, first put them in the same unit, then divide the larger by the smaller.
Each factor of 10 is one order of magnitude. An egg cell 100 µm across is 102 times, or 2 orders of magnitude, bigger than a bacterium 1 µm long. grade 7+
Cheatsheet
Eukaryotic cell: genetic material enclosed in a nucleus (animal and plant cells)
Prokaryotic cell: no nucleus; a single loop of DNA; may have plasmids (bacteria)
Plasmid = small ring of DNA in some bacterial cells
Both kinds of cell have a cell membrane, cytoplasm and ribosomes
Bacterial cells have a cell wall, but no mitochondria or chloroplasts
1 mm = 1000 µm; 1 µm = 1000 nm
1 µm = 10−6 m; 1 nm = 10−9 m
10 times bigger = 1 order of magnitude; 100 times = 2; 1000 times = 3
How to answer each type of question
Classify a cell as prokaryotic or eukaryotic
2 marks4
Look for a nucleus: genetic material inside a nucleus means eukaryotic.
A single loop of DNA, or plasmids, means prokaryotic (a bacterium).
Give your answer, then the reason, using the words 'nucleus' and 'loop of DNA'.
Example. Cell X has a cell wall, a cell membrane and cytoplasm. Its DNA is a single loop and it contains two plasmids. Is cell X prokaryotic or eukaryotic? Give a reason for your answer.
Show the model answer
Prokaryotic (1). Its genetic material is not enclosed in a nucleus / it has a single loop of DNA and plasmids (1).
Describe differences between bacterial and eukaryotic cells
2 to 3 marks5
Make each point clearly about the bacterial cell, or write it as a comparison.
Use the key features: nucleus, single loop of DNA, plasmids, cell wall.
Do not give structures that both cells have, such as cytoplasm or a cell membrane.
Example. Describe two differences between the genetic material in a bacterial cell and the genetic material in a human liver cell.
Show the model answer
In the bacterial cell the DNA is not enclosed in a nucleus, but in the liver cell it is inside a nucleus (1). The bacterial DNA is a single loop, and the bacterium may also have small rings of DNA called plasmids, which the liver cell does not have (1).
Convert units and use standard form
1 to 2 marks6
Write down the conversion you need, e.g. 1 mm = 1000 µm.
Going to a smaller unit makes the number bigger, so multiply.
For metres in standard form, use 1 µm = 10−6 m and check that the first number is between 1 and 10.
Example. A bacterium is 0.0025 mm long. (a) Give this length in micrometres (µm). (b) Give this length in metres. Write your answer in standard form.
Write the answer as a power of ten: the power is the number of orders of magnitude.
Example. A plant cell is 0.08 mm wide. A bacterium is 0.8 µm long. How many orders of magnitude larger is the plant cell than the bacterium?
Show the model answer
0.08 mm = 80 µm (1) 80 ÷ 0.8 = 100 = 102, so the plant cell is 2 orders of magnitude larger (1)
Shortcuts and memory tricks
Prokaryote: 'pro' means 'before' (before a nucleus). Eukaryote: 'eu' means 'true' (a true nucleus).
Units ladder: m → mm → µm → nm. Each step down is × 1000; each step up is ÷ 1000.
Sense check: a bacterium should come out at a few µm and an animal cell at tens of µm. If a bacterium comes out at 2500 µm, check your conversion.
When the ratio is 10, 100, 1000 and so on, the number of zeros is the number of orders of magnitude.
Where marks are lost
Saying bacteria have 'no DNA' or 'no genetic material'. They do: it is just not enclosed in a nucleus.
Giving the cell membrane, cytoplasm or ribosomes as a difference. Both kinds of cell have them.
Saying a bacterial cell wall is made of cellulose.
Dividing instead of multiplying in a unit conversion, e.g. turning 5 µm into 0.005 nm instead of 5000 nm.
Losing the minus sign in standard form: 3 µm is 3 × 10−6 m, not 3 × 106 m.
Exam technique
In 'compare' and 'difference' questions, make it clear which cell each feature belongs to. 'Has a cell wall' on its own may not score.
Use the mark scheme words: 'not enclosed in a nucleus', 'single loop of DNA', 'plasmids'.
Show each step of a unit conversion, so you can still earn a mark if the final answer is wrong.
Give the unit with every size, and write the µm symbol clearly (or write 'micrometres').
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Cells can be classified as eukaryotic or prokaryotic. Name the small rings of DNA found in some bacterial cells.
plasmids
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Cells can be classified as eukaryotic or prokaryotic.
(a) Which cell is a prokaryotic cell? Tick (✓) one box.[1]
Bacterial cell
Cheek cell
Palisade cell
Sperm cell
(b) Name the small rings of DNA found in some bacterial cells.[1]
(c) Give two ways in which the genetic material in a bacterial cell is different from the genetic material in an animal cell.[2]
(d) Bacterial cells and plant cells both have a cell wall. Give one other structure that is found in both bacterial cells and plant cells.[1]
Show the answer and mark scheme
(a)Answer: Bacterial cell
(b)Answer: plasmids
plasmid(s)
(c)
(bacterial) genetic material / DNA is not enclosed in a nucleus
(bacterial) DNA is a single loop / is circular
bacteria may also have plasmids / small rings of DNA
(d)
cell membrane / cytoplasm
Question 2Medium6 marks
{table} shows the lengths of three cells.
[object Object]
(a) Convert the length of the human egg cell into micrometres (µm). 1 mm = 1000 µm[1]
(b) How many times longer is the human egg cell than the bacterium?[1]
(c) By how many orders of magnitude is the egg cell larger than the bacterium?[1]
(d) Give the length of the bacterium in metres. Give your answer in standard form.[1]
(e) Give two structures found in the bacterium that are not found in the human egg cell.[2]
Show the answer and mark scheme
(a)Answer: 120 µm
120 (µm)
(b)Answer: 100 times
100
(c)Answer: 2 (a factor of 102)
2 / 102
(d)Answer: 1.2 × 10−6 m
1.2 × 10−6 (m)
(e)
cell wall
plasmid(s)
a single (circular) loop of DNA (not enclosed in a nucleus)
Question 3Hard8 marks
Cyanobacteria are single-celled prokaryotes that live in ponds. They carry out photosynthesis. One cyanobacterium cell has a diameter of 5 µm.
(a) A student said: 'Cyanobacteria photosynthesise, so they must be plant cells.' Give two reasons why the student is wrong.[2]
(b) Cyanobacteria do not have chloroplasts. Suggest how cyanobacteria are still able to carry out photosynthesis.[1]
(c) A palisade cell from a pond plant is 0.1 mm long. Calculate how many times longer the palisade cell is than the diameter of the cyanobacterium.[2]
(d) Give the diameter of the cyanobacterium in metres. Give your answer in standard form.[1]
(e) Scientists think that the first chloroplasts formed when a small photosynthetic prokaryote was taken inside a larger cell and continued to live there. Chloroplasts contain their own small loops of DNA. Suggest how this observation supports the scientists' idea.[2]
Show the answer and mark scheme
(a)
cyanobacteria have no nucleus / their genetic material is not enclosed in a nucleus
cyanobacteria have a single loop of DNA / may have plasmids
cyanobacteria have no chloroplasts
cyanobacteria are much smaller than plant cells
(b)
they contain chlorophyll (in the cytoplasm / on membranes) to absorb light
(c)Answer: 20 times
0.1 mm = 100 µm
(100 ÷ 5 =) 20
(d)Answer: 5 × 10−6 m
5 × 10−6 (m)
(e)
prokaryotes have their DNA in loops / as a single loop / as plasmids
so chloroplast DNA is like prokaryotic DNA (and is separate from the nucleus), suggesting chloroplasts were once prokaryotic cells