Eukaryotes and prokaryotes — AQA GCSE Biology
Test yourself on Eukaryotes and prokaryotes with AQA GCSE practice questions.
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Eukaryotes and prokaryotes explained
Eukaryotic cells are the building blocks of plants, animals, fungi and protists.
Read the full explanation
Every one is bounded by a partially permeable cell membrane that controls what enters and leaves. Inside is cytoplasm, a watery jelly where most chemical reactions of the cell happen. The genetic material, DNA, sits inside a nucleus bounded by its own membrane, so the DNA is separated from the cytoplasm. This shared plan explains why plant and animal cells look similar under a light microscope: both show a nucleus, cytoplasm and membrane. Plant cells add a cellulose cell wall, a permanent vacuole and chloroplasts, but these are extra features, not part of the definition of a eukaryote. A cheek cell and a root hair cell both fit this description.
Bacterial cells (prokaryotic cells) are much smaller in comparison.
Bacterial cells are prokaryotic and are far smaller than eukaryotic cells. A typical bacterium is about 1 to 5 micrometres across, while a typical animal or plant cell is roughly 10 to 100 micrometres. That size gap matters because it explains why bacteria require higher magnification to observe internal structures compared to eukaryotic cells, and why they pass through filters that trap eukaryotic cells. Size also links to surface area to volume ratio: a small cell has a large surface area relative to its volume, so diffusion alone can supply its needs quickly. When comparing, use the same unit for both cells, for example convert 0.002 mm to 2 micrometres before stating the ratio. A useful method is to divide the larger measurement by the smaller to give a simple size ratio.
They have cytoplasm and a cell membrane surrounded by a cell wall.
A bacterial cell is built from a cytoplasm and a cell membrane, and the membrane is surrounded by a cell wall. The cytoplasm holds the cell's chemical reactions and the ribosomes that make proteins. The cell membrane is partially permeable and controls what enters and leaves. The wall lies outside the membrane and gives the cell shape and protection; it is not selectively permeable, so it does not control entry. Bacterial walls are made of peptidoglycan, a polymer of sugars and amino acids, which differs from the cellulose walls of plant cells. Some bacteria also have a slime capsule outside the wall. When labelling, place the wall outside the membrane and keep the two labels separate, because examiners look for the correct order of layers.
The genetic material is not enclosed in a nucleus.
In a bacterium the genetic material is DNA, but it is not wrapped in a nuclear membrane. Instead the main circular DNA molecule sits free in the cytoplasm, usually as a single loop called the bacterial chromosome. Many bacteria also carry small circular plasmids, which are separate from the main loop and can be passed between cells. Because there is no nucleus, the DNA is in direct contact with the cytoplasm, so transcription and translation can happen at the same time and place. This is a key difference from eukaryotic cells, where the nucleus separates DNA from the cytoplasm. When comparing, state both sides: eukaryotic DNA is enclosed in a nucleus, while prokaryotic DNA lies free in the cytoplasm.
It is a single DNA loop and there may be one or more small rings of DNA called plasmids.
Prokaryotic cells, such as bacteria, store their genetic material differently from eukaryotic cells. Their main chromosome is one circular molecule of DNA, often described as a single DNA loop, which sits free in the cytoplasm rather than inside a nucleus. In addition, many bacteria carry plasmids: small circular rings of DNA that are separate from the main loop. A bacterium may have none, one or several plasmids, so the specification says there may be one or more. Plasmids usually carry extra genes, for example for antibiotic resistance, and can be passed between bacteria. When you label a bacterial cell, show the loop and any plasmid rings clearly, and remember that both are DNA.
Students should be able to demonstrate an understanding of the scale and size of cells and be able to make order of magnitude calculations, including the use of standard form.
Cells vary enormously in size, so biologists compare them using orders of magnitude and standard form. An order of magnitude is a factor of ten: a cell 100 µm across is two orders of magnitude wider than one 1 µm across. Standard form writes a number as A × 10ⁿ, where A is at least 1 and less than 10. For example, 0.005 m becomes 5 × 10⁻³ m. To compare sizes, convert both measurements to the same unit, write them in standard form, then compare the powers of ten. A typical animal cell is about 10–30 µm, a bacterium about 1–5 µm and a virus far smaller. Practise converting between metres, millimetres, micrometres and nanometres before comparing.
Your focus
- Label the cell membrane, cytoplasm and nucleus on diagrams of plant and animal cells.
- State the function of each of these three structures.
- Explain why plant and animal cells are classified as eukaryotic.
Show all 18 objectives
- State an approximate size range for bacterial cells and for eukaryotic cells.
- Convert between millimetres and micrometres and use the values to compare cell sizes.
- Explain how small cell size affects the rate of diffusion into a cell.
- Label the cytoplasm, cell membrane and cell wall on a diagram of a bacterial cell.
- State the function of the bacterial cell wall and of the cell membrane.
- Describe how the bacterial cell wall differs in composition from a plant cell wall.
- State where the genetic material is found in a bacterial cell.
- Describe the shape of the main bacterial DNA molecule and the role of plasmids.
- Compare the location of genetic material in prokaryotic and eukaryotic cells.
- Identify the single DNA loop as the main genetic material of a prokaryotic cell.
- Describe plasmids as small rings of DNA that may be present in one or more copies.
- Explain that plasmids carry additional genes and can be transferred between bacteria.
- Convert between metres, millimetres, micrometres and nanometres.
- Write and interpret numbers in standard form.
- Compare cell sizes by calculating differences in orders of magnitude.
Eukaryotes and prokaryotes exam tips
Marking Points
- Names the cell membrane as the partially permeable boundary controlling entry and exit of substances.
- Identifies cytoplasm as the site of most chemical reactions in the cell.
- States that eukaryotic genetic material is DNA.
- States that the genetic material is enclosed within a nucleus bounded by a membrane.
- Applies the term eukaryotic to plant and animal cells and distinguishes them from prokaryotes.
- Recognises that plant cells also contain a cell wall, vacuole and chloroplasts, while animal cells do not.
- States that bacterial cells are prokaryotic and smaller than eukaryotic cells.
- Gives a sensible order of magnitude, such as bacteria being roughly 1 to 5 micrometres across.
- Uses the same unit for both measurements before comparing cell sizes.
- Calculates a size ratio by dividing the larger measurement by the smaller.
- Links small size to a large surface area to volume ratio and rapid diffusion.
- Names the cytoplasm as the site of the cell's chemical reactions.
- Names the cell membrane as the partially permeable barrier controlling entry and exit.
- States that the cell wall lies outside the cell membrane.
- Gives the wall the function of support, shape or protection rather than control of entry.
- States that bacterial walls are made of peptidoglycan, unlike the cellulose walls of plants.
- Recognises that some bacteria have an additional slime capsule outside the wall.
- States that bacterial genetic material is DNA.
- States that the DNA is not enclosed in a nuclear membrane.
- Describes the main DNA as a circular loop or chromosome lying free in the cytoplasm.
- Names plasmids as small additional circular DNA molecules.
- Contrasts this with eukaryotic cells, where DNA is enclosed in a nucleus.
- Links the absence of a nucleus to DNA being in direct contact with the cytoplasm.
- The main genetic material of a prokaryotic cell is a single DNA loop, not a nucleus.
- The DNA loop is circular and lies free in the cytoplasm.
- Plasmids are small rings of DNA, separate from the main DNA loop.
- A prokaryotic cell may contain no plasmids, one plasmid or several plasmids.
- Plasmids carry additional genes and can be transferred between bacteria.
- Both the DNA loop and plasmids are made of DNA.
- Convert measurements to the same unit before comparing cell sizes.
- Write numbers in standard form as A × 10ⁿ with A between 1 and 10.
- An order of magnitude is a tenfold difference, so compare powers of ten.
- Use the correct unit conversions, for example 1 mm = 1000 µm and 1 µm = 1000 nm.
- State the comparison as a whole-number or simple ratio of orders of magnitude.
- Check that the final answer has the correct unit and a sensible size for a cell.
Examiner Tips
- 💡Label a diagram with straight lines that touch the structure, and spell membrane and cytoplasm correctly.
- 💡When asked for differences, compare plant and animal cells feature by feature rather than listing one cell type alone.
- 💡Use the word eukaryotic only after you have named the nucleus, so the term is clearly earned.
- 💡Convert all lengths to micrometres before you compare, and show the conversion in your working.
- 💡Quote a numerical ratio such as 20:1 rather than saying much smaller.
- 💡If a scale bar is given, measure the cell in the image and use the scale bar to convert to real size.
- 💡On a diagram, draw the wall as a line outside the membrane and label each layer separately.
- 💡When asked for a function, match each structure to one clear job rather than repeating the same job twice.
- 💡Use the term partially permeable for the membrane only, never for the wall.
- 💡Write both halves of a comparison, for example eukaryotic DNA is enclosed in a nucleus whereas prokaryotic DNA is not.
- 💡Use the word circular when describing bacterial DNA, and mention plasmids only if the question asks about extra DNA.
- 💡Avoid saying bacteria have no nucleus membrane without also saying where the DNA actually is.
- 💡Sketch a bacterium with one large loop and two small rings, then label each as DNA.
- 💡Use the phrase single DNA loop when describing the main chromosome of a prokaryote.
- 💡If a question asks why plasmids matter, link them to carrying extra genes such as antibiotic resistance.
- 💡Underline the two sizes and their units before starting any comparison.
- 💡Show each conversion step so the examiner can follow your reasoning.
- 💡Give the final comparison as a number of orders of magnitude, for example about two orders of magnitude larger.
Common Mistakes
- Saying the cell wall controls what enters the cell; correct this by naming the cell membrane as the controlling boundary and the wall as support.
- Describing the nucleus as the site of chemical reactions; correct this by placing reactions in the cytoplasm and giving the nucleus the role of holding genetic material.
- Treating chloroplasts or a vacuole as features of all eukaryotic cells; correct this by restricting those structures to plant cells.
- Comparing a measurement in millimetres with one in micrometres without converting; correct this by converting both to the same unit first.
- Writing that bacteria are smaller because they lack a nucleus; correct this by treating size and nucleus as separate differences.
- Giving a single vague word such as tiny; correct this by quoting approximate measurements or a ratio.
- Saying the cell wall is partially permeable and controls entry; correct this by giving that role to the cell membrane and describing the wall as supportive.
- Placing the cell wall inside the cell membrane; correct this by drawing the wall as the outermost layer of the two.
- Calling the bacterial wall cellulose; correct this by naming peptidoglycan as the bacterial wall material.
- Saying bacteria have no genetic material; correct this by stating that they have DNA but no nucleus around it.
- Describing bacterial DNA as linear; correct this by calling the main molecule circular.
- Confusing plasmids with the main chromosome; correct this by describing plasmids as small extra circles that are not the main genetic material.
- Saying the DNA loop is enclosed in a nucleus: correct this by stating that prokaryotes have no nucleus, so the loop lies free in the cytoplasm.
- Describing plasmids as a protein coat or as food stores: correct this by identifying plasmids as small rings of DNA.
- Assuming every bacterium has exactly one plasmid: correct this by saying there may be one or more plasmids, or none at all.
- Comparing numbers in different units without converting: correct this by converting both to the same unit first.
- Writing standard form with A outside the range 1 to 10, such as 25 × 10³: correct this to 2.5 × 10⁴.
- Treating a tenfold difference as one order of magnitude but a hundredfold difference as one order too: correct this by counting each factor of ten as one order.