Eukaryotes and prokaryotes — AQA GCSE Combined Science
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Eukaryotes and prokaryotes explained
Eukaryotic cells include all plant and animal cells.
Read the full explanation
Whatever their shape or size, they share three features: a cell membrane, cytoplasm and genetic material enclosed in a nucleus. The cell membrane is a partially permeable barrier that controls what enters and leaves the cell. The cytoplasm is the jelly-like fluid where most chemical reactions of the cell take place, and it holds the cell's organelles. The nucleus contains the genetic material, DNA, arranged in chromosomes, and it controls the activities of the cell. Plant and animal cells also contain mitochondria for respiration and ribosomes for protein synthesis, while plant cells additionally have a cell wall, a permanent vacuole and often chloroplasts. Prokaryotic cells, such as bacteria, are much smaller and have no nucleus: their genetic material is a single loop of DNA free in the cytoplasm.
Bacterial cells (prokaryotic cells) are much smaller in comparison. They have cytoplasm and a cell membrane surrounded by a cell wall. The genetic material is not enclosed in a nucleus. It is a single DNA loop and there may be one or more small rings of DNA called plasmids.
Prokaryotic cells, such as bacteria, are typically around 1–5 µm across, far smaller than most eukaryotic cells, which are often 10–100 µm. A bacterial cell still has cytoplasm where reactions occur and a cell membrane controlling what enters and leaves. Outside the membrane is a cell wall that supports and protects the cell. Unlike eukaryotes, the genetic material is not enclosed in a nucleus. Instead there is a single loop of DNA carrying the main genes, and there may also be one or more small rings of DNA called plasmids. Plasmids are additional, separate from the main loop, and can carry genes such as those for antibiotic resistance. A useful comparison is a human cheek cell (eukaryotic, with a nucleus) beside a bacterium such as Escherichia coli (prokaryotic, no nucleus, DNA loop plus possible plasmids).
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 greatly in size, so biologists use a common unit, the micrometre (µm), where 1 µm = 1 × 10⁻⁶ m. A typical bacterium is about 1–5 µm, a human cheek cell about 50–100 µm, and some eukaryotic cells are larger still. An order of magnitude is a factor of ten, so comparing sizes means asking how many powers of ten apart two values are. To do this, write each size in standard form, A × 10ⁿ, then compare the powers n. For example, 2 µm = 2 × 10⁻⁶ m and 80 µm = 8 × 10⁻⁵ m; the powers differ by one, so the cheek cell is about one order of magnitude, roughly ten times, larger. If the powers differ by two, the sizes differ by about one hundred times. Always convert to the same unit before comparing, and give the answer as a factor or a power of ten.
Your focus
- State the three features shared by all plant and animal cells as eukaryotic cells.
- Describe the function of the cell membrane, cytoplasm and nucleus in a eukaryotic cell.
- Compare eukaryotic cells with prokaryotic cells, noting that prokaryotes have no nucleus.
Show all 9 objectives
- Label a bacterial cell diagram with cytoplasm, cell membrane, cell wall, single DNA loop and plasmids.
- Compare a named prokaryotic cell with a named eukaryotic cell, stating at least two structural differences.
- Explain the function of the cell wall, cell membrane, DNA loop and plasmids in a bacterial cell.
- Convert cell measurements between micrometres and metres and write them in standard form.
- Calculate the order of magnitude difference between two cell sizes by comparing powers of ten.
- Explain why a common unit such as the micrometre is needed when comparing the sizes of different cells.
Eukaryotes and prokaryotes exam tips
Marking Points
- Eukaryotic cells are plant and animal cells, and they all possess a cell membrane, cytoplasm and genetic material enclosed in a nucleus.
- The cell membrane controls the movement of substances into and out of the cell and is partially permeable.
- The cytoplasm is where most of the cell's chemical reactions occur and where the organelles are found.
- The nucleus encloses the genetic material, which is DNA organised into chromosomes, and it controls the cell's activities.
- Prokaryotic cells such as bacteria lack a nucleus, so their genetic material lies free in the cytoplasm, usually as a single DNA loop.
- States that prokaryotic cells are much smaller than eukaryotic cells, giving a typical size comparison such as bacteria at roughly 1–5 µm against eukaryotic cells at roughly 10–100 µm.
- Identifies cytoplasm and a cell membrane as present in bacterial cells, with the membrane controlling movement of substances into and out of the cell.
- States that the cell membrane is surrounded by a cell wall, and that this wall is outside the membrane and provides support and protection.
- Explains that the genetic material is not enclosed in a nucleus, so bacterial cells have no nucleus.
- Describes the main genetic material as a single DNA loop, and identifies plasmids as one or more small rings of DNA that may also be present.
- Distinguishes plasmids from the main DNA loop, noting that plasmids are separate small rings and may carry additional genes such as antibiotic-resistance genes.
- Compares a named prokaryote with a named eukaryote, for example Escherichia coli against a human cheek cell, to show the absence of a nucleus in the prokaryote.
- Uses the micrometre as a suitable unit for cell size and converts correctly between micrometres and metres using 1 µm = 1 × 10⁻⁶ m.
- Writes cell sizes in standard form as A × 10ⁿ, where A is between 1 and 10, for example 5 µm = 5 × 10⁻⁶ m.
- Compares two sizes by finding the difference between their powers of ten and expressing the result as an order of magnitude, such as about 10 times or about 100 times larger.
- Converts both measurements to the same unit before dividing or comparing, avoiding mixing micrometres with millimetres or metres.
- Interprets an order of magnitude as a factor of ten and applies this to realistic cell sizes, for example a 2 µm bacterium against an 80 µm cheek cell.
- Communicates the final comparison clearly, stating whether one cell is larger and by roughly how many orders of magnitude.
Examiner Tips
- 💡When listing the shared features of eukaryotic cells, name all three: cell membrane, cytoplasm and genetic material enclosed in a nucleus.
- 💡If asked to compare plant and animal cells, start with the features they share and then give the plant-only structures such as the cell wall and permanent vacuole.
- 💡Use the term partially permeable when describing the cell membrane, and link it to controlling what enters and leaves the cell.
- 💡When asked to compare cell types, structure your answer as paired points, for example eukaryotic cells have a nucleus whereas prokaryotic cells do not, and eukaryotic cells are much larger than prokaryotic cells.
- 💡Use precise terms: write single DNA loop and plasmids rather than vague phrases such as DNA floating around, and name the wall as a cell wall outside the cell membrane.
- 💡If a question asks why a bacterium can still carry out life processes without a nucleus, link each structure to its job: cytoplasm for reactions, membrane for control of entry and exit, wall for support, DNA loop and plasmids for genetic information.
- 💡Show every conversion step, including the unit you are converting to, so the examiner can follow your reasoning even if the final value is slightly out.
- 💡When asked for an order of magnitude, give a factor such as about 10 times or about 100 times, and support it by quoting the powers of ten you compared.
- 💡Check that your standard form has one non-zero digit before the decimal point, and use the correct sign on the power of ten for small measurements.
Common Mistakes
- Saying that plant cells have no cell membrane because they have a cell wall: the error is confusing the two structures; the correction is that plant cells have both a cell membrane and a cell wall outside it.
- Stating that prokaryotic cells have a small nucleus: the error is misapplying the definition; the correction is that prokaryotic cells have no nucleus at all and their DNA is free in the cytoplasm.
- Describing the cytoplasm as empty space: the error is overlooking its role; the correction is that the cytoplasm is where most chemical reactions take place and where organelles are located.
- Saying bacterial cells have no genetic material at all. Correction: they do have genetic material, but it is a single DNA loop not enclosed in a nucleus, and plasmids may also be present.
- Confusing the cell wall with the cell membrane, or placing the wall inside the membrane. Correction: the membrane is the selectively permeable boundary around the cytoplasm, and the wall surrounds the membrane on the outside.
- Describing plasmids as the main genetic material or as a nucleus. Correction: plasmids are small additional DNA rings, separate from the single main DNA loop, and are not a nucleus.
- Claiming all bacteria have plasmids. Correction: plasmids may be present, so use language such as there may be one or more plasmids rather than stating they are always present.
- Mixing units, such as comparing 5 µm with 0.05 mm without converting. Correction: convert both values to the same unit, for example both to metres, before comparing.
- Writing standard form incorrectly, such as 12 × 10⁻⁶ instead of 1.2 × 10⁻⁵. Correction: the number before the power of ten must be at least 1 and less than 10.
- Treating a difference of one power of ten as a difference of one unit rather than a tenfold difference. Correction: each step in the power of ten represents a factor of ten.
- Subtracting the sizes directly, for example 80 − 2 = 78, and calling this the order of magnitude. Correction: divide the larger by the smaller, or compare the powers of ten, to find the factor.