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This lesson covers the two fundamental types of cell — eukaryotic and prokaryotic — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to describe the structures found in both cell types, explain their functions and compare the key differences between them.
All living organisms are made of cells. A cell is the basic structural and functional unit of life. Some organisms are made of just one cell (unicellular, e.g. bacteria), while others are made of many cells working together (multicellular, e.g. animals and plants).
Cells fall into two broad categories:
Exam Tip: The word "eukaryotic" comes from Greek and means "true nucleus". The word "prokaryotic" means "before nucleus". These root words can help you remember the key difference.
Eukaryotic cells are found in animals, plants, fungi and protists. They contain a nucleus enclosed by a nuclear membrane, along with other membrane-bound organelles such as mitochondria. Eukaryotic cells are typically 10–100 μm in diameter.
The following table summarises the organelles you need to know for both animal and plant eukaryotic cells:
| Organelle | Structure | Function |
|---|---|---|
| Nucleus | Large, spherical, surrounded by a nuclear membrane | Contains genetic material (DNA) arranged into chromosomes; controls the cell's activities |
| Cell membrane | Thin, flexible phospholipid bilayer | Controls what enters and leaves the cell; partially permeable |
| Cytoplasm | Jelly-like substance filling the cell interior | Where most chemical reactions take place; contains enzymes |
| Mitochondria | Small, oval-shaped with a folded inner membrane (cristae) | Site of aerobic respiration — energy is transferred from glucose |
| Ribosomes | Very small structures (~25 nm), found free in cytoplasm | Site of protein synthesis — amino acids are assembled into proteins |
Exam Tip: Never write that mitochondria "produce" or "create" energy. Energy cannot be created or destroyed. Say mitochondria are the site of aerobic respiration, where energy is transferred (or released) from glucose.
Prokaryotic cells are found in bacteria. They are much smaller than eukaryotic cells, typically only 1–5 μm in diameter — roughly one-tenth the size of a typical animal cell. They do not have a true nucleus; their genetic material is free in the cytoplasm.
| Structure | Description | Function |
|---|---|---|
| Cell membrane | Phospholipid bilayer | Controls what enters and leaves the cell |
| Cell wall | Made of peptidoglycan (not cellulose) | Provides structural support and protection |
| Cytoplasm | Jelly-like substance | Where chemical reactions occur |
| Ribosomes | Smaller than eukaryotic ribosomes (70S vs 80S) | Site of protein synthesis |
| Chromosomal DNA | A single, circular loop of DNA free in the cytoplasm | Carries the main genetic information |
| Plasmids | Small, extra circles of DNA | Carry additional genes, e.g. antibiotic resistance |
| Flagellum (plural: flagella) | Long, whip-like tail (not all bacteria have one) | Used for movement |
graph TD
A[Bacterial Cell] --> B[Cell wall - peptidoglycan]
B --> C[Cell membrane]
C --> D[Cytoplasm]
D --> E[Circular DNA - no nucleus]
D --> F[Plasmids]
D --> G[70S Ribosomes]
A --> H[Flagellum - movement]
Exam Tip: Do not write that prokaryotic cells have "no DNA". They do have DNA — it is simply not enclosed in a nuclear membrane. Say they have "no true nucleus".
| Feature | Eukaryotic Cell | Prokaryotic Cell |
|---|---|---|
| Size | Larger (10–100 μm) | Smaller (1–5 μm) |
| Nucleus | True nucleus with nuclear membrane | No true nucleus — DNA free in cytoplasm |
| DNA arrangement | Linear chromosomes inside nucleus | Single circular DNA loop + plasmids |
| Membrane-bound organelles | Present (mitochondria, etc.) | Absent |
| Ribosomes | Larger (80S) | Smaller (70S) |
| Cell wall | Present in plants (cellulose); absent in animals | Present (peptidoglycan) |
| Plasmids | Not normally present | Present |
| Examples | Animal, plant, fungal cells | Bacteria |
Understanding relative sizes is important for the exam:
| Object | Approximate Size |
|---|---|
| Animal cell | 10–30 μm |
| Plant cell | 10–100 μm |
| Bacterial cell | 1–5 μm |
| Virus (not a cell) | 20–300 nm |
You need to be able to convert between units of length:
Worked Example:
A bacterium is 3.5 μm long. Convert this to nanometres.
3.5 μm × 1000 = 3500 nm
Convert 3.5 μm to millimetres.
3.5 μm ÷ 1000 = 0.0035 mm
Exam Tip: Remember the conversion chain: mm → μm → nm, each step is × 1000. Going the other way (nm → μm → mm) is ÷ 1000. Show every step of your working for full marks.
The distinction between eukaryotes and prokaryotes is one of the most fundamental divisions in biology. Every living organism on Earth belongs to one of these two categories, and the structural differences between them have profound consequences for how organisms live, grow, and reproduce.
Eukaryotic cells are described as compartmentalised because their membrane-bound organelles keep different chemical reactions separated. For example, respiration takes place inside the mitochondria, while protein synthesis occurs on the ribosomes. This separation allows each organelle to maintain the specific conditions it needs — slightly different pH, ion concentrations, or enzyme concentrations — without interfering with other reactions in the cell.
Prokaryotes, by contrast, carry out all their chemistry in a single space (the cytoplasm). This limits the complexity of reactions they can run simultaneously, which is part of the reason prokaryotes tend to remain small and structurally simple.
Exam Tip: If a 6-mark question asks you to compare eukaryotes and prokaryotes, structure your answer around three themes: (1) size, (2) presence of a true nucleus, (3) membrane-bound organelles. This gives the examiner clear comparison points.
Question: A typical human cheek cell has a diameter of 60 μm. A typical E. coli bacterium has a length of 2 μm. How many times larger is the cheek cell than the E. coli?
Working: 60 ÷ 2 = 30 times larger.
Question: If you placed a row of E. coli across the diameter of a single cheek cell, how many would fit?
Working: 60 μm ÷ 2 μm = 30 bacteria.
This helps you visualise just how different these scales are. A typical virus (say 100 nm) would be even smaller: 60 μm = 60,000 nm, so 60,000 ÷ 100 = 600 viruses across a single cheek cell.
graph TD
A[All cells] --> B[Eukaryotic cells]
A --> C[Prokaryotic cells]
B --> D[Animal cells]
B --> E[Plant cells]
B --> F[Fungal cells]
B --> G[Protist cells]
C --> H[Bacteria]
B --> I[Have: nucleus, mitochondria, ribosomes 80S]
C --> J[Have: circular DNA, plasmids, ribosomes 70S]
Common mistake 1: Writing that prokaryotic cells "have no genetic material". This is wrong. Prokaryotes have DNA; it is just not enclosed within a nuclear envelope. Say they have no true nucleus.
Common mistake 2: Writing that eukaryotic cells "always have a cell wall". This is wrong. Only plant and fungal eukaryotic cells have cell walls; animal eukaryotic cells do not.
Common mistake 3: Confusing plasmids with chromosomes. Plasmids are small circular pieces of DNA that carry extra genes (such as antibiotic resistance). The main chromosomal DNA is the single large circular loop.
| Organelle | Found in | Structure | Function |
|---|---|---|---|
| Nucleus | Eukaryotes | Surrounded by nuclear envelope; contains linear chromosomes | Controls cell; houses DNA |
| Chromosomal DNA (circular) | Prokaryotes | Single loop, free in cytoplasm | Carries genes |
| Plasmid | Prokaryotes | Small circular DNA | Extra genes, e.g. antibiotic resistance |
| Mitochondrion | Eukaryotes | Double membrane, folded cristae | Aerobic respiration |
| Ribosome (80S) | Eukaryotes | Small particle of RNA + protein | Protein synthesis |
| Ribosome (70S) | Prokaryotes | Smaller version of ribosome | Protein synthesis |
| Cell membrane | Both | Phospholipid bilayer | Controls entry and exit |
| Cell wall (peptidoglycan) | Prokaryotes | Mesh of peptidoglycan | Support, protection |
| Cell wall (cellulose) | Plants (Eukaryotes) | Cellulose fibres | Support, prevents bursting |
| Flagellum | Some prokaryotes | Long whip-like tail | Movement |
Question: A virus is 30 nm across. Express this in micrometres and in millimetres.
Working:
Question: A plant cell is 0.08 mm across. Express this in micrometres.
Working: 0.08 mm × 1000 = 80 μm.
Exam Tip: When the question asks for an answer in standard form, write it with a single non-zero digit before the decimal point. For example, 0.00003 mm = 3 × 10⁻⁵ mm.
A cell cannot grow indefinitely large because its surface area to volume (SA:V) ratio decreases as it grows. At a certain point, the cell cannot exchange materials (oxygen, glucose, waste) across its membrane fast enough to support its internal volume. This is one reason prokaryotic cells stay small and eukaryotic cells, once they grow larger, rely on specialised internal membrane systems and transport structures.
Grade 3–4 answer (basic): "Eukaryotic cells are bigger and have a nucleus. Prokaryotic cells are smaller and don't have a nucleus. Bacteria are prokaryotic."
Grade 5–6 answer (detailed): "A eukaryote such as an animal cell contains a nucleus and membrane-bound organelles including mitochondria and ribosomes. A prokaryote such as a bacterium has no true nucleus — instead its DNA is a single circular loop free in the cytoplasm. Prokaryotes also contain plasmids, smaller 70S ribosomes, and a peptidoglycan cell wall. Eukaryotic cells are typically 10–100 μm in diameter whereas prokaryotes are only 1–5 μm."
Grade 7–9 answer (precise, full marks): "Eukaryotic cells are compartmentalised — their organelles, such as the nucleus and mitochondria, are enclosed by membranes, allowing chemical reactions like aerobic respiration to occur in specialised environments. The nucleus is bound by a nuclear envelope and contains linear chromosomes. Prokaryotes lack membrane-bound organelles; their chromosomal DNA exists as a single circular molecule in the cytoplasm, supplemented by plasmids carrying accessory genes such as antibiotic resistance. Prokaryotic ribosomes are 70S (smaller than the eukaryotic 80S). Prokaryotes are also bounded by a peptidoglycan cell wall, a structure chemically distinct from the cellulose wall of plant eukaryotes. The typical size difference (10–100 μm vs 1–5 μm) reflects these structural differences — smaller cells have a greater surface area to volume ratio, enabling efficient exchange without compartmentalisation."
Edexcel alignment: This content is aligned with Edexcel GCSE Combined Science (1SC0) Biology Topic 1 Key concepts in biology — specifically CB1.1 Eukaryotes and prokaryotes, CB1.4 Microscopes and magnification (unit conversions), and supporting material from CB1.2 Plant and animal cells. Assessed on Biology Paper 1.