Eukaryotic cells have a true nucleus enclosing their DNA, whereas prokaryotic cells — such as bacteria — have no nucleus at all. This single structural difference makes prokaryotes simpler and smaller (1–10 µm) than eukaryotes (10–100 µm), and understanding it is the foundation of GCSE cell biology.
What is a eukaryotic cell?
A eukaryotic cell is any cell that contains a true, membrane-bound nucleus. The nucleus encloses the cell's DNA, which is organised into linear chromosomes. All animals, plants, fungi, and protists are made of eukaryotic cells.
Key features found in eukaryotic cells:
- Nucleus — contains linear DNA on chromosomes; separated from the cytoplasm by a nuclear envelope (double membrane with pores)
- Mitochondria — site of aerobic respiration (present in all eukaryotes)
- Ribosomes (80S) — larger than prokaryotic ribosomes; site of protein synthesis
- Endoplasmic reticulum — rough ER (with ribosomes) makes proteins; smooth ER makes lipids
- Golgi apparatus — packages and dispatches proteins
Plant cells additionally contain chloroplasts (photosynthesis), a large permanent vacuole (turgor pressure), and a cellulose cell wall.
What is a prokaryotic cell?
A prokaryotic cell has no true nucleus. Its DNA consists of a single, circular chromosome that floats freely in the cytoplasm. All bacteria are prokaryotes; they are the oldest and simplest form of life on Earth.
Key features of prokaryotic cells:
- No nucleus — circular DNA is in the cytoplasm (called the nucleoid region)
- Plasmids — small extra circles of DNA carrying non-essential genes (e.g. antibiotic resistance genes)
- Cell wall — made of peptidoglycan (not cellulose); provides shape and protection
- Ribosomes (70S) — smaller than eukaryotic ribosomes
- Pili — protein threads that allow attachment to surfaces or other cells
- Flagellum — a rotating tail used for movement (present in some bacteria)
- No mitochondria, chloroplasts, endoplasmic reticulum, or Golgi apparatus
The cell is very small — most bacteria are 0.1–10 µm long — but can reproduce extremely rapidly (as fast as every 20 minutes in ideal conditions).
How do eukaryotic and prokaryotic cells compare?
| Feature | Eukaryotic cell | Prokaryotic cell |
|---|---|---|
| Nucleus | True membrane-bound nucleus | No nucleus; circular DNA in cytoplasm |
| Size | 10–100 µm (typical) | 0.1–10 µm (typical) |
| DNA | Linear chromosomes | Single circular chromosome |
| Extra DNA | — | Plasmids |
| Ribosomes | 80S (larger) | 70S (smaller) |
| Membrane-bound organelles | Yes (mitochondria, ER, Golgi) | No |
| Cell wall | Plant/fungi only (cellulose/chitin) | Yes — peptidoglycan |
| Pili | No | Often present |
| Flagella | Some protists | Some bacteria |
| Examples | Animal, plant, fungal, protist cells | Bacteria |
Why does nucleus presence matter so much?
The nucleus controls the cell's activities because it contains the DNA that codes for every protein the cell makes. In a eukaryote, gene expression is carefully regulated: transcription (DNA → mRNA) happens inside the nucleus; translation (mRNA → protein) happens on ribosomes in the cytoplasm. The nuclear envelope keeps these stages physically separated.
In a prokaryote, transcription and translation occur simultaneously in the same compartment because there is no nuclear membrane to cross. A ribosome can begin translating an mRNA strand before transcription of that strand is even finished — this makes gene regulation faster and simpler, which helps bacteria respond rapidly to changing environments.
What examples does GCSE ask about?
GCSE examiners frequently use bacteria as their standard prokaryote example and may ask you to:
- Identify which structures are present in a prokaryote but not in a eukaryote (plasmids, peptidoglycan cell wall, pili, 70S ribosomes)
- Identify which structures are present in a eukaryote but not in a prokaryote (nucleus, mitochondria, 80S ribosomes, endoplasmic reticulum)
- State one similarity between them (both have a cell membrane, cytoplasm, and ribosomes)
A common error is thinking bacteria have no ribosomes — they do. Antibiotics such as streptomycin target the bacterial 70S ribosome specifically, which is why they can kill bacteria without harming the patient's own 80S ribosomes.
How do the sizes of prokaryotes and eukaryotes compare?
Prokaryotes are typically 1,000–10,000 times smaller in volume than eukaryotic cells. A typical bacterial cell is around 1–5 µm in diameter; a typical animal cell is 10–30 µm. This small size gives bacteria a very high surface-area-to-volume ratio, which means nutrients and waste can diffuse in and out quickly without needing a circulatory system.
Frequently asked questions
What is the main difference between eukaryotic and prokaryotic cells?
The main difference is the presence of a true, membrane-bound nucleus. Eukaryotic cells contain their DNA inside a nucleus enclosed by a nuclear envelope; prokaryotic cells have no nucleus and their circular DNA floats freely in the cytoplasm. This distinction is the basis for dividing all living things into two fundamental domains.
Do prokaryotes have any membrane-bound organelles at all?
No — prokaryotes lack all membrane-bound organelles. They have no mitochondria, no endoplasmic reticulum, no Golgi apparatus, and no chloroplasts. All metabolic processes occur either at the cell membrane or within the cytoplasm itself. Some bacteria carry out photosynthesis using pigments embedded directly in folded regions of the cell membrane (not in chloroplasts).
Why do bacteria have plasmids but eukaryotes do not?
Plasmids are small circular DNA molecules separate from the main chromosome. They are common in bacteria and carry genes that may be advantageous in specific conditions — particularly antibiotic resistance genes. Because plasmids can be passed between bacteria (horizontal gene transfer), resistance spreads rapidly through populations. Most eukaryotes do not naturally carry plasmids, though scientists can introduce them artificially for genetic engineering.
Are all prokaryotes bacteria?
At GCSE level, yes — the two terms are used interchangeably. At a more advanced level, life is divided into three domains: Bacteria, Archaea (which are also prokaryotes with no nucleus but different biochemistry), and Eukarya. GCSE biology only expects students to know about bacteria as the example of prokaryotes.
For Socratic GCSE biology with Professor Darwin — building from cell structure to the systems they form — visit aitutors.me.