Eukaryotic and prokaryotic cells

Eukaryotic cells have their DNA inside a nucleus; prokaryotic cells don't. Animal, plant and fungal cells are eukaryotic. Bacteria are prokaryotic: they are much smaller, and their DNA is a single loop free in the cytoplasm, often with small extra rings of DNA called plasmids.

What is the difference between eukaryotic and prokaryotic cells?

AQA's GCSE specification puts it simply. Eukaryotic cells (plant and animal) have a cell membrane, cytoplasm and genetic material enclosed in a nucleus. Prokaryotic cells (bacteria) are much smaller, with cytoplasm and a cell membrane inside a cell wall. Their DNA has no nucleus around it; it forms a single loop, and there may also be one or more small rings of DNA called plasmids.

The size ranges come from OpenStax Biology. Both kinds of cell have a cell membrane, cytoplasm, DNA and ribosomes, the four features OpenStax lists for all cells.

Prokaryotic cell structure

• Cell wall: supports the cell and stops it bursting. At A-level, AQA notes it contains murein, a glycoprotein (also called peptidoglycan).
• Cell membrane: controls what enters and leaves, as in all cells.
• DNA loop: a single circular DNA molecule in the cytoplasm, not associated with proteins.
• Plasmids: small rings of DNA carrying a few extra genes, which can include genes for antibiotic resistance.
• Ribosomes: smaller than those in eukaryotic cells, but they still make proteins.
• Capsule and flagella: some bacteria have a slimy capsule around the wall that helps them stick to surfaces, and one or more flagella for movement.

Eukaryotic cell structure

Eukaryotic cells are bigger and divided into compartments by membranes. The nucleus holds the DNA; mitochondria release energy in respiration; in plant cells, chloroplasts carry out photosynthesis. The animal and plant cells guide covers each part, and the mitochondria guide shows how one organelle is built for its job.

Comparing sizes: orders of magnitude

AQA expects you to make order of magnitude calculations when comparing cells. Each order of magnitude is a factor of 10.

Example

A bacterium is 2 µm long and a plant cell is 50 µm across. How many times longer is the plant cell, and roughly how many orders of magnitude bigger is it?

1. Ratio: 50 ÷ 2 = 25 times longer.
2. 25 is between 10¹ and 10², and nearer 10¹, so the plant cell is about one order of magnitude bigger.

Small size helps prokaryotes: OpenStax explains that a small cell has a large surface area compared with its volume, so substances can diffuse in and out quickly enough to supply the whole cell.

Where do viruses fit?

Viruses are neither eukaryotic nor prokaryotic, because they aren't cells. They have no cytoplasm or ribosomes of their own and can only reproduce inside a host cell. Bacteria divide by binary fission; viruses are copied by the cells they infect.

Common mistakes

• Saying prokaryotes have no DNA. They have DNA, just not inside a nucleus.
• Saying prokaryotes have no ribosomes. They have smaller ribosomes.
• Calling all cell walls cellulose. Plant cell walls are cellulose; bacterial walls contain murein; fungal walls are different again.
• Saying plant cells are prokaryotic because they have a wall. Plant cells have a nucleus, so they are eukaryotic.
• Calling viruses prokaryotes. Viruses aren't cells at all.

Explore cell structures in the cell simulator or the Cell Biology course. For practice in a short timed paper, try today's GCSE Biology Higher or GCSE Biology Foundation paper.

Read next

Biology · CellsAnimal and plant cells: GCSE structures and differencesAnimal and plant cells for GCSE Biology: labelled diagrams, what each part does, the differences between them, and how bacterial cells compare.