Prokaryotic vs eukaryotic cells (SACE Stage 2 Biology)

What this dot point is asking

You need to compare the two fundamental cell types, identifying the key structural differences and the features they share.

Two types of cell

All cells fall into one of two categories based on whether they have a true nucleus:

• Prokaryotic cells have no membrane-bound nucleus. Bacteria and archaea are prokaryotes.
• Eukaryotic cells have a membrane-bound nucleus. Animals, plants, fungi and protists are eukaryotes.

Features they share

Despite their differences, all cells share certain features, supporting the idea of common ancestry:

• a cell (plasma) membrane controlling what enters and leaves
• cytoplasm, the fluid where reactions occur
• DNA as genetic material
• ribosomes for protein synthesis

Prokaryotic cells

Prokaryotic cells are typically very small (about 1 to 10 micrometres). Their key features:

• No nucleus - the DNA is a single circular chromosome free in the cytoplasm, in a region called the nucleoid.
• No membrane-bound organelles - no mitochondria, no endoplasmic reticulum.
• A cell wall (made of peptidoglycan in bacteria) outside the membrane.
• Smaller (70S) ribosomes.
• Often extra small rings of DNA called plasmids, which can be exchanged between cells.

Eukaryotic cells

Eukaryotic cells are larger (about 10 to 100 micrometres) and far more compartmentalised:

• A true nucleus enclosed by a nuclear membrane, containing linear chromosomes.
• Membrane-bound organelles including mitochondria, endoplasmic reticulum, Golgi apparatus and (in plants) chloroplasts and a large vacuole.
• Larger (80S) ribosomes.
• Plant and fungal cells have a cell wall (cellulose in plants), but animal cells do not.

A point-by-point comparison

SACE comparison questions reward an organised, paired contrast rather than two separate lists. Hold each feature against both cell types:

• Nucleus: prokaryotes have a nucleoid region with no surrounding membrane; eukaryotes have a true nucleus bounded by a double nuclear envelope.
• DNA form: prokaryotic DNA is a single circular chromosome that is not wound around histone proteins; eukaryotic DNA is several linear chromosomes wound around histones.
• Organelles: prokaryotes have no membrane-bound organelles; eukaryotes have mitochondria, endoplasmic reticulum, Golgi apparatus and (in plants) chloroplasts.
• Ribosomes: prokaryotes use the smaller 70S ribosomes; eukaryotes use larger 80S ribosomes in the cytosol (but 70S inside their mitochondria and chloroplasts).
• Size: prokaryotes are roughly $1$ to $10$ $\mu m$; eukaryotes are roughly $10$ to $100$ $\mu m$.
• Cell wall: present in bacteria (peptidoglycan) and in plants (cellulose) and fungi (chitin), absent in animal cells.

Why compartmentalisation matters

Membrane-bound organelles do more than store structures - they create separate chemical environments. The lysosome can hold digestive enzymes at low pH without harming the cytosol, the mitochondrion can build a proton gradient across its inner membrane for ATP synthesis, and the nucleus can keep transcription separate from translation so mRNA is processed before it reaches a ribosome. A prokaryote, lacking these compartments, transcribes and translates in the same space at the same time. This compartmentalisation is what allowed eukaryotic cells to grow larger and run many incompatible reactions at once, and it is the structural theme that links the rest of this topic together.

The endosymbiotic link

Mitochondria and chloroplasts are unusual eukaryotic organelles because they keep prokaryote-like features: their own circular DNA, 70S ribosomes, a double membrane and the ability to divide by binary fission. The endosymbiotic theory explains this by proposing that these organelles descend from free-living prokaryotes engulfed by an ancestral host cell. This is a favourite SACE link because it ties cell structure to the evidence-for-evolution material in Topic 4.

Why the comparison matters

The compartmentalisation of eukaryotic cells underpins the rest of this topic: organelles, membrane transport, respiration and photosynthesis all depend on the membrane-bound structures that prokaryotes lack.