Prokaryotes were the first cells on Earth and they are still the most common. “Simple” is the usual word for them, but simple does not mean primitive — it means the whole cell is one open compartment, and that changes how everything works.
📚 What you need to know
Prokaryotes have no nucleus and no membrane-bound organelles. The name means “before the nucleus”.
They are small: roughly 0.1–5.0 µm across.
Two domains: Bacteria (Eubacteria) and Archaea (Archaebacteria, often found in extreme conditions).
Always present: cell wall, plasma membrane, cytoplasm, 70S ribosomes, and a circular DNA loop in the nucleoid.
Sometimes present: plasmids, capsule, flagella, pili.
The cell wall contains peptidoglycan (also called murein) and stops the cell bursting.
Plasmids are small extra loops of DNA and can be swapped between cells — this is how antibiotic resistance spreads.
The shape of a prokaryotic cell
The DNA sits directly in the cytoplasm in a region called the nucleoid. There is no membrane around it, which is exactly why it is not a nucleus.
Always there, sometimes there
Structure
Always or sometimes
What it does
Cell wall
Always (in almost all)
Protection and shape; stops the cell bursting
Plasma membrane
Always
Controls what enters and leaves
Cytoplasm
Always
Where the cell’s reactions happen
70S ribosomes
Always
Read mRNA and build proteins
Circular DNA (nucleoid)
Always
Carries the genetic information
Plasmid
Sometimes
Extra genes, e.g. antibiotic resistance; can be passed on
Capsule
Sometimes
Stops drying out; protects from the host’s immune cells
Flagellum
Sometimes
Rotates like a propeller to move the cell
Pili
Sometimes
Stick to surfaces and other cells; used in conjugation
Ribosomes: 70S, not 80S
Prokaryotic ribosomes are 70S — structurally smaller than the 80S ribosomes in eukaryotic cells. They do the same job: bind mRNA and build proteins during translation.
This size difference is more useful than it looks. Several antibiotics work by jamming 70S ribosomes but leaving 80S ones alone, which is how they kill bacteria without killing you.
DNA and plasmids
Prokaryotic DNA is a single circular molecule, described as “naked” because it is not wrapped around histone proteins the way eukaryotic DNA is. It sits in a region of the cytoplasm called the nucleoid.
Plasmids are much smaller separate loops. They often carry genes that are useful but not essential, and because they can be transferred from cell to cell, a helpful gene can spread through a bacterial population fast.
The cell wall
Most prokaryotes have a wall containing peptidoglycan (murein). It gives the cell its shape, protects it, and — most importantly — stops it bursting when water moves in.
Gram staining. Bacteria can be sorted by how their wall reacts to a dye called crystal violet. Those that hold the dye and turn blue or violet are Gram positive (for example Bacillus and Staphylococcus). Those that do not are Gram negative. The difference is in the wall structure, and it affects which antibiotics will work.
Worked examples
WORKED EXAMPLE
A prokaryotic cell is 2 µm long. A drawing of it measures 80 mm. Calculate the magnification of the drawing.
Step 1: convert to the same unit80 mm × 1000 = 80 000 µmStep 2: divide image by actual80 000 ÷ 2 = 40 000× 40 000that magnification is well beyond a light microscope – a clue the drawing came from an electron micrograph
WORKED EXAMPLE
Explain how the structure of a prokaryotic cell allows antibiotic resistance to spread through a bacterial population. [3]
Step 1: name the structure
Plasmids — small circular loops of DNA separate from the main chromosome.
Step 2: what they carry
They can carry genes for antibiotic resistance.
Step 3: how they move
Plasmids can be passed between bacteria, so the resistance gene spreads without the bacteria needing to reproduce.
Plasmid transfer moves useful genes sideways through a population
WORKED EXAMPLE
A student writes: “Prokaryotes have no organelles.” Correct this statement. [2]
Step 1: identify what is wrong
Prokaryotes do have ribosomes, which are organelles.
Step 2: give the correct version
They have no membrane-bound organelles — no nucleus, mitochondria, ER or Golgi.
“No membrane-bound organelles” is the accurate phrasethat one missing word is a mark in many papers
💡 Exam tip
Always say membrane-bound organelles when explaining what prokaryotes lack.
Know the size range: 0.1–5.0 µm. Compare it with eukaryotic cells at 10–100 µm.
The DNA is circular, naked, and in the nucleoid — three separate details, often three separate marks.
Learn which structures are optional. A question showing a bacterium without a flagellum is not showing a mistake.
The cell wall is peptidoglycan in prokaryotes, cellulose in plants and chitin in fungi. Do not blur them.
70S for prokaryotes, 80S for eukaryotes. Small cell, small number.
⚠ Common mix-up
Calling the nucleoid a nucleus. There is no membrane around it, so it is not a nucleus.
Saying prokaryotes have no ribosomes. They do — 70S ones.
Confusing the plasmid with the main DNA loop. Plasmids are small, extra and optional.
Mixing up flagella and pili. Flagella are long and rotate for movement; pili are short and thin and stick to things.
Saying the capsule is the cell wall. The capsule is an extra slimy layer outside the wall.
Assuming all bacteria are harmful. Most are harmless or useful, and that framing costs marks in extended answers.
Up next: Eukaryotic Cell Structure — what happens when a cell divides itself into compartments, and what each of those compartments is for.
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