Small cells, and things that are not cells at all.
A bacterium is a complete living cell built on a much simpler plan than yours: no nucleus, no mitochondria, smaller ribosomes. A virus is simpler still, just genes in a protein coat, and cannot do anything until it gets inside a cell.
A prokaryotic cell, such as a bacterium, is unicellular and usually 1 to 5 μm across. It has a peptidoglycan (murein) cell wall, circular DNA free in the cytoplasm with no histones, 70S ribosomes, and no organelles surrounded by double membranes: no nucleus, mitochondria or chloroplasts. A virus is non-cellular: a core of nucleic acid, either DNA or RNA, inside a protein capsid, and in some viruses an outer envelope of phospholipids taken from the host cell. Viruses can only reproduce inside a host cell.
Fig. 1: a typical bacterium and an enveloped virus, with their typical sizes
Section 01
The prokaryotic plan.
Rotate the bacterium and click each part. Notice what is missing: there is no nucleus, and nothing inside the cell is wrapped in a membrane. Use the comparison table in the sim as you go.
Always present: cell wall of peptidoglycan; cell surface membrane; cytoplasm; one circular DNA molecule (the nucleoid region) with no histones; 70S ribosomes.
Sometimes present: a capsule or slime layer; plasmids (small extra rings of DNA, often carrying antibiotic resistance genes); one or more flagella for movement; pili for attachment and passing on plasmids.
Never present: nucleus, mitochondria, chloroplasts, or any other organelle surrounded by a double membrane. Respiration enzymes sit on the cell surface membrane instead.
Feature
Prokaryotic cell
Eukaryotic cell
size
usually 1 to 5 μm
usually 10 to 100 μm
DNA
circular, free in cytoplasm, no histones
linear chromosomes with histones, in a nucleus
ribosomes
70S (smaller)
80S in cytoplasm
double-membrane organelles
none
nucleus, mitochondria, chloroplasts (plants)
other membrane organelles
none (no ER, Golgi body, lysosomes)
ER, Golgi body, lysosomes, vacuoles
cell wall
peptidoglycan
cellulose in plants; none in animals
organisation
unicellular
unicellular or multicellular
Examiner trap
Prokaryotes do have a cell surface membrane, cytoplasm and ribosomes: they lack only the membrane-bound organelles. And their cell wall is peptidoglycan, never cellulose. Writing “bacteria have no ribosomes” or “a cellulose wall” loses the mark.
Section 02
Viruses: non-cellular.
Switch between the three viruses in the sim and tick Cut-away to see the nucleic acid inside. Compare the non-enveloped virus (like adenovirus), the enveloped virus (like influenza) and the bacteriophage.
Nucleic acid core: either DNA or RNA, never both. Adenovirus and phage T4 carry double-stranded DNA; influenza carries single-stranded RNA in segments.
Capsid: a protein coat built from many repeating subunits (capsomeres). It protects the nucleic acid and helps the virus attach to a host cell.
Envelope (some viruses only): a phospholipid membrane taken from the host cell surface membrane as the virus buds out, carrying viral glycoprotein spikes. Influenza and HIV are enveloped; adenovirus is not.
Not cells: no cytoplasm, no ribosomes, no metabolism. Viruses reproduce only inside a host cell, using its ribosomes, enzymes and ATP.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the 3D bacterium, what is the cell wall made of?
Quick check+10 XP
Which structure is found in the bacterium as well as in eukaryotic cells?
Quick check+10 XP
In the virus tab, what forms the capsid?
Quick check+10 XP
Where does the envelope of an enveloped virus such as influenza come from?
Section 03
Are cells the basic unit of life?
Cell theory says that all living organisms are made of cells, that the cell is the smallest unit of life, and that cells come only from pre-existing cells. Viruses test that idea.
For calling viruses non-living: they are not made of cells, cannot respire, grow or reproduce on their own, and show no metabolism outside a host.
Against: they contain genetic material, reproduce (inside a host) and evolve by natural selection, which is why new strains of influenza appear.
The usual conclusion: every independently living organism, prokaryote or eukaryote, is made of cells, so the cell remains the basic unit of life, and viruses sit on the boundary.
Examiner trap
Never describe a virus as a “cell” or as a type of bacterium. A virus is 10 to 100 times smaller than a bacterium, has no cytoplasm or ribosomes, and is not affected by antibiotics, which target bacterial structures such as the peptidoglycan wall and 70S ribosomes.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which feature is present in a typical bacterium but absent from a typical animal cell?
Exam style+20 XP
Which combination correctly describes the genetic material of a prokaryotic cell?
Exam style+20 XP
Which statement applies to all viruses?
Exam style+20 XP
An organelle is about 1 μm wide, has a double membrane and contains 70S ribosomes. Which cell type could contain it?
Skill unlocked
Prokaryotes and viruses, mastered.
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