Biology · Cells, Membranes & Organisation

Bacterial, archaeal and eukaryotic cells

Three diagrams show a bacterium, an archaeon and a yeast cell. Each has an outer boundary and internal contents. If the labels were removed, a small oval outline would tell us remarkably little. The important differences concern organisation and molecular evidence, not an artist's choice of shape.

The earlier biology chapters introduce living systems, evidence, water, ions and biological molecules. Here, polar means a molecule has unevenly distributed electrical charge, and an ion is an electrically charged atom or group of atoms. Concentration means amount of a substance per volume. No microscope or home experiment is required; the examples use explicitly constructed observations.

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By the end, you can

Compare cellular organisation while avoiding the assumption that similar appearance means identical biology.

Start with shared cellular work

Cellular lineages share plasma membranes, internal chemistry, hereditary information in DNA, and ribosomes that assemble proteins. These shared systems support comparison across a great diversity of organisms. They describe basic cellular organisation, rather than a claim that every specialised mature cell retains every component.

Cytosol is the internal fluid region; cytoplasm includes cytosol and cellular structures outside the nucleus in a eukaryotic cell. A ribosome is a protein-making structure, not a membrane-enclosed compartment. Keeping these meanings separate helps us read a diagram without treating every small dot as a separate membrane-bound organelle.

A specialised cell can differ from the general plan of its lineage. For example, a mature mammalian red blood cell has lost its nucleus during development. Such a cell remains part of a eukaryotic organism. Classification considers the organism's cellular history and molecular relationships, not a one-feature sorting rule applied to an isolated mature cell.

Compare the location of genetic information

Bacteria and Archaea are distinct groups whose cells lack a membrane-enclosed nucleus. Their main chromosomal DNA occupies a nucleoid region, which has no nuclear envelope. The term prokaryotic describes this shared organisation; it does not make bacteria and archaea the same group.

Eukaryotic cellular organisation includes a nucleus and an extensive system of membrane-enclosed compartments. Animals, plants, fungi and many single-celled organisms are eukaryotes. Yeast is a single-celled fungus: having a nucleus does not require being multicellular.

The three panels are drawn at a common illustration size so their organisation can be compared. They are not a size chart or a sequence of increasing worth. A small cell can carry out demanding chemistry without the exact compartment arrangement shown in a eukaryotic cell.

Compare internal organisation, not the drawing's size

Not drawn to scale or as an evolutionary sequence. Specialised cells can differ from these general plans.

Read the diagram in words

Three schematic cells contain a plasma membrane, DNA and ribosomes. Bacterial and archaeal DNA lies in a nucleoid region without a nuclear envelope. The typical eukaryotic cell has a nucleus and other membrane-enclosed compartments. Equal drawing sizes are for comparison and do not show actual relative cell sizes.

Plasma membrane
Internal chemistry
DNA
Ribosomes

Compare internal organisation, not the drawing's size
Cell plan	DNA organisation	Nuclear envelope in this model	Other features
Bacterial cell	DNA in a nucleoid	No	Many have a peptidoglycan wall
Archaeal cell	DNA in a nucleoid	No	Distinct membrane and surface chemistry
Typical eukaryotic cell	DNA within a nucleus	Yes	Other membrane-enclosed compartments

Diagram key

Plasma membrane
Ribosomes
Nucleus
Membrane-enclosed compartment

Look beyond a similar outline

Most bacterial walls contain peptidoglycan, a distinctive mesh of sugars and short peptide links. Archaeal cell walls lack that material and can have different surface structures. Some cells lack walls altogether. A wall is outside the plasma membrane; it does not replace the membrane's role in separating the internal solution from the surroundings.

Typical archaeal membrane lipids have ether-linked hydrocarbon chains, unlike the ester-linked fatty-acid chains dominant in many bacterial and eukaryotic membrane lipids. Some archaeal lipids span the membrane in a monolayer. These are useful molecular contrasts, not an absolute rule that any ether lipid identifies an archaeon.

Archaea occur in ordinary environments as well as extreme ones. Nor are bacteria defined by causing disease. Choose a comparison relevant to the question: membrane chemistry for a permeability question, genetic sequences for relationships, or measured shape for a structural description. One observation seldom replaces the others.

Worked example

An unknown cell does not come with a domain label

Suppose a hypothetical record states: 'A small cell has DNA and ribosomes, but no nucleus was detected.' This is consistent with a bacterium or archaeon, yet the record does not discriminate between them. It also needs a method capable of testing the nuclear claim. Add relevant molecular evidence rather than deciding that the cell must be bacterial because it is small.

Now suppose separate evidence identifies a yeast lineage and shows a nucleus in comparable cells. A particular cell section without a visible nucleus does not overturn that identification. The observation, its sampling limits and the wider evidence must be read together.

Check your understanding

Can a single-celled organism be eukaryotic?
Show reasoning
Yes. Yeast and many other single-celled organisms have eukaryotic cellular organisation.
Does the word prokaryotic make Bacteria and Archaea one identical group?
Show reasoning
No. It describes their lack of a membrane-enclosed nucleus. Their molecular features and evolutionary relationships distinguish them.
Is a cell wall a substitute for a plasma membrane?
Show reasoning
No. A wall provides external support or protection, while the membrane forms the selectively permeable boundary of the internal cellular solution.

What to carry forward

Compare shared cellular functions, internal organisation and molecular differences without turning one feature into a universal sorting rule.

References

Resources supporting this lesson’s explanations and examples.