Describe the cell theory and distinguish between prokaryotic and eukaryotic cells, recalling that prokaryotes include bacteria and archaea

What this dot point is asking

QCAA wants you to state the cell theory, classify any cell as prokaryotic or eukaryotic, and recall that the prokaryotic domains are Bacteria and Archaea. This is foundation knowledge for every later Unit 1 dot point on organelles, transport and metabolism, and it returns implicitly in Unit 3 (microbial decomposers, primary producers) and Unit 4 (DNA replication, gene expression).

The answer

The cell is the structural and functional unit of life. Cell theory and the prokaryote vs eukaryote divide are the two organising ideas that everything else in Unit 1 rests on.

The cell theory

The modern cell theory has three core postulates:

1. All living organisms are composed of one or more cells. Viruses are non-cellular and are not considered alive by this definition.
2. The cell is the basic structural and functional unit of life. Each cell can carry out the processes of life (metabolism, response, reproduction).
3. All cells arise from pre-existing cells. Cells divide by mitosis (eukaryotes) or binary fission (prokaryotes); they do not form spontaneously.

Two further extensions are often included:

• Heredity information (DNA) is passed from parent cell to daughter cell.
• All cells share the same basic chemistry and energy currency (ATP).

Prokaryotic cells

Prokaryotes are unicellular organisms whose genetic material is not enclosed in a nucleus. They fall into two domains:

• Bacteria. Includes E. coli, Streptococcus, cyanobacteria. Peptidoglycan cell walls.
• Archaea. Includes methanogens, extremophiles in hot springs and salt lakes. Distinct membrane lipids; no peptidoglycan.

Structural features of prokaryotes:

• A single circular chromosome in the nucleoid region, often supplemented by plasmids.
• 70S ribosomes free in the cytoplasm.
• A plasma membrane, usually surrounded by a cell wall.
• No membrane-bound organelles. Metabolic reactions occur in the cytoplasm or on infoldings of the plasma membrane.
• Typically 1 to 10 micrometres in diameter.
• May have flagella (motility), pili (attachment, conjugation) and a capsule (protection).

Eukaryotic cells

Eukaryotes have a true membrane-bound nucleus containing linear chromosomes wrapped around histone proteins. They include all animals, plants, fungi and protists.

Structural features of eukaryotes:

• Membrane-bound nucleus housing linear chromosomes.
• 80S ribosomes (and 70S inside mitochondria and chloroplasts).
• Membrane-bound organelles: mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, vacuoles and (in plants and algae) chloroplasts.
• A cytoskeleton of microtubules and microfilaments.
• Typically 10 to 100 micrometres in diameter, an order of magnitude larger than prokaryotes.
• Plant cells additionally have a cellulose cell wall and large central vacuole; fungal cells have a chitin cell wall.

Why the size difference

Eukaryotes can be larger because they compartmentalise functions into organelles, increasing internal membrane surface area for reactions. Prokaryotes rely on diffusion across the plasma membrane, which limits their size (see surface area to volume ratio).

The endosymbiotic theory

Mitochondria and chloroplasts have their own circular DNA, 70S ribosomes and double membranes, and divide by binary fission. The endosymbiotic theory proposes that they originated from free-living bacteria engulfed by an ancestral eukaryotic cell. This is consistent with cell theory (organelles arise from pre-existing organelles).

Common traps

Calling viruses prokaryotes. Viruses are non-cellular: no plasma membrane, no ribosomes, no metabolism of their own. They are not classified by cell theory.

Treating archaea as a kind of bacteria. Archaea are a separate prokaryotic domain with distinct biochemistry (membrane lipids, RNA polymerase). The "three-domain" view (Bacteria, Archaea, Eukarya) replaced the older "five-kingdom" view in the 1990s.

Saying prokaryotes have no organelles. They have no membrane-bound organelles. Ribosomes, nucleoid and cytoskeletal proteins are still present.

Cross-link to Year 12 assessment

You will not be examined on this dot point in IA1, IA2 or IA3 directly, but cell-type fluency is assumed throughout Unit 3 ecology (microbial decomposers, primary producers) and Unit 4 genetics (binary fission vs mitosis, plasmid transformation in biotechnology IA3 contexts).

In one sentence

All living organisms are made of one or more cells, each cell arising from a pre-existing cell, with prokaryotes (Bacteria and Archaea) lacking a membrane-bound nucleus and organelles, and eukaryotes possessing both along with a larger, compartmentalised cell plan.