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).

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).

Examples in context

Example 1. Mt Isa lead-tolerant bacteria. Soils around the Mt Isa lead and zinc mines in north-west Queensland contain bacteria such as Cupriavidus metallidurans whose cells exemplify the prokaryotic plan: a single circular chromosome in the nucleoid, no membrane-bound organelles, 70S ribosomes, and metal-efflux pumps on the plasma membrane. Each new cell arises by binary fission from a pre-existing cell, consistent with the third postulate of cell theory. These bacteria precipitate dissolved lead as insoluble lead sulfide and lead phosphate inside the cell, a process bioremediation researchers at James Cook University have tested on Mt Isa tailings to immobilise lead before run-off reaches the Leichhardt River.

Example 2. Halobacterium in Lake Hart and Australian salt pans. Pink salt lakes such as Lake Hart and Lake Eyre support archaeal cells (Halobacterium salinarum) that thrive at 4 mol/L NaCl. Archaea are prokaryotic but biochemically distinct from bacteria: ether-linked membrane lipids, no peptidoglycan in the cell wall, and a chromosome packaged with histone-like proteins. The pink colour is bacteriorhodopsin in the plasma membrane, a light-driven proton pump that supports cell theory postulate 2 because the entire light-energy harvest happens in one cell. The cells reproduce by binary fission only when salt and light allow, satisfying postulate 3.

Try this

Q1. State the three postulates of the cell theory and identify which postulate is supported by the observation that viruses cannot replicate outside a host cell. [3 marks]

• Cue. Three postulates listed; viruses lack cell structure, so they do not satisfy postulate 2 (the basic unit of life).

Q2. A microscope image of two cells, X and Y, shows X is 2 micrometres long with a single circular chromosome free in the cytoplasm, while Y is 25 micrometres long with a nucleus and visible mitochondria. Classify each cell, identifying two distinguishing features used. [4 marks]

• Cue. X prokaryotic (size, circular DNA, no organelles); Y eukaryotic (size, nucleus, mitochondria).

Q3. Refer to the Mt Isa lead-tolerant bacterial example. (a) Identify the domain to which Cupriavidus metallidurans belongs. (b) Explain why the absence of membrane-bound organelles does not prevent the cell from concentrating lead. (c) Justify whether viruses observed in the same soil samples are cells. [2+2+2 marks]

• Cue. (a) Bacteria. (b) Plasma-membrane pumps and cytoplasmic precipitation. (c) Not cells; lack structural features required by cell theory.