How do cells function?
cells as the basic structural feature of life on Earth, including the distinction between prokaryotic and eukaryotic cells
A focused answer to the VCE Biology Unit 1 dot point on cells as the basic unit of life. Covers the cell theory, the distinction between prokaryotic and eukaryotic cells, and the structural features (nucleus, membrane-bound organelles, ribosomes, cell wall) that separate them.
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What this dot point is asking
VCAA wants the cell theory, the recognition that cells are the basic unit of life, and a clean comparison between prokaryotic and eukaryotic cells.
The answer
A cell is the smallest structural and functional unit of life. The cell theory has three modern principles:
- All living organisms are composed of one or more cells.
- The cell is the basic structural and functional unit of life.
- All cells arise from pre-existing cells by division.
All known life on Earth fits into one of two cell types: prokaryotic or eukaryotic.
Prokaryotic cells
Prokaryotes (from the Greek for "before nucleus") are typically 1 to 5 micrometres in diameter. They include all Bacteria and Archaea.
Key features:
- No membrane-bound nucleus. A single, circular DNA chromosome sits in the cytoplasm in a region called the nucleoid.
- No membrane-bound organelles. No mitochondria, no chloroplasts, no endoplasmic reticulum, no Golgi apparatus, no lysosomes.
- 70S ribosomes in the cytoplasm carry out protein synthesis.
- Cell wall made of peptidoglycan (in Bacteria) outside the plasma membrane gives the cell shape.
- Plasmids: small circular DNA molecules separate from the main chromosome; carry genes such as antibiotic resistance.
- Pili and flagella in some species for attachment and movement.
Prokaryotes reproduce by binary fission (a simple DNA copy and split into two), not by mitosis.
Eukaryotic cells
Eukaryotes (from the Greek for "true nucleus") are typically 10 to 100 micrometres in diameter. They include all animals, plants, fungi, and protists.
Key features:
- Membrane-bound nucleus containing linear chromosomes wrapped around histone proteins.
- Membrane-bound organelles that compartmentalise function: mitochondria (cellular respiration), chloroplasts in plants and algae (photosynthesis), endoplasmic reticulum (protein and lipid synthesis), Golgi apparatus (sorting and packaging), lysosomes (in animals; digestion), peroxisomes, vacuoles.
- 80S ribosomes in the cytosol; smaller 70S ribosomes inside mitochondria and chloroplasts.
- Cytoskeleton of microfilaments, intermediate filaments and microtubules.
- Cell wall of cellulose in plants, chitin in fungi; absent in animals.
Eukaryotes reproduce by mitosis (somatic cells) and meiosis (gametes).
Side-by-side summary
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Nucleus | None (DNA in nucleoid) | Membrane-bound |
| DNA | Single circular chromosome + plasmids | Multiple linear chromosomes |
| Membrane-bound organelles | No | Yes |
| Ribosomes | 70S | 80S in cytosol, 70S in mitochondria and chloroplasts |
| Cell wall | Peptidoglycan (Bacteria) | Cellulose (plants), chitin (fungi), none (animals) |
| Size | 1 to 5 micrometres | 10 to 100 micrometres |
| Reproduction | Binary fission | Mitosis (somatic), meiosis (gametes) |
| Examples | E. coli, Streptococcus | Yeast, plants, animals |
Examples in context
Example 1. Cyanobacteria in the Murray-Darling system. Each summer the Murray-Darling Basin Authority issues blue-green algae alerts for stretches of the Darling River near Menindee. The "algae" responsible are actually cyanobacteria (Microcystis aeruginosa), which are prokaryotic. They have no membrane-bound nucleus, no chloroplasts and no mitochondria, yet they photosynthesise because thylakoid-like membranes sit free in the cytoplasm and respire across the plasma membrane. Their small size, circular DNA and 70S ribosomes make them sensitive to antibiotics targeting bacterial ribosomes, which is one tool used by water managers studying the blooms. Photosynthetic eukaryotic algae in the same river, in contrast, carry chloroplasts and a true nucleus.
Example 2. Yeast in Yarra Valley wineries. Saccharomyces cerevisiae, the yeast used by Yarra Valley winemakers, is a single-celled eukaryote. Under the microscope at the Melbourne University microbiology teaching lab, students see a clear nucleus, mitochondria around the cell periphery, a vacuole and a cell wall made of chitin and glucans. Despite being single-celled like a bacterium, the yeast is roughly ten times larger than Escherichia coli, has linear chromosomes wrapped around histones, divides by mitosis rather than binary fission, and uses 80S cytosolic ribosomes. It is a textbook contrast that VCE students can see in the lab.
Try this
Q1. Compare prokaryotic and eukaryotic cells with respect to three structural features. [3 marks]
- Cue. Award one mark each for nucleus (absent vs membrane-bound), organelles (absent vs present) and ribosome size (70S vs 80S).
Q2. A cell viewed under an electron microscope is 2 micrometres long, has a single circular chromosome in a nucleoid region, no membrane-bound organelles, and 70S ribosomes. Identify the cell type and justify your classification using two pieces of evidence. [3 marks]
- Cue. Prokaryotic; circular DNA in nucleoid and 70S ribosomes are diagnostic features.
Q3. Refer to two species in the Murray-Darling system: Microcystis aeruginosa (cyanobacterium) and Chlamydomonas (green alga). (a) Classify each as prokaryotic or eukaryotic. (b) Explain how each photosynthesises despite the structural difference. (c) Suggest one reason cyanobacteria can form rapid blooms that out-compete eukaryotic algae. [2+2+2 marks]
- Cue. (a) Microcystis prokaryote, Chlamydomonas eukaryote. (b) Cyanobacterium uses free thylakoids; alga uses chloroplasts. (c) Faster binary fission, no need for mitosis, smaller size means higher SA:V and faster nutrient uptake.
Exam-style practice questions
Practice questions written in the style of VCAA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2022 VCE3 marksCompare the structure of a prokaryotic cell and a eukaryotic cell.Show worked answer →
A 3-mark answer needs three clear structural differences.
- Nucleus. Prokaryotic cells have no membrane-bound nucleus; their DNA is a single circular chromosome free in the cytoplasm (in the nucleoid region). Eukaryotic cells have a membrane-bound nucleus containing linear chromosomes wrapped around histones.
- Membrane-bound organelles. Prokaryotic cells lack membrane-bound organelles (no mitochondria, no chloroplasts, no ER, no Golgi). Eukaryotic cells contain the full set of membrane-bound organelles for compartmentalised function.
- Ribosomes and size. Prokaryotic ribosomes are smaller (70S); eukaryotic ribosomes in the cytosol are larger (80S). Prokaryotes are typically 1 to 5 micrometres across; eukaryotes are typically 10 to 100 micrometres.
Markers reward three explicit comparisons, not three facts about each cell type in isolation.
2024 VCE2 marksState the three principles of the cell theory.Show worked answer →
A 2-mark answer needs the three modern principles.
- All living things are made of one or more cells.
- The cell is the smallest unit of life (the basic structural and functional unit).
- All cells arise from pre-existing cells by cell division.
Related dot points
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A focused answer to the VCE Biology Unit 1 dot point on surface area to volume ratio. Covers why SA:V decreases as cells get larger, why diffusion becomes inefficient, and why eukaryotes rely on internal membrane compartments (organelles) to maintain rapid exchange.
- the structure and specialisation of plant and animal cell organelles for distinct functions, including chloroplasts and mitochondria, and the suggested origins of mitochondria and chloroplasts as described by the endosymbiotic theory
A focused answer to the VCE Biology Unit 1 dot point on cell organelles. Covers the structure and function of the nucleus, ribosomes, ER, Golgi, mitochondria, chloroplasts, lysosomes, vacuole, cytoskeleton and cell wall, and the endosymbiotic theory for the origin of mitochondria and chloroplasts.
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A focused answer to the VCE Biology Unit 1 dot point on cellular reproduction. Covers prokaryotic binary fission, the eukaryotic cell cycle (G1, S, G2, M), the four phases of mitosis (PMAT), cytokinesis in plant and animal cells, and the checkpoints that regulate the cycle.