← Unit 1: How do organisms regulate their functions?
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 |
Worked example
An electron micrograph shows a cell about 2 micrometres long with a region of dense DNA but no membrane-bound nucleus, ribosomes spread through the cytoplasm, and an external peptidoglycan wall. This is a prokaryote (Bacterium). A second micrograph shows a 20-micrometre cell with a distinct nucleus, mitochondria and a Golgi stack: this is a eukaryote (an animal cell, because there is no cell wall and no chloroplast).
Common traps
Saying prokaryotes "have no DNA". They have DNA; it just is not enclosed in a nucleus.
Saying eukaryotes are always larger. Almost always, but some large bacteria (such as Thiomargarita namibiensis) reach 0.75 mm. Use the structural definition, not the size.
Saying "no organelles in prokaryotes". Prokaryotes have ribosomes, which are organelles in the broad sense; they just have no membrane-bound organelles.
Calling plasmids "small organelles". Plasmids are small circular DNA molecules, not organelles.
In one sentence
A cell is the smallest unit of life; prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles and carry their DNA as a circular chromosome in the cytoplasm, while eukaryotic cells have a true nucleus, membrane-bound organelles, and linear chromosomes wrapped around histones.
Past exam questions, worked
Real questions from past VCAA papers on this dot point, with our answer explainer.
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
- surface area to volume ratio as an important factor in the limitations of cell size and the need for internal compartments (organelles) with specific cellular functions
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.
- the characteristics of the plasma membrane as a semi-permeable boundary between the internal and external environments of a cell and the movement of hydrophilic and hydrophobic substances across it, including water (osmosis), simple diffusion, facilitated diffusion, active transport, endocytosis and exocytosis
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- the binary fission of prokaryotic cells and the eukaryotic cell cycle, including interphase (G1, S and G2), mitosis (prophase, metaphase, anaphase and telophase) and cytokinesis in plant and animal cells, with reference to checkpoints that regulate the cycle
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.