The cell is the structural and functional unit of life; prokaryotes lack a membrane-bound nucleus and organelles, while eukaryotes compartmentalise their cytoplasm with membranes.
How it's asked: Who contributed what to cell theory (Schleiden, Schwann, Virchow), the layers of the bacterial cell envelope, mesosomes and inclusion bodies, 70S vs 80S ribosomes, and plant vs animal cell differences.
Why this is true
Cell theory grew in two steps: Schleiden (1838) and Schwann (1839) showed that plants and animals are built of cells, but they could not say where cells came from. Rudolf Virchow (1855) completed it by stating that new cells arise only from pre-existing cells (omnis cellula-e cellula), which is why the modern theory has two parts. Prokaryotes are small and simple because they never evolved internal membranes: their DNA lies naked in the cytoplasm as a nucleoid, and jobs that eukaryotes give to organelles are handled by infoldings of the plasma membrane called mesosomes. Eukaryotes, by contrast, wall off reactions inside membrane-bound compartments, which lets incompatible chemistry run side by side in one cell and allows much larger cell size. Plant and animal cells share this plan but differ in what their lifestyle needs: plants have a cell wall, plastids and a large central vacuole, while animal cells have centrioles.
Key points
Prokaryotic vs eukaryotic cell
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | Nucleoid, no nuclear envelope | True nucleus with double nuclear envelope |
| Membrane-bound organelles | Absent | ER, Golgi, mitochondria, plastids (plants), lysosomes, vacuoles |
| Ribosomes | 70S | 80S in cytoplasm; 70S in mitochondria and chloroplasts |
| Genetic material | Single circular DNA + plasmids | Linear DNA in several chromosomes with histones |
| Membrane infolding | Mesosomes; chromatophores in cyanobacteria | Absent as such |
| Examples | Bacteria, cyanobacteria, mycoplasma, PPLO | Protists, fungi, plants, animals |
Common traps
- Schwann, not Schleiden, recognised the plasma membrane and concluded that the cell wall is a unique plant feature; Virchow, not either of them, explained the origin of cells.
- Mitochondria and chloroplasts are inside eukaryotic cells yet carry 70S ribosomes; a question asking 'which ribosome type occurs in eukaryotic cells' can therefore have two right answers.
Worked example
A student examines an unknown cell under the electron microscope. It has a cell wall, 70S ribosomes, a nucleoid and membrane infoldings, but no nuclear envelope or mitochondria.
High-yield
Omnis cellula-e cellula — Virchow (1855). Prokaryotes: 70S ribosomes, nucleoid, mesosomes; Mycoplasma about 0.3 μm is the smallest cell.
Quick check
Q1.Which structure in a bacterial cell helps in cell wall formation, DNA replication and respiration?
Q2.Who first explained that new cells arise from pre-existing cells?