Eukaryotic and Prokaryotic Cells
Two types of cell
All cells are either eukaryotic (with a true, membrane-bound nucleus and organelles — animals, plants, fungi, protists) or prokaryotic (no nucleus, no membrane-bound organelles — bacteria). A-Level requires detailed knowledge of organelle structure and function.
Eukaryotic organelles
| Organelle | Structure | Function |
|---|---|---|
| Nucleus | Double membrane (nuclear envelope) with pores; contains chromatin & nucleolus | Stores DNA; controls the cell; nucleolus makes ribosomes |
| Mitochondrion | Double membrane; inner folded into cristae; fluid matrix | Site of aerobic respiration (ATP production) |
| Chloroplast (plants) | Double membrane; thylakoids stacked into grana; stroma | Site of photosynthesis |
| Ribosomes | 80S (eukaryotic); made of rRNA + protein | Protein synthesis |
| Rough ER | Membranes studded with ribosomes | Folds and transports proteins |
| Smooth ER | Membranes, no ribosomes | Synthesises lipids |
| Golgi apparatus | Stacked flattened sacs | Modifies, packages and sorts proteins into vesicles |
| Lysosome | Vesicle of digestive enzymes | Breaks down waste/pathogens |
| Cell wall (plants/fungi) | Cellulose (plants) / chitin (fungi) | Support and shape |
Prokaryotic cells
Smaller and simpler:
- No nucleus — DNA is a single circular loop, free in the cytoplasm.
- No membrane-bound organelles; smaller 70S ribosomes.
- Have a cell wall (murein/peptidoglycan), and may have plasmids (small DNA loops), a capsule, and flagella.
Viruses
Not cells at all — acellular. Just genetic material (DNA/RNA) in a protein coat (capsid), with attachment proteins; they can only replicate inside a host cell.
Studying cells
- Magnification = image size ÷ actual size; resolution = the smallest distance at which two points appear separate.
- Electron microscopes have much higher resolution than light microscopes (TEM = internal detail; SEM = 3D surface).
- Cell fractionation & ultracentrifugation: homogenise tissue in a cold, isotonic, buffered solution, then spin at increasing speeds to separate organelles by density (nuclei first, then mitochondria, etc.).
Worked example
Why is the solution used in cell fractionation cold, isotonic and buffered?
- Cold slows enzyme activity that could digest organelles; isotonic prevents organelles bursting/shrinking by osmosis; buffered keeps pH stable so proteins aren't denatured. ✓
Common mistakes
- Giving prokaryotes membrane-bound organelles — they have none.
- Confusing ribosome sizes: 80S (eukaryotic) vs 70S (prokaryotic/mitochondria).
- Mixing up magnification (how much bigger) with resolution (level of detail).
Exam tips
- Learn each organelle's structure and function together.
- Know the cell-fractionation conditions and the order organelles settle out.
- Distinguish eukaryotic vs prokaryotic features clearly; remember viruses are acellular.
Key facts to remember
- Eukaryotic = nucleus + membrane-bound organelles (80S ribosomes); prokaryotic = no nucleus, circular DNA, plasmids, 70S ribosomes, murein wall.
- Learn organelle structure → function (mitochondria/respiration, chloroplast/photosynthesis, Golgi/packaging, RER/protein transport…).
- Cell fractionation: cold, isotonic, buffered; centrifuge separates organelles by density; EM gives high resolution.