Knowledge Organiser: Cell Structure
Key Terms
- Cell: Basic unit of life; smallest structure capable of all life processes
- Organelle: Specialised structure within a cell with a specific function
- Prokaryotic: Cell type with no true nucleus; DNA free in cytoplasm (e.g. bacteria)
- Eukaryotic: Cell type with a true membrane-bound nucleus (e.g. animal, plant, fungal cells)
- Magnification: Number of times larger an image is than the real object
- Resolution: Ability to distinguish two separate points as distinct structures
Must-Know Facts
- All cells share: cell membrane, cytoplasm, ribosomes, genetic material (DNA)
- Prokaryotic cells are 1–5 μm; eukaryotic cells are 10–100 μm
- Plant cells have cell wall (cellulose), chloroplasts, and permanent vacuole — animal cells do not
- Bacterial cells have plasmids (small rings of DNA) and may have flagella
- Red blood cells have no nucleus — more space for haemoglobin
- Mitochondria are the site of aerobic respiration in ALL living cells, including plant cells
- Ribosomes are found in ALL cell types (prokaryotic and eukaryotic)
- Specialised cells are adapted in structure to match their function
Key Formulas
- Magnification = Image size ÷ Real size
- Real size = Image size ÷ Magnification
- Image size = Magnification × Real size
- 1 mm = 1,000 μm (× 1,000 to convert mm → μm)
- 1 μm = 1,000 nm (× 1,000 to convert μm → nm)
- 1 mm = 1 × 10⁻³ m
- 1 μm = 1 × 10⁻⁶ m
- 1 nm = 1 × 10⁻⁹ m
Quick Reference: Cell Types
| Feature | Animal | Plant | Bacterial |
|---|---|---|---|
| Nucleus | Yes | Yes | No |
| Cell membrane | Yes | Yes | Yes |
| Cell wall | No | Yes (cellulose) | Yes (not cellulose) |
| Mitochondria | Yes | Yes | No |
| Chloroplasts | No | Yes | No |
| Permanent vacuole | No | Yes | No |
| Ribosomes | Yes | Yes | Yes (smaller) |
| Plasmids | No | No | Yes |
Common Mistakes
- Confusing prokaryotic and eukaryotic cells: Prokaryotic cells have NO nucleus — the DNA floats free in the cytoplasm. Eukaryotic cells (animal, plant, fungal) always have a membrane-bound nucleus.
- Saying plant cells always have chloroplasts: Only cells that photosynthesise have chloroplasts — root cells do not. Always check whether the plant cell you are describing carries out photosynthesis.
- Mixing up magnification formula: Magnification = Image size ÷ Real size. If the image is bigger than the real object, magnification must be greater than 1 — check your answer makes sense.
- Forgetting ribosomes in bacterial cells: All living cells contain ribosomes (they make proteins). Bacterial ribosomes are smaller (70S) than eukaryotic ribosomes (80S), but they are still present.
Memory Aid
- Nucleus = Manager's office — contains all the instructions (DNA) and controls what the factory does
- Mitochondria = Power station — generates the energy (ATP) that keeps the whole factory running
- Ribosomes = Assembly line — where the actual products (proteins) are made, following the instructions
- Cell membrane = Security gate — controls what comes in and what goes out; only lets authorised substances through
- Endoplasmic reticulum = Corridors and conveyor belts — transports products around the factory
- Golgi apparatus = Post office / packaging department — packages finished proteins and sends them to the right destination
- Cell wall (plants only) = Outer walls of the factory — rigid structure that gives shape and protection
- Chloroplasts (plants only) = Solar panels — capture energy from the sun to power the factory
- PRO-karyote = PRImitive — no true nucleus, no membrane-bound organelles, came first in evolution
- EU-karyote = EUvolved / has a trUE nucleus — has a true nucleus enclosed by a membrane; more complex