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

Unit conversions (scale):

• 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

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.