Cell StructureEdexcel GCSE Biology: Revision notes
Section 1
What sub-cellular structures do eukaryotic cells have?
Eukaryotic cells (animal and plant) contain a nucleus enclosing genetic material as chromosomes. Structures and their functions:
Animal cells:
- Nucleus — contains DNA (as chromosomes), controls the cell's activities
- Cell membrane — controls what substances enter and leave the cell
- Mitochondria — site of aerobic respiration, releases energy
- Ribosomes — site of protein synthesis
Plant cells have all of the above, plus:
- Cell wall — made of cellulose, provides structural strength
- Chloroplasts — contain chlorophyll, site of photosynthesis
- Permanent vacuole — filled with cell sap, helps maintain cell turgidity
In exams, always link structure to function — e.g. 'mitochondria have folded membranes to increase surface area for respiration reactions', not just 'mitochondria release energy'.
Do not say plant cells 'don't have mitochondria' — plant cells respire too and have both chloroplasts and mitochondria.
Section 2
How are bacterial cells different?
Bacteria are prokaryotic cells — smaller and simpler than eukaryotic cells, with no nucleus or membrane-bound organelles.
| Feature | Prokaryotic (bacteria) | Eukaryotic (animal/plant) |
|---|---|---|
| Genetic material | Chromosomal DNA (single loop, free in cytoplasm) + plasmid DNA (small circular DNA) | DNA enclosed in a nucleus |
| Membrane-bound organelles | None | Nucleus, mitochondria (plant: also chloroplasts) |
| Ribosomes | Present (smaller) | Present (larger) |
| Movement | Flagellum — rotates to move the cell | Not applicable in most human/plant cells |
Bacterial cells also have a cell membrane and cell wall, and the cell membrane controls substance movement in and out.
Plasmid DNA is not the same as chromosomal DNA — plasmids are extra, separate loops, not part of the main chromosome.
Section 3
How are specialised cells adapted to their function?
Cells are adapted with specific sub-cellular structures suited to their job.
Sperm cell (adapted for reaching and fertilising an egg):
- Acrosome — contains digestive enzymes to break down the egg cell membrane
- Haploid nucleus — contains half the normal number of chromosomes, so fertilisation restores the full number
- Many mitochondria — provide energy for swimming
- Tail (flagellum) — enables swimming towards the egg
Egg cell (adapted to nurture a fertilised embryo and prevent multiple fertilisation):
- Nutrients in the cytoplasm — feed the developing embryo
- Haploid nucleus — contributes half the chromosomes
- Changes in the cell membrane after fertilisation — membrane structure changes immediately after one sperm enters, preventing any further sperm from fertilising the egg
Ciliated epithelial cells (e.g. lining the airways):
- Covered in cilia — hair-like structures that beat in a coordinated way to move mucus (and trapped particles/bacteria) away from the lungs
Worked example: 'Explain how the sperm cell is adapted to its function.' Answer: it has an acrosome to digest the egg's membrane allowing entry; many mitochondria to release energy for the tail to swim; a haploid nucleus so the fertilised egg has the correct chromosome number.
Section 4
How has microscopy changed our understanding of cells?
Light microscopes were developed first — they use light and lenses, giving lower magnification and resolution. They allowed scientists to see cells, nuclei and chloroplasts.
Electron microscopes use beams of electrons instead of light. They have a much higher magnification and resolution (the ability to distinguish between two separate points), allowing scientists to see much smaller sub-cellular structures in far greater detail — such as internal mitochondrial and chloroplast membranes, ribosomes and plasmids.
As microscope technology has improved over time, our understanding of sub-cellular structures and their functions has increased accordingly — many organelles were unknown before electron microscopy existed.
Examiners reward the distinction between magnification and resolution — a bigger image isn't useful unless resolution also increases enough to show detail clearly.
Section 5
How do we handle number, size and scale in cell biology?
Cells and organelles are measured using very small units, so calculations often involve standard form and unit conversions.
Unit prefixes (largest to smallest):
- milli (m) = ×10⁻³
- micro (μ) = ×10⁻⁶
- nano (n) = ×10⁻⁹
- pico (p) = ×10⁻¹²
Magnification formula:
magnification = image size ÷ actual size
Rearranged as needed:
- actual size = image size ÷ magnification
- image size = actual size × magnification
Always convert units to be consistent (e.g. mm to μm: ×1000) before calculating.
Estimations are used when an exact count or measurement isn't practical or possible.
Worked example: An image of a cell measures 40 mm across on the page and is drawn at ×2000 magnification. actual size = image size ÷ magnification = 40,000 μm ÷ 2000 = 20 μm.
A common error is forgetting to convert mm to μm (or vice versa) before applying the magnification formula.
Section 6
Core Practical: Investigating biological specimens using microscopes
Method outline:
- Prepare a slide, adding a stain such as iodine to increase contrast.
- Place the specimen on the stage and view under the lowest magnification objective lens first to locate the specimen.
- Adjust focus using coarse, then fine focus.
- Switch to a higher-power objective lens once focused.
- Produce a labelled scientific drawing: clear single lines, no shading, labels with straight lines that do not cross, title and magnification.
Magnification calculation: total magnification = eyepiece lens magnification × objective lens magnification
Scientific drawings must never be shaded or coloured — examiners mark for clean single outlines, proportion, and correct labelling.
Must Know
- Animal cells: nucleus, cell membrane, mitochondria, ribosomes. Plant cells: same plus cell wall, chloroplasts, permanent vacuole.
- Bacteria are prokaryotic: chromosomal DNA + plasmid DNA (no nucleus), cell membrane, ribosomes, flagellum.
- Sperm cells: acrosome, haploid nucleus, many mitochondria, tail. Egg cells: nutrients in cytoplasm, haploid nucleus, membrane changes after fertilisation.
- Electron microscopes have much higher magnification and resolution than light microscopes.
- magnification = image size ÷ actual size — always convert units consistently.
- Total magnification = eyepiece × objective; scientific drawings use clear unshaded outlines with labels.
That's the notes covered.
Carry on to the next subtopic.