Cells and organisation · GCSE Biology
Cell biology
Eukaryotic and prokaryotic cells, organelles, specialised cells, microscopy and magnification — the flagship opening of GCSE Biology.
If it has a nucleus it is eukaryotic. If the DNA is a loop in the cytoplasm it is prokaryotic. Then ask: which extra structures does this cell need for its job?
The important bits
What you need to know
- 1
Animal and plant cells are eukaryotic: DNA is stored as chromosomes inside a nucleus. Bacterial cells are prokaryotic: there is no nucleus, chromosomal DNA is a single loop, and plasmids are extra rings of DNA.
- 2
All cells have a cell membrane (controls entry and exit), cytoplasm (site of most chemical reactions) and ribosomes (protein synthesis). Mitochondria are the site of aerobic respiration.
- 3
Plant cells also have a cellulose cell wall for strength, chloroplasts containing chlorophyll for photosynthesis, and a permanent vacuole of cell sap that keeps the cell turgid.
- 4
Bacterial cells have a cell wall, cytoplasm, cell membrane, chromosomal DNA, plasmids and often flagella. They do not have mitochondria or a nucleus.
- 5
Magnification = size of image ÷ size of real object. Rearrange to find the real size. Convert units first: 1 mm = 1000 µm, 1 µm = 1000 nm.
- 6
Specialised cells have adaptations that match their function: sperm (streamlined, mitochondria, acrosome), nerve cells (long axon, synapses), palisade cells (chloroplasts, packed tightly), root hair cells (large surface area, no chloroplasts).
- 7
Light microscopes use light and lenses and can view living cells. Electron microscopes have much higher magnification and resolution, so they reveal organelles, but specimens are dead and the equipment is expensive.
- 8
The cell cycle includes growth and DNA replication, then mitosis, which produces two genetically identical daughter cells for growth, repair and asexual reproduction.
Go deeper
Read the cell before you name it
Examiners start with a diagram and expect you to name structures and give functions, not a memorised paragraph. Begin with the nucleus test. A nucleus means eukaryotic: animal or plant. No nucleus and a loop of DNA means prokaryotic: a bacterium. Next hunt for the plant extras — cellulose wall, chloroplasts, permanent vacuole. Mitochondria appear in eukaryotic cells that respire aerobically; they are not in bacteria. Ribosomes are too small to see with a school light microscope, which is why electron micrographs are used to justify their presence. Once the type is named, the function question is usually a structure–function link: more mitochondria where energy demand is high, chloroplasts where light is captured, a long axon where a signal must travel.
Go deeper
Magnification is a rearrangement, not a guess
The formula is reliable if the units match. Measure the image in millimetres, convert to micrometres by multiplying by 1000, then divide by the magnification to get the real size in micrometres. Students lose marks by mixing millimetres and micrometres, or by dividing the wrong way round. Write the triangle: image size on top, real size and magnification below. Light microscopes resolve objects about 0.2 micrometres apart; electron microscopes resolve far smaller distances, which is why you can see mitochondria, ribosomes and plasmid loops on electron images but not in a typical school microscope. Resolution is the ability to distinguish two points, not the same as magnification.
Go deeper
Specialisation is the course in miniature
A specialised cell keeps the same genetic information but expresses the genes it needs for one job. A sperm cell is streamlined, with a flagellum and packed mitochondria for swimming, and an acrosome of enzymes to digest the egg membrane. A palisade mesophyll cell is column-shaped, packed with chloroplasts and placed at the top of the leaf. A root hair cell is a long projection that increases surface area for water and mineral ions, and it has no chloroplasts because it is underground. A nerve cell has a long axon and synaptic endings. In the exam, name the feature and the advantage. “Lots of mitochondria so more energy is released for movement” scores; “it is adapted” does not.
See the idea in action
A student measures a nucleus as 8 mm on a micrograph taken at ×2000. Convert 8 mm to micrometres: 8 × 1000 = 8000 µm. Real size = image size ÷ magnification = 8000 ÷ 2000 = 4 µm. That is a sensible order of magnitude for a nucleus. If the answer had been 4 mm, the units were not converted. If it had been 16 000 µm, the formula was upside down.
Exam technique
Turn knowledge into marks
On labelled diagrams, give the function as well as the name when the command word is explain. “Chloroplast — contains chlorophyll to absorb light for photosynthesis” is a full answer. For microscope practicals, mention focus, stain, and why you start on the lowest power objective.
Common mistakes
Do not give these marks away
- 01
Calling bacterial DNA a nucleus, or putting mitochondria in prokaryotic cells.
- 02
Forgetting to convert millimetres to micrometres before using the magnification formula.
- 03
Writing “it is adapted” without naming the structural feature and the job it helps the cell do.
Which structure is found in a bacterial cell but not in an animal cell?
AMitochondria
BNucleus
CPlasmid DNA
DChloroplast
Show the answer
Plasmid DNA. Bacteria are prokaryotic: they have chromosomal DNA and plasmids, but no nucleus or mitochondria. Chloroplasts are plant organelles, not bacterial ones.
Quick questions
If this is the bit you searched
What is the difference between eukaryotic and prokaryotic cells?
Eukaryotic cells (animals, plants) have a nucleus. Prokaryotic cells (bacteria) have no nucleus; their DNA is a loop in the cytoplasm, often with plasmids.
How do you calculate magnification in GCSE Biology?
Magnification = size of image ÷ size of real object. Convert both measurements to the same unit first, usually micrometres.
Why can electron microscopes see more than light microscopes?
They have much higher magnification and resolution, so organelles such as mitochondria and ribosomes can be distinguished. Specimens must be dead.