Every living organism on Earth is made of one of two fundamentally different types of cell: eukaryotic or prokaryotic.
The single most important difference between them is whether the cell has a TRUE NUCLEUS — a membrane-bound compartment containing the DNA.
Eukaryotic cells HAVE a nucleus. Prokaryotic cells DO NOT.
This distinction is so fundamental that it defines two of the largest groupings of life on Earth.
Eukaryotic Cells
Eukaryotic cells are larger, more complex cells that have:
• A true nucleus — DNA enclosed in a nuclear membrane.
• Membrane-bound organelles — mitochondria, endoplasmic reticulum, Golgi apparatus etc.
• A cytoskeleton — internal protein framework.
All animals, plants, fungi and protists are made of eukaryotic cells.
Typical size: 10–100 micrometres (µm).
The nucleus protects DNA and allows its activity to be carefully regulated.
Prokaryotic Cells — Bacteria
Prokaryotic cells are smaller and simpler. All bacteria are prokaryotes.
Key features:
• NO nucleus — DNA is a SINGLE CIRCULAR LOOP floating free in the cytoplasm.
• NO membrane-bound organelles — no mitochondria, no chloroplasts.
• DO have: cytoplasm, a cell membrane, a cell wall (made of peptidoglycan — not cellulose), and ribosomes (smaller than eukaryotic ribosomes).
• May also have: PLASMIDS (small extra circular loops of DNA, not essential for survival), a FLAGELLUM (rotating tail for movement), and a CAPSULE (slimy outer layer for protection and attachment).
Typical size: 1–10 µm — roughly 10× smaller than a typical animal cell.
Why Size Matters — Surface Area to Volume Ratio
The small size of prokaryotic cells gives them a very HIGH surface area to volume ratio (SA:V).
This means that relative to their volume, bacteria have a lot of membrane surface available for absorbing nutrients and removing waste.
This is one reason bacteria can grow and reproduce extremely fast — every 20 minutes under ideal conditions.
Larger eukaryotic cells have a lower SA:V ratio — they need specialised exchange surfaces and transport systems (e.g. lungs, circulatory system) to move substances efficiently.
Units and Scale
Working with cells requires understanding very small units:
• 1 metre (m) = 1,000 millimetres (mm)
• 1 mm = 1,000 micrometres (µm)
• 1 µm = 1,000 nanometres (nm)
So: 1 m = 1,000,000 µm = 1,000,000,000 nm
Typical sizes to remember:
• Animal cell: ~20 µm
• Plant cell: ~40–80 µm
• Bacterium: ~1–5 µm
• Virus: ~0.1 µm (100 nm)
• Ribosome: ~20 nm
You need to be able to convert between these units and use them in magnification calculations.
⚠️ Common Mistake
Prokaryotic cells have NO nucleus and NO membrane-bound organelles. Students often write 'bacteria have a nucleus' — they do not. The DNA is a single circular loop loose in the cytoplasm. Also: the bacterial cell wall is made of PEPTIDOGLYCAN, not cellulose (cellulose = plant cell walls).
📐 Variables
MMagnification (M) is measured in no unit (×)
IImage size (I) is measured in mm or µm (mm / µm)
AActual size (A) is measured in mm or µm (mm / µm)
📐 Key Equations
Magnification (M) = Image size (I) ÷ Actual size (A)
📌 Key Note
Eukaryote = nucleus present. Prokaryote = no nucleus, DNA floats free. Size: bacterium ~1–5 µm; animal cell ~20 µm; plant cell ~40–80 µm. Units: 1 mm = 1000 µm.
🎯 Matching Activity — Eukaryote or Prokaryote?
Sort each feature — does it belong to eukaryotic cells, prokaryotic cells, or both? — drag the symbols on the right to match the component names on the left.
Eukaryote only
Drop here
Prokaryote only
Drop here
Prokaryote only
Drop here
Eukaryote only
Drop here
Both
Drop here
Both
Drop here
Has a cell membrane and cytoplasm
DNA is a single circular loop floating in cytoplasm
Contains mitochondria for aerobic respiration
Contains ribosomes for protein synthesis
May have plasmids — small extra circular DNA rings
Has a true membrane-bound nucleus containing DNA
⚽ FIFA Worked Examples
Magnification Calculation
A bacterium is drawn 30 mm long in a diagram. Its actual length is 0.003 mm. Calculate the magnification.
F
Magnification = Image size ÷ Actual size
I
M = 30 ÷ 0.003
F
Both values already in mm — no conversion needed
A
Magnification = ×10,000
Finding Actual Size
A cell is shown at ×500 magnification and measures 10 mm in the image. Calculate the actual size in µm.
F
Actual size = Image size ÷ Magnification
I
A = 10 mm ÷ 500 = 0.02 mm
F
Convert to µm: 0.02 mm × 1000 = 20 µm
A
Actual size = 20 µm
🎯 Test Yourself
Question 1 of 5
1. What is the key structural difference between eukaryotic and prokaryotic cells?
2. A bacterium is approximately 2 µm in length. How does this compare to a typical animal cell?
3. Which of the following is found in a bacterial cell but NOT in a typical animal cell?
4. 1 mm is equal to how many micrometres (µm)?
5. What is a flagellum in a bacterial cell?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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