Cell Structure
AQA GCSE Combined Science > Biology > Cell Biology > Cell Structure > Eukaryotes and prokaryotes
4.1.1.1 EUKARYOTES AND PROKARYOTES
WHAT IS A CELL?
Cells are the basic building blocks of all living organisms. Every living thing, from a human to a sunflower to a bacterium, is made of cells.
There are two main types of cell:
- Eukaryotic cells (plant and animal cells)
- Prokaryotic cells (bacterial cells)
The key difference?
Whether or not the genetic material is enclosed inside a nucleus.
EUKARYOTIC CELLS (PLANT AND ANIMAL CELLS)
What defines a eukaryotic cell?
Plant and animal cells are classified as eukaryotes. In these cells, the DNA is arranged into chromosomes and is kept inside a clearly defined nucleus, separated from the rest of the cell.
Eukaryotic cells:
- Have a cell membrane
- Contain cytoplasm
- Have genetic material enclosed in a nucleus
- Are much larger than bacterial cells
- Contain membrane-bound organelles
| Structure | Function |
|---|---|
| nucleus | contains dNA and activities of the cell |
| cytoplasm | where most of the chemical reactions happen |
| cell membrane | controls movement of substances in and out of the cell |
| mitochondria | site of aerobic respiration |
| ribosomes | protein synthesis |
Plant cells also contain:
Cell wall (made of cellulose)
Chloroplasts (site of photosynthesis)
Large permanent vacuole
PROKARYOTIC CELLS (BACTERIA)
Prokaryotic cells are much smaller and simpler than eukaryotic cells.
Key features:
- No nucleus
- DNA is a single circular loop
- May contain plasmids (small extra rings of DNA)
- Have a cell membrane
- Surrounded by a cell wall
- Contain cytoplasm
- Have ribosomes (but smaller than in eukaryotes)
Some bacteria also have:
- Flagella for movement
- A slime capsule for protection
COMPARING EUKARYOTIC AND PROKARYOTIC CELLS
| FEATURE | EUKARYOTIC CELLS | PROKARYOTIC CELLS |
|---|---|---|
| Size | Larger (10-100 μm) | Smaller (0.2–2 μm) |
| Nucleus | Present | absent |
| DNA | Linear chromosomes inside nucleus | Single circular DNA loop |
| Plasmids | Not normally present | May be present |
| Organelles | protein synthesis | No membrane-bound organelles |
SCALE AND SIZE OF CELLS
Typical sizes:
- Animal cell ≈ 10–30 micrometres (μm)
- Plant cell ≈ 10–100 μm
- Bacterium ≈ 1 μm
- Ribosome ≈ 20 nanometres (nm)
Unit conversions
| Pre-fix | Symbol | Meaning |
|---|---|---|
| centi | C | 10⁻² |
| milli | m | 10⁻³ |
| micro | μ | 10⁻⁶ |
Key conversions:
- 1 mm = 1000 μm
- 1 μm = 1000 nm
- 1 mm = 1 × 10⁻³ m
- 1 μm = 1 × 10⁻⁶ m
- 1 nm = 1 × 10⁻⁹ m
ORDER OF MAGNITUDE
An order of magnitude is a factor of 10 difference.
Example:
- A 10 μm cell is 10 times larger than a 1 μm bacterium.
- That is one order of magnitude bigger.
- Using standard form helps compare very small numbers clearly:
- 0.000001 m = 1 × 10⁻⁶ m