Cell: The Building Block of Life
Prokaryotic and eukaryotic cells
Tell the two kinds of cells apart: prokaryotic and eukaryotic.
Most cells have three basic parts:
- the plasma membrane on the outside;
- the cytoplasm, a jelly-like filling;
- the nucleus, the control centre.
Floating in the cytoplasm are tiny working parts called organelles ("little organs"). Most can be seen only with an electron microscope.
Cells come in two big types. The difference is whether the genetic material is kept in its own covered room.
| Prokaryotic | Eukaryotic | |
|---|---|---|
| Name means | pro = early, karyon = nucleus | eu = true nucleus |
| Nucleus | No proper nucleus; the genetic material lies loose in a region called the nucleoid | A proper nucleus with a membrane around it |
| Organelles with membranes | None | Yes |
| Usual size | 1 to 10 µm | 10 to 100 µm |
| Examples | Bacteria (usually one-celled) | Plants and animals (one-celled or many-celled) |
Eukaryotic cells also have a cytoskeleton: a network of fine fibres, like the poles of a tent. It gives the cell support and shape and helps with movement and transport.
Cells can store things too, called cell inclusions: starch grains, or crystals of calcium oxalate or silica in some plant cells.
"Are viruses cells?" No. Some things that cause disease are acellular (not made of cells):
- Viruses: genetic material inside a protein coat.
- Viroids: genetic material with no protein coat.
- Prions: wrongly folded proteins with no genetic material at all.
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