Every living thing is made of cells, and there are two kinds.

• Eukaryotic cells keep their genetic material (DNA) inside a nucleus. Animals, plants, fungi and protists are made of them.
• Prokaryotic cells are bacteria. They are much smaller, about a tenth of the size of a typical animal cell, and have no nucleus: their DNA is one loop floating in the cytoplasm, often with extra small rings called plasmids.

Animal and plant cells have:

• nucleus: holds the genetic material that controls the cell
• cytoplasm: a jelly where most chemical reactions happen
• cell membrane: controls what goes in and out
• mitochondria: where aerobic respiration releases energy
• ribosomes: where proteins are made

Plant cells also have a cell wall of cellulose (strength), chloroplasts (absorb light for photosynthesis) and a permanent vacuole full of cell sap (keeps the cell firm). Bacteria have a cell wall too, but it is not made of cellulose.

Not every plant cell has chloroplasts: root cells get no light, so they have none.

Cells are measured in micrometres (μm) and their smallest parts in nanometres (nm).

1 mm = 1000 μm
1 μm = 1000 nm

To compare two sizes, put them in the same unit and divide the bigger by the smaller. Each power of ten is one order of magnitude.

1. Plant cell 100 μm; virus 100 nm = 0.1 μm
2. 100 ÷ 0.1 = 1000 times bigger
3. 1000 = 10³: three orders of magnitude

Going to a smaller unit, the number gets bigger (0.05 mm = 50 μm). If yours got smaller, you divided instead of multiplying.