Cell structures are measured in very small units. Each unit is made with a prefix that gives a power of ten. Milli means 10-3, one thousandth. Micro (µ) means 10-6, one millionth. Nano (n) means 10-9, one billionth. Pico (p) means 10-12, one trillionth.
Each of these units is 1000 times smaller than the one before. So 1 mm = 1000 µm, 1 µm = 1000 nm and 1 nm = 1000 pm. To change to a smaller unit you multiply by 1000, and to change to a larger unit you divide by 1000. For example, 2.5 µm = 2500 nm and 3.5 mm = 3500 µm.
Very small numbers are easier to write in standard form. Standard form is a number between 1 and 10 multiplied by a power of ten. For example 0.0000035 m = 3.5 × 10-6 m. Since 1 µm = 1 × 10-6 m, this is 3.5 µm, and 4.2 × 10-6 m is 4.2 µm. When you multiply numbers in standard form, you multiply the numbers and add the powers of ten.