Measuring size in Biology

The relative size of cell components and different biological units of size.

How many µm make a mm? This activity gives students clear examples of biological objects to cover sizes from 1mm to 1nm. Students watch a short video clip, investigate a visual biological data base using a slider and make a display which illustrate the huge range of sizes between tiny fleas and the miniscule molecules.

Lesson Description

Guiding Questions

How many nanometres (nm) make a micrometre (µm)?

How much bigger than a bacterium is an onion cell?

Why is it better to use µm and not mm to measure a cell?

Activity 1 - Introduction to size and its units.

Look at the interactive Utah university - slider of cell sizes and scale which shows objects from a coffee bean to a carbon atom.

Create a poster showing the the three units of size for biological objects. The collection should include something from each of the following sizes; 1mm, 100µm; 10µm, 1µm, 100nm, 10nm and 1nm.

Questions to answer:

  1. Name an object which is best measured in mm, what it it's typical size?
  2. Name three objects measured in µm and give their typical size in both mm and µm.
  3. Name three objects measured in nm and give their sizes in all three units.

Activity 2 - Visualizing powers of ten and size.

Watch the following short video which shows objects getting smaller by powers of ten before they get increasingly bigger as it zooms into the molecules of a human hand. Notice the dimension of the objects and the notation at the right of the screen. This is a good introduction to µm and scientific notation using powers of ten.

This is a new version of the same idea. Very nice too.

Teachers notes

  • Timing.
    This short activity could easily be a 30 minute homework task. To make a poster would take a single student about 20 minutes.
    If the students shared the task of making a set of posters this could become a wall display.
    It may save time to begin with a simple Students' Worksheet - template of a poster for students to adapt.
  • Aims
    This is a simple lesson to introduce students to the relative sizes of some important biological objects and to the units µm and nm which will be used later in the topic.
  • Extension / Creative idea:
    Student may enjoy designing a Pokemon or Top Trump Card to show object sizes. Size is of course the most important criteria but the cards could also include categories like, 'importance to the cell', 'beauty', etc.
    Try this online free card maker:
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