Principles Module 4

Cell Size and Scale (requires Flash plugin)


  • Identify the essential elements of life and the role of trace elements
  • Describe the simplified atomic model
  • Explain the role of valence electrons in determining the behavior of an atom

Key points

  • atoms are the smallest particle of an element that has its properties
  • elements are substances having a single kind of atom
  • compounds are substances made up of 2 or more elements
  • H, C, N, O make up 96% of the matter in living things, Ca and P also in high concentrations
  • other elements are required in smaller quanitites
  • parts of atom: proton, neutron, electron
  • valence shells determine reactivity (2–8–8)

In-class activities

Glucose is an important metabolite in all cells. Using the simplified atomic model and the concept of valence electrons, explain each of the bonds in this molecule.


Fischer projection of glucose
Fischer projection of glucose


As a scientist, having a sense of scale is extremely important. Because of the microscopic scales involved in cell biology, though, having this sense requires some cultivation. Use this scenario to begin to train your sense of scale.

  • If you were holding the nucleus of a H atom that appeared to you to be the size of a golf ball, what distance away would the nearest electron be?