Chemistry of Living Things
The following table summarizes the relative abundance (in percent) of elements
found in the sun, the Earth’s crust, living organisms and the Earth’s atmosphere. It
lists the percentages based on the mass of each element. This takes into account the
mass of each atom of an element rather than simply counting the number of atoms.
For example, since cells are largely water and water is two hydrogen atoms and one
oxygen atom, cells have a lot of hydrogen atoms, but not much hydrogen mass
because hydrogen atoms have an extremely low mass (compared to other atoms).
Relative Abundance of Elements
Table 1: Chemical composition (in percentages) by mass of different systems. Not all
elements are listed in this table.
Element
Sun
Earth’s Crust
0.14
Earth’s
Atmosphere
Trace
Living
Organisms
9.9
Hydrogen (H)
73.4
Helium (He)
25
Trace
Trace
____
Carbon (C)
Nitrogen (N)
Oxygen (O)
Phosphorus
(P)
Silicon (Si)
0.2
0.09
0.8
____
0.03
Trace
46
0.1
0.01
75.6
23
_____
12.1
3
73.7
0.6
0.09
28
______
Trace
Sulfur (S)
0.05
Trace
Trace
0.3


“trace” means that the element is found in quantities too small to count
“_____” means that the element is not found in that system
Step 1 – Prepare a bar graph of the information contained in the table above.
 Using colored pencils, assign different colors to each category (sun,
earth’s crust, etc. – total of four colors)
 List the different elements along the x-axis and the percentages along
the y-axis
 Include all the elements listed above on your graph
 Include labels on the axes, a title and a key that identifies the colors
you used.
Analysis Questions
1. How does the composition of living organisms compare with the composition
of the sun, the Earth’s atmosphere and the Earth’s crust?
2. The ancient Earth’s atmosphere is thought to have been very low in oxygen
and quite high in carbon dioxide. How is this early atmosphere different from
the one we know today? Use data from your graph to support your answer.
3. Can you predict why the atmosphere today is so different? (Hint: think of the
different types of organisms that produce oxygen today, that weren’t around
on the early earth. How does the oxygen get into our atmosphere?)
4. According to your graph, could the composition of an object be used to tell
whether it is “alive/living” or not? Give reasons for your answer.
5. Give one reason that living things would have so much oxygen and hydrogen.