Elemental Composition of Life
Here's a puzzle: if you did a full chemical breakdown of a rock and a full chemical breakdown of your own body, you'd find almost the same list of elements in both. So what actually makes living matter chemically different from a lump of granite? The answer isn't which elements are present — it's how much of each one is present.
Same elements, wildly different proportions
When biologists compare the elemental make-up of the Earth's crust with that of the human body, the contrast is striking:
| Element | % of Earth's crust | % of human body |
|---|
| Oxygen (O) | 46.6 | 65.0 |
| Silicon (Si) | 27.7 | negligible |
| Carbon (C) | 0.03 | 18.5 |
| Hydrogen (H) | 0.14 | 9.5 |
| Nitrogen (N) | very little | 3.3 |
| Calcium (Ca) | 3.6 | 1.5 |
| Sodium (Na) | 2.8 | 0.2 |
The crust's chemistry is dominated by oxygen and silicon — the raw material of silicate minerals, quartz and sand. The body's chemistry is dominated by carbon, hydrogen, oxygen and nitrogen — the exact four elements that build amino acids, sugars, fatty acids and nucleotides. Silicon, despite being nearly a third of the crust by weight, barely registers in living tissue at all.
This is precisely why life is described as "carbon-based chemistry." It isn't that carbon is rare elsewhere and special to life — it's that living systems concentrate carbon, hydrogen, oxygen and nitrogen into a huge variety of ring and chain structures, something silicate rock chemistry simply doesn't do.
A common exam trap
A classic exemplar question asks: since the same major elements occur in both living and non-living matter, what is the real difference? The tempting wrong answers usually point to something true but irrelevant — say, that living organisms contain more water, or more calcium. Those are real differences, but they aren't what "elemental composition" is asking about. The actual answer is quantitative: living organisms carry a much higher proportion of carbon, oxygen and hydrogen per unit mass than non-living matter does. Presence isn't the test — proportion is.
Don't confuse "found in living tissue" with "abundant in living tissue." Trace elements like iron, zinc and magnesium genuinely occur in the body, just at very low percentages — they're real biological constituents, not silicon-style outsiders. Silicon is different in kind: it's not just rare, it's essentially absent from biological chemistry.
How elemental composition is actually measured
This whole comparison comes from a genuine lab procedure. A tissue sample is weighed fresh (its wet weight), then dried until all the water evaporates (its dry weight), and then the dried residue is completely burnt. Burning oxidises every carbon compound away as CO2 and water vapour, leaving behind only the ash — the inorganic residue containing elements like calcium, magnesium, sodium and potassium. This is called elemental analysis, and it's run alongside a separate analysis for actual compounds (which tells you not just which elements are present, but which molecules they're locked up in — sulphates, phosphates, sodium chloride, calcium carbonate, and so on).
Keep the two analyses distinct in your head: elemental analysis answers "which atoms?" (C, H, O, N, Na, Ca...); compound analysis answers "which molecules?" (amino acids, NaCl, CaCO3...). A question about "elemental composition" is always about the first list, not the second.
So the next time you see a question comparing living and non-living matter, remember: it's never a story about exotic elements found only in life. It's a story about the same handful of elements, redistributed in dramatically different proportions because life runs on carbon chemistry, while rock runs on silicate chemistry.