The four elements that together make up about 99 per cent of the mass of living matter are
- A. carbon, hydrogen, oxygen and nitrogen
- B. carbon, hydrogen, oxygen and sulphur
- C. sodium, potassium, calcium and chlorine
- D. carbon, nitrogen, phosphorus and sulphur
Explanation
Carbon, hydrogen, oxygen and nitrogen are the backbone of carbohydrates, lipids, proteins and nucleic acids, with phosphorus and sulphur adding roughly another one per cent. Sodium, potassium and calcium are essential but are needed only in small quantities as ions. Carbon is central because it forms four stable covalent bonds and so builds long chains and rings.
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