A teacher asks students to compare the most abundant elements found in the human body with those found in the universe as a whole. Describe the similarities and differences between these two sets of elements, and explain why living organisms contain large amounts of carbon, hydrogen, oxygen, and nitrogen.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The four most abundant elements in the universe by mass are hydrogen, helium, carbon, and oxygen. The four most abundant elements in the human body by mass are oxygen, carbon, hydrogen, and nitrogen.
Model answer (5 marks)
Hydrogen, carbon and oxygen are common to both lists – they are the most abundant elements in the universe and also the most abundant in the human body.
Helium is abundant in the universe but is almost absent from the body.
Nitrogen is abundant in the body but does not appear in the universe’s top four.
Living organisms contain large amounts of C, H, O and N because these elements form the backbone of biological molecules such as proteins, DNA and carbohydrates.
Carbon’s ability to form long chains and rings, together with the versatile bonding of H, O and N, allows the synthesis of a wide variety of stable, complex molecules essential for life.
Helium is abundant in the universe but is almost absent from the body.
Nitrogen is abundant in the body but does not appear in the universe’s top four.
Living organisms contain large amounts of C, H, O and N because these elements form the backbone of biological molecules such as proteins, DNA and carbohydrates.
Carbon’s ability to form long chains and rings, together with the versatile bonding of H, O and N, allows the synthesis of a wide variety of stable, complex molecules essential for life.
Examiner tips
- Use the exact wording from the mark scheme – e.g. ‘hydrogen, carbon and oxygen are common to both lists’. Show the two differences explicitly. Explain the role of C, H, O, N in biological molecules and mention carbon’s bonding flexibility. Keep the answer concise – 5 marks, not more than 5–6 short sentences.
Common mistakes
- Mixing up the order of elements or omitting helium/nitrogen. Failing to explain why C, H, O, N are important – just listing them. Using vague phrases like ‘important for life’ without linking to molecules.
Mark scheme (5 marks)
- Hydrogen, carbon, and oxygen appear in both the universe's most abundant elements and the human body's most abundant elements (similarity).
- Helium is abundant in the universe but is not significantly present in the human body (difference).
- Nitrogen is abundant in the human body but is not in the top four elements of the universe (difference).
- Carbon, hydrogen, oxygen, and nitrogen are the main elements that make up biological molecules such as proteins, DNA, and carbohydrates, which are essential for life.
- These elements (C, H, O, N) have properties that allow them to form a wide variety of stable compounds / molecules; carbon in particular can form long chains and rings, making it ideal for complex biological molecules.
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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