# 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.

> OCR A-Level Chemistry B: Salters (H433) — EL Elements of life · Describe · 5 marks

> 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.

## Mark scheme (5 marks)

1. Hydrogen, carbon, and oxygen appear in both the universe's most abundant elements and the human body's most abundant elements (similarity).
2. Helium is abundant in the universe but is not significantly present in the human body (difference).
3. Nitrogen is abundant in the human body but is not in the top four elements of the universe (difference).
4. 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.
5. 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

- [abundant](https://www.gradenine.co.uk/glossary/abundant)

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-teacher-asks-students-to-compare-01ca6ab4) · Published by Druglandscape Ltd.