Which of the following elements is the most abundant element in living organisms?

UP CHO - 10 September 2021

Biochemistry and Nutrition

Easy

Appeared in: UP CHO - 10 September 2021

Explanation

Oxygen is the most abundant element in the human body, constituting approximately 65% of total body mass.
The primary reason for this high abundance is its presence in water (H₂O), which makes up about 60-70% of the body.
Oxygen's high atomic mass (16 amu) compared to hydrogen's (1 amu) means it contributes the vast majority of water's weight.
It is also a fundamental component of all major organic molecules, including carbohydrates, lipids, proteins, and nucleic acids.

Option A: Although Hydrogen is the most abundant element by atom count, its very low atomic mass makes it only the third most abundant element by mass (~9.5%).
Option C: Nitrogen is a critical component of proteins (in amino acids) and nucleic acids (in nitrogenous bases), but it only makes up about 3.2% of the body's mass, making it the fourth most abundant among the major elements.
Option D: Carbon is the second most abundant element by mass (~18.5%) and forms the structural backbone of all organic molecules, which is why life on Earth is considered 'carbon-based'. However, it is significantly less abundant by mass than oxygen.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Elemental composition of living organisms as background academic context rather than a clinical decision trigger.
Understanding the body's elemental composition is fundamental to physiology and medicine. For instance, the high prevalence of oxygen and hydrogen highlights the critical importance of water and hydration for all cellular processes.
Metabolic disorders often involve the improper processing of carbon-based molecules (like carbohydrates in diabetes), while nutritional deficiencies can relate to insufficient intake of elements like nitrogen (protein malnutrition).
What if? If a patient is severely dehydrated, the total body percentage of water decreases. This would lead to a relative increase in the percentage composition of other elements like carbon and nitrogen, but oxygen would still remain the most abundant element by mass overall due to its presence in organic molecules.

How to Approach the Question

First, identify the core question: It asks for the 'most abundant element' in 'living organisms'.
Recognize that 'abundance' in a biological context, unless specified otherwise, typically refers to abundance by mass, not by the number of atoms.
Recall the major elements that make up the human body. The acronym CHON (Carbon, Hydrogen, Oxygen, Nitrogen) represents the top four.
Remember the hierarchy of abundance by mass: Oxygen > Carbon > Hydrogen > Nitrogen.
The high percentage of water (H₂O) in the body is the key reason for oxygen's top position. Oxygen is much heavier than hydrogen, so it dominates the mass of water.
Select 'Oxygen' as the correct answer based on this reasoning.

Concept Tested & Keywords

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E6 Harper's Illustrated Biochemistry2023 (pp 26-793 of 813) p. 81-83

E6 Textbook of Biochemistry for medical StudentsDM Vasudevan Part 1 (pp 26-370 of 370) p. 25-27