Which gas in Earth's atmosphere is essential for living organisms to synthesize proteins?

• A. Nitrogen (\(N_2\))
• B. Oxygen (\(O_2\))
• C. Methane (\(CH_4\))
• D. Carbon dioxide (\(CO_2\))

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the role of proteins in living organisms. Proteins are essential macromolecules that perform various functions, including structural support, enzymatic activity, and cellular communication.
2. Step 2: Recognize that proteins are composed of amino acids, which contain nitrogen atoms. Nitrogen is a key element in the amino group (-NH2) of amino acids.
3. Step 3: Identify the source of nitrogen in Earth's atmosphere. Nitrogen gas (\(N_2\)) makes up approximately 78% of the atmosphere and is the primary reservoir of nitrogen for living organisms.
4. Step 4: Explain how nitrogen is made available to living organisms. Most organisms cannot use \(N_2\) directly; it must be converted into biologically usable forms like ammonia (\(NH_3\)) or nitrate (\(NO_3^-\)) through processes such as nitrogen fixation, carried out by certain bacteria.
5. Step 5: Conclude that nitrogen (\(N_2\)) is the essential gas in Earth's atmosphere for synthesizing proteins, as it provides the nitrogen atoms required for amino acid and protein formation.