Question

Arachidonic acid is with highest number of carbon atoms, because

• A. It contains 18 carbons in the 'R' group
• B. It contains 15 carbons in the 'R' group
• C. It contains 19 carbons in the 'R' group
• D. It contains 20 carbons in the 'R' group

Hint:

Arachidonic acid is a 20-carbon fatty acid derivative involved in eicosanoid synthesis.

Answer 1

The Correct Option is C

Arachidonic acid is a 20 carbon fatty acid, so once you know the total carbon count you can work out how many of those carbons sit in the R group attached to the carboxyl end.

1. 18 carbons: If the R group held 18 carbons, adding back the carboxyl carbon would give a 19 carbon acid, one short of arachidonic acid's real chain length, so this undercounts.
2. 15 carbons: This would only add up to a 16 carbon fatty acid, far shorter than arachidonic acid, so it is well off.
3. 19 carbons: A fatty acid's carboxyl group (-COOH) itself uses up one carbon of the chain. Arachidonic acid has 20 carbons in total, so subtracting that one carboxyl carbon leaves 19 carbons in the R group attached to it. This matches the acid's known chain length.
4. 20 carbons: This double counts, since the carboxyl carbon is already part of the 20 total and should not also be counted again inside the R group.

Counting the whole molecule as 20 carbons and setting aside the one carboxyl carbon leaves 19 carbons in the R group, so option (3) is correct.

Let's summarize:

• Arachidonic acid: 20 carbon polyunsaturated fatty acid.
• Total carbons minus the one carboxyl carbon equals the R group carbon count.
• 20 minus 1 gives 19 carbons in the R group.

So the R group of arachidonic acid contains 19 carbons.