Which of the following vitamin groups contains nitrogen in its molecules and is therefore often called the B complex of nitrogen containing vitamins?

Introduction / Context:
Vitamins are organic compounds required in small quantities for normal metabolism. Some vitamins contain nitrogen atoms in their molecular structure, while others do not. The B complex vitamins are a group of water soluble vitamins that often contain nitrogen and function as coenzymes in many metabolic pathways. This question asks you to identify which vitamin group specifically contains nitrogen in its molecules from among several common vitamin groups.

Given Data / Assumptions:

• The options list fat soluble and water soluble vitamin groups: A, B, C, D, and E.

• The question focuses on the presence of nitrogen in the vitamin molecules.

• We assume basic knowledge that B complex vitamins include compounds like thiamine and niacin that have nitrogen containing groups.

• Only one of the listed groups is widely described as nitrogen containing coenzymes.

Concept / Approach:
The B complex vitamins are a family of chemically distinct compounds, many of which contain nitrogen in functional groups like amines. Examples include thiamine (vitamin B1), riboflavin (B2), niacin (B3), pyridoxine (B6), and cobalamin (B12), all of which have nitrogen in their structures. Vitamin A, C, D, and E are mainly composed of carbon, hydrogen, and oxygen atoms and are not characterised by nitrogen containing rings or amine groups in the same way. Therefore, the collective group that clearly contains nitrogen is the B complex vitamins.

Step-by-Step Solution:
Step 1: Recall that vitamin B complex consists of multiple members such as B1, B2, B3, B5, B6, B7, B9, and B12. Step 2: Recognise that many of these vitamins have nitrogen containing heterocyclic rings or amine groups which help them function as coenzymes. Step 3: Consider vitamin A, which includes retinol and related compounds that are fat soluble and primarily composed of carbon, hydrogen, and oxygen. Step 4: Note that vitamin C (ascorbic acid) and vitamin E (tocopherols) are also water soluble or fat soluble antioxidants without nitrogen in their basic structures. Step 5: Vitamin D compounds are steroid like molecules involved in calcium metabolism and do not feature nitrogen as a key element in their structure. Step 6: Based on these structural differences, confirm that the B complex vitamins are the group most clearly identified as nitrogen containing vitamins.

Verification / Alternative check:
Biochemistry texts describe B complex vitamins as coenzymes in metabolic reactions and often display their chemical structures. These structures reveal nitrogen atoms in thiamine rings, in the flavin of riboflavin, in the pyridine ring of niacin, and in the corrin ring of vitamin B12. Diagrams of vitamins A, C, D, and E lack such nitrogen containing features and emphasise their roles as pigments, antioxidants, or steroid derivatives. This consistent chemical evidence supports the statement that vitamin B group contains nitrogen.

Why Other Options Are Wrong:

Vitamin A: Mainly fat soluble retinoids without a characteristic nitrogen containing structure.

Vitamin C: A water soluble antioxidant (ascorbic acid) composed of carbon, hydrogen, and oxygen.

Vitamin D: A group of fat soluble secosteroids involved in calcium and phosphate homeostasis.

Vitamin E: Fat soluble tocopherols and tocotrienols, again lacking nitrogen in the core structure.

Common Pitfalls:
Students may focus on vitamin function rather than chemical structure and accidentally choose vitamin A or D because they are well known. Another confusion stems from the fact that the term vitamin is used broadly for many unrelated molecules. To answer questions specifically about nitrogen containing vitamins, it is helpful to remember that B complex vitamins act largely as nitrogen containing coenzymes in metabolic pathways, unlike the other groups.

Final Answer:
The vitamin group that contains nitrogen in its molecules is the Vitamin B complex.