Final answer:
The jejunum is the portion of the small intestine where most nutrient absorption occurs. It comes after the duodenum and before the ileum, which is longer but mainly absorbs vitamin B12 and bile salts.
Step-by-step explanation:
The longest portion of the small intestine, where most absorption of nutrients occurs, is the jejunum. The small intestine consists of three parts: the duodenum, jejunum, and ileum. The duodenum is the first part, where most chemical digestion takes place. The jejunum, the second part of the small intestine, is the site of the bulk of nutrient absorption, including carbohydrates and amino acids. It is in the jejunum that most of the digestion products are absorbed into the bloodstream. The ileum is the longest part in terms of length, but it is mainly involved in the absorption of vitamin B12, bile salts, and the remaining nutrients that were not absorbed in the jejunum. The large intestine, also known as the colon, is not involved in nutrient absorption, as it primarily absorbs water and salts from the remaining waste material.