Biology 2e is designed to cover the scope and sequence requirements of a typical two-semester biology course for science majors. The text provides comprehensive coverage of foundational research and core biology concepts through an evolutionary lens. Biology includes rich features that engage students in scientific inquiry, highlight careers in the biological sciences, and offer everyday applications. The book also includes various types of practice and homework questions that help students understand—and apply—key concepts. The 2nd edition has been revised to incorporate clearer, more current, and more dynamic explanations, while maintaining the same organization as the first edition. Art and illustrations have been substantially improved, and the textbook features additional assessments and related resources.

By the end of this section, you will be able to do the following:

Explain the processes of digestion and absorption
Compare and contrast different types of digestive systems
Explain the specialized functions of the organs involved in processing food in the body
Describe the ways in which organs work together to digest food and absorb nutrients

The duodenum is the proximal part of the small intestine and extends from the pylorus of the stomach to the jejunum. It has descending and ascending portions and both portions have digestive and absorptive functions.

The ileum is the terminal portion of the small intestine and continues from the jejunum. It opens into the caecum at the ileocaecal orifice. The intestinal epithelium is mainly absorptive, with much less digestion occurring compared to the duodenum and the jejunum.

The jejunum continues from the duodenum and leads into the ileum. It is the longest part of the small intestine and is highly coiled. It has digestive and absorptive functions.

The small intestine extends from the pylorus of the stomach to the caecum. The small intestine recieves chyme from the stomach. It is the main site of chemical degradation and absorption of chyme. Fats are exclusively broken down in this part of the alimentary tract. Carbohydrates and proteins that are not degraded in the small intestine are available for microbial fermentation in the large intestine. The small intestine produces enzymes for digestion of protein, carbohydrate and fat and absorbs the products of their digestion. Enzymes are produced by glands in the intestinal wall and the pancreas. The gall bladder produces bile which emulsifies fats for digestion. Absorption is facilitated by ridges in the small intestine and by the presence of villi and microvilli.