3 5.9 Chapter 5 Summary
Learning Objectives
- Connect between macromolecule catabolism and anabolism and organismal growth
- Interpret models of membrane transport presented as cell diagrams.
5.1 Digestive Processes in Animals
Digestion involves the breakdown of complex macromolecules (carbohydrates, proteins, lipids) into smaller, absorbable units (monosaccharides, amino acids, fatty acids/glycerides) through enzymatic hydrolysis.
- Carbohydrate Digestion: Begins with enzymes like amylase and continues with disaccharidases like maltase, sucrase, and lactase, breaking down polysaccharides and disaccharides into monosaccharides (like glucose, fructose, and galactose) for absorption.
- Protein Digestion: Starts in the stomach with the enzyme pepsin, which cleaves intact proteins into smaller peptides. In the small intestine, enzymes called peptidases further break these peptides down into individual amino acids, which are then absorbed into the bloodstream.
- Lipid Digestion and Absorption:
- Since lipids are hydrophobic, bile salts (amphipathic molecules) surround large lipid globules to separate them into smaller droplets, a process called emulsification, which greatly increases the surface area.
- Lipases break down the emulsified lipids into fatty acids and monoglycerides.
- Bile salts then form small spheres called micelles to carry these digested products to the brush border of the intestinal lining and aid with absorption.
- Elimination: Waste that is undigested and not absorbed remains in the gut lumen must be eliminated, following the critical reabsorption of water in the large intestine. Problems like constipation (excess water removal) and diarrhea (insufficient water removal) relate to this final step.
5.2 The Cellular Basis of Nutrient Absorption in Animals
Digestive system size varies on diet; for example, carnivorous mammals have a shorter large intestine compared to herbivorous mammals. The primary adaptation for nutrient absorption is the extensive surface area of the small intestine, achieved through large folds, finger-like projections called villi, and microscopic projections on epithelial cells called microvilli.
- Stomach Digestion & Protection: The stomach is the main site for protein digestion, where the enzyme pepsin breaks down proteins into smaller polypeptides. A highly acidic environment (hydrochloric acid, HCl) is maintained by parietal cells, which use primary active transport protons (H+) into the stomach lumen. The stomach lining protects itself by synthesizing pepsin in an inactive form and maintaining a thick mucus layer.
- Nutrient Transport: The absorption of nutrients across the intestinal epithelial cell membranes requires various methods of membrane transport, depending on the molecule.
- Small Intestine Function: This is the main organ where the digestion of all macromolecules (protein, fats, carbohydrates) is completed. Absorbed nutrients enter the blood where they can be distributed throughout the body. This is further illustrated in the Celiac Disease case study discussed in section 5.3.
- Large Intestine Function: The large intestine’s primary roles are to reabsorb water and mineral salts from the undigested food material and to store waste. It also houses beneficial bacteria (intestinal/gut flora) that aid in some digestive processes.
