95 Physiology of digestion

Chemical Digestion and Enzymatic Hydrolysis

Chemical digestion involves breaking down macromolecules into their constituent building blocks through enzymatic hydrolysis. Various enzymes sourced from different organs and glands catalyze these reactions, each targeting specific substrates to produce distinct products.

Enzymes, Their Sources, Substrates, and Products

Carbohydrate Digestion Carbohydrate digestion begins in the mouth with the enzyme amylase, produced by the salivary glands. Amylase breaks down starches into smaller molecules like maltose, maltotriose, and alpha-dextrins. This process continues in the small intestine with pancreatic amylase, further hydrolyzing starches. The brush border enzymes of the small intestine, including maltase, sucrase, and lactase, complete carbohydrate digestion. Maltase acts on maltose to yield glucose, sucrase breaks down sucrose into glucose and fructose, and lactase hydrolyzes lactose into glucose and galactose.

Protein Digestion Protein digestion begins in the stomach, where the enzyme pepsin, activated by hydrochloric acid (HCl) secreted by the stomach’s parietal cells, breaks down proteins into smaller peptides. This process continues in the small intestine with the action of pancreatic enzymes like trypsin, chymotrypsin, and carboxypeptidase. These enzymes further degrade peptides into smaller peptides and amino acids. Finally, the brush border enzymes, including aminopeptidases and dipeptidases, complete the breakdown into individual amino acids ready for absorption.

Lipid Digestion Lipid digestion primarily occurs in the small intestine, where bile salts play a crucial role in emulsification. Produced by the liver and stored in the gallbladder, bile salts emulsify large fat globules into smaller droplets, increasing the surface area for the action of pancreatic lipase. Pancreatic lipase hydrolyzes triglycerides into monoglycerides and free fatty acids, which can be absorbed into the intestinal cells.

Nucleic Acid Digestion Nucleic acids are digested by nucleases, enzymes produced by the pancreas. These enzymes break down DNA and RNA into nucleotides, which are further broken down into nitrogenous bases, pentose sugars, and phosphate ions by brush border enzymes like nucleotidases and phosphatases.

Locations of Chemical Digestion

The digestion of macromolecules occurs at various locations within the digestive tract. Carbohydrate digestion begins in the mouth and continues in the small intestine. Protein digestion starts in the stomach and is completed in the small intestine. Lipid digestion primarily occurs in the small intestine, facilitated by bile salts from the liver and lipases from the pancreas. Nucleic acid digestion also takes place in the small intestine.

Functions, Production, and Regulation of Hydrochloric Acid (HCl)

Hydrochloric acid (HCl) plays a critical role in digestion within the stomach. Produced by the parietal cells of the stomach lining, HCl creates an acidic environment with a pH of 1.5 to 3.5, which is essential for the activation of pepsinogen to pepsin, an enzyme that digests proteins. The acidity also helps break down food particles, kills ingested bacteria, and provides an optimal pH for enzyme activity.

The production of HCl is regulated by neural, hormonal, and paracrine mechanisms. The sight, smell, and taste of food trigger the release of acetylcholine from the vagus nerve, stimulating parietal cells. Gastrin, a hormone released from G cells in the stomach in response to food, also stimulates HCl secretion. Additionally, histamine released from enterochromaffin-like cells binds to H2 receptors on parietal cells, further promoting acid production.

Emulsification and the Role of Bile Salts

Emulsification is the process of breaking down large fat globules into smaller droplets, increasing their surface area for enzyme action. Bile salts, produced by the liver and stored in the gallbladder, facilitate this process in the small intestine. When fat enters the small intestine, bile salts are released into the duodenum, where they surround fat globules and break them into smaller droplets, forming micelles. This emulsification is crucial for the efficient digestion of fats by pancreatic lipase, which can then hydrolyze triglycerides into monoglycerides and free fatty acids.

The Role of Bacteria in Digestion

The human gut microbiome, consisting of trillions of bacteria, plays a vital role in digestion. These bacteria reside primarily in the large intestine and contribute to the breakdown of complex carbohydrates, fibers, and other substances that human digestive enzymes cannot digest. Bacteria ferment these substrates, producing short-chain fatty acids (SCFAs) like acetate, propionate, and butyrate, which serve as an energy source for colon cells and have various health benefits.

Bacteria also synthesize essential vitamins, such as vitamin K and certain B vitamins, and aid in the metabolism of bile acids, sterols, and xenobiotics. Moreover, the gut microbiome interacts with the immune system, promoting the development and function of the gut-associated lymphoid tissue (GALT), which plays a crucial role in immune responses.

In conclusion, the digestive system relies on a complex interplay of enzymes, hormones, bile salts, and gut bacteria to break down food, absorb nutrients, and maintain overall health. Each component, from the initial enzymatic hydrolysis to the intricate role of the microbiome, plays a vital role in ensuring efficient digestion and nutrient assimilation.