What are intestinal glands? Describe the roles of intestinal juices in digestion and absorption of food.

Intestinal Glands: Structure and Location

Intestinal glands are tubular glands located within the mucosa of the small intestine. They are found in the crypts between the villi. Each gland contains several types of cells:

• Paneth cells: Secrete lysozyme, an antibacterial enzyme.
• Goblet cells: Secrete mucus, which lubricates the intestinal lining and protects it from self-digestion.
• Enterocytes: Absorptive cells that line the villi and absorb nutrients.
• Enteroendocrine cells: Secrete hormones like secretin and cholecystokinin (CCK) that regulate digestion.
• Stem cells: Continuously divide to replenish the intestinal epithelium.

Composition of Intestinal Juice

Intestinal juice is not produced by a single gland but is a collective secretion from the intestinal glands and the duodenal glands (Brunner's glands). Its primary components include:

• Peptidases: Enzymes that break down peptides into amino acids (e.g., aminopeptidase, dipeptidase).
• Sucrase, Maltase, Lactase: Enzymes that hydrolyze disaccharides (sucrose, maltose, lactose) into monosaccharides (glucose, fructose, galactose).
• Lipase: Enzyme that breaks down fats into fatty acids and glycerol.
• Nucleases: Enzymes that break down nucleic acids (DNA and RNA) into nucleotides.
• Mucus: Provides lubrication and protection.
• Water and Electrolytes: Facilitate the transport of nutrients.

Roles of Intestinal Juices in Digestion

Carbohydrate Digestion

The digestion of carbohydrates primarily occurs in the small intestine. Pancreatic amylase, secreted into the duodenum, breaks down starch into smaller oligosaccharides. Intestinal juice contains enzymes like sucrase, maltase, and lactase, which further hydrolyze these oligosaccharides into monosaccharides – glucose, fructose, and galactose – which are then absorbed.

Protein Digestion

Protein digestion begins in the stomach but is completed in the small intestine. Pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase) break down proteins into peptides. Intestinal juice contains peptidases (aminopeptidase, dipeptidase) that further break down these peptides into individual amino acids, ready for absorption.

Fat Digestion

Fat digestion is more complex. Bile salts, produced by the liver and stored in the gallbladder, emulsify fats, increasing their surface area for enzymatic action. Pancreatic lipase breaks down emulsified fats into fatty acids and glycerol. Intestinal juice contains intestinal lipase, which contributes to this process. These products are then absorbed with the help of bile salts forming micelles.

Roles of Intestinal Juices in Absorption

Intestinal juice plays a crucial role in creating an optimal environment for absorption:

• Solubilization: The watery component of intestinal juice helps solubilize digested nutrients, making them easier to transport across the intestinal epithelium.
• Brush Border Enzymes: Enzymes located on the microvilli of enterocytes (the brush border) complete the final stages of digestion, ensuring nutrients are in their absorbable form.
• Active Transport: Intestinal juice provides the necessary ions and co-transporters for active transport mechanisms, allowing for the absorption of nutrients against their concentration gradients.
• Facilitated Diffusion: Some nutrients are absorbed via facilitated diffusion, aided by carrier proteins present in the intestinal epithelium.

The absorbed nutrients then enter the bloodstream or lymphatic system for distribution throughout the body.