Digestion:
Digestion is the process of breaking down food into smaller, absorbable components.
It occurs in several stages and involves both mechanical and chemical actions:
a. Mechanical digestion:
This process begins in the mouth, where food is physically broken down into smaller pieces by chewing.
Mechanical digestion continues in the stomach, where food is mixed with gastric secretions and churned by the muscular contractions of the stomach walls.
b. Chemical digestion:
This process involves breaking down macronutrients (proteins, carbohydrates, and fats) into their smaller building blocks using enzymes and other substances.
1. Carbohydrate digestion:
Begins in the mouth with the enzyme salivary amylase, which breaks down starches into maltose (a disaccharide).
In the small intestine, pancreatic amylase continues this process, breaking down the remaining starches.
Brush border enzymes (maltase, isomaltase, sucrase, and lactase) in the small intestine break down disaccharides into monosaccharides (glucose, fructose, and galactose), which are then absorbed.
2. Protein digestion:
Starts in the stomach, where the enzyme pepsin breaks proteins into smaller peptides.
In the small intestine, pancreatic enzymes (trypsin, chymotrypsin, and elastase) further break down peptides into smaller peptides and amino acids.
Brush border enzymes (aminopeptidases and dipeptidases) in the small intestine complete the process, breaking down small peptides into individual amino acids that are then absorbed.
3. Fat digestion:
Begins in the stomach, where the enzyme gastric lipase initiates the breakdown of triglycerides.
The majority of fat digestion occurs in the small intestine, where pancreatic lipase breaks down triglycerides into monoglycerides and free fatty acids.Bile, produced by the liver and stored in the gallbladder, emulsifies fats, increasing their surface area for lipase action.
Absorption:
Absorption is the process of taking up the digested nutrients into the bloodstream or lymphatic system for transport to cells throughout the body.
Most absorption occurs in the small intestine, which has a large surface area due to the presence of finger-like projections called villi and microvilli.
Carbohydrate absorption:
Monosaccharides (glucose, fructose, and galactose) are absorbed by specific transport proteins in the small intestine's enterocytes.
Glucose and galactose are taken up via sodium-dependent glucose transporter 1 (SGLT1), while fructose is absorbed through facilitated diffusion using GLUT5.
Once inside the enterocytes, monosaccharides are transported into the bloodstream via GLUT2.
Protein absorption:
Amino acids are absorbed into enterocytes using specific transporters, depending on their chemical properties (e.g., neutral, acidic, or basic).
Small peptides are absorbed via cotransport with hydrogen ions using the peptide transporter PepT1.
Once inside the enterocytes, amino acids and peptides are transported into the bloodstream.
Fat absorption:
Monoglycerides and free fatty acids are absorbed into enterocytes by simple diffusion.
Inside the enterocytes, they are reassembled into triglycerides and incorporated into lipoprotein particles called chylomicrons.
Chylomicrons are then released into the lacteals (lymphatic vessels) and eventually enter the bloodstream via the thoracic duct.