Complex carbohydrate digestion (starches and glycogen) entails:
Amylases produced by the salivary glands and pancreas.
Brush-border enzymes in small intestine.
Carbohydrate digestion starts from mouth.
In the mouth, amylase from the parotid and submandibular salivary glands begins carbohydrate digestion.
Salivary amylase converts starch and glycogen into following products; maltose (disaccharide), maltotriose (trisaccharide) and alpha-dextrins (starch fragments)
But before they reach the small intestine, only few molecules of starch or glycogen undergo full digestion into maltose. Carbohydrate digestion in pancreas and small intestine
The pancreas secretes amylase into the duodenum.
In the small intestine, bicarbonate ions from pancreatic juice neutralizes gastric acid.
Amylase plays a key role in further dismantling starch and glycogen into simpler sugars like maltose, maltotriose, and alpha-dextrins. However, it has no effect on cellulose—a tough plant fiber we cannot digest due to the absence of a specific enzyme, cellulase.
Carbohydrate digestion reaches its final phase at the brush border of the small intestine, where enzymes embedded in the microvilli membranes carry out the last steps, converting remaining carbohydrates into absorbable simple sugars. Among these, alpha-dextrinase specifically targets alpha-dextrin fragments, cleaving them to release individual glucose molecules.
Sucrase breaks sucrose into glucose and fructose.
Maltase breaks maltose and maltotriose into glucose.
Lactase breaks lactose into glucose and galactose.
The last byproducts of the breakdown of carbohydrates are glucose, fructose, and galactose.
The three byproducts of the metabolism of carbohydrates, glucose, fructose, and galactose, are all absorbed as monosaccharides.
Ultimately, the capillaries of the villi absorb the sugars.
Fructose is absorbed from the intestinal lumen into the epithelial cells of the villi through facilitated diffusion, a process that uses specific transport proteins to move the sugar across the cell membrane without energy expenditure.
Epithelial cells absorb transported monosaccharides and release them into the interstitial fluid.
The monosaccharide eventually diffuses into the bloodstream without the need for ATP.
Secondary active transport is used to absorb transported glucose and galactose.
Glucose and galactose are transported into the epithelial cells of the intestinal villi through secondary active transport, which relies on the movement of sodium ions to drive their uptake.
The transport of glucose or galactose is coupled with that of sodium ions.
These are transported in the same direction, down the concentration gradient for at least one substance.
Through enhanced diffusion, glucose and galactose are subsequently moved from epithelial cells into the interstitial fluid and ultimately into the circulation.
Protein digestion, absorption and transport:
The stomach and small intestine are where protein digestion takes place.
The digestive process is started by the stomach enzyme pepsin and finished by intestinal brush border and pancreatic enzymes.
When pH-lowering hydrochloric acid (HCl) is present in the stomach, inactive pepsinogen is converted to active pepsin.
Pepsin molecules start to break down proteins into smaller peptides; Newly generated pepsin molecules then catalyze the creation of more pepsin.
Protein digestion progresses further in the small intestine.
In the duodenum, acidic chyme from the stomach blends with pancreatic juice—a combination of enzymes and fluid. Within this mixture, enzymes such as trypsin, chymotrypsin, elastase, and carboxypeptidase act on proteins.
Each of these enzymes targets specific peptide bonds, breaking them to yield smaller peptide fragments or free amino acids.
Chymotrypsin, trypsin, and elastase help break down larger peptides into smaller ones.
Carboxypeptidase separates the terminal amino acid from the carboxyl terminus of the peptide. Two active enzymes—aminopeptidase, which breaks the peptide bond that binds the terminal amino acid to the amino end of the peptide, and dipeptidase, which separates dipeptides into single amino acids—complete digestion in the brush boundary.
End product of protein digestion are amino acids, dipeptide and tripeptides
Protein digestion occurs by brush border enzymes.
End product of protein digestion are absorbed at the intestinal villus.
Active transport is used to absorb the majority of proteins as amino acids. The absorption of nutrients relies on three key mechanisms: primary active transport, sodium-dependent secondary active transport, and hydrogen ion-dependent secondary active transport.
Once inside the absorptive cells, dipeptides and tripeptides are converted into individual amino acids. Most amino acids cross into the epithelial cells through active transport and typically leave these cells via simple diffusion.
Some amino acids utilize sodium-coupled secondary active transport to enter epithelial cells. In contrast, dipeptides and tripeptides are taken up using hydrogen ion-linked secondary active transport and are then enzymatically broken down into their single amino acid components within the cells.
Through the intestinal fluid and the villus’s blood capillaries, amino acids can permeate through the epithelial cells.
Lipid digestion, absorption and transport:
The small intestine is where lipid digestion mostly occurs. The mouth and stomach are where some lipid digestion takes place. Triglycerides and phospholipids are broken down by enzymes called lipases.
Fatty acids and monoglycerides are the byproducts of the slight hydrolysis of triglycerides by lingual and stomach lipases.
Triglycerides interact with pancreatic juice and bile salts in the duodenum.
Triglyceride emulsion droplets are produced when fat globules break up, and bile salts bind to the mono, di, and triglycerides of the fat globules.
Within the duodenum of the small intestine, triglyceride digestion is largely carried out as pancreatic lipase—secreted by the acinar cells—attaches to fat droplets.
There, it catalyzes the breakdown of triglycerides into monoglycerides and free fatty acids. This process is crucial for converting bulky fat molecules into forms that the body can readily absorb.
The end products fatty acids monoglycerides, depend on bile salts for absorption.
Micelles are micelle-shaped spheres formed by bile salts that transport monoglycerides and fatty acids to the absorptive cells for absorption.
Micelles also help to solubilize and absorb other large hydrophobic molecules such as Vitamin A, D, E, K and cholesterol.
Epithelial cells receive bile salts from micelles that include free fatty acids, monoglycerides, certain phospholipids, and cholesterol molecules. These substances easily diffuse into the cells. Micelles continue to transfer end products after diffusing back into the chyme.
Lipases frequently break down monoglycerides further, resulting in glycerol and fatty acids, which then recombine to form triglycerides. Chylomicrons are created when triglycerides combine with cholesterol phospholipids.
Chylomicrons are the coated with proteins and leave the epithelial cell via exocytosis.
Chylomicrons are too bulky to enter blood capillaries directly.
They enter lacteals, travel through lymphatic vessels and enter the bloodstream at the left subclavian vein.
Chylomicrons are quickly removed from the blood and broken down by lipoprotein lipases in capillary endothelial cells in the liver and adipose tissue.
