As monoglycerides and fatty acids are liberated through the action of lipase, they retain their association with bile acids and complex with other lipids to form structures called micelles. Micelles are essentially small aggregates nm in diameter of mixed lipids and bile acids suspended within the ingesta.
As the ingesta is mixed, micelles bump into the brush border of small intestinal enterocytes, and the lipids, including monoglyceride and fatty acids, are taken up into the epithelial cells. The major products of lipid digestion - fatty acids and 2-monoglycerides - enter the enterocyte by simple diffusion across the plasma membrane.
A considerable fraction of the fatty acids also enter the enterocyte via a specific fatty acid transporter protein in the membrane. Lipids are transported from the enterocyte into blood by a mechanism distinctly different from what we've seen for monosaccharides and amino acids.
Once inside the enterocyte, fatty acids and monoglyceride are transported into the endoplasmic reticulum, where they are used to synthesize triglyeride. Beginning in the endoplasmic reticulum and continuing in the Golgi, triglyceride is packaged with cholesterol, lipoproteins and other lipids into particles called chylomicrons. Remember where this is occurring - in the absorptive enterocyte of the small intestine.
Chylomicrons are extruded from the Golgi into exocytotic vesicles, which are transported to the basolateral aspect of the enterocyte. The vesicles fuse with the plasma membrane and undergo exocytosis, dumping the chylomicrons into the space outside the cells.
Because chylomicrons are particles, virtually all steps in this pathway can be visualized using an electron microscope, as the montage of images below demonstrates.
It involves the physical breakdown of food but does not alter its chemical makeup. Chemical digestion, on the other hand, is a complex process that reduces food into its chemical building blocks, which are then absorbed to nourish the cells of the body.
In this section, you will look more closely at the processes of chemical digestion and absorption. Figure 1. Digestion begins in the mouth and continues as food travels through the small intestine.
Most absorption occurs in the small intestine. Large food molecules for example, proteins, lipids, nucleic acids, and starches must be broken down into subunits that are small enough to be absorbed by the lining of the alimentary canal. This is accomplished by enzymes through hydrolysis. The many enzymes involved in chemical digestion are summarized in Table 1. Glucose, galactose, and fructose are the three monosaccharides that are commonly consumed and are readily absorbed.
Your bodies do not produce enzymes that can break down most fibrous polysaccharides, such as cellulose. While indigestible polysaccharides do not provide any nutritional value, they do provide dietary fiber, which helps propel food through the alimentary canal. Three brush border enzymes hydrolyze sucrose, lactose, and maltose into monosaccharides. Sucrase splits sucrose into one molecule of fructose and one molecule of glucose; maltase breaks down maltose and maltotriose into two and three glucose molecules, respectively; and lactase breaks down lactose into one molecule of glucose and one molecule of galactose.
Insufficient lactase can lead to lactose intolerance. Figure 2. Carbohydrates are broken down into their monomers in a series of steps. Proteins are polymers composed of amino acids linked by peptide bonds to form long chains. Digestion reduces them to their constituent amino acids. You usually consume about 15 to 20 percent of your total calorie intake as protein.
The digestion of protein starts in the stomach, where HCl and pepsin break proteins into smaller polypeptides, which then travel to the small intestine. Chemical digestion in the small intestine is continued by pancreatic enzymes, including chymotrypsin and trypsin, each of which act on specific bonds in amino acid sequences.
At the same time, the cells of the brush border secrete enzymes such as aminopeptidase and dipeptidase , which further break down peptide chains. This results in molecules small enough to enter the bloodstream. Figure 3. The digestion of protein begins in the stomach and is completed in the small intestine.
Figure 4. Proteins are successively broken down into their amino acid components. A healthy diet limits lipid intake to 35 percent of total calorie intake. The most common dietary lipids are triglycerides, which are made up of a glycerol molecule bound to three fatty acid chains.
Small amounts of dietary cholesterol and phospholipids are also consumed. The three lipases responsible for lipid digestion are lingual lipase, gastric lipase, and pancreatic lipase. However, because the pancreas is the only consequential source of lipase, virtually all lipid digestion occurs in the small intestine.
Pancreatic lipase breaks down each triglyceride into two free fatty acids and a monoglyceride. The fatty acids include both short-chain less than 10 to 12 carbons and long-chain fatty acids. Two types of pancreatic nuclease are responsible for their digestion: deoxyribonuclease , which digests DNA, and ribonuclease , which digests RNA. Villi that line the walls of the small intestine absorb nutrients into capillaries of the circulatory system and lacteals of the lymphatic system.
Villi contain capillary beds, as well as lymphatic vessels called lacteals. Fatty acids absorbed from broken-down chyme pass into the lacteals. Other absorbed nutrients enter the bloodstream through the capillary beds and are taken directly to the liver, via the hepatic vein, for processing. Chyme passes from the small intestine through the ileocecal valve and into the cecum of the large intestine. Any remaining nutrients and some water are absorbed as peristaltic waves move the chyme into the ascending and transverse colons.
This dehydration, combined with peristaltic waves, helps compact the chyme. The solid waste formed is called feces. It continues to move through the descending and sigmoid colons. The inner wall, or mucosa, of the small intestine is covered in wrinkles or folds called plicae circulares that project microscopic finger-like pieces of tissue called villi, which in turn have finger-like projections known as microvilli.
Each villus transports nutrients to a network of capillaries and fine lymphatic vessels called lacteals close to its surface. Key Terms villi : Tiny, finger-like projections that protrude from the epithelial lining of the intestinal wall. The Small Intestine The small intestine is the part of the gastrointestinal tract between the stomach and the large intestine where much of the digestion of food takes place. Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions: Iron is absorbed in the duodenum.
Vitamin B12 and bile salts are absorbed in the terminal ileum. Water and lipids are absorbed by passive diffusion throughout the small intestine.
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