8.2Digestion and Absorption
Sugar, protein, and fat have long been viewed as the three main nutrients, for they are deemed important as bioenergy sources and biogenic materials. Sugar is also called carbohydrate, including monosaccharides, oligosaccharides, and polysaccharides, as well as glycoconjugates. Starch consists of several thousand units of glucose (grape sugar) linked together. Amylases produced in and secreted from the salivary glands and the pancreas eventually hydrolyze starch into oligosaccharides such as maltose. These oligosaccharides are decomposed further into glucose by small intestinal enzymes and absorbed from microvilli in the small intestine. The glucose then enters the blood vessels via epithelial cells (Fig. 8-2). Hydrolyzation of starch involves various types of enzymes, such as ones that break the middle of glucose chains, ones that snip off branches, and ones that cut the chains from their termini. These enzymes act in a synergistic manner to enhance digestion.
Fig. 8-2. Small Intestine
The inner side of the small intestine is corrugated, bristled with innumerable villi protruding toward the lumen. The surface of the villi is covered with single-layer epithelial cells, and peripheral lymphatic vessels called capillaries and lacteal vessels run inside the villi. The surface area of the epithelial cells is enormous on account of their brush-like structure referred to as microvillus. Nutrients absorbed from here pass through the epithelial cells and are transported to the portal vein via the capillaries or to the lymphatic vessels via the lacteal vessels.
Although enzymes that hydrolyze proteins are named generically as proteases, the structures of substrates to be most easily severed and their optimum conditions differ depending on each enzyme. Pepsin—a protease contained in gastric juice—functions well in the acidic environment of gastric acid. Pancreatic juice, which is produced in the pancreas and secreted from duodenum, is weak alkaline and neutralizes gastric acid, thus keeping the inside of the small intestine neutral. Pancreatic juice contains various digestive enzymes functioning under neutral conditions, for example, amylases and proteases including trypsin and chymotrypsin, lipases that hydrolyze lipids, and nucleolytic enzymes. The duodenum also receives the bile, which is secreted in the liver and temporarily stored in the gallbladder before being released. Bile contains surfactants that assist digestion through emulsification and dispersion of lipids in food.
Fig. 8-3. Blood Flow in the Intestine and Liver
Arteries branching from the aorta supply blood to the organs and digestive tracts. The portal veins are blood vessels located in the region between 2 capillary systems and transport nutrients and toxins absorbed from the intestines and stomach to the liver. The blood, after being treated in the liver, returns to the heart via the hepatic veins and is pumped into the whole body from there.
After being decomposed by proteases into amino acids or peptides consisting of several amino acids linked together, proteins enter the blood vessels via the epithelial cells of the microvilli protruding from the small intestine. Their decomposition into amino acids progresses in the cellular membranes or epithelial cells as well. Neutral lipids (see Column Fig. 8-1), meanwhile, are absorbed into the epithelial cells along with other fatty acids after partial hydrolyzation by lipases. In the epithelial cells, they are resynthesized into neutral lipids, which form complexes with proteins there (referred to as chylomicrons) and then enter the lymphatic vessels. They are transported to the whole body, and eventually, to the liver.
Blood that contains nutrients absorbed from the stomach and intestine converges at the portal vein and is conveyed to the liver. Hepatic cells in the liver decompose harmful substances in the blood. For instance, ethanol is detoxified in the hepatic cells by oxidation to acetaldehyde and further to acetic acid, which is ultimately reduced to carbon dioxide and water after being circulated and decomposed in tissues throughout the body. In addition to the portal vein, the hepatic artery branching from the aorta also flows into the liver. After being filtrated in the liver, the blood exits the liver via the hepatic vein and is pumped into the whole body from the heart (Fig. 8-3).
Why Are the Digestive Organs Not Digested?
Despite the presence of many strong proteases such as pepsin produced in the stomach and trypsin/chymotrypsin produced in the pancreas, how are the cells of these organs not digested? These proteases are first biosynthesized in the cells and secreted into the lumen of the digestive tracts as inactive precursor proteins called pepsinogen, trypsinogen, and chymotrypsinogen, which include superfluous parts of proteases. It is not before these parts are cut off by already-active proteases in the lumen that these enzymes form active steric structures. The cells therefore can synthesize and secrete a copious amount of proteases without being digested until then. Besides, mucus is secreted profusely from the surface of the gastric wall, serving as a barrier to protect the wall from gastric acid and pepsin.
What Happens to DNA in Food
We consume genes of various animals, plants, and microorganisms in huge quantity in the form of food every day. Do our bodies take in the genes of other organisms then? Since the pancreas secretes nuclease (enzymes to hydrolyze RNA) and deoxyribonuclease (enzymes to hydrolyze DNA), and the small intestine secretes other nucleolytic enzymes, almost all nucleic acids present in the food are decomposed into bases or sugar phosphates before being absorbed into the blood vessels via epithelial cells. Although some base moieties are reutilized, other bases are discharged as uric acid in the case of humans, which sometimes causes gout due to crystallization owing to its low solubility. In any event, food-derived DNA is digested and never gets incorporated into our cells. Besides, in order for genes to function, DNA macromolecules with genetic information need to be absorbed intact into cells and integrate into their chromosomes. Moreover, such a phenomenon must occur in reproductive cells if the offspring are to inherit the genetic information. Considering this, it is unlikely that food-derived DNA is passed down to future generations among humans; factually, no trace for such a phenomenon has been discovered in the human genome.