Chemical Digestion and Absorption

Objectives

When you have completed this section, you should be able to

• describe how each major class of nutrients is chemically digested, name the enzymes involved, and discuss the functional differences among these enzymes; and

• describe how each type of nutrient is absorbed by the small intestine.

Chemical digestion and nutrient absorption are essentially finished by the time food residue leaves the small intes tine and enters the cecum. But before going on to the functions of the large intestine, we trace each major class of nutrients—especially carbohydrates, proteins, and fats— from the mouth through the small intestine to see how it is chemically degraded and absorbed.

Carbohydrates

Most digestible dietary carbohydrate is starch. Cellulose is indigestible and is not considered here, although its importance as dietary fiber is discussed in chapter 26. The amount of glycogen in the diet is negligible, but it is digested in the same manner as starch.

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Carbohydrate Digestion

Starch is digested first to oligosaccharides up to eight glucose residues long, then into the disaccharide maltose, and finally to glucose, which is absorbed by the small intestine. The process begins in the mouth, where salivary amylase hydrolyzes starch into oligosaccharides. Salivary amylase functions best at pH 6.8 to 7.0, typical of the oral cavity. It is quickly denatured upon contact with stomach acid, but it can digest starch for as long as 1 to 2 hours in the stomach as long as it is in the middle of a food mass and escapes contact with the acid. Amylase therefore works longer when the meal is larger, especially in the fundus, where gastric motility is weakest and a food bolus takes longer to break up. As acid, pepsin, and the churning contractions of the stomach break up the bolus, amylase is denatured; it does not function at a pH any lower than 4.5. Being a protein, amylase is then digested by pepsin along with the dietary proteins. About 50% of the dietary starch is digested before it reaches the small intestine.

Starch digestion resumes in the small intestine when the chyme mixes with pancreatic amylase (fig. 25.26). Starch is entirely converted to oligosaccharides and maltose within 10 minutes. Its digestion is completed as the chyme contacts the brush border of the absorptive cells. Two brush border enzymes, dextrinase and glucoamylase, hydrolyze oligosaccharides that are three or more residues long. The third, maltase, hydrolyzes maltose to glucose.

Maltose is also present in some foods, but the major dietary disaccharides are sucrose (cane sugar) and lactose (milk sugar). They are digested by the brush border enzymes

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sucrase and lactase, respectively, and the resulting mono-saccharides (glucose and fructose from the former; glucose and galactose from the latter) are immediately absorbed. In most humans, however, lactase is no longer produced after the age of four and lactose is indigestible past that age (see insight 25.4).

Carbohydrate Absorption

The plasma membrane of the absorptive cells has transport proteins that absorb monosaccharides as soon as the brush border enzymes release them (fig. 25.27). About 80% of the absorbed sugar is glucose, which is taken up by a sodium-glucose transport protein (SGLT) like that of the kidney tubules (see p. 894). After a high-carbohydrate meal, however, solvent drag absorbs two to three times as much glucose as the SGLT. Sugars entering the extracellular fluid (ECF) at the base of the intestinal epithelium increase its osmolarity. Water then passes osmotically from the lumen, through the now-leaky tight junctions between the epithelial cells, and into the ECF, carrying glucose and other nutrients with it.

The SGLT also absorbs galactose, whereas fructose is absorbed by facilitated diffusion using a separate carrier that does not depend on Na+. Inside the epithelial cell, most fructose is converted to glucose. Glucose, galactose, and the small amount of remaining fructose are then transported out the base of the cell by facilitated diffusion and are absorbed by the blood capillaries of the villus. The hepatic portal system delivers them to the liver; chapter 26 follows the fate of these sugars from there.

Starch Digestion The Small Intestine

Figure 25.26 Starch Digestion in the Small Intestine.

Pancreatic amylase digests starch into maltose and small oligosaccharides. Brush border enzymes digest these to glucose, which is absorbed by the epithelial cells.

Epithelial cell of

Core of villus small intestine Lumen of small intestine

Epithelial cell of

Core of villus small intestine Lumen of small intestine

Small Intestine Osmosis

OFacilitated diffusion

|<Glucosei

OActive transport

Osmosis

Solvent drag

Figure 25.27 Monosaccharide Absorption by the Small Intestine. Glucose and galactose are absorbed by the SGLT cotransporter in the apical membrane of the absorptive cell (right). Glucose is also absorbed along with water through the paracellular route (between cells) by solvent drag. Fructose is absorbed by a separate facilitated diffusion carrier. Most fructose is converted to glucose within the epithelial cell. The monosaccharides pass through the basal membrane of the cell by facilitated diffusion (left).

Figure 25.26 Starch Digestion in the Small Intestine.

Pancreatic amylase digests starch into maltose and small oligosaccharides. Brush border enzymes digest these to glucose, which is absorbed by the epithelial cells.

Figure 25.27 Monosaccharide Absorption by the Small Intestine. Glucose and galactose are absorbed by the SGLT cotransporter in the apical membrane of the absorptive cell (right). Glucose is also absorbed along with water through the paracellular route (between cells) by solvent drag. Fructose is absorbed by a separate facilitated diffusion carrier. Most fructose is converted to glucose within the epithelial cell. The monosaccharides pass through the basal membrane of the cell by facilitated diffusion (left).

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Insight 25.4 Clinical Application

Lactose Intolerance

Humans are a strange species. Unique among mammals, we go on drinking milk in adulthood, and moreover, we drink the milk of other species! This odd habit is largely limited, however, to Europeans, a few pastoral tribes of Africa, and their descendants. Only they have an ancestral history of milking domestic animals, a practice that goes back about 10,000 years and has coincided with the continued production of lactase into adulthood.

People without lactase have lactose intolerance. If they consume milk, lactose passes undigested into the large intestine, increases the osmolarity of the intestinal contents, and causes colonic water retention and diarrhea. In addition, lactose fermentation by intestinal bacteria produces gas, resulting in painful cramps and flatulence.

Lactose intolerance occurs in about 15% of American whites, nearly all people of Asian descent, and about 90% of American blacks, who are predominantly descended from nonpastoral African tribes. People with lactose intolerance can consume products such as yogurt and cheese, in which bacteria have broken down the lactose, and they can digest milk with the aid of lactase drops or tablets.

Proteins

The amino acids absorbed by the small intestine come from three sources: (1) dietary proteins, (2) digestive enzymes digested by each other, and (3) sloughed epithelial cells digested by these enzymes. The endogenous amino acids from the last two sources total about 30 g/day, compared with about 44 to 60 g/day from the diet.

Enzymes that digest proteins are called proteases (peptidases). They are absent from the saliva but begin work in the stomach. Here, pepsin hydrolyzes any peptide bond between tyrosine and phenylalanine, thus digesting 10% to 15% of the dietary protein into shorter polypeptides and a small amount of free amino acids (fig. 25.28). Pepsin has an optimal pH of 1.5 to 3.5; thus it is inactivated when it passes into the duodenum and mixes with the alkaline pancreatic juice (pH 8).

In the small intestine, the pancreatic enzymes trypsin and chymotrypsin take over protein digestion by hydrolyz-ing polypeptides into even shorter oligopeptides. Finally, these are taken apart one amino acid at a time by three more enzymes: (1) carboxypeptidase removes amino acids from the -COOH end of the chain; (2) aminopeptidase removes them from the -NH2 end; and (3) dipeptidase splits dipep-tides in the middle and releases the last two free amino acids. All three of these are brush border enzymes, while carboxypeptidase also occurs in the pancreatic juice.

Amino acid absorption is similar to that of monosac-charides. There are several sodium-dependent amino acid cotransporters for different classes of amino acids. Dipep-tides and tripeptides can also be absorbed, but they are hydrolyzed within the cytoplasm of the epithelial cells before their amino acids are released to the bloodstream. At the basal surfaces of the cells, amino acids behave like the monosaccharides discussed previously—they leave the cell by facilitated diffusion, enter the capillaries of the villus, and are thus carried away in the hepatic portal circulation.

The absorptive cells of infants can take up intact proteins by pinocytosis and release them to the blood by exo-cytosis. This allows IgA from breast milk to pass into an infant's bloodstream and confer passive immunity from mother to infant. It has the disadvantage, however, that intact proteins entering the infant's blood are detected as foreign antigens and sometimes trigger food allergies. As the intestine matures, its ability to pinocytose protein declines but never completely ceases.

Lipids

The hydrophobic quality of lipids makes their digestion and absorption more complicated than that of carbohydrates and proteins (fig. 25.29). Fats are digested by enzymes called lipases. Lingual lipase, secreted by the intrinsic salivary glands of the tongue, is activated by acid in the stomach, where it digests as much as 10% of the ingested fat. In infants, the stomach also secretes gastric lipase. Most fat digestion, however, occurs in the small intestine through the action of pancreatic lipase.

As chyme enters the duodenum, its fat is in large globules exposed to lipase only at their surfaces. Fat digestion would be rather slow and inefficient if it remained this way. Instead, it is broken up into smaller emulsification droplets by certain components of the bile—lecithin (a phospholipid) and bile acids (steroids). These agents have hydrophobic regions attracted to the surface of a fat globule and hydrophilic regions attracted to the surrounding water. The agitation produced by intestinal segmentation breaks the fat up into droplets as small as 1 ^m in diameter, and a coating of lecithin and bile acids keeps it broken up, exposing far more of its surface to enzymatic action.

There is enough pancreatic lipase in the small intestine after a meal to digest the average daily fat intake in as little as 1 or 2 minutes. When lipase acts on a triglyceride, it removes the first and third fatty acids from the glycerol backbone and usually leaves the middle one. The products of lipase action are therefore two free fatty acids (FFAs) and a monoglyceride. Bile acids coat these and other lipids and form micelles28 (my-SELLS), droplets about 5 nm in diameter containing bile acids, FFAs, monoglycerides, cholesterol, and fat-soluble vitamins. Micelles pass amid the microvilli of the brush border and release their lipids, which diffuse freely through the plasma membranes (which are themselves mostly lipid) into the absorptive cells. The absorptive cells also have transport proteins that take up cholesterol and fatty acids.

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Mouth

HO

No digestion occurs.

Stomach

Stomach

Small intestine

Actions of pancreatic enzymes

Pepsin ( £ ) hydrolyzes certain peptide bonds, breaking protein down into smaller polypeptides.

Hy OH

Trypsin ( ) and chymotrypsin ( A ) hydrolyze other peptide bonds, breaking polypeptides down into smaller oligopeptides.

Carboxypeptidase ( £ ) removes one amino acid at a time from the carboxyl (-COOH) end of an oligopeptide.

Small intestine

Actions of brush border enzymes (contact digestion)

Carboxypeptidase ^ Aminopeptidase

* Dipeptidase l

Blood capillary— of intestinal villus

Carboxypeptidase ( ^ ) of the brush border continues to remove amino acids from the carboxyl (-COOH) end.

Aminopeptidase ( A ) of the brush border removes one amino acid at a time from the amino (-NH2) end.

Dipeptidase ( 4 ) splits dipeptides ( CO ) into separate amino acids ( o ).

Figure 25.28 Protein Digestion and Absorption.

Within the cell, the FFAs and monoglycerides are transported into the smooth endoplasmic reticulum and resynthesized into triglycerides. The Golgi complex combines these with a small amount of cholesterol and coats the complex with a film of phospholipids and protein, forming droplets about 60 to 750 nm in diameter called chylomicrons29 (KY-lo-MY-crons). It packages chylomi-crons into secretory vesicles that migrate to the basal surface of the cell and release their contents into the core of chyl = juice + micr = small

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972 Part Four Regulation and Maintenance

Emulsification

Fat globule

Lecithin

Hydrophilic region Hydrophobic region

Bile acid

Hydrophilic region Hydrophobic region

Bile acid

Emulsification î^s droplets

Fat globule is broken up and coated by lecithin and bile acids.

Fat hydrolysis jiM .

Pancreatic lipase

Lecithin

Bile acid

Dietary lipid i i-AA/W i-AA/W

Pancreatic lipase /WW Free fatty acid A/VW Monoglyceride Free fatty acid

Emulsification droplets are acted upon by pancreatic lipase, which hydrolyzes the first and third fatty acids from triglycerides, usually leaving the middle fatty acid.

Triglyceride

Micelle formation i

-VWV

Monoglycerides Cholesterol

Lipid core

AAAA/

Fatty acids

QAAA/

Fat-soluble vitamins

Chylomicron formation

AAAA/ Fatty acids jAAAA/ Monoglycerides

Micelles

Triglycerides jptttt Phospholipids

0$° Cholesterol

Protein shell-./Tjl

Chylomicron-

Absorptive cell

Intestinal cells absorb lipids from micelles, resynthesize triglycerides, and package triglycerides, cholesterol, and phospholipids into protein-coated chylomicrons.

Micelles

Several types of lipids form micelles coated with bile acids.

Chylomicron exocytosis and lymphatic uptake

Chylomicrons in secretory vesicles

Lacteal

Chylomicrons in lymph

Chylomicrons in lymph

Golgi complex packages chylomicrons into secretory vesicles; chylomicrons are released from basal cell membrane by exocytosis and enter the lacteal (lymphatic capillary) of the villus.

Figure 25.29 Fat Digestion and Absorption.

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Physiology: The Unity of Companies, 2003 Form and Function, Third Edition the villus. Although some FFAs enter the blood capillaries, chylomicrons are too large to penetrate the endothe-lium. They are taken up by the more porous lacteal into the lymph. The white, fatty intestinal lymph (chyle) flows through larger and larger lymphatic vessels of the mesenteries, eventually reaching the thoracic duct and entering the bloodstream at the left subclavian vein. The further fate of dietary fat is described in chapter 26.

_Think About It_

Explain why the right lymphatic duct does not empty dietary fat into the bloodstream.

Nucleic Acids

The nucleic acids, DNA and RNA, are present in much smaller quantities than the polymers discussed previously. The nucleases (ribonuclease and deoxyribonucle-ase) of pancreatic juice hydrolyze these to their constituent nucleotides. Nucleosidases and phosphatases of the brush border then decompose the nucleotides into phosphate ions, ribose (from RNA) or deoxyribose (from DNA), and nitrogenous bases. These products are transported across the intestinal epithelium by membrane carriers and enter the capillary blood of the villus.

Vitamins

Vitamins are absorbed unchanged. The fat-soluble vitamins A, D, E, and K are absorbed with other lipids as just described. Therefore, if they are ingested without fat-containing food, they are not absorbed at all but are passed in the feces and wasted. Water-soluble vitamins (the B complex and vitamin C) are absorbed by simple diffusion, with the exception of vitamin B12. This is an unusually large molecule that can only be absorbed if it binds to intrinsic factor from the stomach. The B12-intrinsic factor complex then binds to receptors on absorptive cells of the distal ileum, where it is taken up by receptor-mediated endocytosis.

Minerals

Minerals (electrolytes) are absorbed along the entire length of the small intestine. Sodium ions are cotrans-ported with sugars and amino acids. Chloride ions are actively transported in the distal ileum by a pump that exchanges them for bicarbonate ions, thus reversing the chloride-bicarbonate exchange that occurs in the stomach. Potassium ions are absorbed by simple diffusion. The K+ concentration of the chyme rises as water is absorbed from it, creating a gradient favorable to K+ absorption. In diarrhea, when water absorption is hindered, potassium ions remain in the intestine and pass with the feces; thus chronic diarrhea can lead to hypokalemia.

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Iron and calcium are unusual in that they are absorbed in proportion to the body's need, whereas other minerals are absorbed at fairly constant rates regardless of need, leaving it to the kidneys to excrete any excess. The absorptive cells bind ferrous ions (Fe2+) and internalize them by active transport; they are unable to absorb ferric ions (Fe3+). Fe2+ is transported to the basal surface of the cell and there taken up by the extracellular protein trans-ferrin. The transferrin-iron complex diffuses into the blood and is carried to such places as the bone marrow for hemoglobin synthesis, muscular tissue for myoglobin synthesis, and the liver for storage (see fig. 18.5, p. 687). Excess dietary iron, if absorbed, binds irreversibly to ferritin in the epithelial cell and is held there until that cell sloughs off and passes in the feces.

_Think About It_

Young adult women have four times as many iron transport proteins in the intestinal mucosa as men have. Can you explain this?

Calcium is absorbed especially by the duodenum and jejunum. Parathyroid hormone induces the kidneys to release active vitamin D (calcitriol), and vitamin D stimulates calcium absorption by the small intestine. Ca2+ diffuses through calcium channels into the epithelial cells and binds to a cytoplasmic protein called calbindin. At the basal side of the epithelial cell, calcium is pumped out by a calcium-ATPase and a Na+-Ca2+ antiport. Vitamin D works by stimulating the synthesis of both calbindin and calcium-ATPase.

Water

The digestive system is one of several systems involved in water balance. The digestive tract receives about 9 L of water per day—0.7 L in food, 1.6 L in drink, 6.7 L in the gastrointestinal secretions: saliva, gastric juice, bile, pancreatic juice, and intestinal juice. About 8 L of this is absorbed by the small intestine and 0.8 L by the large intestine, leaving 0.2 L voided in the daily fecal output. Water is absorbed by osmosis, following the absorption of salts and organic nutrients that create an osmotic gradient from the intestinal lumen to the ECF.

Diarrhea occurs when the large intestine absorbs too little water. This occurs when the intestine is irritated by bacteria and feces pass through too quickly for adequate reabsorption, or when the feces contain abnormally high concentrations of a solute such as lactose that opposes osmotic absorption of water. Constipation occurs when fecal movement is slow, too much water is reabsorbed, and the feces become hardened. This can result from lack of dietary fiber, lack of exercise, emotional upset, or long-term laxative abuse.

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_Think About It_

Magnesium sulfate (epsom salt) is poorly absorbed by the intestines. In light of this, explain why it has a laxative effect.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

21. What three polymers account for most of the dietary calories? What are the end products of enzymatic digestion of each?

22. What two nutrients are digested by saliva? Why is only one of them digested in the mouth?

23. Name as many enzymes of the intestinal brush border as you can, and identify the substrate or function of each.

24. Explain the distinctions between an emulsification droplet, a micelle, and a chylomicron.

25. What happens to digestive enzymes after they have done their job? What happens to dead epithelial cells that slough off the gastrointestinal mucosa? Explain.

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Responses

  • LANA
    How pancreatic amylase digest starch and oligosaccharides?
    8 years ago
  • austin
    What nutreints are chemically digested by Disaccharide hydrolase?
    7 years ago
  • janis
    Why do young adult women have iron transport proteins in intestinal mucosa compared to men?
    5 years ago
  • elbert
    What happend tobdead epithelial cell that slough off the gastrointestinal mucosa?
    5 years ago
  • teighan mitchell
    What are chemical inncerals which inhabites nutrient absorbation and impaire digestablity?
    2 months ago

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