Digestion And Absorption

Question 1

Carbohydrate digestion

Answer 1

begins with salivary amylase in the mouth.

There are no carbohydrases in gastric juice. Most intraluminal digestion of carbohydrates takes place in the small
intestine by the action of pancreatic α-amylase.

Digestion by α-amylase is rapid and occurs by hydrolysis of the α-1,4
linkages only. products are maltose, maltotriose and α-limit dextrins which contain the α-1,6 linkages.

Question 2

Carbohydrate absorption

Answer 2

1. Brush border digestion: The major brush border carbohydrases are maltase, sucrase, lactase and α-limit dextrinase (isomaltase).
2. Enterocytes: can only absorb monosaccharides. All starch and oligosaccharides must be completely hydrolyzed by brush border carbohydrases. The final products are glucose, galactose and fructose.

Question 3

Fructose absorption

Answer 3

Fructose moves into the cell along its concentration gradient by facilitated diffusion. The carrier involved is GLUT-5, which is located in the apical membrane of the enterocyte. This process does not depend on Na+ or energy.

Question 4

Glucose/Galactose absorption

Answer 4

Glucose and galactose share a common transport process
which is linked to Na+ transport (secondary active
transport) called SGLT-1. Uptake across the apical membrane is energized by the electrochemical Na+ gradient, maintained by extrusion of Na+ across the basolateral membrane by a Na-K pump. The entry of monosaccharides into the cell is the rate-limiting step in the absorption of carbohydrates.

Question 5

Normally all carbohydrates are absorbed by the time they reach the

Answer 5

Mid jejunum

Question 6

Unabsorbed fiber fx

Answer 6

Unabsorbed fiber reaching the colon serves as a fuel for colonic bacteria. The bacterial products of fiber metabolism
include short-chain fatty acids, hydrogen, and methane.

Question 7

Why are dietary proteins required?

Answer 7

Dietary proteins are required for two reasons: (1) to provide the amino acids that are nutritionally
essential under all conditions and those that are conditionally essential under specific physiological and
pathological conditions; and (2) to provide nitrogen for the synthesis of nutritionally nonessential amino acids and other metabolically important nitrogen-containing compounds.

Question 8

Protein digestion

Answer 8

Pepsin starts it in stomach (but other than with collagen, it is not very significant).

Most occurs in upper small intestine (jejunum).

Protein digestion products in duodenum stimulate CCK secretion, which stimulates secretion of proteolytic enzymes from pancreas.

Proteases are secreted as inactive pro-enzymes, and are activated in duodenum by enterokinase (trypsinogen converted to trypsin, and trypsin converts all other pro-enzymes to endopeptidases and exopeptidases.

Question 9

Endopeptidases

Answer 9

trypsin, elastase and chymotrypsin which act on specific bonds on the interior of the protein molecule.

Question 10

Exopeptidases

Answer 10

carboxypeptidases A and B which hydrolyze, in sequence, the carboxy terminal peptide bond of the peptide chain.

Question 11

Protein absorption (free AA)

Answer 11

Free amino acids are absorbed by several different Na+-dependent active transport mechanisms. Transport of amino acids is slow and is the rate-limiting step in their absorption.

Question 12

Protein absorption (di and tripeptides)

Answer 12

Most protein is absorbed in the form of di- and tripeptides.
The peptide transporter shows a preference for di- and
tripeptides containing proline and glycine residues. There are at least two mechanisms for peptide transport, these include a Na+-dependent and an H+/oligopeptide cotransporter called PepT1.

Once inside the cell, cytoplasmic peptidases hydrolyze these small peptides into their constituent amino acids. Amino acids leave the cell by amino acid transporters or passive diffusion. Most amino acids and
peptides are absorbed in the jejunum but some may reach the ileum.

Question 13

Dietary lipids

Answer 13

Acylglycerol is a fat and is formed by the attachment of a fatty acid to glycerol. Attachment of three fatty acids to glycerol forms triglyceride which accounts for more than 90% of dietary lipid. Phospholipids are the major components of cell membranes. Most of the phospholipid in the diet is derived from cells that are sloughed from the intestinal mucosa. Steroids are primarily cholesterol. Bile acids are also derivatives of cholesterol. Small amounts of fat-soluble vitamins (vitamins A, D, E & K) are also found in the diet

Question 14

Lipid breakdown

Answer 14

Starts in mouth with lingual lipase and continues in stomach with gastric lipase.

Due to low pH of stomach, lipids are not emulsified until they reach the duodenum and interact with bile salts and lecithin.

Question 15

Pancreatic lipase

Answer 15

Pancreatic lipase is secreted in its active form!!!

It hydrolyzes triglycerides to one molecule of monoglyceride and two molecules of fatty acid and acts only at the oil/water interface.

Lipase is inactivated in the presence of bile salts. To overcome this problem pancreatic co-lipase is secreted as procolipase which has no lipase action itself. It is activated by trypsin. In the presence of bile salts colipase binds to triglyceride at the oil/water interface allowing lipase to work without being inactivated by bile salts.

Triglyceride hydrolysis is rapid because the amount of pancreatic lipase secreted into the duodenum is in excess of the fat present.

Question 16

Dietary phospholipids breakdown

Answer 16

Dietary phospholipids are hydrolyzed by phospholipase A2 which is secreted as a pro-enzyme prophospholipase A2. In the presence of calcium and bile salts phospholipase A2 cleaves the fatty acid from the phospholipid.

Question 17

Cholesterol breakdown

Answer 17

Cholesterol is present in the diet primarily as cholesterol ester. Cholesterol ester hydrolase (cholesterol esterase) is secreted in the pancreatic juice and cleaves the fatty acid from cholesterol ester to yield free cholesterol.

Question 18

Solubilization by Bile Salts & Diffusion to the Mucosa:

Answer 18

The emulsified fat products must form micelles with bile salts so that they can be absorbed. Micelles containing lipid
digestion products are called mixed micelles.

The bile salts are on the exterior of the micelle and the 2-monoglycerides and lysophosphatides have their hydrophobic chains facing the interior and their polar (hydrophilic) ends facing the water phase. Cholesterol and fat-soluble vitamins are in the fat-soluble interior of the micelle.

Formation of mixed micelles facilitates the movement of fat-soluble
substances through the aqueous chyme.

Question 19

Mucosal phase of lipid digestion/absorption

Answer 19

The micelles move along the microvilli allowing their lipids to passively diffuse across the microvillus membrane and into the enterocyte. Once the micelle has delivered its lipid contents the micelle itself is recycled back to the lumen.

The rate-limiting step in lipid absorption is the migration of micelles to the microvilli surface.

Bile salts are absorbed in the terminal ileum by a Na+-dependent active transport process.

Question 20

Uptake of lipid products

Answer 20

The uptake of the lipid products is passive and their rate of exchange between the micelle and the lipid layer is dependent upon the concentration gradient and the lipid solubility of the lipid product in the cell membrane. This process is generally rapid

Question 21

Intracellular processing of lipids

Answer 21

After the lipid products enter the enterocyte they are resynthesized in the smooth endoplasmic reticulum:

• Free fatty acids are re-esterified into acyl CoA by the action of CoA ligase. The acyl CoA serves as a donor of fatty acids.
• 2-monoglycerides are combined with fatty acids to re-form triglycerides.
• Cholesterol is re-esterified to cholesterol ester.
• Lysophosphatides are combined with fatty acids to form phospholipid.

Question 22

Bile acids absorption

Answer 22

Bile acids which are not absorbed in the ileum enter the colon and are extensively metabolized by the bacteria. Bile salts will then inhibit the reabsorption of Na+ and water by the proximal colon.

Question 23

Calcium functions, sources, and absorption

Answer 23

Calcium is the most common mineral in the body. It is
important for the body’s skeleton in bones and teeth and
also serves essential functions in blood clotting, muscle
contraction, nerve transmission and intracellular signaling
during hormone action. It is also an essential cofactor for
many enzymes and proteins.

Calcium is found in dairy products (eg. milk, yogurt, cheese) beans and some vegetables (eg. broccoli and spinach).

Absorption of calcium takes place in all segments of the intestine via a passive, paracellular route. In addition, Ca2+ is actively absorbed in the duodenum and jejunum by a membrane bound carrier that is activated by 1,25-dihydroxycholecalciferol. This compound is a derivative of vitamin D and it is produced in the proximal tubules of the kidney under the influence of parathyroid hormone.

Question 24

Calcium regulation

Answer 24

Calcium levels within the body are regulated tightly via the activation of vitamin D. This will increase levels of calcium via actions on the calcium channels, calbindin and the basolateral transporters.

Question 25

Iron absorption

Answer 25

The absorption of iron occurs primarily in the proximal small bowel, in the duodemum and jejunum. There are at least 2 mechanisms of Fe uptake; one for heme iron and a second for free iron. On a typical Western diet ~30% of iron absorption is in the form of heme. It appears that most free iron (~70%) is absorbed in the form of Fe2+ (ferrous).

Ferrous iron binds to a divalent metal transporter-1 (DMT1) on the brush border membrane and, once inside the cell, iron that is being stored is oxidized to the Fe3+ form (Ferric) before binding to apoferritin to form ferritin. Some is stored inside the cell in this form.

Question 26

Iron transport

Answer 26

To leave the enterocyte iron must dissociate from ferritin and bind to an intracellular carrier protein (transferrin), which takes it to the basolateral membrane to be transported out of the cell.

Iron transport at the basolateral membrane occurs via an iron transport protein Ferroportin-1 (iron-regulated transporter 1; IREG1). Hephaestin (Hp) helps to facilitate the basolateral transport. Ferroportin is inhibited by hepcidin. Once in the plasma the Fe2+ is converted to Fe3+ and bound to plasma transferrin.

Question 27

LACTOSE INTOLERANCE

Answer 27

is caused by an absence of the brush border enzyme lactase.

The individual will have a reduced ability to hydrolyze lactose to glucose and galactose. The presence of unmetabolized lactose produces an osmotic diarrhea. In addition, this non-absorbed lactose is an excellent substrate for the colonic bacteria and the resulting gas production gives a feeling of severe bloating.

Question 28

SPRUE: (celiac disease)

Answer 28

is most commonly caused by an adverse mucosal response to dietary gluten which is present in wheat- or rye-based foods. Hence it is also referred to as “gluten-sensitive enteropathy”.

The gluten has a destructive effect on the intestinal enterocytes, which results in a stunting of the villi and a decreased absorptive mucosal surface area.

Tropical sprue is found in the tropics - appears to be caused by an unidentified infectious agent.

Question 29

ABETALIPROTEINEMIA:

Answer 29

Apo-B is missing from serum lipoproteins. There is decreased absorption of lipids into blood and a buildup of lipids within the intestinal cells.

Question 30

RICKETS:

Answer 30

Vitamin D deficiency or failure to convert to 1,25-dihydroxycholecalciferol results in decreased calcium absorption -> bone softening (osteomalacia).

With aging calcium absorption decreases leading to bone loss (osteoporosis).

Question 31

IRON DEFICIENCY:

Answer 31

is usually due to lack of iron in the diet or poor iron bioavailablity, also may be due to chronic bleeding of the intestinal tract. In adults, deficiency can lead to lethargy
and impaired immune function. In children, cognitive and motor function deficits can occur.

Question 32

IRON EXCESS:

Answer 32

may result from chronically high intake (Hemochromatosis) or may be due to increased absorption due to an autosomal dominant disorder (Hereditary hemochromatosis).