Human Biochemistry Volume 2 - Murray R. 1993
Special Topics
Nutrition, Digestion, and Absorption
Carbohydrate Absorption
Carbohydrate Digestion products are absorbed from the jejunum into the Blood of the portal Venous system in the form of Monosaccharides, predominantly hexoses (glucose, fructose, mannose, and galactose) and pentoses. Oligosaccharides (compounds derived from starch that yield 3–10 monosaccharide units upon Hydrolysis) and Disaccharides are hydrolyzed by specific Enzymes located on the mucosal surface of the Small Intestine, which may include pancreatic amylase adsorbed onto the mucosa. The activity of free disaccharidases within the intestinal lumen is low, with the majority of their activity associated with small microvillar protrusions (the brush border) of the intestinal epithelial Cells.
The absorption of monosaccharides occurs via two primary mechanisms: Active Transport against a concentration gradient, and simple diffusion. However, the absorption of certain sugars cannot be strictly attributed to either mechanism alone. The molecular configuration features apparently required for active transport, which are characteristic of glucose and galactose, are as follows: the hydroxyl group at the C-2 position must have the same configuration as in glucose; a pyranose ring must be present; and a methyl or substituted methyl group must be located at the C-5 position. Fructose is absorbed more slowly than glucose and galactose, a process that appears to proceed via concentration-gradient-driven diffusion.
Class="center">Table 53.11. Summary of Digestive Processes
|
Source of Secretion and Stimulus |
Enzyme |
Activation Mechanism and Optimal Conditions |
Substrate |
End Product of Action |
|
Salivary Glands: reflex secretion of saliva in response to food in the Oral Cavity |
Salivary amylase |
Requires chloride ion, pH 6.6–6.8 |
Starch |
Maltose plus 1:6 glucosides (oligosaccharides) plus maltotriose |
|
Lingual glands |
Lingual lipase |
pH range 2.0–7.5; optimum 4.0–4.5 |
Short-chain primary esters, sn-3 bond |
Fatty acids plus 1,2-diacylglycerols |
|
Gastric glands: chief and parietal cells secrete gastric juice in response to reflex stimulation and gastrin |
Pepsin A (fundus) Pepsin B (pylorus) |
Pepsinogen is converted to active pepsin by HCl, pH 1.0–2.0 |
Protein |
|
|
Rennin |
Requires calcium for activity, pH 4.0 |
Milk casein |
Curdled milk |
|
|
entry of acidic chyme from The Stomach triggers the duodenum to produce 1) secretin, a hormone that stimulates pancreatic juice secretion; 2) cholecystokinin, which stimulates enzyme secretion |
Trypsinogen is converted to active trypsin by intestinal enterokinase at pH 5.2–6.0. Autocatalytic conversion occurs at pH 7.9 |
Protein Peptides |
Dipeptides |
|
|
Secreted as chymotrypsinogen and activated by trypsin, pH 8.0 |
Protein Peptides |
Same as for trypsin. Higher milk-clotting activity |
||
|
Secreted as proelastase and converted to its active form by trypsin |
Protein Peptides |
Polypeptides Dipeptides |
||
|
Carboxypeptidase |
Secreted as procarboxypeptidase, activated by trypsin |
Polypeptides with a free C-terminal end |
Smaller peptides, free Amino Acids |
|
|
Pancreatic amylase |
pH 7.1 |
Starch Glycogen |
Maltose plus 1:6 glucosides (oligosaccharides) plus maltotriose |
|
|
Lipase |
Activated by Bile salts, Phospholipids, colipase, pH 8.0 |
Primary ester bonds of triacylglycerols |
Fatty acids, monoacylglycerols, diacylglycerols, glycerol |
|
|
Ribonucleic acid |
||||
|
Deoxyribonuclease |
Deoxyribonucleic acid |
Nucleotides |
||
|
Cholesterol ester hydrolase |
Activated by bile salts |
Cholesterol esters |
Free cholesterol plus fatty acids |
|
|
Phospholipase A2 |
Secreted as a proenzyme, activated by trypsin and Ca2+ |
Phospholipids |
Fatty acids, lysophospholipids |
|
|
Liver and Gallbladder: cholecystokinin (an intestinal mucosal hormone) and, potentially, gastrin and secretin stimulate the gallbladder and hepatic bile secretion |
(Bile salts and alkalis) |
Fats; also neutralize acidic chyme |
Fatty acid–bile salt conjugates, finely emulsified micelles of neutral fat and bile salts, and Liposomes |
|
|
Small intestine: secretions of Brunner's glands in the duodenum and Lieberkühn's crypts |
Aminopeptidase |
Polypeptides with a free N-terminal end |
Smaller peptides, free amino acids |
|
|
Dipeptidases |
Dipeptides |
Amino acids |
||
|
Sucrase |
pH 5.0–7.0 |
Sucrose |
Fructose, glucose |
|
|
Maltase |
pH 5.8–6.2 |
Maltose |
Glucose |
|
|
Lactase |
pH 5.4–6.0 |
Lactose |
Glucose, galactose |
|
|
Trehalase |
Trehalose |
Glucose |
||
|
Phosphatase |
pH 8.6 |
Organic phosphates |
Free phosphate |
|
|
Isomaltase or 1:6 glucosidase |
1:6 glucosides |
Glucose |
||
|
Polynucleotidase |
Nucleotides |
|||
|
Nucleosidases (nucleoside phosphorylases) |
Purine or pyrimidine nucleosides |
Purine or pyrimidine bases, pentose phosphate |
Last update: 06/08/2026
Editorial and Educational Adaptation: This material has been compiled based on the primary/original source text. The project team performed an editorial review, corrected technical inaccuracies, structured sections, and adapted the content for an educational format.
What was processed:
- elimination of formatting defects (OCR errors, structural breaks, corrupted characters);
- editorial organization of content;
- standardization of terminology in accordance with academic sources;
- verification of factual statements against the original source text.
All mentions of the author, publication year, and origin of the primary text have been preserved in accordance with the source.