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Difficulty: MediumAnimal Nutrition and Digestive System

The chemical breakdown of dietary proteins requires sequential enzymatic activity across different regions of the mammalian alimentary canal. What is the correct sequence of these processes from the start of chemical digestion to nutrient absorption?

  1. 1Denaturation of complex proteins and cleavage into polypeptides by pepsin in an acidic environment
  2. 2Hydrolysis of polypeptides into short oligopeptides by pancreatic trypsin and chymotrypsin in an alkaline lumen
  3. 3Cleavage of oligopeptides into free amino acids by brush-border peptidases
  4. 4Active transport of free amino acids across intestinal epithelial cells into hepatic portal capillaries

Answer

The correct sequence of protein digestion and absorption is: acidic cleavage into polypeptides in the stomach, alkaline breakdown into oligopeptides in the duodenum, brush-border cleavage into free amino acids in the ileum, and active absorption of amino acids into intestinal capillaries.
Protein digestion follows a strict anatomical and biochemical progression. First, stomach hydrochloric acid and pepsin denature and break complex proteins into polypeptides under acidic conditions. Next, pancreatic trypsin and chymotrypsin in the alkaline environment of the duodenum break polypeptides into short oligopeptides. Then, brush-border peptidases in the ileum convert oligopeptides into absorbable free amino acids. Finally, these free amino acids are actively absorbed across microvilli into blood capillaries.

Step-by-Step Solution

1
Identify the initial organ of chemical protein digestion.
Chemical digestion of protein begins in the stomach where gastric juice containing hydrochloric acid activates pepsinogen to pepsin, hydrolyzing native proteins into polypeptides.
Salivary amylase in the mouth acts only on carbohydrates; no protein-digesting enzymes are secreted in the buccal cavity.
2
Determine the subsequent region of the digestive tract and its corresponding enzymatic activity.
Chyme enters the duodenum where pancreatic juice neutralizes acid. Pancreatic proteases (trypsin and chymotrypsin) hydrolyze polypeptides into short oligopeptides.
Pancreatic endopeptidases require an alkaline pH optimal for their catalytic activity in the duodenum.
3
Identify the terminal stage of enzymatic hydrolysis.
Intestinal peptidases (erepsin) on the brush border of the ileum hydrolyze oligopeptides into individual amino acids.
Final breakdown to amino acid monomers must occur before absorption across cell membranes can take place.
4
Identify the final nutrient absorption process.
Free amino acids are actively transported across villi epithelial membranes into blood capillaries of the hepatic portal system.
Absorbed amino acids travel via the hepatic portal vein directly to the liver for metabolic processing.

Key Concept

Sequential Protein Digestion and Absorption Pathway
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Topics

Animal Nutrition and Digestive System

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