BPK 105 Study Guide - Final Guide: Active Transport, Vagus Nerve, Parasympathetic Nervous System
27 Apr 2018
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Module 11 - Review Questions - Part 6
11. Describe the digestion and absorption of proteins through the entire digestive
system. Include the site, stimulation and function of important secretions and describe
the details of absorption across the cell membrane. [7 marks]
- When digestive enzymes break the covalent bonds within organic molecules, it is called
chemical digestion. Proteins are broken down into amino acids. Amino acid chains make
up proteins.
- The enzyme pepsin is produced in the gastric glands.
- The stomach activates pepsinogen when pepsinogen is secreted, converting it into
pepsin, which is active.
- Pepsin is ultimately activated by hydrochloric acid.
- The stomach secretes pepsin, which breaks down proteins to form polypeptides, which
are chains of amino acids that are shorter.
- Other enzymes carry on with the digestion as the rest of the protein and chains of
polypeptide travel from the stomach and into the small intestine. These enzymes -
carboxypeptidase, chymotrypsin and trypsin - are in inactive forms when the pancreas
produces them; cholecystokinin, which is released from the duodenum, stimulates the
release of these enzymes from the pancreas.
- When parasympathetic stimulation occurs through the vagus nerves, the secretion of
pancreatic fluids that contain many pancreatic enzymes is also stimulated. The enzymes
are secreted as their inactive forms and when they’re in the small intestine, they’re
activated. The polypeptides are then broken down into small peptides by these
enzymes.
- Peptidases are produced in the small intestine and are secreted there as well.
Peptidases, which are digestive enzymes bound to the small intestine microvilli, break
down the small peptides into individual amino acids, dipeptides, which consist of two
amino acids, or tripeptides, which contain three amino acids when these amino acids
reach the small intestine. They stay in the microvilli.
- The stomach is where absorption starts; it is where some molecules that are lipid-soluble
and not large are able to diffuse into circulation through the epithelium of the stomach. In
general, the majority of absorption takes place in the jejunum and duodenum. Some
absorption also takes place in the ileum.
- Using several cotransport methods, tripeptides, dipeptides and lone amino acids are
absorbed through the intestinal epithelial cells.
- Specifically, acidic amino acids, as well as the majority of neutral acids, are absorbed
into these cells by a transport mechanism called symport; symport is a type of
cotransport. Many amino acids enter the intestinal epithelial cells by cotransport with
sodium ions; this results due to the sodium gradient that exists due to a
Sodium-Potassium pump.
