The vertebrate gastrointestinal system is a tubular gut that is adapted to ingest food, fragment it, digest it, and absorb nutrients. Peristalsis moves food through the gut. Segmentation movements mix the gut contents. Enzymatic digestion followed by absorption of nutrients occur mostly in the small intestine; water and ions are absorbed in the large intestine.

Question 1

The tongue pushes the chewed food to the back of the mouth stimulating the swallowing reflex that through activation of many muscles pushes the food into the esophagus. Stretching of the smooth muscle in the esophagus stimulates contraction of that smooth muscle pushing the food toward the stomach. The directionality of this movement is facilitated by the enteric nervous system causing the smooth muscle ahead of the bolus of food to relax. When the bolus of food reaches the esophageal sphincter, that anticipatory wave of relaxation opens the sphincter allowing the food to enter the stomach. The coordination of smooth muscle contraction and relaxation creates the waves of peristalsis that generally moves the food in the esophagus towards the stomach even in the absence of gravity.

Question 2

Stomach acid is produced by chief cells in the gastric pits of the stomach. In these cells, carbonic anhydrase catalyzes the hydration of CO₂ to produce H₂CO₃, which dissociates into HCO₃ and H⁺. The H⁺ is transported across the pit lumen side of the cell in exchange for K⁺. The HCO₃ is exchanged across the opposite end of the cell into the interstitial fluid in exchange for Cl^– ions. The excess K⁺ in the cell leaks out the luminal end and is pumped back in via the H⁺/K⁺ exchanger. Thus the concentration of H⁺ in the stomach lumen and the separation of HCO₃^– in the interstitial fluid depend on the anatomical integrity of the stomach wall. If that integrity is destroyed, the H⁺ cannot be separated from the HCO₃^- and there will be no pH change.

Question 3

Bile emulsifies fats in the diet, creating tiny micelles that present a large surface area for the action of water-soluble lipases. Bile prevents the fatty micelles from coalescing into larger fat globules with a smaller surface area. Equivalent molecules are not required in the lymph and blood because the fats in lymph and blood are incorporated into lipoproteins that are coated with water-soluble proteins.

Question 4

The gut microbiota is a significant source of nutrition for ruminants because their microbiota grow on the ingested food in the rumen and reticulum before the semi-digested food is exposed to the HCl^– and digestive enzymes of the stomach and small intestine. In humans, abundant microbiota are present in the small and large intestine and therefore are not killed by the stomach acid and digested in the upper region of the small intestine, where they could otherwise serve as a major source of nutrition.

Question 5

The ruminant does not produce the cellulose hydrolyzing enzymes necessary to digest the plant materials it eats. The rumen and reticulum have cultures of microorganisms that produce cellulases and break down the plant matter. The resulting fermenting mixture of plant matter and microorganisms moves into the omasum, where water is reabsorbed. From the omasum, the mass of semi-digested plant matter and the associated microorganisms move to the abomasum (true stomach) that secretes HCl, which aids digestion and kills the microorganisms that are an important component of the animal’s nutrition.

Question 6

If even a small amount of pepsinogen is activated by hydrolytic cleavage to produce the active enzyme pepsin, that pepsin will act on additional pepsinogen to release more pepsin molecules creating an autocatalytic cascade. This is an example of positive feedback in which a product of a reaction (pepsin) stimulates still more reaction (pepsinogen→ pepsin), which amplifies formation of the product of that reaction (pepsin).