Lizards and snakes cannot breathe while chewing food because the connection between their external nostrils and their esophagus is in the mouth. In contrast, mammals can continue breathing through the nostrils while chewing food in the mouth. However, choking sometimes occurs when the paths of air and food cross each other. Thinking about the high oxygen demand of active endotherms, explain how the concept of descent with modification explains this “imperfect” anatomy of some amniotes (see Figure 34.2 to review vertebrate phylogeny).

If you put the following events in the order they occur in the human digestive system, which would be the third event in the series?
(A) Cells in gastric pits secrete protons.
(B) Pepsin activates pepsinogen.
(C ) HCl activates pepsinogen.
(D) Partially digested food enters the small intestine.

Describe the results observed for pairing (d). Note how these results differ from those for pairing (b). Suggest a hypothesis to explain this difference. How could you test your hypothesis using the kinds of mice in this study?

In which digestive system organ does nearly all nutrient absorption occur?

(A) stomach

(B) small intestine

(C) large intestine

(D) pancreas

Which of the following is not a major activity of the stomach?

a. mechanical digestion

b. $\mathrm{HCl}$ production

c. nutrient absorption

d. enzyme secretion

Compare the results observed for pairing (c ) to those observed for pairing (b). Based on these results, does the ob$^+$ gene product appear to promote or suppress appetite? Explain your answer.

$\text{\blue{WHAT IF?}}$ If a zoo animal eating ample food shows signs of malnutrition, how might a researcher determine which nutrient is lacking in its diet?

$\text{\blue{WHAT IF?}}$ Thinking in broad terms, what similarities can you identify between digestion in an animal body and the breakdown of gasoline in an automobile engine? (You don’t have to know about auto mechanics.)

In which organ does almost all enzymatic digestion of food occur?

(A) stomach

(B) small intestine

(C) large intestine

(D) pancreas

Which of the following organs is incorrectly paired with its function?

a. stomach-protein digestion

b. large intestine-bile production

c. small intestine-nutrient absorption

d. pancreas-enzyme production

$\text{\blue{WHAT IF?}}$ People who have lactose intolerance have a shortage of lactase, the enzyme that breaks down lactose in milk. As a result, they sometimes develop cramps, bloating, or diarrhea after consuming dairy products. Suppose such a person ate yogurt that contains bacteria that produce lactase. Why would eating yogurt likely provide at best only temporary relief of the symptoms?

Compare the results observed for pairing (a) and pairing (b) in terms of phenotype. If the results had been identified for these two pairings, what would that outcome have implied about the experimental design?

An insulinoma is a cancerous mass of pancreatic beta cells that secrete insulin but do not respond to feedback mechanisms. How you would expect an insulinoma to affect blood glucose levels and liver activity?

Owls, which have a varied diet, periodically cough up pellets composed of indigestible material, such as bones, feathers, and fur.

In which digestive compartment do you predict owl pellets form? Explain.