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

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.

Thinking about our nutritional needs and feeding behavior, propose an evolutionary explanation for why amylase, unlike other digestive enzymes, is secreted into the mouth.

Suppose you were studying two groups of obese people with genetic abnormalities in the leptin pathway. In one group, the leptin levels are abnormally high; in the other group, they are abnormally low. How would each group's leptin levels change if they ate a low-calorie diet for an extended period? Explain.

$\text{\blue{MAKE CONNECTIONS}}$ Considering how enzymes function (see Concept $8.4$), explain why vitamins are required in very small amounts.

What features of a mammal’s digestive system make it an attractive habitat for mutualistic microorganisms?

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?

Suppose you were studying two groups of people who are obese and have genetic abnormalities in the leptin pathway. In one group, the leptin levels are abnormally high; in the other group, they are abnormally low. How would each group’s leptin levels change if they ate a low-calorie diet for an extended period? Explain.

The mammalian trachea and esophagus both connect to the ( A ) pharynx. ( B ) stomach. ( C ) large intestine. ( D ) rectum.

In what sense are nutrients from a recently ingested meal not really “inside” your body prior to the absorption stage of food processing?

Hummingbirds are well adapted to obtain sugary nectar from flowers, but they use some of the energy obtained from nectar when they forage for insects and spiders. Explain why this foraging is necessary.

The acids in bile salts have both lipid-soluble (hydrophobic) and water-soluble (hydrophilic) surfaces. How is such an organization beneficial for the role of bile salts in digestion? A barley mutant lacking a gibberellic acid receptor would

(A) fail to make GA.

(B) catalyze starch more quickly.

(C) fail to make alpha-amylase.

(D) fail to take up water.

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.

$\text{\blue{WRITE ABOUT A THEME: ORGANIZATION}}$ Hair is largely made up of the protein keratin. In a short essay (100-150 words), explain why a shampoo containing protein cannot replace the protein in damaged hair.