$\text{\blue{WHAT IF?}}$ 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?

Phosphorus has the atomic number 15 and an atomic mass of 31 amu. How many protons, neutrons, and electrons are in a neutral phosphorus atom?

When a wet dog shakes itself, people standing nearby tend to get wet. Why does the water fly outward from the dog in this way?

Compare and contrast eukaryotes and prokaryotes.

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

$\text{\blue{WHAT IF?}}$ Predict what would happen if you mixed gastric juice with crushed food in a test tube.

$\text{\blue{SCIENTIFIC INQUIRY}}$ In human populations of northern European origin, the disorder called hemochromatosis causes excess iron uptake from food and affects one in 200 adults. Among individuals 15 to 50 years old, men are ten times as likely as women to suffer from iron overload. Taking into account this range of ages, propose a hypothesis that explains this difference.

$\text{\blue{WHAT IF?}}$ 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.)

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?