Lactose and Lactase

Lactose and Lactase

Got Milk?

Although we can easily digest milk as babies, most of us lose this ability as adults. This is because we usually stop producing lactase, an enzyme that breaks down the main sugar in milk. However, about one-third of people worldwide still produce lactase as adults. 

Poll

PREDICTION: How do adults lose the ability to digest milk?

• Individuals were born without the gene to digest milk

• Adults lose the gene for producing lactase

• Adults stop using the gene for producing lactase

• The enzyme lactase wears off as we age.

Background

Milk is packed with proteins, fats, and carbohydrates that support the growth, development, and survival of baby mammals. The main carbohydrate in milk is a sugar called lactose. To digest milk, lactose must be cleaved, or broken down, by lactase, an enzyme produced in the small intestine. Lactase cleaves lactose into two smaller sugars, glucose and galactose, which are easily absorbed through the walls of the small intestine. Once these sugars are absorbed into the bloodstream, they can be delivered to the cells of the body and used for energy.

Multiple Choice

Which molecule listed is most likely an enzyme?

• lactose

• sucrose

• lactase

• galactose

Multiple Choice

Which molecule is most likely a carbohydrate?

• amylose

• amylase

• protease

• lipase

Open Ended

Why does the disaccharide lactose need to be broken down before it can be absorbed?

aka: Lactose Intolerant

As baby mammals grow up and stop drinking their mother’s milk, their bodies usually stop producing the lactase enzyme (presumably because it is a waste of energy to produce). Individuals that do not produce lactase as adults are called lactase nonpersistent. Most mammals are lactase nonpersistent and do not drink milk as adults. Humans are unusual in that some adults continue to drink milk from other mammals, such as cows.  

Multiple Choice

Why would natural selection favor lactose non-persistence?

• not making lactase saves energy

• not raising cows saves energy

• humans are natural herbivores

When an individual who is lactase nonpersistent drinks milk, they cannot easily break down the lactose in the milk. The lactose passes from their small intestine to their large intestine, where it is fermented by bacteria. Fermentation produces various gases in the large intestine, which can cause abdominal pain, bloating, flatulence, and diarrhea. Most adults are lactase nonpersistent and thus typically lactose intolerant (although some may not know it because their symptoms are mild). However, about 35% of the global human population continues to produce lactase into adulthood. These individuals are called lactase persistent and are typically lactose tolerant, meaning that they can digest lactose easily and drink milk without problems. 

Multiple Choice

What is fermentation?

• anaerobic respiration

• aerobic respiration

Multiple Choice

Which is considered "normal" for mammals?

• lactase persistent

• lactase non-persistent

Testing for Tolerance

There are a few ways to test of lactase persistence. One way is by testing the level of glucose in the blood stream of an individual 0, 15, 45 and 60 min after drinking milk. The results from 7 individuals are show in the graph.

Multiple Choice

If an individual that is lactase persistent drinks milk, the level of glucose in their blood will...

• increase

• decrease

• stay the same

Multiple Choice

If an individual that is lactase non-persistent drinks milk, the level of glucose in their blood will...

• increase

• decrease

• stay the same

Multiple Select

Check all the individuals that are likely lactase persistent. (Click the graph to enlarge)

• a

• b

• c

• d

• e

Multiple Choice

Remember: Making lactase as an adult is normally a waste of energy. Under what conditions would Natural Selection favor lactase persistence?

• very warm environments with plenty of fruit available.

• Warm environments with clean water available

• Areas experiencing drought with a herding culture

• Cold area with a culture of farming.

Poll

I have not DIRECTLY said this, but I want to know your ideas: How do adults lose the ability to digest milk?

• Individuals were born without the gene to digest milk

• Adults lose the gene for producing lactase

• Adults stop using the gene for producing lactase

• The enzyme lactase wears off as we age.