Study Guide

Biomolecules and the Chemistry of Life In the Real World

  • Health

    Lactose Intolerance in Human Populations

    Who doesn’t love a scoop of ice cream on a hot day, a cheesy slice of pizza, or a tall, refreshing glass of milk with some cookies? If you are like most people, milk products are some of your favorite foods, and yet the milk sugar, lactose—a disaccharide made from glucose and galactose—can cause serious digestive issues for a lot of the world’s population. Bloating, cramps, and diarrhea….those "Got Milk?" commercials definitely made a smart move leaving those symptoms out of their marketing campaign. Got Bloating?

    What exactly is lactose intolerance, and what causes it? First of all, it’s important to realize that your small intestine, which absorbs most of your nutrients, cannot readily absorb lactose. Ideally, your intestinal walls should house an enzyme called lactase that breaks down the lactose into glucose and galactose. These monosaccharides can then be absorbed into your bloodstream. Most mammals, including people, have plenty of lactase when they are little babies. This makes sense because mammals initially rely on milk from their mothers. As baby mammals get older, they produce less and less lactase, until, in adulthood, they can no longer digest lactose. These folks can eat all the ice cream, butter, yogurt, cheese, and triple chocolate pudding they want, and they digest it fine. How’d they get so lucky? Why do they have this capability, when other populations don’t?

    It seems that lactose tolerance probably evolved relatively recently – within the last 10,000 years, after humans began to keep domestic animals for agriculture. Archaeological evidence suggests that the first Europeans to store milk lived in Central Europe about 7,500 years ago, and they may have traveled from there to other parts of Europe. A similar genetic change may have arisen completely independently in African and Asian populations that are also lactose-tolerant. We don’t really know the reason why lactose tolerance evolved, but there are lots of possible reasons why this trait may have been beneficial to ancient people, and therefore, favored by natural selection (read: the process by which beneficial characteristics are selected to pass from one generation to the next).

    Suddenly, evolving the ability to digest milk would provide a huge survival advantage – people would have a new source of calories, protein, calcium, and vitamin D. Milk would be a fresh beverage for them to drink if water were scarce or contaminated. Milk would also be a pretty steady and predictable source of nutrition and energy5. What's more, people would have milk moustaches, and we all know how cool those look.

    As for why some populations do not show this ability, one explanation is that, in areas where people could not keep dairy animals for one reason or another, lactose tolerance did not provide a particular survival advantage. If there was no advantage to digesting lactose, the trait might not have become as widespread as it did in the populations where having lactose tolerance was extremely beneficial4.

    If you are one of those carefree people who can chow down on a hot fudge sundae or blow milk bubbles with a straw (come on, admit it – it’s a little more than awesome), just remember to thank your lucky lactase for helping you out with the digestive process.

  • Pop Culture

    Jurassic Park – Science Fiction or Biological Reality?

    We can answer this one right away: Jurassic Park, for all its special effects, suspenseful moments, and heart-stopping dinosaur chases through the forest, had some major scientific flaws. For one, T. rex lived in the late Cretaceous period—almost 80 million years after the Jurassic ended. Oops. Perhaps more importantly, the whole premise of the movie—that dinosaurs can be cloned from DNA extracted from mosquitoes preserved in amber—does not hold any water. For one, DNA is pretty fragile and degrades quickly once an organism dies. Even if there were enough surviving DNA to look at, it would probably be all mixed with the mosquito’s DNA. On that note, can you imagine what a dinosaur-mosquito hybrid would look like? One thing is for sure: it would have a wicked bite.

    Additionally, the DNA would be so fragmentary and damaged that it would be impossible to put the pieces back together correctly. The movie dealt with this problem by suggesting that frog DNA could fill in the gaps, but frogs are completely different animals. Putting random pieces of their genetic code into dino DNA would not work. It would be like taking isolated sentences from The Sound and the Fury, filling in the gaps with sentences from Twilight, and then expecting a coherent story to emerge. Not gonna happen.

    OK, so Hollywood’s scientific accuracy is…umm…questionable (the understatement of the century right there). We may not have any dino DNA, but as it turns out, a team of researchers at North Carolina State University, led by Dr. Mary Schweitzer, recently discovered dinosaur collagen, a protein common in animal muscle tissue, preserved in fossilized bone! Normally, when an animal dies, its body decays. Obvious, right? However, in very rare circumstances, an organism may become fossilized, in which case the original organic material in the body gradually is replaced by minerals. Dinosaurs lived so long ago that scientists always figured the mineralization process would be complete by the time we dug them up, but now we know that the original protein can still be intact after all this time.

    If we are starting to find dinosaur collagen in well-preserved fossils, does this mean that Jurassic Park will be a reality in the future? Probably not. Apart from the ethical and practical limitations on bringing dinosaurs back from extinction, it is still scientifically impossible to do this. Collagen is just one of thousands of proteins in the body. It’s an interesting and important piece of the dinosaur biology puzzle, but certainly not enough to build a whole amusement, or terror, park.

  • Environment

    Water’s Special Properties + Pollution = Trouble

    We know water has loads of unique traits that make it a critical substance for the survival of life on Earth. However, when people put mass quantities of pollutants into the environment, water’s ability to dissolve these pollutants can actually make the damage to living organisms and the environment worse.

    Wet acid deposition, such as acid rain, snow, sleet, or fog, is a good example of this problem. Acid precipitation forms when sulfur dioxide, SO2, and nitrogen oxides, NO and NO2, in the air dissolve in water and form acids. How do they get into the air in the first place? Sulfur naturally occurs in small amounts in fossil fuels, and when they are burned, it combines with oxygen in the air to form SO2. SO2 then undergoes some additional chemical reactions, forming SO3, which dissolves in water and forms sulfuric acid, H2SO4. Youch.

    As for nitrogen oxides, nitrogen constitutes 78% of the gas in the atmosphere. During the process of combustion, like the burning of fossil fuels, nitrogen in the air combines with oxygen to form either NO or NO2. These molecules dissolve in water to form nitric and nitrous acids, HNO3 and HNO2, respectively. The result is that water droplets in the air are acidified, and when they fall to the ground as precipitation, they cause streams, lakes, marshes, and soils to become acidified as well. Sounds like bad news, and it is.

    Most surface—as opposed to underground—bodies of water normally have a pH of between 6 and 8, which is pretty close to neutral. With acid deposition, however, the pH can decrease to much lower values. Little Echo Pond in Franklin, New York, has a pH of 4.2.

    Taking calcium away from corals means that they will not be as healthy and will not grow. Coral reefs are some of the most diverse ecosystems on the planet, and losing them would be a major blow to biodiversity, or the diversity of life. In addition, reefs help protect shorelines from violent storms. If reefs were to suffer substantial damage, it could have significant economic implications for humans.

    We mentioned before that water’s ability to dissolve molecules is one of its special properties and one of the qualities that makes it critically important to life. In this case, the fact that it’s such a good solvent—of sulfur dioxide, nitrogen oxides, and carbon dioxide—leads to the formation of acid deposition and decalcification of corals. Too bad water can’t be a tad more selective in what it fraternizes with!