This post is the third in a three-part series on how living creatures use the elements of the periodic table. Read earlier posts here and here, and learn more about the elements on NOVA's two-hour special, "Hunting The Elements."

Calcium (Ca) is all around us and even within us: from rocks to shells, pearls, antacids, bones, teeth (in the form of a mineral appropriately named apatite), nerves, and our beating hearts. Simply reading and thinking about this post requires the shuttling of calcium ions through special calcium-ion channels in our bodies for cardiac muscle contractions and the release of neurotransmitters.

Yet one organism plays a large role in removing calcium from watery environments and trapping it in the form of calcite, which forms rocks. This organism is called a coccolithophore, a photosynthesizing single-celled marine plant.

For the past 230 million years, coccolithophores have been protecting themselves with calcium armor. This armor is made up of hubcap-shaped structures called coccoliths which are composed of calcite molecules, which contain one atom of calcium with one carbon and three oxygen. Each coccolith is just a fraction of a millimeter across, so coccolithophores combine dozens of them to create protective scales, as shown in the image below.

Coccolithus pelagicus. Credit: Richard Lampitt, Jeremy Young, The Natural History Museum, London. Via the Wikimedia Commons and planktonnet.

Over time, these scales flake off into the water, shedding as much as 1.5 million tons of calcite each year. This makes coccolithophores the largest calcite producers in the world's oceans.

Coccolithophores have a complex effect on the biosphere. In the short term, coccolithophores photosynthesize, taking in carbon dioxide from the atmosphere and emitting oxygen. When they create their coccoliths, they take carbon, oxygen, and calcium from the water, removing a carbon atom that could potentially become carbon dioxide. However, in the process of making their coccoliths, they also emit carbon dioxide.

A large-scale effect of coccolithophores is that the carbon used to create the coccoliths is trapped as calcite. When the scales fall off of the organism, the calcite sinks to the ocean floor, where it mixes with silt and clay to form chalk. Over time, the deposits of coccolithophores can accumulate and create geological wonders like the White Cliffs of Dover.

Credit: Remi Jouan, via the Wikimedia Commons

When coccolithophores find the right mixture of sunlight and nutrients, they quickly proliferate, leading to large "blooms." These blooms are so large and dramatic that they even temporarily change the color the local seawater. In this image, taken from space, the calcite scales turn the water brighter and more turquoise, causing it to reflect more sunlight.

A phytoplankton bloom in the Barents Sea. Credit: NASA image courtesy Jeff Schmaltz, MODIS Rapid Response Team at NASA GSFC.

Iron (Fe) is the most common element by mass on Earth. But it isn't just the stuff of pots, pans, and fences: It is also one of life's essential nutrients. The iron-containing hemoglobin in our red blood cells carries oxygen, and iron helps plants create chlorophyll. But when iron combines with oxygen to make the magnetic compound magnetite, it becomes a built-in compass for living creatures.

Magnetite has been found in the brains of termites, bees, fish, birds, dolphins and even humans. Creatures like bats, sea turtles, pigeons, and salmon are able to sense the planet's weak magnetic field, which helps guide them on their migrations. Even tiny bacteria (called magnetotactic bacteria) use the Earth's magnetic field to orient themselves.

Magnetotactic bacteria were first discovered in 1975 when a researcher noticed that his cells naturally kept moving north. Curious, he put a magnet near them and, viola, they aligned themselves with the magnet, just like in this video.

When a magnetotactic bacterium develops, it grabs three iron atoms and four oxygen atoms from its environment and uses them to make magnetite crystals. Once the bacterium has created enough magnetite crystals, it links them together in create a magnetosome. This turns the cell into a sensitive living compass. But why would a bacterium need a compass in first place?

Magnetic bacteria are very picky about where they live. They prefer to live in deep water, where there is little oxygen but plenty of the ions they need for their metabolism. Furthermore, there are two varieties of magnetotactic bacteria: one that points north and one that points south, corresponding to the hemisphere they live in. By following Earth's the magnetic field lines, they find their way to the deep water in which they thrive.

The magnetosome automatically directs the movement of the bacteria, even after the bacteria are dead. When magnetotactic bacteria die, their denser-than-water magnetosomes cause them to sink to the seafloor, where they become embedded in marine sediments. Their fossils remain oriented with the magnetic field, leaving a historical record of the Earth's changing magnetic field.

This is just one of many ways that organisms transform the elements, literally bringing chemistry and physics to life. If only we had a large, relatively powerful magnet permanently within us--we'd probably never have to ask for directions again.

When you think of silicon (Si), you may think about Silicon Valley and the fact that modern computing would not be possible without this element in computer circuits. You may also think about how silicon is the second most abundant element in the Earth's crust, after oxygen. You even see it (or don't see it) every time you look through a piece of glass.

What you probably won't think about are diatoms: tiny photosynthesizing algae that are the primary movers of silicon in the world's oceans. Diatoms depend on silicon. They flock to locations where silicon is available. In the process, they generate enormous blue-green blooms.

A bird's eye view of diatom blooms in the ocean. Credit: Norman Kuring/NASA Ocean Color Group, via NASA Earth Observatory.

Every year, diatoms use almost seven trillion kilograms of silicon. They collect silicon in the form of silicic acid--that's silicon plus four hydrogen atoms and four oxygen atoms--and convert it into silica, the primary constituent of glass. What could algae possibly be doing with all of this amorphous, natural glass?

Diatoms use silica to create protective cell walls called frustules that are strong and chemically inert. Essentially, this is glass armor--cheap glass armor. It costs a diatom only two percent of its total energy budget needed for growth to make its shell.

Their beautifully geometric and symmetric shells are tiny marvels of natural art. Furthermore, each of the estimated 100,000 of species of diatoms has its own unique glass shell design. Diatoms are extremely efficient micro- and nano-architects. They consistently create the same 3D structure over and over again.

A collection of various diatom species. Credit: Wipeter, via the Wikimedia Commons.

The intricate armor protects the diatoms and also acts as a chamber that helps the diatoms photosynthesize by boosting their surface area, facilitating the exchange of gasses between the air, seawater, and organism. The silica in the glass also speeds up the conversion of ocean bicarbonate to carbon dioxide by changing the acidity of the water, which facilitates the exchange of protons in vital chemical reactions. The diatom then uses this carbon dioxide for photosynthesis. Thanks to silicon, diatoms have been able to carve out a unique biological niche in Earth's oceans.

With such efficient photosynthesis thanks to their miniature greenhouses (silica shells), diatoms produce about one quarter of the world's oxygen--almost as much as all tropical forests combined. This process also removes great amounts of carbon dioxide from the atmosphere, potentially helping combat global warming. When diatoms die, their heavy shells cause them to sink down to the depths of the ocean, taking that carbon with them. So, not only do they play a vital role in the global silicon cycle, they are also responsible for removing carbon dioxide and producing the oxygen that we breath.

Since they have been using silicon for more than 110 million years, diatoms are a vital part of Earth's biogeochemical silicon cycle. Diatoms are the world's way of moving silicon between rocks, water, and life. They are a form of life that changes the chemistry of the world's oceans.

In a beautiful cycle, the tiny glass masterpieces created by living diatoms sink to watery depths when the diatom dies. Many of the silica shells become part of the rock record when they fall to a seabed. Over time, these rocks dissolve and become orthosilicic acid, which is reused by new diatoms. These rocks also erode and the old silica shells end up on beachy shores in the form of sand. For 5,500 years, humans have been making glass from the same sand that all came from diatoms.

It's final exam season again, and for students across the country, the midnight oil is burning bright. Yet a question looms: It's the morning after an intense all-night cram session, and there's a little bit of time before that big test. Should you take a short nap? Or is it better to just stay up? I decided to ask Dr. Christopher Landrigan, Director of the Sleep and Patient Safety Program at Brigham and Women's Hospital.

For more with Dr. Landrigan on the science of sleep and sleep deprivation, check out the Q&A below.

NOVA: How did you get interested in studying sleep deprivation?

Landrigan: My interest in patient safety and sleep deprivation began in my own training as a resident at Children's Hospital. Fifteen years ago, there were no limits on how much doctors in training could work, and I often found myself at the tail end of a 36 hour shift, not performing my best. I got interested in this later. Was there an impact this might have on the safety of our patients?

NOVA: Why do we feel sleepy?

Landrigan: There are a number of different factors that drive human alertness and performance. The first is something called the sleep homeostat. It's a seesaw system: The longer you've been asleep, the greater the drive to wake up, and the longer you've been awake, the greater the drive to sleep. There's also the circadian system, a 24 hour biological clock that tends to drive maximum performance in the daytime, and drives maximum sleepiness in the early hours of the morning. If we stay up past the point where our bodies are telling us to go to sleep, then the sleep homeostat system and the circadian system both start working together to force you to go to sleep.

NOVA: What happens when you don't sleep?

Landrigan: We know that reaction time goes down, that your ability to put together complex puzzles in your mind, even to do simple math seems to deteriorate. It's not exactly understood how sleepiness contributes to degradation of cognitive function, but we know from imaging studies that the ability of the brain to metabolize glucose, to do what it's supposed to do, tends to decrease after you've been up too many hours in a row.

There have been a number of studies that have tried to look at the effects of sleep deprivation on performance and compare those to the effect that's induced by alcohol. It's been consistently shown that wakefulness of about 17-20 consecutive hours leads to performance decrements that are more or less equivalent to those induced by a BAC [blood alcohol content] of 0.05. And at the 24 hour mark, on average we will perform as if we had a BAC of 0.1, which is beyond the legal limit. With sleep inertia, sometimes the sleep decrements can be even greater than that 24 hour level.

NOVA: What's sleep inertia?

Landrigan: It's a slowness in the brain, an inability to react as quickly or to perform cognitive functions as well in the first few minutes after awakening. Sleep inertia is really the idea that the brain doesn't go from zero to 60 in six seconds. In fact it can take many minutes to even a couple hours for it to wear off.

If somebody is sleep deprived on a regular basis, there's a tendency to go into a deeper stage of sleep more quickly, and when woken from deeper stages of sleep, it appears that sleep inertia is most profound. So if you are sleep deprived and you're rapidly woken from sleep for whatever reason, the tendency to have sleep inertia where performance is really impaired for a long period of time is at its worst.

NOVA: What's so bad about not getting enough sleep?

Landrigan: We're only beginning to understand what the adverse consequences of this may be. From long term studies, it's pretty clear that not only do performance failures increase after sleep depriving ourselves, but there's a really a very substantially increased risk of safety problems--for drivers, for people in high risk occupations in risky industries, as well as long term health consequences of this sort of thing. We know that people who deprive themselves of sleep or work shift work on a regular basis are at higher risk of diabetes, metabolic syndrome, heart disease, hypertension, obesity, and so on for years to come.

To the extent that one can, thinking a little bit constructively about schedules and planning a little bit ahead to minimize all nights and minimize sleep deprivation. It's going to do you a world of good down the road.