Life Needs Matter to Move

Reading time ~6 min · Bring your three answers to Block 2

Energy and matter behave completely differently on this planet, and almost every misunderstanding about how Earth works starts by mixing them up.

Energy passes through. Sunlight arrives, plants capture a little of it, it moves up through whatever eats what, and it leaves again as heat radiating back to space. It is a one-way trip. Turn off the Sun and the flow stops.

Matter does not pass through. It arrives essentially once and then stays. Dust and meteorite fall on Earth every day, and estimates of how much vary a great deal depending on how you measure it. Whatever the true figure, it is nothing at all against the mass of the planet. For the purposes of life, Earth is a closed box. No meaningful delivery of new material is coming.

Now put those two facts together and something uncomfortable follows. Life is built out of a short list of elements: carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. Every organism that has ever lived was assembled from that list, and the supply has not been topped up in four billion years.

So the atoms have to be used again. And again. The carbon in the breath you just took has been in a limestone cliff, in seawater, in something's body. The nutrients that make you were all secondhand. There was no alternative.

What a cycle actually is

The route an element takes as it moves around the planet and back is called a biogeochemical cycle. It is an ugly word built out of three honest ones: bio for the living part, geo for the rock, chemical for what happens in between.

Every cycle has the same two pieces. Matter sits still in a reservoir, which is anywhere an element accumulates and waits: the ocean, the atmosphere, a forest, a layer of rock. And processes move matter from one reservoir to another. That is all a cycle is. Places where matter rests, and routes between them.

Five cycles carry most of the traffic, and you will meet all of them this year.

Water is the courier. It carries everything else, which is why it turns up inside every other cycle. Unit 5 takes it apart properly.

Carbon is the frame that living things are built on, and it moves between air, ocean, life and rock. Unit 4 does the full accounting, and it is the cycle we have disturbed most.

Nitrogen is in your proteins and your DNA. The atmosphere is nearly four-fifths nitrogen and almost none of it is usable, because the molecule is bound too tightly for most life to break. Bacteria do the breaking. Almost every nitrogen atom in your body passed through one.

Phosphorus is the odd one out. It has no gas phase worth mentioning, so it cannot travel by air. It moves by water and rock only, which makes it slow, local, and the first thing an ecosystem tends to run short of.

Sulfur holds proteins in the shapes that make them work, and moves through rock, water and air.

The part that matters for this unit

Here is the question worth carrying into class. What keeps the loops closed?

Because matter does not return on its own. Left alone, it settles. Things sink, get buried, get compressed into rock, and stay there. A cycle only stays a cycle if something keeps hauling material back up where life can reach it.

On Earth, four things do that work. Volcanoes return buried gas and mineral to the surface. Plate tectonics drags old seafloor down and pushes new rock up, recycling the crust itself over millions of years. Liquid water dissolves, carries and redeposits. And life does an enormous share of it, because decomposition is a return route as much as decay is an ending.

Notice what all four have in common. They need a planet that is still doing something. A hot interior. A crust that moves. Water that runs. Take those away and the loops open into one-way streets, and matter ends up locked in places it cannot come back from.

Which brings you to Mars, one block early. Mars has carbon. It has nitrogen, phosphorus, sulfur, oxygen, hydrogen. It has the whole list. What it does not have is anything still turning the crank.

So when you write your habitability framework in Block 7, you have a criterion available that is not on the suggested list: does this planet have a mechanism to put matter back into circulation? A planet can hold every ingredient life needs and still be a dead end, if nothing returns them.

Sources

• Cycles, reservoirs, and the six elements: OpenStax, Concepts of Biology, §20.2 "Biogeochemical Cycles" [open-licensed university textbook]
• Return mechanisms and the geosphere's role: NASA Earth Observatory, "The Carbon Cycle" (slow-cycle section), earthobservatory.nasa.gov [government science agency]

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