Where Do All Those Leaves Come From?!



I wanted to link my leaf raking drudgery to some scene

in a famous movie. No go. Raking leaves is so mundane

that I could only find one movie that showed someone

raking leaves – Disney’s The Odd Life of Timothy Green.

And heck, he sprang up from a garden, all those leaves

are his cousins!

It’s Fall if you hadn’t noticed. Apple cider, football, pumpkin, wonderful colors….. and raking leaves. I spent last weekend with a rake in my hand and hate in my heart.

Maybe that’s a little strong. But when you spend time trying to put the little beggars in piles and then have to watch the wind scatter them and blow more leaves off the trees – well, you know it’s exasperating.

My efforts resulted in seven 50 gallon bags stuffed with botanical death. All told, more than 400 pounds of biomass. I stare up at the bare branches and wonder, “where did all that matter come from?”

Matter is made of atoms, so the leaves in the trash bags represent literally trillions upon trillions of atoms joined together in specific molecules. So the question is really, what was the source(s) of those particular atoms?

The tree made the leaves, but it didn’t get smaller due to its effort to produce leaves, so the material in leaves didn’t come from the mass of the tree. They were certainly organized by the tree into those biomolecules (proteins, carbohydrates, DNA and RNA, and fats) that make up the leaves, but they didn’t come from the tree originally.

The tree has roots in the soil and pulls water and nutrients into its trunk via those roots. Could the soil be where all the mass comes from? Consider this. If you have a potted plant, do you have to add soil to it every year? Nope.

Jan Baptiste van Helmont was a successful scientist, even

though it may not look like it at first. He was wrong about

digestion’s stages, but did include a description of

something that sounds a lot like the enzymes we have.

He said that trees didn’t get there mass from the soil, but

said they did get it from water – wrong. However, he said

air was involved and proposed that air was made of gas –

a word he coined.

Six hundred years ago, a scientist named Jan Baptist von Helmont measured this more carefully. He grew a tree in 200 pounds of soil from a seed until it weighed 190 pounds. Then he weighed the soil again. He found all his original dirt except for 2 ounces. I very much doubt that 190 pounds of tree mass came from two ounces of soil.

Everyone knows that trees need water. During a drought, your trees die and your grass turns brown. If water is that crucial, maybe my 400 hundred pounds of dead leaves came from water.

Water is made up of hydrogen and oxygen (H₂O).  Can all the leaves be made from just water? No way; trees (and every other living thing on Earth) are carbon based. The proteins sugars, lipids, and nucleic acids are all built on a backbone of carbon atoms, with oxygen, hydrogen, and nitrogen atoms in some specific spots.

Carbon, hydrogen, oxygen, and nitrogen are the main elements of life. You can’t change one atom into another (with the exception of radioactive elements) so all the mass in my bags of leaves couldn’t have come from just water. Certainly some of the mass came from water, after all, living things are mostly water, but there’s no way the carbon came from water.

When a tree uses sugars to make energy, some water is produced. This is called metabolic water, and we do it too. You can prove this by breathing on a mirror. See that condensation? Much of that is metabolic water. Metabolic water is important in producing the molecules of life, so important that some animals, like the kangaroo rat, can live only on metabolic water, they never drink!

Kangaroo rats can live their lives without drinking even

though they live in Death Valley. Another fun fact – they

have fur-lined cheek pouches for carrying seeds back to

their den. No drinking and fur in their oral cavity – worst

case of dry mouth ever!

So a bit comes from water, even less from soil, and none of the leaf mass springs from the tree itself. What do we have left? Only one choice comes to my mind - and your breathing it right now. 

Air? Really? How could 400 hundred pounds of leaves come from the air? It seems silly, but it’s the basis of life on Earth. Air is 78% nitrogen (N₂), 21% oxygen (O₂), and 0.00397% carbon dioxide (CO₂) – plus some water vapor. This is almost everything a growing tree needs, except for some trace minerals that can be found in that 2 ounces of disappeared soil that von Helmont measured 600 years ago. 

Basically, those things we humans breathe out (CO₂, H₂O, O₂, and N₂) are exactly what plants need to grow. We actually use only a small portion of the oxygen that we breathe in, but we do add some carbon dioxide to the air when we exhale. This is a big reason why talking to your plants makes them grow better; you're increasing the concentration of carbon dioxide in their immediate vicinity.

Plants take carbon dioxide from the air, and using the energy of sunlight, turn it into carbohydrates – this is photosynthesis. But they don’t just use the carbs for energy. The sugars are the carbon basis for synthesizing every biomolecule the plant will need in order to build leaves and wood.

We have covered carbon (from CO₂), hydrogen (from rain and metabolic water), oxygen (from carbon dioxide and metabolic water), but what about the nitrogen in DNA and proteins? Believe it or not, that comes from the air too.

N₂ in the air is hard to tear apart so the nitrogen can be used in building biomolecules. Plants can “fix” carbon themselves, turning gaseous carbon to solid carbon during photosynthesis, but they can’t fix nitrogen. To turn nitrogen into a form they can use, plants rely on nitrogen fixing bacteria in the soil.

Many plants form a symbiotic relationship with nitrogen fixing bacteria, letting them live inside nodules of their roots. Therefore, the nitrogen the trees use comes from the air, even if it passes through the bacteria first.

The top image is from art student Melchiorri. He claims

that the silk mesh stabilizes the chloroplasts and lets them

produce oxygen via photosynthesis even though they aren’t

in a cell. There are problems here, like chloroplasts don’t live

very long. The bottom image is of an artificial leaf that can

produce hydrogen and oxygen gases from water in the

presence of light. Leaves use 1% of available light; this “leaf”

already uses 7x as much of the light.

Most trees don’t harbor nitrogen-fixing bacteria in their roots, they rely on the nitrogen that the bacteria spread around and leave in the soil, or that nitrogen that comes from dead plant material. One exception is the black locust tree. It is estimated that a black locust stand of trees (and their bacteria) can add 40-60 kg (88 –133 lb.s) of nitrogen to the soil every year.

Now I can rake my leaves confident in the knowledge that I'm raking up a true miracle. They can’t weigh too much, they’re basically nothing but air, water vapor and a bit of bacterial waste. So why does my back hurt so much?

Soon, we may have artificial leaves to deal with. One recent project by art school graduate Julian Melchiorri has used chloroplasts embedded in silk protein mesh in order to carry out photosynthesis. These can be used in space, where plants have a harder time growing in zero gravity. This might be our source of oxygen on the way to Mars.

In addition, new research describes a synthetic leaf made form metal films on either side of a silicon mesh can be used to split water as occurs in photosynthesis. These synthetic leaves might be helpful in producing hydrogen and oxygen for use in fuel cells. I just hope we don’t have to rake them up.



Contributed by Mark E. Lasbury, MS, MSEd, PhD
As Many Exceptions As Rules

Pijpers, J., Winkler, M., Surendranath, Y., Buonassisi, T., & Nocera, D. (2011). Light-induced water oxidation at silicon electrodes functionalized with a cobalt oxygen-evolving catalyst Proceedings of the National Academy of Sciences, 108 (25), 10056-10061 DOI: 10.1073/pnas.1106545108