Soakin' Up the Sun

Photosynthesis in plants is an incredible process. Like solar cells, they get their energy directly from the Sun.

Humans don’t photosynthesize the way plants do, but we do make good use of the Sun’s energy anyway. One of the most direct ways we do this is with vitamin D.

Vitamin D is unique. Sunlight reacts directly with a type of cholesterol we have in our skin, triggering a chemical process that produces vitamins D2 and D3 inside our bodies.

We make it inside our bodies, and the Sun’s light is a lot like an on switch. We need this switch to turn on so that our bodies can regulate the amount of calcium in our gut and in our bones. Vitamin D also plays a big role in our immune system, and it can even improve your mood.

Incidentally, dogs don’t process Vitamin D the same way we do (in spite of Dink-Dink’s insistence on being in the sunlight for much of the day). They get it from their diet instead, although their fur can synthesize a small amount.

So yeah, we can’t photosynthesize material in the way plants can, but we certainly all rely on the Sun’s energy in one way or another. In fact, virtually all of the energy living things use on Earth comes from the Sun—everything from our diet to our sources for renewable energy.

How?

Let’s start with the food we eat. That’s the main source of the energy we use every day, and we measure the energy of this food in terms of the calorie. We’re talking about how much energy the food contains when we talk about how many calories there are.

Why should food have energy, though? Sure, mass and energy are equivalent, but we’re not like going to use nuclear fission to unlock this potential power inside our bodies… so what gives?

It all starts with the Sun.

After trillions of trillions of photons travel millions of miles to us after making their way out from the inside of the Sun, they land all over the surface of our planet. Spread out all over this surface are, of course, tons and tons of plants.

Lots of these energetic particles from the Sun land on these plants. As the chlorophyll molecules absorb this energy, the electrons inside these molecules becomes energized (“excited” in physics jargon). Part of this energy is used to split water molecules into oxygen and hydrogen.

Incidentally, this is why we can breathe the air: oxygen is released into the atmosphere as a byproduct, while the hydrogen is what plants crave.

The remaining energy from the electrons then make use of the hydrogen, creating glucose. The plant uses glucose for energy, and of course, so do we.

Vegan humans eat only plants and plant byproducts as a rule, and omnivores add animals and animal byproducts to their diet. Of course, all those animals initially got their energy by eating plants, and plants got their energy from photons from the Sun.

You might be thinking: all right, I can see your point about the food we eat, but you mentioned that “virtually all of the energy on the planet living things use on Earth comes from the Sun.” But what about the power we use to heat our homes, or to turn sand into glass, or literally anything else besides our bodies themselves?

It turns out that the Sun is responsible for all of this, too. You’ve probably already figured out that fossil fuels are already accounted for; after all, we already get the idea that every living thing gets its energy from the Sun. So, any time we burn coal or oil, we’re unlocking all this potential energy.

Well sure, you’re thinking, but what about renewables?

Solar energy is the most obvious and direct example, but there are two other renewables we use much more than solar right now. Hydropower, like the falling water that powers Hoover Dam, seems completely unrelated to the Sun, but… well hey, I probably wouldn’t be writing about this if it was just that simple.

The Sun’s energy ultimately drives the water cycle: it causes water to evaporate and then flow into rivers, giving us the chance to capture the gravitational potential energy as the water falls. Wind, too, is caused by the uneven heating of the Earth by the Sun, which ultimately creates those air currents we can capture with wind turbines.

It’s certainly fair to say that we have sources of energy here on Earth that aren’t directly from the Sun, too, although these sources are much, much smaller today than the other category.

Geothermal energy has pretty much nothing to do with Sunlight—we tap straight into radioactive decay and the heat from the planet itself. Similarly, nuclear energy (fission today, but maybe eventually controlled fusion as well) doesn’t have anything to do with the Sun.

So, there are some very small exceptions right now, but the big picture is this: the overwhelming majority of all of the energy on Earth, both in our bodies and in our industries, comes from our Sun. The Sun's dominance in our energy landscape is undeniable.

The Sun is like the king of planets, and it occurs to me that many of you might not get that reference. Let’s see if we can’t fix that today: