Photosynthesis is a way that nature observes the first law of thermodynamics.

As we all learn in school, the sun is the primary source of energy on Earth, but only a fraction of Earth’s residents can tap into that energy directly. The rest of us, the heterotrophs (from hetero- other and troph – food), get our energy indirectly. We either eat the plants (or other organisms) that produce their own food, or we eat the things that somewhere down the line got their energy from eating autotrophs (from auto- self).

But, because the first law of thermodynamics states that energy cannot be created or destroyed, but only converted from one form to another, these autotrophs could not make their food from nothing. Instead, they converted (solar) energy from the sun into chemical energy via photosynthesis.

Solar energy, which comes to Earth as photons, has characteristics of both particles and waves (as it turns out everything does). These waves have energy that is inversely proportional to the wavelength of the light- shorter wavelengths transfer more energy than longer ones. I like to think of it this way: Shorter wavelengths mean more waves per unit time. If you were one the beach watching waves come in to shore, if more waves crash on the beach in an hour on Saturday than on Sunday, then more energy was transferred per hour from the ocean waves to the shore on Saturday.

Absorption spectrum of pigments

The visible light we can see only a small slice of the broader electromagnetic spectrum. Because we see only the light that bounces of things, if those things absorb some of that light (such as plants that use the light for photosynthesis), then we see only what they reflect back because it is not absorbed. This explains precisely why most leaves appear green – all but the green light is absorbed by pigment molecules that are collecting energy in the chloroplasts.

We can see this clearly by looking at an absorption spectrum of several pigments found in leaves.

What’s really interesting, is the beauty of flowers. These parts of the plant are not photosynthetic*, but they also contain pigment molecules. Why?

Of course we know this. Flowers are the reproductive organs of plants, and they often require assistance from insects or other animals for pollination. The way they attract pollinators is by giving a reward (nectar) and providing visual cues about where that reward can be found (the colorful flower).

Visual spectrum comparison

But, it turns out that bees (a common pollinator) don’t see the same visual spectrum as we humans do. Instead, their spectrum is shifted slightly in the ultraviolet direction.

Naturally, this would have consequences. If bees can see UV light, it would be reasonable to expect that some flowers use pigments that make them visible at UV wavelengths. In fact, this is exactly what we see – well, what we would see if we could see UV. Here’s a representative flower shown as we see it and as a bee may see it – with a UV colored landing area right where the pollen and nectar are found.

Natural Light / UV Light

*At least I think they aren’t. If anyone can provide an example of flower petals that photosynthesize, that would be greatly appreciated.