Why is the Grass Green?
I was outside talking with Alemi last week and we were both startled to realize that the frozen white tundras of Ithaca had somehow transformed into fields of green. Apparently the snow was a temporary fixture that covered real live grass. Neato, gang! The joy at seeing green grass led quickly to surprise, confusion and then anger. Why the heck is grass green? Well, things look a color when they reflect back that color. So grass is green because its pigments (chlorophyll) absorb only a certain range of the visible spectrum, reflecting back the greenish bits. But if I know anything about approximating the sun as a blackbody, I know that it has a peak output of around 550 nm (i.e. green) light. So what’s going on? Why are plants blatantly rejecting the most abundant kind of light? Since my initial confusion rested on the assumption of the sun as a blackbody, I decided to take a closer look at the actual spectrum of the sun. Below is a graph showing the frequency dependence of solar radiation incident on the top of the atmosphere and at sea level. Since plants typically don’t live in space, we are most concerned with the sea level plot. From this plot, it looks like the incident radiation from the sun is fairly level beyond about 450nm or so. Just going on this graph alone, it looks like plants could just absorb reddish light and do alright for themselves. But do they? Let’s take a look at the absorption spectrum of chlorophyll. As it turns out there are a bunch of different “flavors” of chlorophyll (chlorophyll a, b, c, and d). As far as I could gather, only a and b are important (a is found in just about everything that plays the photosynthesis game and b is found in plants and green algae). So we need to find the absorption spectrum of chlorophyll a and b. After looking for a while at very qualitative drawings, I found this neato-toledo site, which actually gives real live data. Plotting the results, we get the figure below. Comparing with our handy dandy wavelength-to-color converter below we see that there is a big peak in absorption of both chlorophyll a and b in the dark blue and lesser peaks in the red. So how do these absorption lines correspond to the incident light at sea level given above. Well, its kind of tough to check by eye, but it looks like chlorophyll has its biggest peaks right below the plateau in the solar spectrum. Marking the wavelengths of the absorption peaks makes this clear (a = blue, b = green). It seems like plants are using a sub-optimal band of the spectrum! So how do we reconcile this? Well, let’s first start at The Beginning. The first photosynthetic organisms developed and spent the first billion or so years of their existence living and evolving in the oceans. The solar spectrum we have been using so far has been accurate only in air at sea level. Presumably it would be favorable for the organism to be able to survive at some finite depth in the ocean and not merely at the surface. Thus we must consider the effects of water on our incident solar radiation. A plot of the absorption spectrum of water is shown below (note the log-log scale). Lo and behold, the minimum absorption of visible light in water occurs towards the far end in the blue. And this is exactly where our biggest absorption peak in of chlorophyll is! Comparing our two graphs we see that the ratio of incident blue light to incident green light at see level is at worst about a third. But we see that for each meter traveled in water, green light is absorbed almost ten times more than blue light. Thus, an organism that lives a few meters underwater and wants to harness solar energy would probably do best to focus on that blue light. [WARNING: The next bit is speculative and I haven’t taken a bio class since high school] As much as I gather about chlorophyll from Wikipedia and other semi-reputable sites, chlorophyll a is found in just about anything that photosynthesizes, BUT chlorophyll b is only found in plants and green algae. And apparently, land plants are largely descended from green algae (which are aquatic, but typically around the surface and around the shoreline). Now take a look at the chlorophyll absorption spectra again. The chlorophyll b spectrum is sort of squished in more towards the middle (towards 550 nm maybe?) than chlorophyll a. In fact, on the graph where I have drawn lines on the solar spectra graph where the chlorophyll peaks are, we see that chlorophyll b just barely gets up to that plateau region. This suggests to me that chlorophyll a was working just fine for the early aquatic plants, but once they reached land and got out of all that water it became an advantage to utilize light closer to the peak solar output. Thus, plants that had chlorophyll b in addition to a had a slight advantage over their b-less brethren. Or so I shall continue to shamelessly speculate (and, apparently, alliterate). Anyway, I thought that was kind of cool, but if I have made some horrible error or mangled some biology, please let me know!Tweet