This week in class we discussed how pH can change the structure of chlorophyll, and by doing so alter the molecule’s properties. When the addition of acid removed some of the magnesium ions in the center of the chlorophyll molecules, the green beans we were observing became more yellowish. I decided to do a little research on how other changes to chlorophyll’s structure can change its properties, and how those changes affect the organisms where chlorophyll is found.
Chlorophyll, the pigment found in plants and other photosynthetic organisms, is green in color due to the pattern of light that it absorbs. The pigment absorbs light in the blue and red regions of the visible spectrum, which are found at around 450 nm and 700 nm, respectively. This leaves a gap at around 550 nm where light is reflected rather than absorbed. This is the wavelength that corresponds to the color green, which is why chlorophyll appears green to the human eye. However, changes in the structure of chlorophyll can result in changed properties, e.g. different patterns of absorbance. Such alterations in the properties of the chlorophyll can have either beneficial or extremely damaging results on photosynthetic organisms.
One way that the structure of chlorophyll can be altered is for different functional groups to be substituted on the edges of the molecule. A functional group is a portion of an organic molecule with a specific structure and related properties. For instance, the most common form of chlorophyll, chlorophyll a, has a vinyl group attached to one of its rings. This group is made up of two carbons double-bonded to each other. If this vinyl group is replaced with a formyl group (a carbon double-bonded to an oxygen and single-bonded to a hydrogen) then the molecule becomes chlorophyll d instead. Chlorophyll d performs nearly all the same functions as chlorophyll a, but has the additional quality of being “red-shifted.” This means that it is able to absorb light with longer wavelengths; chlorophyll a can absorb light up to 700 nm, but chlorophyll d has an absorbance maxima at around 710 nm.
The fact that chlorophyll d can absorb longer wavelengths of light comes in handy for organisms living in environments with only long-wavelength light available. For instance, a cyanobacteria called Acaryochloris lives below a layer of other photosynthetic organisms whose chlorophyll a takes up most of the 700 nm light. Without the photons from this light, Acaryochloris would be unable to photosynthesize and survive. Since its chlorophyll d can absorb the longer wavelengths that the other organisms don’t use and use those photons for photosynthesis, Acaryochloris is able to get by despite the limits of its environment.
On the other hand, changing the structure of the chlorophyll molecule can also be very bad for the organisms that use it for survival. For instance, if a plant takes in metals like copper or zinc, they can replace the magnesium ion in the center of the chlorophyll molecule. This substitution makes the molecule unstable during photosynthesis, often causing the process to halt completely and therefore cutting off the organism’s source of food. However, some of the pigments formed from this substitution are stable enough to continue absorbing red/blue and reflecting green wavelengths of light. This means that a plant with Cu- or Zn-chlorophyll might continue to have a vibrant, healthy green color despite the fact that it is dying or dead.
Overall, there are a lot of interesting ways that changing the structure of a molecule can affect its functions and properties, both for better and for worse.
This is really interesting. I hadn’t taken into account the different wavelengths of light and their affect on chlorophyll and how each different type of chlorophyll functions. What was really striking was your commentary on how chlorophyll bonded with a zinc ion can retain its healthy green color despite the fact that it is dying or dead. If color is such an important characteristic on how we judge the healthiness and freshness of our food, the ability to alter color could be potentially dangerous; you would never know what you were eating! Overall, a very interesting piece of research.
Hi Zoe, I think you have made some really interesting points, but I am curious why placing values on whether a structural change is good or bad is important. It seems that something that could be seen as a good structural change in one moment could be deemed bad in another as we discussed in class about cooking in copper pots–people did it and thought it was great until it was shown to be bad for the liver. I would be interested to discuss this in class because it seems that so many studies that come out today make these calls and categorize things into good or bad but I’m not convinced something has to be categorized as such.
I think you’re right in saying that what we define as “good” or “bad” depends entirely on context. There’s also plenty of shades of grey between those black and white labels. In this blog, I’m referring directly to how structural changes to chlorophyll effect an organism’s chances of survival. For instance, the red-shifted chlorophylls are beneficial to an organism’s chances of survival because they allow bacteria to comfortably exist in a broader range of environments. Whether this is “good” or “bad” is another discussion entirely.
This relates back to when we discussed an artist’s representation of organic and genetically modified tomatoes in class. The artist was clearly putting a “good/bad” spin on the research, but the research itself is only an exploration of scientific facts. A scientific fact or theory isn’t good or bad, it’s simply an observation. 1 + 1 = 2: fact. The bacteria has a better chance of survival: fact. We humans are the ones who put positive or negative significance on those facts, and that’s very important to recognize when viewing things from a scientific perspective.
In conclusion, I wasn’t attempting to put positive or negative spins on structural changes; I was simply trying to explain the observations that researchers have made. It’s definitely an interesting point for discussion though, and something that we should all keep an eye on when approaching the sciences 🙂