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.
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