It isn’t easy being green: the light problem

Photosynthetic organisms have a complicated relationship with light. Because they use it as their primary energy source for producing biochemical energy, plants and other photosynthetic organisms have elaborate systems for collecting as much light as possible. Very few pigment molecules (chlorophylls) are involved in actually driving the movement of electrons in the photosystems to fuel the light reactions. Alone, the light-harvesting power of these chlorophylls is insufficient to sustain life. Plants and other photosynthetic organisms have additional pigment-protein complexes that serve as antennae that funnel a steady stream of light energy into the photosystem chlorophylls.

Having lots of antenna complexes (green) increases the chances of absorbing light, whose energy can be transferred to photosystems (red).

Having lots of antenna complexes (green) increases the chances of absorbing light, whose energy can be transferred to photosystems (red).

Across different photosynthetic organisms, there is quite a bit of diversity in antenna complex structure and the pigments used within them. The light available to species depends on the environment and each species has tailored their antennae accordingly to maximize the amount of light harvested. For example, plants have antenna complexes made up of proteins and lots of chlorophyll. Aquatic photosynthesizers like cyanobacteria have an antenna complex with bilin pigments that can take advantage of light at wavelengths chlorophyll does not absorb well. Other photosynthetic organisms like purple bacteria and green sulfur bacteria use bacteriochlorophyll pigments that can absorb far-red light. Because of these differences in antenna strategy, these organisms can coexist without competing with one another for the same narrow region of the light spectrum. In other words, each of these guys has their favorite color of the rainbow (vs. all of them competing for blue or red light only).  The different antenna structures for various photosynthetic organisms are structurally very beautiful, so stay tuned for future posts on them.

When too much light energy is funneled into the photosystems, they become damaged and non-functional.

When too much light energy is funneled into the photosystems, they become damaged and non-functional.

However, light energy is also potentially dangerous to photosynthetic organisms. When the light reactions of photosynthesis are functioning at maximum capacity, the normal route for energy transfer in the light reactions is not available. At this point, the extra light energy can cause oxidative damage to the proteins performing the light reactions. This damage is irreversible and the photosynthetic organisms must spend a great deal of energy removing the damaged proteins and replacing them with newly made functional ones. Continually recycling these proteins is very expensive (biochemically speaking) for plant cells, so they have a number of strategies aimed at reducing the need for this process.

NPQ pathways prevent excess light energy from getting to the photosystems by modifying antenna function.

NPQ pathways prevent excess light energy from getting to the photosystems by modifying antenna function.

Collectively these strategies are known as NPQ (nonphotochemical quenching). NPQ pathways are the way plants and other photosynthetic organisms switch from funneling light energy into photosynthesis to safely dumping that energy elsewhere. The dangers of excess light make NPQ a necessity, but it must be tightly regulated because its sole function is to siphon energy away from photosynthesis. Therefore NPQ must be turned off when excess light energy is no longer a problem. Scientists are still unraveling the mysteries behind the various NPQ strategies and their regulation.

Because this all comes down to light energy, plants and other photosynthetic organisms must be able to deal with this problem on a wide ranging timescale. Some pathways must work at the speed of light. Think light intensity changes when clouds move in front of or away from the sun. Other pathways must work on longer timescales like over the course of a growing season when plants or parts of the same plants are shaded by other leaves above them. Still other pathways must work on even longer timescales, over the course of seasonal changes. The light harvesting and NPQ pathways must work coordinately; antennae for capturing additional light and NPQ for getting rid of excess light energy. The complementary processes of light harvesting and NPQ are amazingly complex to fine tune the fundamental photosynthetic reactions within plants under varied conditions.

Ideally, plants would be able to shift efficiently back and forth between light harvesting and NPQ, but research suggests there is room for improvement in this area. As humans place greater demands on productivity, one way to increase the potential of photosynthetic organisms is to streamline these pathways even further. In order to do this, scientists are studying the details of how antenna complexes function, identifying the molecular components of the NPQ pathways and integrating how both halves of the solution to the ‘light problem’ are regulated.

Johnna

Blankenship, Molecular Mechanisms of Photosynthesis. 2002 Blackwell Publishing.

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3 thoughts on “It isn’t easy being green: the light problem

  1. Pingback: Something new in NPQ | New Under The Sun Blog

  2. Pingback: Toxic Cyanobacteria | New Under The Sun Blog

  3. Pingback: Adamantium, for plants! | New Under The Sun Blog

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