Carbon dioxide is trapped and reduced to carbohydrate in the
light-independent reactions of photosynthesis, using ATP and reduced
NADP from the light-dependent reactions.
During these reactions the photosynthetic pigments of the chloroplast
absorb light energy and give out excited electrons used to synthesise
Other bacteria use sunlight for different chemical purposes. Splitting water — as plants do — requires a lot of input energy, so some organisms "make a living" off less demanding reactions. For example, green sulfur bacteria use hydrogen sulfide as an "electron donor" in place of water. This process and others like it are referred to as anoxygenic photosynthesis because no oxygen is produced. The associated pigments are called bacteriochlorophylls.
Plants and other photosynthetic organisms use special molecules for absorbing light. These pigments have a distinctive color, or spectrum, that is known to leave an imprint on the light reflected off our planet's surface. A new database catalogues the diverse palette of light-absorbing biological molecules on Earth in order to better predict what the photosynthetic signature might look like on other planets.
The Biological Pigment Database contains spectral information for chlorophylls and bacteriochlorophylls, as well as for other accessory pigments that absorb light energy and transfer it to the main chlorophyll pigments responsible for photosynthesis. In addition, it includes biological sunscreen compounds that photosynthetic organisms produce to protect against excessive or harmful radiation, and carotenoids that serve the roles of both anti-oxidants and light harvesting pigments.
Imagine a leaf as a solar collector crammed full of photosynthetic cells where the raw materials of photosynthesis (water and carbon dioxide) enter the cells of the leaf, and the products of photosynthesis (sugar and oxygen) leave the leaf.
Chlorophylla-a is the primary for in plants. Its structure is shown at left. It has the composition C55H72O5N4Mg. It exhibits a grass-green visual color and absorption peaks at 430nm and 662nm. It occurs in all photosynthetic organisms except photosynthetic bacteria.
Life on our planet has adapted to the light from our Sun. Plant photosynthetic pigments, for example, use the Sun's abundant visible light — most strongly absorbing in the blue and red part of the spectrum, which is why they appear green in reflected light.
But plants aren't the only organisms that use sunlight on Earth. Kiang studies various photosynthetic bacteria that have an entirely different set of pigments for absorbing light. She compares the spectra from these pigments to try to understand what mechanisms drive the evolution of the light harvesting ability.
What this tells us is that photosynthesis is not a one-size-fits-all solution, and the pigments used on a different planet will likely be adapted to local conditions. To help imagine what these alien pigments might be, Kiang has started the Biological Pigments Database. This contributes a biological component to the NASA Astrobiology Institute Virtual Planetary Laboratory’s Spectral Database, which brings together stellar radiation spectra, molecular line lists for atmospheric radiative transfer, and now biological pigments.
"I want [the pigment database] to be a community resource that can help in modeling the potential biosignatures from other planets," Kiang says.
The most familiar pigment is chlorophyll. There's actually a handful of different kinds of chlorophyll, but the most essential one is chlorophyll a, used by plants, algae, and cyanobacteria. Chlorophyll a absorbs most strongly in the violet-blue and orange-red part of the spectrum, which is a natural choice for plants growing in direct sunlight.
Being an evergreen plant it has the advantage of being able to
photosynthesis during the winter months whereas deciduous trees are
Plants and algae use the energy absorbed by chlorophyll a to split water molecules. This splitting allows electrons from the water to be "donated" towards the reduction of carbon dioxide into carbohydrates (i.e. sugars). The byproduct of these reactions is oxygen, which is why the process is called oxygenic photosynthesis.