Plant Cell Structures: Chloroplasts, Chemical Factories Powered By The Sun
Photosynthesis in Bacteria and Algae
Photosynthesis does not only occur in the higher green plants; it occurs in algae and bacteria as well, albeit a little differently. Bacteria are unique in that they do not have chloroplasts, which, as we well know by now, are the main sites of photosynthesis in the green plants. They utilize different pigments, and sometimes their products differ significantly. For example, modern archaebacteria photosynthesize with the pigment bacteriorhodopsin (which, by the way, is structurally related to the visual pigments in our eyes). Green and purple sulfur bacteria owe their respective colors to bacteriochlorophyll, their main photosynthetic pigment. Plants which perform green plant photosynthesis as opposed to bacterial photosynthesis have chlorophylls as common pigments having major absorption peaks in the visible region of the light spectrum. Bacteriochlorophylls, however, absorb near infrared radiation; thus, photosynthetic bacteria grow in regions where higher plants cannot.
One very important chlorophyll is chlorophyll a. All photosynthetic organisms (except a few groups of bacteria) have this as their main photosynthetic pigment. It is important because only it can participate directly in the light reactions, which convert solar energy to chemical energy. But chlorophyll a is not the only pigment in chloroplasts important to photosynthesis. Other pigments can absorb light and transfer the energy to chlorophyll a, which then initiates the light reactions. We refer to these other pigments as accessory pigments. One of these is another form of chlorophyll, chlorophyll b. It is very similar to chlorophyll a, but different enough so that each has its own unique color. The former is blue-green, while the latter is yellow-green. So the point is, if chlorophyll b absorbs a photon of sunlight (this discrete packet of energy), it conveys the energy to chlorophyll a, which then acts as if it had directly absorbed the energy itself.
Many other chlorophylls exist along with the two we have just emphasized. For example, chlorophyll c is present in the brown algae, the dinoflagellates, and the diatoms. It is generally present along with chlorophyll a in most marine photosynthetic organisms. Another example is chlorophyll d, which is almost exclusive to the red algae; it acts as a minor pigment to chlorophyll a.
Well, what makes leaves orange and yellow? The chloroplast also has many other accessory pigments that aid in the process of photosynthesis. There are a group of pigments called carotenoids, which unlike chlorophyll, are different shades of yellow and orange. Carotenoids reside with the two types of chlorophylls in the thylakoid membrane. Carotenoids are important because they can absorb certain wavelengths of light that chlorophylls cannot. These are the structures that cause leaves to appear orange and yellow. Carotenoids are also important because they are involved in a function known as photoprotection. Excessive light intensity can damage the chlorophyll pigments, so instead of transmitting energy to chlorophyll, some carotenoids use photoprotection to accept energy from chlorophyll, thereby protecting them from harm. A variety of carotenoids are also found in the photosynthetic bacteria.
Carotenoids are divided into two classes: carotenes and xanthophylls. The major xanthophylls of higher plants are also present in the green algae and brown algae. The main xanthophyll of diatoms, in specific, and brown algae, in general, is called fucoxanthin, which is what gives these organisms their brownish color. Fucoxanthin, however, is not present outside the kingdom Chromista (brown algae and relations) and the dinoflagellates (diatoms). Several other xanthophylls are also prevalent in other algal divisions, but these will not be discussed here. The important point is that the many different xanthophylls are what give some algae their unique colors.
A final group of pigments, in addition to the chlorophylls and carotenoids, are the phycobilins. Unlike the chlorophylls and carotenoids, which are rather widely distributed among various plants, phycobilins have a relatively narrow distribution and are found only in the red algae and cyanobacteria. Phycoerythrin, an accessory pigment belonging to this family, is what makes red algae commonly red. Conversely, phycocyanin, another accessory pigment, is what causes the cyanobacteria to appear blue-green. The phycobilins and other accessory pigments are what also make possible the absorption of filtered blue and green wavelengths in deep waters.