CHLOROPLASTS AND PIGMENTS

Light plays a very important role as the driving force in the process of photosynthesis. Knowing this, we can observe where photosynthesis actually occurs in a photosynthetic organism.

Plant Cell Structures: Chloroplasts, Chemical Factories Powered By The Sun


This is a typical plant cell. The many structures present in the cell cytoplasm (the entire contents of the cell, not including the nucleus, and surrounded by the plasma membrane) are organelles, which you can think of as "little organs," each having a specialized function of their own. Plastids are a type of membrane-bound organelle, the most important of which is the chloroplast, the site of photosynthesis in plants. Chloroplasts carry out photosynthesis, converting sunlight to chemical energy stored in sugar and other organic molecules. An easy way to remember this is to think of chloroplasts as chemical factories powered by the sun.
All green parts of a plant contain chloroplasts. The leaves, however, are the major organs of photosynthesis in most plants. A leaf is comprised of a waxy upper and lower layer, as well as a middle layer (the mesophyll) in between, kind of like a sandwich. This middle layer is the portion that is specialized for photosynthesis. Actually, there are two parts to this middle layer: there are cells that are long and columnar in shape on top, and there are spongy cells with lots of air spaces right below. The top layer filters light to the spongy layer, where photosynthesis occurs. Other important structures on a leaf (or more specifically, the waxy layers) are the stomata, tiny pores with specialized cells called guard cells on each side. Each stoma is actually a gap between a pair of guard cells. In addition, there are also specialized cells for the transport of nutrients and water. Within a leaf, veins subdivide repeatedly and branch throughout the middle layer of the leaf. Veins consist of water-conducting tissue on top of nutrient-conducting tissue, surrounded by a sheath. Hence the term, bundle-sheath cells. The location of these veins brings the conducting tissues in close contact with the photosynthetic tissue, which obtains water and minerals from the water-conducting tissue and loads it sugars and other organic products into the nutrient-conducting tissue for shipment to other parts of the plant.
Chloroplasts, found mainly in the middle layer of the leaf, are bound by a double membrane that encloses the stroma, the dense fluid content of the chloroplast. Another important structure to become familiar with is a thylakoid, which is a flattened membrane sac inside the chloroplasts, used to convert light energy to chemical energy. Membranes of the thylakoid system separate the stroma from the thylakoid space. Thylakoids are concentrated in stacks called grana. You can picture thylakoids as stacks of pancakes, immersed in heavy maple syrup (which you can think of as the stroma).

PIGMENT TYPES


Chlorophylls
As we have just mentioned, leaves are the major sites of photosynthesis. What makes a leaf green is chlorophyll, the green pigment located within the chloroplasts. More specifically, chlorophyll resides in the thylakoid membranes. The chlorophyll absorbs energy from sunlight, and it is this energy that drives the synthesis of food molecules in the chloroplast. The chloroplasts make food via photosynthesis, and veins in the leaves and other photosynthetic parts export sugar to the roots and other nonphotosynthetic parts (parts that have no chloroplasts and thus are not green) of the plant.
So what function do these pigments have? Well, photosynthetic pigments act as light receptors. The process of photosynthesis depends upon the efficient capture of light energy by aggregates of pigments present in photosynthetic tissues. We already know that as light comes into contact with matter, it may be reflected, transmitted, or absorbed. Substances that selectively absorb visible light are called pigments. Different pigments absorb light of different wavelengths, and the wavelengths that are absorbed disappear. This is important because light has to be absorbed before it can be of any use in a photobiological reaction. The pigments of chloroplasts absorb blue and red light most effectively, and transmit or reflect green light, which is why leaves appear green.

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.

Chorophylls Revisited
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.

Carotenoids
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.

Xanthophylls
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.

Phycobilins
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.

Material mainly taken from Campbell, 4th edition textbook



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