deep in the ocean sunlight and I'll reach plants such as brown and red algae found how do you think they are able to synthesise their food
Answers
Phycobiliproteins play the role of assistants to Chl in aquatic (water) environments. Since light has a difficult time penetrating into the oceans, phycobiliproteins make this job easier by absorbing whatever light is available; they absorb the green portion of the light and turn it to red light, which is the color of light required by Chl [2]. However, changing the color of light is not as easy as it seems. The green light has to pass through different phycobiliprotein molecules, which absorb light of one color and give out light of another color. The color that is given out is then taken up by a second phycobiliprotein, which turns it into a third color. This process continues until the emitted light is red, which can finally be taken up by Chl. For this whole process to take place, we have three different kinds of phycobiliprotein molecules arranged as a sort of a hat over the Chl molecule, as you can see in Figure 3. These three kinds of phycobiliproteins are:
(a) C-phycoerythrin (CPE), pinkish-red in color and responsible for absorbing the green portion of sunlight.
(b) C-phycocyanin (CPC), deep blue in color and responsible for absorbing the orange-red portion of sunlight.
(c) Allophycocyanin (APC), light blue in color and responsible for absorbing the red portion of sunlight.
Figure 3 - Hat-like arrangement of phycobiliproteins and chlorophyll (Chl) in cyanobacteria.
The green light is first absorbed by C-phycoerythrin which passes it on to C-phycocyanin (CPC). CPC further passes the light energy to allophycocyanin (APC) which transfers it to Chl for photosynthesis, using the red light.
The reason phycobiliproteins absorb light of different colors is that they contain chemical molecules called bilins inside them, which give them their bright colors. These bilins are responsible for absorbing light of one color and emitting light of another color, thus causing a change in the color of light. Advanced instruments have let us analyze the arrangement of these molecules and proteins in the cyanobacteria. We know that phycobiliproteins are shaped like disks [3], and the disks are stacked on top of each other to form the hat-like structure. One end of the stack is made of CPE, whereas the other end is made of CPC. This assembly joins to the core, made of APC. This entire structure is linked to Chl, which accepts the red light emitted by APC. The arrangement of the hat-like structure has been shown in Figure 3.
HOW DOES THE LIGHT ENERGY TRANSFER TAKE PLACE IN PHYCOBILIPROTEINS?
The change in light color from green to red takes place through a process known as fluorescenceThe property of certain compounds to absorb one color of light and to give off another color. Phycobiliproteins use this property to change the color of light they absorb so that the light can be used for photosynthesis.. Let us see what fluorescence is. Imagine a transparent container filled with a pink-colored liquid that, when illuminated with a flashlight, shines a bright orange! That is exactly what CPE does (Figure 4). All phycobiliproteins possess this exciting property of giving off visible light of a color different from the color of light that is shone on them. After CPE changes green light to yellow-orange, CPC takes up the yellow-orange light and changes it to light red. APC takes up this light-red light and changes it to a deep red light for Chl. So, now we have the green light changed to red, which is the color of light that nature intended Chl to absorb. The entire process is a sort of a relay race, where each participant picks up where the previous one left off (Figure 5). These phycobiliproteins are an important part of the tiny microscopic organisms called cyanobacteria, which carry out photosynthesis in much the same way as land plants do. The only difference is that they use a different set of chemical molecules—cyanobacteria use phycobiliproteins while land plants use Chl.
Figure 4 - Fluorescence property of C-phycoerythrin (CPE).
The white color of the light produced by the flashlight is changed to yellowish-orange light by CPE, to be taken up by C-phycocyanin.
Figure 5 - Phycobiliproteins change the color of light from green to red, so that it can be used for photosynthesis.
The green-colored light is taken up by C-phycoerythrin (CPE), which changes the color of the light to yellowish orange. The orange light is taken up by C-phycocyanin (CPC), which further changes it to light red. The light red color is absorbed by allophycocyanin (APC), which changes it to red color. The red color is finally absorbed by chlorophyll, for producing food through photosynthesis.