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. The change in light color from green to red takes place through a process known as fluorescence. 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. 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.
So, we now know that photosynthesis is the process by which plants produce their food, using Chl. We also know that the reduced amount of light available in the oceans decreases this photosynthetic process. Nature has evolved some helper chemical molecules known as phycobiliproteins, which are able to absorb the colors of light available in the oceans and turn this light into a color that Chl molecules can use.
These phycobiliproteins are found in tiny, invisible-to-the-naked-eye cyanobacteria, whose photosynthesis is responsible for providing food for the living organisms in the oceans and also for making the oxygen in our atmosphere that we breathe every second. In the future, we hope to gain more understanding of the functions of phycobiliproteins and the roles that they may play for the benefit of mankind. Phycobiliproteins use this property to change the color of light they absorb so that the light can be used for photosynthesis.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Phycobilisome and phycobiliprotein structure. In: Bryant, D. The Molecular Biology of Cyanobacteria. Chlorophyll, the green pigment common to all photosynthetic cells, absorbs all wavelengths of visible light except green, which it reflects.
This is why plants appear green to us. Black pigments absorb all wavelengths of visible light that strike them. White pigments reflect most of the wavelengths striking them. Each pigment has a characteristic absorption spectrum describing how it absorbs or reflects different wavelengths of light.
The light-dependent reactions begin in a grouping of pigment molecules and proteins called a photosystem. Photosystems exist in the membranes of thylakoids. A photon of light energy travels until it reaches a molecule of chlorophyll. To replace the electron in the chlorophyll, a molecule of water is split.
Technically, each breaking of a water molecule releases a pair of electrons, and therefore can replace two donated electrons. The replacing of the electron enables chlorophyll to respond to another photon. The oxygen molecules produced as byproducts find their way to the surrounding environment. The hydrogen ions play critical roles in the remainder of the light-dependent reactions. Keep in mind that the purpose of the light-dependent reactions is to convert solar energy into chemical carriers that will be used in the Calvin cycle.
In eukaryotes and some prokaryotes, two photosystems exist. The first is called photosystem II, which was named for the order of its discovery rather than for the order of the function. After the photon hits, photosystem II transfers the free electron to the first in a series of proteins inside the thylakoid membrane called the electron transport chain.
As the electron passes along these proteins, energy from the electron fuels membrane pumps that actively move hydrogen ions against their concentration gradient from the stroma into the thylakoid space. This is quite analogous to the process that occurs in the mitochondrion in which an electron transport chain pumps hydrogen ions from the mitochondrial stroma across the inner membrane and into the intermembrane space, creating an electrochemical gradient.
After the energy is used, the electron is accepted by a pigment molecule in the next photosystem, which is called photosystem I Figure 5. The energy that these molecules carry is stored in a bond that holds a single atom to the molecule. Recall that NADH was a similar molecule that carried energy in the mitochondrion from the citric acid cycle to the electron transport chain. This potential energy is harvested and stored as chemical energy in ATP through chemiosmosis, the movement of hydrogen ions down their electrochemical gradient through the transmembrane enzyme ATP synthase, just as in the mitochondrion.
The hydrogen ions are allowed to pass through the thylakoid membrane through an embedded protein complex called ATP synthase. The energy generated by the hydrogen ion stream allows ATP synthase to attach a third phosphate to ADP, which forms a molecule of ATP in a process called photophosphorylation.
The flow of hydrogen ions through ATP synthase is called chemiosmosis, because the ions move from an area of high to low concentration through a semi-permeable structure. The remaining function of the light-dependent reaction is to generate the other energy-carrier molecule, NADPH. As the electron from the electron transport chain arrives at photosystem I, it is re-energized with another photon captured by chlorophyll.
Now that the solar energy is stored in energy carriers, it can be used to make a sugar molecule. As an Amazon Associate we earn from qualifying purchases. Want to cite, share, or modify this book? This book is Creative Commons Attribution License 4. Skip to Content Go to accessibility page. Concepts of Biology 5. My highlights.
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