Chlorophyll is the green pigment found in plants that allows them to convert sunlight into usable energy through a process called photosynthesis. More specifically, chlorophyll molecules are described as photoreceptors due to their light absorption properties. There are two main types of chlorophyll, named chlorophyll a and chlorophyll b. These two different chlorophyll molecules are characterized by their varying chemical structure and specific infrared light that they absorb.
Structure
Chlorophyll a and b differ in structure only at the third carbon position. Chlorophyll b has an aldehyde (-CHO) side chain at this carbon position as compared to the methyl group (-CH3) for chlorophyll a. This difference in structure contributes to their varying light absorption properties.
Chlorophyll A
Chlorophyll a is the most commonly used photosynthetic pigment and absorbs blue, red and violet wavelengths in the visible spectrum. It participates mainly in oxygenic photosynthesis in which oxygen is the main by-product of the process. All oxygenic photosynthetic organisms contain this type of chlorophyll and include almost all plants and most bacteria.
Chlorophyll B
Chlorophyll b primarily absorbs blue light and is used to complement the absorption spectrum of chlorophyll a by extending the range of light wavelengths a photosynthetic organism is able to absorb. Both of these types of chlorophyll work in concert to allow maximum absorption of light in the blue to red spectrum; however, not all photosynthetic organisms have the chlorophyll b pigment.
Role in Photosynthesis
Both of these chlorophyll molecules capture light energy and transfer it to the reaction center of the cell. From here, electrons are passed from this absorbed light energy to water molecules resulting in the formation of hydrogen ions and oxygen. The oxygen is released as a by-product; whereas the hydrogen ions are transferred across the plant’s thylakoid membrane resulting in the phosphorylation of adenosine diphosphate (ADP) into adenosine triphosphate (ATP). ATP then subsequently reduces a coenzyme called nicotinamide adenine dinucleotide phosphate (NADP) to NADPH2, which is then used to convert carbon dioxide into a sugar.
Pigments are colorful chemical compounds that reflect light of a specific wavelength and absorb other wavelengths. Leaves, flowers, coral, and animal skins contain pigments that give them color. Photosynthesis is a process taking place in plants and can be defined as a conversion of light energy to chemical energy. It is a process by which green plants produce carbohydrates from carbon dioxide and water by the help of chlorophyll (green pigment in plants) in the presence of light energy.
Chlorophyll a
Chlorophyll a appears green in color. It absorbs blue and red light and reflects green light. It is the most abundant type of pigment in leaves and thus the most important type of pigment in chloroplast. At a molecular level it has a porphyrin ring that absorbs light energy.
Chlorophyll b
Chlorophyll b is less abundant than chlorophyll a but has ability to absorb a wider wavelength of light energy.
Chlorophyll c
Chlorophyll c is not found in plants but is found in some microorganisms capable of performing photosynthesis.
Carotenoid and Phycobillin
Carotenoid pigments are found in many photosynthetic organisms, as well as in plants. They absorb light between 460 and 550 nm and hence appear orange, red, and yellow. Phycobillin, a water-soluble pigment, is found in chloroplast.
Mechanism of Energy Transfers
The importance of pigment in photosynthesis is that it helps absorb the energy from light. The free electrons at the molecular level in the chemical structure of these photosynthetic pigments revolve at certain energy levels. When light energy (photons of light) falls on these pigments, the electrons absorb this energy and jump to the next energy level. They cannot continue to stay in that energy level, as it is not the state of stability for these electrons, so they must dissipate this energy and come back to their stable energy level. During photosynthesis these high-energy electrons transfer their energy to other molecules, or these electrons themselves get transferred to other molecules. Hence, they release the energy they had captured from light. This energy is then used by other molecules to form sugar and other nutrients by using carbon dioxide and water.
Facts
In an ideal situation the pigments must be capable of absorbing light energy of the entire wavelength, so that the maximum energy can be absorbed. To do so, they should appear black, but chlorophylls are actually green or brown in color and absorb light wavelengths in the visible spectrum. If the pigment starts absorbing wavelength away from the visible light spectrum, such as ultraviolet or infrared rays, the free electrons may gain so much energy that they will either get knocked off their orbit or may soon dissipate energy in the form of heat, thus damaging the pigment molecules. So it is the visible wavelength energy absorbing capability of pigment that is important for photosynthesis to take place.
Chlorophyll a known as universal pigments which is found in all living green cells. Both pigments helps in photosynthesis as well as other metabolic pathways in plant cells. Constituent of these pigments, mainly Mg, act as co-factors and responsible for developmental process in plants.
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