What Are the Roles of Chlorophyll A & B?

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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.

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