Photosynthetically Active Radiation

Photosynthetically active radiation (PAR) is the specific segment of the solar radiation spectrum ranging from 400 to 700 nanometers, within which photosynthetic organisms can effectively engage in photosynthesis. This spectral interval closely aligns with the light range visible to the human eye. Shorter-wavelength photons possess higher energy levels, making them potentially harmful to cells and tissues; however, these are largely filtered out by the ozone layer in the stratosphere.

Certain living organisms, such as cyanobacteria, purple bacteria, and heliobacteria, have the remarkable ability to exploit solar light in slightly extended spectral regions, including the near-infrared. This allows them to harness energy from a broader range of wavelengths beyond the typical PAR photosynthetic active radiation. 

1.0Photosynthetically Active Radiation Range

The range of photosynthetically active radiation (PAR) is 400 to 700 nanometers (nm). This is the range of solar radiation that photosynthetic organisms, like plants and microalgae, can use for photosynthesis.The amount of PAR absorbed depends on the time of day and altitude. PAR is important for many fields, including: Agricultural land evaluation, Forestry, and Oceanography. Shorter wavelengths are damaging to cells and are filtered by the ozone layer and longer wavelengths have less energy to carry out photosynthesis. Some bacteria like cyanobacteria can utilize extended spectral regions.

2.0Photosynthetically Active Radiation Measurement

Photosynthetically active radiation (PAR) is measured using two methods:

Photosynthetic photon flux density (PPFD)

This measures the number of photons in the 400–700 nanometer range that a surface receives over a specific amount of time. PPFD is measured in moles per square meter per second (mol/m2/s). 

Yield photon flux (YPF)

This method takes into account the different weights of different wavelengths. 

PAR is the part of electromagnetic radiation that green plants can use for photosynthesis. It's important to measure PAR because it's a prerequisite for productive farmland. PAR measurements are also used in forestry and oceanography.

PAR Absorbing Pigments

Photosynthetically active radiation for plants and other photosynthetic organisms have particularly Chlorophyll-a, which is responsible for major absorbance, operating within a specific range of wavelengths. However, light energy at other wavelengths is simply lost. Additionally, within the spectrum range of 450 to 650 nm, many photons escape without being absorbed, suggesting a lower density of chlorophyll-a.

Accessory pigments play a crucial role in enhancing absorbance at different wavelengths beyond the visible range. These include Chlorophyll-b and carotenoids(carotene and xanthophyll), serving as vital light-harvesting molecules that efficiently transfer energy to the reaction centers. Chlorophyll-b differs from chlorophyll-a by having a formyl group in place of a methyl group in its molecular structure. This slight variation causes a shift in absorbance peaks, directing it toward the center of the visible region. Chlorophyll-b absorbs light at wavelengths between 450 and 500 nm, while carotenoids exhibit absorbance between 400 and 500 nm.

The yellow and red hues observed in flowers and fruits are attributed to the presence of carotenoids, creating vibrant shades and colors. Beyond their role in energy transfer to reaction centers, carotenoids perform other functions, such as suppressing photochemical reactions, especially those involving oxygen, in intense sunlight conditions. This ability helps shield plants from excessive light and oxygen exposure, and plants lacking carotenoids face threats and rapid deterioration.

The pigments within photosynthetic organisms are organized into two distinct photochemical light-harvesting complexes (LHC) present within Photosystem I (PS-I) and Photosystem II (PS-II). Interestingly, their nomenclature follows the order of their discovery rather than the sequence in which they function during the light reaction.

Each LHC comprises hundreds of pigment molecules, typically ranging from 250 to 400, tightly bound to proteins. Within each photosystem, these pigments, excluding one chlorophyll-a molecule, collectively form a light-harvesting system often referred to as an antenna. The solitary chlorophyll-a molecule serves as the reaction center.

The nature of the reaction center differs between the two photosystems. In PS-I, the chlorophyll-a molecule within the reaction center has an absorption peak at 700 nm, earning it the designation P 700. Conversely, in PS-II, the absorption maximum occurs at 680 nm, leading to its identification as P 680.