Bachelor’s Degree Programme (BSCG) – Plant Physiology and Metabolism Assig. Answer | BBYCT–137(Q-3)
3. a) What is PAR? Describe the absorption and action spectra of Chlorophyll a.
b) Describe the C4 cycle with the help of an outline diagram.
a) Photosynthetically Active Radiation (PAR) is the amount of light available for photosynthesis, which is light in the 400 to 700 nanometer wavelength range. PAR changes seasonally and varies depending on the latitude and time of day.
Levels are greatest during the summer at mid-day. Factors that reduce the amount of PAR available to plants include anything that reduces sunlight, such as cloud cover, shading by trees, and buildings. Air pollution also affects PAR by filtering out the amount of sunlight that can reach plants. Photosynthetically Active Radiation (PAR) is measured by a silicon photovoltaic detector.
-> Absorption Spectra – i) It is the graphical representation of the different wavelengths of light absorbed by the different pigments in the leaf during the process of photosynthesis.
ii) It infers the relationship between the quality of light and the absorbing capacity of pigments.
iii) The absorption of wavelengths of different pigments can be measured with a spectrophotometer.
-> Action Spectra – i) It is the graphical representation of the effectiveness of the different wavelengths of light in photosynthesis.
ii) It infers the relationship between photosynthetic activity in relation to different wavelengths of light.
iii) The amount of photosynthesis can be measured with the amount of carbon dioxide fixation, oxygen production, NADP Reduction, etc.
b) C4 Pathway (Hatch and Slack Pathway) – Every photosynthetic plant follows Calvin cycle but in some plants, there is a primary stage to the Calvin Cycle known as C4 pathway. Plants in tropical desert regions commonly follow the C4 pathway. Here, a 4-carbon compound called oxaloacetic acid (OAA) is the first product by carbon fixation. Such plants are special and have certain adaptations as well.
The C4 pathway initiates with a molecule called phosphoenolpyruvate (PEP) which is a 3-carbon molecule. This is the primary CO2 acceptor and the carboxylation takes place with the help of an enzyme called PEP carboxylase. They yield a 4-C molecule called oxaloacetic acid (OAA).
Eventually, it is converted into another 4-carbon compound known as malic acid. Later, they are transferred from mesophyll cells to bundle sheath cells. Here, OAA is broken down to yield carbon dioxide and a 3-C molecule.
The CO2 thus formed is utilized in the Calvin cycle whereas 3-C molecule is transferred back to mesophyll cells for regeneration of PEP.
Corn, sugarcane and some shrubs are examples of plants that follow the C4 pathway. Calvin pathway is a common pathway in both C3 plants and C4 plants but it takes place only in the mesophyll cells of the C3 Plants but not in the C4 Plants.
The balanced chemical equation for the photosynthesis process is as follows:
6CO2 + 6H2O —> C6H12O6 + 6O2
Sunlight is the ultimate source of energy. Plants use this light energy to prepare chemical energy during the process of photosynthesis.