Differences Between Chlorophyll A and B
There is only one difference between chlorophyll a and b which is one of the functional groups bonded to the porphyrin. In chlorophyll a methyl group (CH) while in chlorophyll aldehyde group (-CHO) is present.
Role of Pigments in Photosynthesis:
The clusters of photosynthetic pigments are called photos. Each pigment complex is composed of chlorophyll a and b molecules with accessory pigments. When these pigments absorb light they are said to be excited. The light energy is used to boost electrons to a higher energy level which is transferred into chemical energy.
The excited state is unstable and molecules will tend to return to their unexcited state.
The energy that is released during this process can be passed from one chlorophyll molecule to another chlorophyll molecule.
The instrument that is used to measure the relative abilities of different pigments to absorb different wavelengths of light is called a Spectrophotometer.
4.1.3 Absorption Spectrum :
It is a measure that exhibits the absorbed amount of light of different wavelengths (different colors) from the visible spectrum of light. The main photoreceptors are chlorophyll a and chlorophyll b which absorb violet blue (430 nm) and red light (670 nm).
The green light (550 nm) is the least absorbed. The carotenoids absorb light between 430-470 nm of the light spectrum and transfer it to chlorophyll b and then to chlorophyll a. The chlorophyll a and b show different absorption spectra as shown in Figure 4.2 (a).
Chlorophyll shows absorption peaks at about 680 and 700 nm while chlorophyll b's absorption peaks range between 450- 475 nm.
4.1.4 Action Spectrum
A graph showing the measure of the effectiveness of light of various wavelengths in driving photosynthesis is called the action spectrum. Some of the absorbed light is released as heat and the rest of the light is stored in organic compounds as chemical energy.
The action spectrum of a particular pigment can be calculated by measuring the rate of photosynthesis at each type of wavelength of light. A plant is illuminated with light of different wavelengths. During photosynthesis plants give off oxygen.
As photosynthesis produces oxygen and consumes CO2, the rate of production of oxygen, or consumption of CO, can be used as a measure of the rate of photosynthesis.
4.1.5 Arrangement of Photosynthetic Photosystems I and II
It is initiated when photosynthetic pigments capture light energy. The clusters of photosynthetic pigment complex are composed of. chlorophyll a and b molecules and accessory pigments (carotenoid pigments). There are two photosystems, namely photosystem I (PSI) and photosystem II (PSII) named so in order of their discovery.
Each photosystem contains several hundred pigment molecules. There are two parts of each photosystem i.e. antenna complex and reaction center. The antenna complex possesses many molecules of chlorophyll a, b, and carotenoids.
All these pigment molecules in the photosystem serve as an antenna for gathering solar energy, which is passed from one pigment to the other and finally transferred to the reaction center.
The reaction center contains one more molecule of chlorophyll a with primary electron acceptor and electron carriers of the electron transport system. The electron transport system plays a role in the generation of ATP by chemiosmosis. The PSI absorbs light of 700 nm and is called P700 while the PSII absorbs light of P680 nm and is called P680.
The primary electron acceptor traps the electrons from the reaction center and then passes them on to the series of electron carriers. Electrons have two pathways in the light reaction of photosynthesis; The non-cyclic electron pathway (flow) and the cyclic electron pathway.
The cyclic is less common and generates only ATP while the non-cyclic is predominant and generates both ATP and NADPH,
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