Basics of Photosynthesis: Absorption and Action spectrum
Channel: YashFame
Date: 2020-05-08

In this video, Students will easily understand about basic concepts of photosynthesis and principal light harvesting pigments. Various types of chlorophyll molecules viz a,b,c and d. Absorption and action spectrum
Photosynthesis is a redox reaction in which CO2 is reduced and H20 is oxidized to O2. Reduction of CO2 to carbohydrate and O2 is released from photo-splitting of water. Transformation of light energy into chemical energy satisfies the First Law of thermodynamics that states " Energy neither be created  nor destroyed but changes its form from one to another. Light energy into chemical energy by plants transforms in the Light dependent phase/light reaction. This takes place in thylakoid of chloroplast with the help of green pigments i.e. chlorophylls. 

Chemical energy includes ATP and NADP+ molecules which in turn reduces atmospheric carbon dioxide (CO2) into carbohydrate.ATP is Adenosine Triphosphate while NADPH is reduced form of NADP+ (Nicotinamide Adenine diphosphate hydrogen)

Chlorophyll molecules in plants predominantly absorb light energy. Chlorophylls are of different types; Chl a, b c and d. Each chlorophyll molecule has four pyrrole rings arranged in the porphyrin manner where pyrroles rings are interconnected with help of =CH- (Methine bridge). This forms the head of the chlorophyll with which 20 carbon tail is attached. Ring C is connected with the help of ester bond to the 20 C phytol tail. chlorophyll is a lipid molecule. 

Ring A/II if has CH3 group that it is called chlorophyll a, becomes Chl b when Ch3 is replaced with -CHO at the same ring. Chlorophyll c and d are also important pigments present in the in diatoms and some special class of algae. 

Besides, these photosynthetic pigments, there are other pigments which are present as accessory pigments like carotenoids. They absorbs light of shorter wavelength (400-450) and transfers energy to the reaction center of photosynthetic components involved in the photochemical reaction. These carotenoids ; Carotene and xanthophyll and the important constituent of Light harvesting Complexes I and II ( Antennae Complexes) channelised energy through resonance to the reaction centers. The LHCs are loosely associated with Photosystems (PS) I & II.

Photochemical reaction is the important term when photosynthesis is studied.
Light energy converted to chemical energy means that electron reaches to higher energy level and stays there and when comes to the ground state, releases its energy in as follows:
a. Releases as Heat
b. Releases as fluorescence
c. Releases as Phosphorescence (stays in the triplet state)
d . In the triplet state, electron stays more than the singlet state. Hence it has time to talk to the other molecule and due to this conversation thoughts are shared i.e electron gives its electron to other molecule. The molecule that releases its electron, gets oxidized and that receives this electron is reduced. Or we can say that system has entered into chemical energy wherein oxidation -reduction (Redox) has been initiated. The reaction which is powered by the light energy is called called photochemical reaction.

This reaction is also done when electrons are ejected from the reaction center P680 and P700 and thereby accepted by the acceptor molecule and charge separation takes place and which continued and one form of energy is transformed into another.

Can one measure degree of absorbance by chlorophyll molecules in relation to wavelength of light? obviously yes and that is known as absorption spectrum. When a plot is graphed between wavelength and pigment under investigation the obtained curve is called absorption spectrum. This is the absorption signature of a particular pigment. 

Chlorophylls molecules absorbs light optimally in blue and red range and if we see absorption spectrum it is evident that why plants look green because green light is ineffectively absorbed by chlorophyll molecules. And this reflected on our retina and hence plants look green to us.

Similarly, action spectrum can also be plotted when we put performance triggered by light on Y axis and visible range spectrum on the X axis, the obtained graph or curve is called action spectrum.

So next question is what is relevance of having these spectrums. When we compare these two curves and find both are superimposing each other then we can say that particular absorption is responsible for particular action.

The overall equation of photosynthesis we must write as under:
6CO2  + 12H2O = C6H12O6 +6O2 + 6H2O
in which six molecules of CO2 are reduced to carbohydrate and 12 molecules of water photo-oxidised to sic molecules of O2.
The above equation is of the Calvin cycle/ C3 cycle which is present in all plants whether C4 plants or CAM plants.