Light reactions and dark reactions
During his experiments to study the limiting factors of Photosynthesis as light, heat, and Carbon dioxide, Blackman concluded that Photosynthesis process is divided into two types of reactions:
1. Light reactions:
In which light acts as the limiting factor of the process. Light reactions take
place in grana.
2. Dark reactions:
Dark reactions are sensitive for temperature and are not affected by light. These
reactions take place in both light and darkness. And temperature is the limiting
factor for the rate of the process. Dark reactions take place in stroma.
1. Light reactions of Photosynthesis:
a. When light falls on chlorophyll of the grana inside the chloroplast, some electrons in the atoms of chlorophyll molecule gain light energy.
b. These electrons are shifted up from their low energy levels to higher ones. So, the kinetic light energy is stored as potential chemical energy in the chlorophyll. Molecules of chlorophyll are therefore said to be in an excited or activated state.
c. When the stored energy is released, the electrons fall once more to the lower energy levels. So, The chlorophyll returns to the stable state ready to receive another influx of light, to become excited once more.
d. Part of the energy stored in chlorophyll is used in splitting up water molecules into Hydrogen and Oxygen.
e. Another part of the energy of the excited chlorophyll is stored in ATP (Adenosine Tri-Phosphate) molecules which are called the energy currency in living cells {ATP is the result of combination of ADP (Adenosine Di-Phosphate) molecule, a phosphate group, and an amount of the released energy by means of high-energy bond, which is marked by a squiggle}:
A
DP
+ P + energy ATP
f. Hydrogen resulting from decomposition of the water molecules combines with a co-enzyme present in the chloroplast which is called NADP which is a Hydrogen receptor (Nicotinamide dinucleotide phosphate) to give NADPH2. In this way Hydrogen will not escape or recombine with Oxygen once more.
g. Oxygen releases as a bi-product for the decomposition of water.
Light
reactions
Melvin
Calvin Experiment
To reveal the nature of
Dark reactions
2. Dark reactions of Photosynthesis:
A group of reactions that takes place in the stroma of the chloroplast outside the grana, in which:
Hydrogen carried on NADPH2 is used to fix CO2 gas into carbohydrates with the help of the energy stored in ATP molecules.
Melvin Calvin and his associates in California University revealed together the nature of dark reactions by using the radio-active isotope C14 :
a. They placed the chlorella alga in the apparatus shown, and supplied it with CO2 gas containing radio-active C14
b. A lamp was lighted very briefly in order for photosynthesis to take place.
c. The chlorella alga is then immersed in a beaker containing hot alcohol to kill the alga and to stop the biochemical reactions.
d. They separate the product of photosynthesis by special means. And they tested the presence of radio-active C14 in these products.
The results:
When photosynthesis is proceeded with the briefest flash possible of light, the 1st. stable compound to be produced is PGAL ( Phospho-glyceraldehyde ) which is a 3-carbon compound. PGAL acts as crossroads in the metabolic network, from which glucose, starch, proteins, and fats are formed. It can be utilized in the cellular respiration. The Hexose’s sugar is formed throughout several enzymatic intermediate reactions.