Light and dark reactions of the photosynthesis process

In Blackman’s experiments on the limiting factors of photosynthesis , He concluded that photosynthesis consists of two kinds of reactions which are Light reactions ( light acts as the limiting factor for the rate of photosynthesis ) , Dark reactions or enzymatic reactions ( temperature is the limiting factor for the rate of photosynthesis ) .

Light reactions

Light reactions is a group of reactions that occurs in the granum inside the green plastid , as it contains the chlorophyll pigments and the light is the limiting factor for the rate of photosynthesis .

Steps of light reactions

Light falls on the chlorophyll of grana inside the chloroplast , some electrons in the atoms of chlorophyll will gain energy , these excited electrons are shifted up from their low-energy levels to the higher ones .

The kinetic light energy is stored as a potential chemical energy in the chlorophyll , so , the chlorophyll molecules are said to be in an excited state or activated state .

When the stored potential chemical energy is released , the electrons fall once again to the lower energy levels and the chlorophyll will return to the stable state , ready for another influence of light to become excited once more .



Part of the energy released from the chlorophyll is used in splitting up the water molecule into hydrogen and oxygen , where : hydrogen combines with a co-enzyme present in the chloroplast ( NADP ) to give NADPH2 , in this way hydrogen will not escape or recombine with oxygen again , Oxygen is released as a secondary product .

The other part of energy from the excited chlorophyll is stored in ATP molecule ( energy currency in the cell ) by the combination of ADP molecule ( which is present in chloroplast ) with a phosphate group ( P ) by means of a high-energy bond marked by a squiggle ( ∼ ) , this process is called photosynthetic phosphorylation .


Adenosine − P ∼P + PAdenosine − P∼P ∼ P 

ADP : Adenosine DiPhosphate .

ATP : Adenosine TriPhosphate that carries the energy to the dark reactions .

NADP : Nicotinamide Adenine Dinucleotide Phosphate which acts as a hydrogen receptor .

Dark reactions

Dark reactions ( Enzymatic reactions ) is a group of reactions that occurs in the stroma of chloroplast in which the temperature is the limiting factor for the rate of photosynthesis , so , these reactions can occur in both the light and darkness .

In these reactions , hydrogen carried on NADPH2 is used to fix CO2 gas by reducing it into carbohydrates with the help of energy stored in ATP molecules .

Malvin Calvin and his associates at California university revealed in 1949 , the nature of dark reactions by using the newly discovered radioactive isotope of carbon 14C .

Malvin Calvin experiment

steps :

They placed the Chlorella alga in the apparatus .

They supplied the alga with CO2 gas that is containing the radioactive carbon 14C .

The apparatus was exposed to a very brief light of lamp to allow the photosynthesis process to take place .

Chlorella was then immersed in a beaker containing hot alcohol to kill the protoplasm by stopping its biochemical reactions .

They manged to separate the products of photosynthesis by special means and tested for the radioactive carbon in these compounds .

Results :

A 3-carbon compound was formed which is PGAL , phosphoglyceraldehyde , This is :

  • The first stable compound produced from photosynthesis .
  • Used in building glucose , starch , proteins and fats .
  • Used in the cellular respiration as a high-energy compound .

Calvin also pointed out that the synthesis of hexose sugar ( glucose ) is not completed in one step , but through out several intermediate reactions catalyzed by certain specific enzymes .

Structure of leaf & green plastid , Mechanism of photosynthesis in green plants

You may also like...

Leave a Reply

Your email address will not be published. Required fields are marked *