Question

Describe the main chemical changes which occur during photosynthesis in:
(i) Light reaction
(ii) Dark reaction.

Answer 1

(i) In this phase, light plays a key role. A series of chemical reactions occur in very quick succession, initiated by light and therefore, the phase is called the photochemical phase.
The light reaction occurs in two main steps.
Step 1:- Activation of chlorophyll. The chlorophyll on exposure to light energy becomes activated by absorbing photons.
Step 2:- Splitting of water. The absorbed energy is used in splitting the water molecule $\left(H_{2}O\right)$ into its two components (Hydrogen and Oxygen) and releasing electrons.
$2H_{2}O\stackrel{\text{energy of 4 photons}}{\to }4H^{+}+4e^{-}+O_2$
The reaction is known as photolysis.
(a) The hydrogen ions $\left(H^{+}\right)$ are picked up by a compound NADP (Nicotinamide adenine dinucleotide phosphate) to form NADPH.
$NAD{P}^{+}+e^{-}+H^{+}\stackrel{enzyme}{\to }NADPH$

(b) The oxygen $\left(O\right)$ component is given out as molecular oxygen $\left(O_2\right)$.

$2O\to O_2$.

(C) The electrons $\left(e^{-}\right)$ are used in converting ADP (adenosine diphosphate) into energy-rich compound ATP (adenosine triphosphate) by adding one phosphate group $P_i$ (inorganic phosphate)

$ADP+Phosphate\to ATP$

(Inorganic)

(ii) The reaction is the phase do not require light energy and occur simultaneously with the light reaction (time gap between the two being less than even one-thousandth of a second)

The dark reaction takes place in three major steps:

1. Carboxylation: RuBP is carboxylated to 3-phosphoglycerate.(12 molecules)

2. Reduction: Phosphoglycerate is reduced to Glyceraldehyde - 3- phosphate (12 molecules)

3. 2 molecules of Glyceraldehyde - 3- phosphate forms glucose

4. Regeneration: 10 molecules of Glyceraldehyde - 3-phosphate is used for regenerating RuBP molecules.